Jean Schils, MD

Sarcomas are rare neoplasms arising from connective tissue elements of the body. Approximately 80% arise in soft tissue, while the remainder originate in bone.¹ Soft-tissue sarcomas are tumors of the mesenchymal system, and about half develop in the extremities. Bone sarcomas are characterized by their location in bone and sometimes produce osteoid, or immature bone.

The most common types of soft-tissue sarcomas are malignant fibrous histiocytoma (although this term has lost favor with some pathologists) and liposarcoma.

The most common types of bone sarcomas are osteosarcoma (a sarcoma that makes osteoid or bone), Ewing sarcoma (composed of small round blue cells with a characteristic chromosomal translocation), and chondrosarcoma (a sarcoma that makes chondroid tissue, or cartilage).

EPIDEMIOLOGY AND PRIMARY BODY SITES

Approximately 10,600 new cases of soft-tissue sarcoma and 2,570 new cases of bone sarcoma were estimated to have been diagnosed in the United States in 2009.^2,3 For perspective, the annual incidence of soft-tissue sarcoma is approximately 5.5% that of breast cancer and approximately 5% that of lung cancer.³

Most sarcomas develop in the lower extremities, although the age groups at greatest risk vary among sarcoma types.⁴

Soft-tissue sarcomas develop most commonly in the thigh and occur primarily in adults.²

Osteosarcomas are the most common bone sarcoma and develop most frequently in 10- to 20-year-olds; their most common location is the distal femur.^5–8 Metastatic osteosarcoma is found in approximately 20% of patients at the time of osteosarcoma diagnosis. Osteosarcomas mainly spread hematogenously, and the lungs are the most common initial site of metastases, being affected in up to 90% of patients with metastatic disease.⁹

Ewing sarcomas develop most often in the long bones of the extremities or bones of the pelvis. The large majority of cases develop in patients aged 10 to 15 years. ^5–8

Chondrosarcomas represent approximately 20% of all bone sarcomas and primarily affect older adults, with a peak incidence in the sixth decade of life.¹⁰

OVERVIEW OF PRESENTATION AND EVALUATION

Presentation is highly variable

The clinical presentation of patients with bone or soft-tissue sarcoma is highly variable. Patients often present with a mass, typically one that is increasing in size. In general, bone sarcomas are painful and soft-tissue sarcomas are not, but there are exceptions to this general rule. Constitutional symptoms are rare in patients with bone or soft-tissue sarcomas, but symptoms such as fever, malaise, and weight loss can be seen, especially in patients with Ewing sarcoma.¹¹

Delayed presentation and diagnosis are common

Particularly when a sarcoma is painless, patients sometimes do not seek medical attention until a suspicious mass becomes quite large. Certain tumors, such as synovial sarcoma, a high-grade soft-tissue sarcoma often seen in young adults, may present as a slowly growing or stable-appearing mass over several years. In one study of 33 children with synovial sarcoma, the mean duration of symptoms was 98 weeks (range, 2–364), the mean patient delay before a doctor was seen was 43 weeks (0–156), the mean doctor delay before a correct diagnosis was made was 50 weeks (0–362), and the mean number of doctors seen before referral was 3 (1–6).¹² For nearly half the patients in this study (15), the diagnosis was obtained only after unplanned excision, meaning that the surgeon did not expect a malignancy at the time of biopsy. Because delayed presentation is not uncommon in cases of bone or soft-tissue sarcoma, every patient with a mass with indeterminate imaging findings should be referred to or reviewed by an orthopedic or musculoskeletal oncologist.

Biopsy is gold standard for diagnosis

A comprehensive medical history and physical examination are essential at the initial presentation of patients with masses and/or pain suggestive of bone or soft-tissue sarcoma. Sarcoma simulators such as hematoma, metastatic disease, or infection can sometimes be ruled out by careful clinical examination, laboratory work-up, and appropriate imaging, but the gold standard for diagnosis is a biopsy. Moreover, an index of suspicion is required to rule out primary malignancy in any soft-tissue or bone lesion, and this index of suspicion will allow for referral or appropriate selection of the site for biopsy.

Biopsy considerations, as well as further detail on clinical presentation, are provided in the second and third articles in this supplement, which focus, respectively, on bone sarcoma and soft-tissue sarcoma. The remainder of this article reviews the use of imaging for the evaluation of suspected sarcomas, as imaging findings typically prompt or guide biopsy of a suspicious mass. Choosing the right imaging modality is critical to the diagnosis and management of patients with suspected sarcoma.

CONVENTIONAL IMAGING MODALITIES

Radiography. Despite recent advances in cross-sectional imaging with computed tomography (CT) and magnetic resonance imaging (MRI), radiographs remain valuable for characterizing osseous lesions and enabling accurate differential diagnosis of benign and malignant tumors (Figure 1). Multiple lytic lesions in an adult older than 40 years almost always suggest metastases or multiple myeloma.¹³ Bone sarcomas are rare, and the most common cause of a solitary destructive lesion in an adult is metastasis.

Despite their utility for evaluating osseous lesions, radiographs have limited to no value in the evaluation of soft-tissue sarcomas but can demonstrate matrix mineralization and erosion or destruction of adjacent bone.

Angiography. In the past, angiography was frequently used to assess the vascularity of sarcomas preoperatively. Diagnostic angiography has been replaced by conventional MRI and magnetic resonance angiography, but some vascular sarcomas may require presurgical embolization to prevent excessive bleeding during surgery.

Radionuclide bone scans have long been a reliable tool for detecting multifocal or disseminated osseous lesions and remain the mainstay for evaluation of osseous metastases. They also are helpful in identifying skip lesions of osteosarcoma (ie, smaller discrete foci of osteosarcoma occurring in the same bone or on the opposing side of a joint).¹⁴ Advantages of this modality include whole-body scanning and low radiation at relatively low cost. Radionuclide bone scans demonstrate areas of bony repair and thus could be negative in purely lytic/destructive processes such as renal cell carcinoma metastases and multiple myeloma.

Chest radiographs are typically obtained in the initial stages of patient evaluation and are helpful in demonstrating large nodules or masses resulting from metastatic disease. In a patient with known sarcoma, a negative or equivocal chest radiograph should be followed by chest CT to definitively assess for metastasis.

 

 

CROSS-SECTIONAL IMAGING WITH MRI AND CT

MRI preferred for evaluation of most masses

MRI is the examination of choice in the evaluation of soft-tissue masses in light of its superior contrast resolution and ability to demonstrate subtle changes in soft tissues.

Predicting the histology of most soft-tissue masses is difficult, with the exception of some benign vascular lesions (eg, hemangioma), ganglia, neurogenic lesions, and well-differentiated lipomatous lesions. Aggressive features of a soft-tissue neoplasm include size greater than 5 cm,¹⁵ deep location, and absence of central enhancement, which is suggestive of necrosis (Figure 1). Yet one third of soft-tissue sarcomas are either superficial or smaller than 5 cm, which highlights the relative nonspecificity of these features.¹⁵

A pseudocapsule or capsule is commonly seen around a sarcoma in the soft tissues, producing well-defined margins on MRI that do not reflect benignity (Figure 2). MRI may show T2-weighted signal changes surrounding soft-tissue sarcomas. These abnormalities are considered to represent peritumoral edema. Tumor cells beyond the sarcoma margin were found within areas of peritumoral edema seen on preoperative MRI in 9 of a series of 15 cases.¹⁶ Hematomas and abscesses typically have marked edema of surrounding soft tissue (Figure 3) and no internal enhancement after contrast injection. If MRI features of a suspected hematoma are equivocal, a follow-up study is warranted to exclude hemorrhagic sarcoma.

Nowadays, CT of primary soft-tissue tumors is rarely performed. Rare exceptions are patients with a contraindication to MRI (eg, presence of cardiac pacemaker) or for detecting subtle matrix mineral, if it will alter management.

MRI is also the preferred modality in the evaluation of the majority of bone sarcomas, given its ability to accurately define the extent of marrow changes and soft-tissue involvement. MRI should be performed prior to a biopsy to prevent misinterpretation of biopsy-related signal changes in the surrounding tissues, which may negate the value of MRI in sarcoma staging.

Several distinct roles for CT

CT is better at detecting subtle matrix mineral and for establishing the presence or absence of a thin rim of cortex around expansive lesions or masses containing calcific densities—findings that may have a bearing on the differential diagnosis (Figure 4). Abdominal and chest CT remain particularly important tools in the staging process. CT is critical for evaluation of pulmonary nodules.

Chest CT should be obtained in all cases of known malignant neoplasms to evaluate for pulmonary nodules, masses, and lymphadenopathy. Despite the recent advances in MRI, CT remains the imaging modality of choice to evaluate the retroperitoneum, abdomen, and pelvis for masses, lymphadenopathy, or other signs of metastatic disease.

Post-treatment monitoring for recurrence

After definitive treatment, MRI remains the most sensitive imaging tool for diagnosing local recurrences of either bone or soft-tissue sarcomas. After surgery, hematomas or seromas often present as palpable soft-tissue masses and are easily diagnosed with contrast MRI (Figure 5). Baseline examination is critical for visualizing the postoperative distortion of tissues. The presence of metallic hardware in postsurgical patients is not an absolute contraindication, and artifacts may be minimized by new MRI techniques (Figure 6).¹⁷

ULTRASONOGRAPHY

Ultrasonography has a limited role in the initial diagnosis and follow-up of musculoskeletal tumors. Its main advantages are a lack of ionizing radiation and dynamic imaging capabilities. Doppler ultrasonography allows direct visualization of tumor vascularity, which may be important for diagnosis and presurgical planning. Unfortunately, bone lesions cannot be evaluated with ultrasonography, owing to the inability of sound waves to penetrate the bony cortex. Poor sound wave penetration may prevent visualization of deep-seated lesions, such as retroperitoneal sarcomas.

Ultrasonography is best used for differentiating solid masses from cystic structures and can provide image guidance in solid tumor biopsy and cyst aspiration. It also may play a role in detecting suspected tumor recurrence in patients in whom artifact from implanted hardware precludes cross-sectional imaging, and it can be reliably used for following up unequivocal soft-tissue masses such as ganglia near joints.

POSITRON EMISSION TOMOGRAPHY

Positron emission tomography (PET) is a functional imaging technique that allows evaluation of tumor or physiologic tissue metabolism in vivo with positron-emitting radionuclides. [¹⁸F]2-Deoxy-2-fluoro-d-glucose (FDG), the most commonly used radiolabeled tracer for PET imaging, demonstrates increased accumulation in several different types of benign and malignant neoplasms. PET/CT scanning of lesions offers a combination of concurrent anatomical and functional imaging findings (Figure 7A), and PET/CT scanner sales now account for more than 80% of the PET scanner market.¹⁸ In the setting of sarcoma, FDG-PET is useful for detection of local recurrence and metastatic disease¹⁹ as well as for evaluation of response to neoadjuvant chemotherapy.²⁰ Some initial studies have examined the ability of ¹⁸F-FDG-PET to differentiate between benign and malignant bone and soft-tissue tumors,²¹ which has been controversial because of the considerable number of false-positive and false-negative interpretations of lesions.¹⁹ In addition to benign tumors, non-neoplastic findings such as fractures (Figure 7), sarcoid, synovial chondromatosis, pigmented villonodular synovitis, and infection may demonstrate the maximum standardized uptake value (SUV_max) levels that are characteristic of  malignant tumors.¹⁹

IMAGING-GUIDED INTERVENTIONS

Percutaneous imaging-guided procedures have increasingly replaced open surgical biopsies for bone and soft-tissue tumors. CT guidance is commonly used for percutaneous biopsy, whereas ultrasonographic guidance is sometimes used for superficial soft-tissue lesions. Although the shortest and most direct approach is desirable, this may not be possible in all cases due to the presence of nearby vital structures or the risk of contamination. Seeding of malignant cells along the biopsy tract is a well-known possible complication of image-guided biopsies, and en bloc resection of the needle tract is typically performed at the definitive surgery.

Knowledge of compartmental anatomy is paramount in planning the approach for these biopsies, and consultation with the referring orthopedic surgeon is recommended for optimal management. Expert histopathological interpretation of bone and soft-tissue specimens is essential for the efficacy and high success rates of percutaneous imaging-guided biopsies. Such expertise is integral to the broader interdisciplinary collaboration that is needed to arrive at the most plausible diagnosis, especially in the setting of uncommon or atypical neoplasms.

Currently, MRI-guided interventions are in the initial stage of evolution and could provide valuable guidance for subtle marrow or soft-tissue lesions visible on MRI but not well seen on CT.²² In the future, MRI could play an increasingly important role in imaging-guided procedures because of its lack of ionizing radiation and its ability to demonstrate subtle soft-tissue and bone marrow changes. Imaging-guided therapeutics are growing in their applications in musculoskeletal oncology. CT-guided radiofrequency ablation and cryoablation have been used in the treatment of a variety of tumors²³ as well as in the palliation of metastatic bone pain.²⁴

SUMMARY AND CONCLUSION

Bone and soft-tissue sarcomas are rare neoplasms with variable clinical presentations. A high index of suspicion is required for any unexplained mass with indeterminate imaging findings. Recent advances in imaging technology, including cross-sectional MRI and CT, have significantly refined the diagnosis and management of bone and soft-tissue sarcomas. When faced with a possible sarcoma, the clinician’s selection of imaging modalities has a direct impact on diagnosis, staging, and patient management.

Sarcomas are rare neoplasms arising from connective tissue elements of the body. Approximately 80% arise in soft tissue, while the remainder originate in bone.¹ Soft-tissue sarcomas are tumors of the mesenchymal system, and about half develop in the extremities. Bone sarcomas are characterized by their location in bone and sometimes produce osteoid, or immature bone.

The most common types of soft-tissue sarcomas are malignant fibrous histiocytoma (although this term has lost favor with some pathologists) and liposarcoma.

The most common types of bone sarcomas are osteosarcoma (a sarcoma that makes osteoid or bone), Ewing sarcoma (composed of small round blue cells with a characteristic chromosomal translocation), and chondrosarcoma (a sarcoma that makes chondroid tissue, or cartilage).

EPIDEMIOLOGY AND PRIMARY BODY SITES

Approximately 10,600 new cases of soft-tissue sarcoma and 2,570 new cases of bone sarcoma were estimated to have been diagnosed in the United States in 2009.^2,3 For perspective, the annual incidence of soft-tissue sarcoma is approximately 5.5% that of breast cancer and approximately 5% that of lung cancer.³

Most sarcomas develop in the lower extremities, although the age groups at greatest risk vary among sarcoma types.⁴

Soft-tissue sarcomas develop most commonly in the thigh and occur primarily in adults.²

Osteosarcomas are the most common bone sarcoma and develop most frequently in 10- to 20-year-olds; their most common location is the distal femur.^5–8 Metastatic osteosarcoma is found in approximately 20% of patients at the time of osteosarcoma diagnosis. Osteosarcomas mainly spread hematogenously, and the lungs are the most common initial site of metastases, being affected in up to 90% of patients with metastatic disease.⁹

Ewing sarcomas develop most often in the long bones of the extremities or bones of the pelvis. The large majority of cases develop in patients aged 10 to 15 years. ^5–8

Chondrosarcomas represent approximately 20% of all bone sarcomas and primarily affect older adults, with a peak incidence in the sixth decade of life.¹⁰

OVERVIEW OF PRESENTATION AND EVALUATION

Presentation is highly variable

The clinical presentation of patients with bone or soft-tissue sarcoma is highly variable. Patients often present with a mass, typically one that is increasing in size. In general, bone sarcomas are painful and soft-tissue sarcomas are not, but there are exceptions to this general rule. Constitutional symptoms are rare in patients with bone or soft-tissue sarcomas, but symptoms such as fever, malaise, and weight loss can be seen, especially in patients with Ewing sarcoma.¹¹

Delayed presentation and diagnosis are common

Particularly when a sarcoma is painless, patients sometimes do not seek medical attention until a suspicious mass becomes quite large. Certain tumors, such as synovial sarcoma, a high-grade soft-tissue sarcoma often seen in young adults, may present as a slowly growing or stable-appearing mass over several years. In one study of 33 children with synovial sarcoma, the mean duration of symptoms was 98 weeks (range, 2–364), the mean patient delay before a doctor was seen was 43 weeks (0–156), the mean doctor delay before a correct diagnosis was made was 50 weeks (0–362), and the mean number of doctors seen before referral was 3 (1–6).¹² For nearly half the patients in this study (15), the diagnosis was obtained only after unplanned excision, meaning that the surgeon did not expect a malignancy at the time of biopsy. Because delayed presentation is not uncommon in cases of bone or soft-tissue sarcoma, every patient with a mass with indeterminate imaging findings should be referred to or reviewed by an orthopedic or musculoskeletal oncologist.

Biopsy is gold standard for diagnosis

A comprehensive medical history and physical examination are essential at the initial presentation of patients with masses and/or pain suggestive of bone or soft-tissue sarcoma. Sarcoma simulators such as hematoma, metastatic disease, or infection can sometimes be ruled out by careful clinical examination, laboratory work-up, and appropriate imaging, but the gold standard for diagnosis is a biopsy. Moreover, an index of suspicion is required to rule out primary malignancy in any soft-tissue or bone lesion, and this index of suspicion will allow for referral or appropriate selection of the site for biopsy.

Biopsy considerations, as well as further detail on clinical presentation, are provided in the second and third articles in this supplement, which focus, respectively, on bone sarcoma and soft-tissue sarcoma. The remainder of this article reviews the use of imaging for the evaluation of suspected sarcomas, as imaging findings typically prompt or guide biopsy of a suspicious mass. Choosing the right imaging modality is critical to the diagnosis and management of patients with suspected sarcoma.

CONVENTIONAL IMAGING MODALITIES

Radiography. Despite recent advances in cross-sectional imaging with computed tomography (CT) and magnetic resonance imaging (MRI), radiographs remain valuable for characterizing osseous lesions and enabling accurate differential diagnosis of benign and malignant tumors (Figure 1). Multiple lytic lesions in an adult older than 40 years almost always suggest metastases or multiple myeloma.¹³ Bone sarcomas are rare, and the most common cause of a solitary destructive lesion in an adult is metastasis.

Despite their utility for evaluating osseous lesions, radiographs have limited to no value in the evaluation of soft-tissue sarcomas but can demonstrate matrix mineralization and erosion or destruction of adjacent bone.

Angiography. In the past, angiography was frequently used to assess the vascularity of sarcomas preoperatively. Diagnostic angiography has been replaced by conventional MRI and magnetic resonance angiography, but some vascular sarcomas may require presurgical embolization to prevent excessive bleeding during surgery.

Radionuclide bone scans have long been a reliable tool for detecting multifocal or disseminated osseous lesions and remain the mainstay for evaluation of osseous metastases. They also are helpful in identifying skip lesions of osteosarcoma (ie, smaller discrete foci of osteosarcoma occurring in the same bone or on the opposing side of a joint).¹⁴ Advantages of this modality include whole-body scanning and low radiation at relatively low cost. Radionuclide bone scans demonstrate areas of bony repair and thus could be negative in purely lytic/destructive processes such as renal cell carcinoma metastases and multiple myeloma.

Chest radiographs are typically obtained in the initial stages of patient evaluation and are helpful in demonstrating large nodules or masses resulting from metastatic disease. In a patient with known sarcoma, a negative or equivocal chest radiograph should be followed by chest CT to definitively assess for metastasis.

 

 

CROSS-SECTIONAL IMAGING WITH MRI AND CT

MRI preferred for evaluation of most masses

MRI is the examination of choice in the evaluation of soft-tissue masses in light of its superior contrast resolution and ability to demonstrate subtle changes in soft tissues.

Predicting the histology of most soft-tissue masses is difficult, with the exception of some benign vascular lesions (eg, hemangioma), ganglia, neurogenic lesions, and well-differentiated lipomatous lesions. Aggressive features of a soft-tissue neoplasm include size greater than 5 cm,¹⁵ deep location, and absence of central enhancement, which is suggestive of necrosis (Figure 1). Yet one third of soft-tissue sarcomas are either superficial or smaller than 5 cm, which highlights the relative nonspecificity of these features.¹⁵

A pseudocapsule or capsule is commonly seen around a sarcoma in the soft tissues, producing well-defined margins on MRI that do not reflect benignity (Figure 2). MRI may show T2-weighted signal changes surrounding soft-tissue sarcomas. These abnormalities are considered to represent peritumoral edema. Tumor cells beyond the sarcoma margin were found within areas of peritumoral edema seen on preoperative MRI in 9 of a series of 15 cases.¹⁶ Hematomas and abscesses typically have marked edema of surrounding soft tissue (Figure 3) and no internal enhancement after contrast injection. If MRI features of a suspected hematoma are equivocal, a follow-up study is warranted to exclude hemorrhagic sarcoma.

Nowadays, CT of primary soft-tissue tumors is rarely performed. Rare exceptions are patients with a contraindication to MRI (eg, presence of cardiac pacemaker) or for detecting subtle matrix mineral, if it will alter management.

MRI is also the preferred modality in the evaluation of the majority of bone sarcomas, given its ability to accurately define the extent of marrow changes and soft-tissue involvement. MRI should be performed prior to a biopsy to prevent misinterpretation of biopsy-related signal changes in the surrounding tissues, which may negate the value of MRI in sarcoma staging.

Several distinct roles for CT

CT is better at detecting subtle matrix mineral and for establishing the presence or absence of a thin rim of cortex around expansive lesions or masses containing calcific densities—findings that may have a bearing on the differential diagnosis (Figure 4). Abdominal and chest CT remain particularly important tools in the staging process. CT is critical for evaluation of pulmonary nodules.

Chest CT should be obtained in all cases of known malignant neoplasms to evaluate for pulmonary nodules, masses, and lymphadenopathy. Despite the recent advances in MRI, CT remains the imaging modality of choice to evaluate the retroperitoneum, abdomen, and pelvis for masses, lymphadenopathy, or other signs of metastatic disease.

Post-treatment monitoring for recurrence

After definitive treatment, MRI remains the most sensitive imaging tool for diagnosing local recurrences of either bone or soft-tissue sarcomas. After surgery, hematomas or seromas often present as palpable soft-tissue masses and are easily diagnosed with contrast MRI (Figure 5). Baseline examination is critical for visualizing the postoperative distortion of tissues. The presence of metallic hardware in postsurgical patients is not an absolute contraindication, and artifacts may be minimized by new MRI techniques (Figure 6).¹⁷

ULTRASONOGRAPHY

Ultrasonography has a limited role in the initial diagnosis and follow-up of musculoskeletal tumors. Its main advantages are a lack of ionizing radiation and dynamic imaging capabilities. Doppler ultrasonography allows direct visualization of tumor vascularity, which may be important for diagnosis and presurgical planning. Unfortunately, bone lesions cannot be evaluated with ultrasonography, owing to the inability of sound waves to penetrate the bony cortex. Poor sound wave penetration may prevent visualization of deep-seated lesions, such as retroperitoneal sarcomas.

Ultrasonography is best used for differentiating solid masses from cystic structures and can provide image guidance in solid tumor biopsy and cyst aspiration. It also may play a role in detecting suspected tumor recurrence in patients in whom artifact from implanted hardware precludes cross-sectional imaging, and it can be reliably used for following up unequivocal soft-tissue masses such as ganglia near joints.

POSITRON EMISSION TOMOGRAPHY

Positron emission tomography (PET) is a functional imaging technique that allows evaluation of tumor or physiologic tissue metabolism in vivo with positron-emitting radionuclides. [¹⁸F]2-Deoxy-2-fluoro-d-glucose (FDG), the most commonly used radiolabeled tracer for PET imaging, demonstrates increased accumulation in several different types of benign and malignant neoplasms. PET/CT scanning of lesions offers a combination of concurrent anatomical and functional imaging findings (Figure 7A), and PET/CT scanner sales now account for more than 80% of the PET scanner market.¹⁸ In the setting of sarcoma, FDG-PET is useful for detection of local recurrence and metastatic disease¹⁹ as well as for evaluation of response to neoadjuvant chemotherapy.²⁰ Some initial studies have examined the ability of ¹⁸F-FDG-PET to differentiate between benign and malignant bone and soft-tissue tumors,²¹ which has been controversial because of the considerable number of false-positive and false-negative interpretations of lesions.¹⁹ In addition to benign tumors, non-neoplastic findings such as fractures (Figure 7), sarcoid, synovial chondromatosis, pigmented villonodular synovitis, and infection may demonstrate the maximum standardized uptake value (SUV_max) levels that are characteristic of  malignant tumors.¹⁹

IMAGING-GUIDED INTERVENTIONS

Percutaneous imaging-guided procedures have increasingly replaced open surgical biopsies for bone and soft-tissue tumors. CT guidance is commonly used for percutaneous biopsy, whereas ultrasonographic guidance is sometimes used for superficial soft-tissue lesions. Although the shortest and most direct approach is desirable, this may not be possible in all cases due to the presence of nearby vital structures or the risk of contamination. Seeding of malignant cells along the biopsy tract is a well-known possible complication of image-guided biopsies, and en bloc resection of the needle tract is typically performed at the definitive surgery.

Knowledge of compartmental anatomy is paramount in planning the approach for these biopsies, and consultation with the referring orthopedic surgeon is recommended for optimal management. Expert histopathological interpretation of bone and soft-tissue specimens is essential for the efficacy and high success rates of percutaneous imaging-guided biopsies. Such expertise is integral to the broader interdisciplinary collaboration that is needed to arrive at the most plausible diagnosis, especially in the setting of uncommon or atypical neoplasms.

Currently, MRI-guided interventions are in the initial stage of evolution and could provide valuable guidance for subtle marrow or soft-tissue lesions visible on MRI but not well seen on CT.²² In the future, MRI could play an increasingly important role in imaging-guided procedures because of its lack of ionizing radiation and its ability to demonstrate subtle soft-tissue and bone marrow changes. Imaging-guided therapeutics are growing in their applications in musculoskeletal oncology. CT-guided radiofrequency ablation and cryoablation have been used in the treatment of a variety of tumors²³ as well as in the palliation of metastatic bone pain.²⁴

SUMMARY AND CONCLUSION

Bone and soft-tissue sarcomas are rare neoplasms with variable clinical presentations. A high index of suspicion is required for any unexplained mass with indeterminate imaging findings. Recent advances in imaging technology, including cross-sectional MRI and CT, have significantly refined the diagnosis and management of bone and soft-tissue sarcomas. When faced with a possible sarcoma, the clinician’s selection of imaging modalities has a direct impact on diagnosis, staging, and patient management.

Sarcomas are rare neoplasms arising from connective tissue elements of the body. Approximately 80% arise in soft tissue, while the remainder originate in bone.¹ Soft-tissue sarcomas are tumors of the mesenchymal system, and about half develop in the extremities. Bone sarcomas are characterized by their location in bone and sometimes produce osteoid, or immature bone.

The most common types of soft-tissue sarcomas are malignant fibrous histiocytoma (although this term has lost favor with some pathologists) and liposarcoma.

The most common types of bone sarcomas are osteosarcoma (a sarcoma that makes osteoid or bone), Ewing sarcoma (composed of small round blue cells with a characteristic chromosomal translocation), and chondrosarcoma (a sarcoma that makes chondroid tissue, or cartilage).

EPIDEMIOLOGY AND PRIMARY BODY SITES

Approximately 10,600 new cases of soft-tissue sarcoma and 2,570 new cases of bone sarcoma were estimated to have been diagnosed in the United States in 2009.^2,3 For perspective, the annual incidence of soft-tissue sarcoma is approximately 5.5% that of breast cancer and approximately 5% that of lung cancer.³

Most sarcomas develop in the lower extremities, although the age groups at greatest risk vary among sarcoma types.⁴

Soft-tissue sarcomas develop most commonly in the thigh and occur primarily in adults.²

Osteosarcomas are the most common bone sarcoma and develop most frequently in 10- to 20-year-olds; their most common location is the distal femur.^5–8 Metastatic osteosarcoma is found in approximately 20% of patients at the time of osteosarcoma diagnosis. Osteosarcomas mainly spread hematogenously, and the lungs are the most common initial site of metastases, being affected in up to 90% of patients with metastatic disease.⁹

Ewing sarcomas develop most often in the long bones of the extremities or bones of the pelvis. The large majority of cases develop in patients aged 10 to 15 years. ^5–8

Chondrosarcomas represent approximately 20% of all bone sarcomas and primarily affect older adults, with a peak incidence in the sixth decade of life.¹⁰

OVERVIEW OF PRESENTATION AND EVALUATION

Presentation is highly variable

The clinical presentation of patients with bone or soft-tissue sarcoma is highly variable. Patients often present with a mass, typically one that is increasing in size. In general, bone sarcomas are painful and soft-tissue sarcomas are not, but there are exceptions to this general rule. Constitutional symptoms are rare in patients with bone or soft-tissue sarcomas, but symptoms such as fever, malaise, and weight loss can be seen, especially in patients with Ewing sarcoma.¹¹

Delayed presentation and diagnosis are common

Particularly when a sarcoma is painless, patients sometimes do not seek medical attention until a suspicious mass becomes quite large. Certain tumors, such as synovial sarcoma, a high-grade soft-tissue sarcoma often seen in young adults, may present as a slowly growing or stable-appearing mass over several years. In one study of 33 children with synovial sarcoma, the mean duration of symptoms was 98 weeks (range, 2–364), the mean patient delay before a doctor was seen was 43 weeks (0–156), the mean doctor delay before a correct diagnosis was made was 50 weeks (0–362), and the mean number of doctors seen before referral was 3 (1–6).¹² For nearly half the patients in this study (15), the diagnosis was obtained only after unplanned excision, meaning that the surgeon did not expect a malignancy at the time of biopsy. Because delayed presentation is not uncommon in cases of bone or soft-tissue sarcoma, every patient with a mass with indeterminate imaging findings should be referred to or reviewed by an orthopedic or musculoskeletal oncologist.

Biopsy is gold standard for diagnosis

A comprehensive medical history and physical examination are essential at the initial presentation of patients with masses and/or pain suggestive of bone or soft-tissue sarcoma. Sarcoma simulators such as hematoma, metastatic disease, or infection can sometimes be ruled out by careful clinical examination, laboratory work-up, and appropriate imaging, but the gold standard for diagnosis is a biopsy. Moreover, an index of suspicion is required to rule out primary malignancy in any soft-tissue or bone lesion, and this index of suspicion will allow for referral or appropriate selection of the site for biopsy.

Biopsy considerations, as well as further detail on clinical presentation, are provided in the second and third articles in this supplement, which focus, respectively, on bone sarcoma and soft-tissue sarcoma. The remainder of this article reviews the use of imaging for the evaluation of suspected sarcomas, as imaging findings typically prompt or guide biopsy of a suspicious mass. Choosing the right imaging modality is critical to the diagnosis and management of patients with suspected sarcoma.

CONVENTIONAL IMAGING MODALITIES

Radiography. Despite recent advances in cross-sectional imaging with computed tomography (CT) and magnetic resonance imaging (MRI), radiographs remain valuable for characterizing osseous lesions and enabling accurate differential diagnosis of benign and malignant tumors (Figure 1). Multiple lytic lesions in an adult older than 40 years almost always suggest metastases or multiple myeloma.¹³ Bone sarcomas are rare, and the most common cause of a solitary destructive lesion in an adult is metastasis.

Despite their utility for evaluating osseous lesions, radiographs have limited to no value in the evaluation of soft-tissue sarcomas but can demonstrate matrix mineralization and erosion or destruction of adjacent bone.

Angiography. In the past, angiography was frequently used to assess the vascularity of sarcomas preoperatively. Diagnostic angiography has been replaced by conventional MRI and magnetic resonance angiography, but some vascular sarcomas may require presurgical embolization to prevent excessive bleeding during surgery.

Radionuclide bone scans have long been a reliable tool for detecting multifocal or disseminated osseous lesions and remain the mainstay for evaluation of osseous metastases. They also are helpful in identifying skip lesions of osteosarcoma (ie, smaller discrete foci of osteosarcoma occurring in the same bone or on the opposing side of a joint).¹⁴ Advantages of this modality include whole-body scanning and low radiation at relatively low cost. Radionuclide bone scans demonstrate areas of bony repair and thus could be negative in purely lytic/destructive processes such as renal cell carcinoma metastases and multiple myeloma.

Chest radiographs are typically obtained in the initial stages of patient evaluation and are helpful in demonstrating large nodules or masses resulting from metastatic disease. In a patient with known sarcoma, a negative or equivocal chest radiograph should be followed by chest CT to definitively assess for metastasis.

 

 

CROSS-SECTIONAL IMAGING WITH MRI AND CT

MRI preferred for evaluation of most masses

MRI is the examination of choice in the evaluation of soft-tissue masses in light of its superior contrast resolution and ability to demonstrate subtle changes in soft tissues.

Predicting the histology of most soft-tissue masses is difficult, with the exception of some benign vascular lesions (eg, hemangioma), ganglia, neurogenic lesions, and well-differentiated lipomatous lesions. Aggressive features of a soft-tissue neoplasm include size greater than 5 cm,¹⁵ deep location, and absence of central enhancement, which is suggestive of necrosis (Figure 1). Yet one third of soft-tissue sarcomas are either superficial or smaller than 5 cm, which highlights the relative nonspecificity of these features.¹⁵

A pseudocapsule or capsule is commonly seen around a sarcoma in the soft tissues, producing well-defined margins on MRI that do not reflect benignity (Figure 2). MRI may show T2-weighted signal changes surrounding soft-tissue sarcomas. These abnormalities are considered to represent peritumoral edema. Tumor cells beyond the sarcoma margin were found within areas of peritumoral edema seen on preoperative MRI in 9 of a series of 15 cases.¹⁶ Hematomas and abscesses typically have marked edema of surrounding soft tissue (Figure 3) and no internal enhancement after contrast injection. If MRI features of a suspected hematoma are equivocal, a follow-up study is warranted to exclude hemorrhagic sarcoma.

Nowadays, CT of primary soft-tissue tumors is rarely performed. Rare exceptions are patients with a contraindication to MRI (eg, presence of cardiac pacemaker) or for detecting subtle matrix mineral, if it will alter management.

MRI is also the preferred modality in the evaluation of the majority of bone sarcomas, given its ability to accurately define the extent of marrow changes and soft-tissue involvement. MRI should be performed prior to a biopsy to prevent misinterpretation of biopsy-related signal changes in the surrounding tissues, which may negate the value of MRI in sarcoma staging.

Several distinct roles for CT

CT is better at detecting subtle matrix mineral and for establishing the presence or absence of a thin rim of cortex around expansive lesions or masses containing calcific densities—findings that may have a bearing on the differential diagnosis (Figure 4). Abdominal and chest CT remain particularly important tools in the staging process. CT is critical for evaluation of pulmonary nodules.

Chest CT should be obtained in all cases of known malignant neoplasms to evaluate for pulmonary nodules, masses, and lymphadenopathy. Despite the recent advances in MRI, CT remains the imaging modality of choice to evaluate the retroperitoneum, abdomen, and pelvis for masses, lymphadenopathy, or other signs of metastatic disease.

Post-treatment monitoring for recurrence

After definitive treatment, MRI remains the most sensitive imaging tool for diagnosing local recurrences of either bone or soft-tissue sarcomas. After surgery, hematomas or seromas often present as palpable soft-tissue masses and are easily diagnosed with contrast MRI (Figure 5). Baseline examination is critical for visualizing the postoperative distortion of tissues. The presence of metallic hardware in postsurgical patients is not an absolute contraindication, and artifacts may be minimized by new MRI techniques (Figure 6).¹⁷

ULTRASONOGRAPHY

Ultrasonography has a limited role in the initial diagnosis and follow-up of musculoskeletal tumors. Its main advantages are a lack of ionizing radiation and dynamic imaging capabilities. Doppler ultrasonography allows direct visualization of tumor vascularity, which may be important for diagnosis and presurgical planning. Unfortunately, bone lesions cannot be evaluated with ultrasonography, owing to the inability of sound waves to penetrate the bony cortex. Poor sound wave penetration may prevent visualization of deep-seated lesions, such as retroperitoneal sarcomas.

Ultrasonography is best used for differentiating solid masses from cystic structures and can provide image guidance in solid tumor biopsy and cyst aspiration. It also may play a role in detecting suspected tumor recurrence in patients in whom artifact from implanted hardware precludes cross-sectional imaging, and it can be reliably used for following up unequivocal soft-tissue masses such as ganglia near joints.

POSITRON EMISSION TOMOGRAPHY

Positron emission tomography (PET) is a functional imaging technique that allows evaluation of tumor or physiologic tissue metabolism in vivo with positron-emitting radionuclides. [¹⁸F]2-Deoxy-2-fluoro-d-glucose (FDG), the most commonly used radiolabeled tracer for PET imaging, demonstrates increased accumulation in several different types of benign and malignant neoplasms. PET/CT scanning of lesions offers a combination of concurrent anatomical and functional imaging findings (Figure 7A), and PET/CT scanner sales now account for more than 80% of the PET scanner market.¹⁸ In the setting of sarcoma, FDG-PET is useful for detection of local recurrence and metastatic disease¹⁹ as well as for evaluation of response to neoadjuvant chemotherapy.²⁰ Some initial studies have examined the ability of ¹⁸F-FDG-PET to differentiate between benign and malignant bone and soft-tissue tumors,²¹ which has been controversial because of the considerable number of false-positive and false-negative interpretations of lesions.¹⁹ In addition to benign tumors, non-neoplastic findings such as fractures (Figure 7), sarcoid, synovial chondromatosis, pigmented villonodular synovitis, and infection may demonstrate the maximum standardized uptake value (SUV_max) levels that are characteristic of  malignant tumors.¹⁹

IMAGING-GUIDED INTERVENTIONS

Percutaneous imaging-guided procedures have increasingly replaced open surgical biopsies for bone and soft-tissue tumors. CT guidance is commonly used for percutaneous biopsy, whereas ultrasonographic guidance is sometimes used for superficial soft-tissue lesions. Although the shortest and most direct approach is desirable, this may not be possible in all cases due to the presence of nearby vital structures or the risk of contamination. Seeding of malignant cells along the biopsy tract is a well-known possible complication of image-guided biopsies, and en bloc resection of the needle tract is typically performed at the definitive surgery.

Knowledge of compartmental anatomy is paramount in planning the approach for these biopsies, and consultation with the referring orthopedic surgeon is recommended for optimal management. Expert histopathological interpretation of bone and soft-tissue specimens is essential for the efficacy and high success rates of percutaneous imaging-guided biopsies. Such expertise is integral to the broader interdisciplinary collaboration that is needed to arrive at the most plausible diagnosis, especially in the setting of uncommon or atypical neoplasms.

Currently, MRI-guided interventions are in the initial stage of evolution and could provide valuable guidance for subtle marrow or soft-tissue lesions visible on MRI but not well seen on CT.²² In the future, MRI could play an increasingly important role in imaging-guided procedures because of its lack of ionizing radiation and its ability to demonstrate subtle soft-tissue and bone marrow changes. Imaging-guided therapeutics are growing in their applications in musculoskeletal oncology. CT-guided radiofrequency ablation and cryoablation have been used in the treatment of a variety of tumors²³ as well as in the palliation of metastatic bone pain.²⁴

SUMMARY AND CONCLUSION

Bone and soft-tissue sarcomas are rare neoplasms with variable clinical presentations. A high index of suspicion is required for any unexplained mass with indeterminate imaging findings. Recent advances in imaging technology, including cross-sectional MRI and CT, have significantly refined the diagnosis and management of bone and soft-tissue sarcomas. When faced with a possible sarcoma, the clinician’s selection of imaging modalities has a direct impact on diagnosis, staging, and patient management.

Radiography plays a key role in the initial evaluation of acute knee pain in adults. Yet conflicting studies and the absence of clear guidelines may leave the primary care physician uncertain as to which imaging test to order—ie, whether radiography is sufficient, and when computed tomography (CT) or magnetic resonance imaging (MRI) is needed. This article reviews the indications for radiologic examination of the knee and discusses indications for cross-sectional imaging studies. Imaging in oncology patients is not discussed here.

ACUTE KNEE PAIN: A TYPICAL SCENARIO

A 47-year-old woman presents to the emergency department with left knee pain after a motor vehicle accident that occurred the day before. The car she was driving hit a tree, and she hit her knee on the dashboard. She was wearing a seatbelt at the time of the accident. She says she was unable to walk immediately after the accident because of knee pain.

The initial examination in the emergency room reveals swelling and pain throughout the range of motion. The anterior drawer test and the Lachman test are negative (see below).

Initial radiographs (Figure 1) reveal no acute fracture or effusion, but focal ossification adjacent to the proximal medial femoral condyle may indicate a past injury to the medial collateral ligament.

The patient is discharged home with a knee immobilizer, pain medication, and crutches, with instructions for a follow-up visit in the orthopedics clinic.

Five days later, she returns to the emergency department complaining of continuing knee pain. She says the knee gives way when she puts weight on it. The physical findings are unchanged, and she is discharged home with a follow-up appointment with orthopedics in 3 days.

At the follow-up visit, she complains of persistent knee pain in the medial aspect of the knee joint. Physical examination is difficult because of pain and swelling, and it reveals mild joint effusion with no gross instability. She has pain on the medial side with valgus stress, but there appears to be a hard end point. There is no posterior sag, and the Lachman test is negative.

Based on the physical examination and the patient’s complaints, she receives a diagnosis of medial collateral ligament strain and injury. She is given a hinged brace and is instructed to undergo a physical rehabilitation program.

Three weeks after the initial evaluation, she returns to the orthopedics clinic with continuing knee problems. Mild knee effusion persists, but she has less pain and swelling, allowing a more complete examination. The examination reveals less limitation of range of motion and a hint of positivity on the Lachman test. The knee is diffusely tender, and the pain seems out of proportion with the maneuvers used during the examination. She requests more pain medication. You suspect internal derangement of the knee. Which imaging test should you order to further evaluate this patient?

A SYSTEMATIC AND COST-EFFECTIVE APPROACH IS NEEDED

The case presented above represents a typical scenario for the presentation of acute knee pain and illustrates the diagnostic challenges.

Knee pain is a common reason for emergency room visits, and it accounts for approximately 1.9 million visits to primary care clinics annually.¹ In the emergency department, most patients undergo plain radiography to assess for fracture, yet approximately 92% of radiographic studies do not show a fracture.² Clearly, the evaluation of knee pain requires a systematic, accurate, and cost-effective approach.

Key elements of the physical examination

In acute knee pain, accurate diagnosis begins with a detailed history and physical examination.

The anterior drawer test is done to evaluate the anterior cruciate ligament. With the relaxed knee flexed to approximately 80° and the foot stabilized in a neutral position, the examiner grasps the proximal tibia in a firm yet gentle grip, and then applies anterior force, noting the degree of anterior displacement compared with the other knee.

The Lachman test, a variation of the anterior drawer test, is more definitive for the anterior cruciate ligament and is carried out with the knee in 15° of flexion and external rotation, in order to relax the iliotibial band. The upper hand grasps the distal thigh, and the lower hand, with the thumb on the tibial tubercle, pulls the tibia forward. The degree of anterior motion in millimeters is noted and compared with that on the other side, and the end point is graded as “soft” or “hard.” An end point is considered hard when a ligament abruptly halts the motion of the bone being tested against the stabilized bone. An end point is considered soft when the ligament is disrupted and the restraints are the more elastic secondary stabilizers.

Debate continues

Some authors contend that in skilled hands a thorough history, physical examination, and radiographic examination are sufficient to diagnose trauma-related intra-articular knee disorders.³ Others contend that MRI plays a key role in the initial evaluation. A number of studies^4–8 have shown that using MRI in the initial evaluation not only identifies key lesions, but also may eliminate the need for an invasive diagnostic procedure (ie, arthroscopy).

For example, MRI can reveal fracture, stress fracture, insufficiency fracture, and transient patellar dislocation—conditions that may satisfactorily explain knee symptoms.

 

PLAIN RADIOGRAPHY STILL THE FIRST STEP IN KNEE EVALUATION

Radiography is the first step in the evaluation of knee pain. It is quick and inexpensive and can yield many diagnostic clues. It can readily reveal fractures, osteochondral defects, bony lesions, joint effusions, joint space narrowing, and bone misalignment.

In patients with knee trauma, supine anteroposterior and cross-table lateral radiographic images are generally obtained. In patients whose knee pain is not due to trauma, standing projections are done, as well as dedicated projection of the patellofemoral articulation. A standing series is most helpful for evaluating joint space and alignment.

Applying the Ottawa rules

When a patient presents to the emergency room with acute knee pain, the immediate concern is whether he or she has a fracture. The Ottawa knee rules⁹ for when to order radiography in adults with knee pain are highly sensitive for detecting a clinically important fracture. If any one of the five Ottawa criteria applies—ie, the patient is age 55 or older, has tenderness at the head of the fibula, has patellar tenderness, is unable to flex the knee to 90°, or is unable to bear weight—then radiography is indicated.

While studies have validated the ability of the Ottawa rules to detect important fractures in acute knee injury,²,¹⁰ fracture is the cause of only a small percentage of knee complaints in the primary care setting. More common causes include osteoarthritis, meniscal injury, ligamental injury, and crystal arthropathy, and these account for approximately half of all diagnoses. Sprain and strain account for most of the rest of knee injuries.¹

Acute exacerbations of osteoarthritis

Osteoarthritis is a chronic problem, yet it is not unusual for a patient to present to the primary care physician with an acute exacerbation of joint pain. The clinical hallmarks include age over 50, stiffness lasting less than 30 minutes, bony enlargement and tenderness, and crepitus. The radiographic hallmarks, according to the Kellgren-Lawrence grading scale, are joint space narrowing, osteophytes, subchondral cysts, and sclerosis. These radiographic findings correlate well with clinical findings in these patients.¹¹

Situations in which radiography is less helpful

In some cases the radiographic findings may not explain the patient’s clinical signs and symptoms. For example, in suspected crystalline and septic arthritis, the clinical presentation may include warmth, erythema, and effusion. Arthrocentesis would be indicated in such a patient. Indeed, in the case of suspected pseudogout, chondrocalcinosis may be radiographically evident. However, it is also present in many patients without symptoms or with osteoarthritis, so radiographic evidence does not provide a definite diagnosis.

While radiography may not always identify the cause of knee pain, it is useful in excluding serious problems such as fractures, advanced degenerative changes, and neoplasms, and it may help direct further management. Radiography is not useful in the evaluation of the cruciate and collateral ligaments, the menisci, and the hyaline cartilage of the knee and may fail to show an insufficiency or stress fracture. To evaluate these structures and associated soft tissues, MRI is preferable.

COMPUTED TOMOGRAPHY IN ACUTE KNEE PAIN

CT is the imaging method of choice when patients have knee trauma but radiographs are negative for fracture. CT can detect and help analyze fracture better, and it can better define fractures seen on conventional radiographs (Figure 2). CT is fast, the procedure lasting only a few minutes. It costs less than MRI and provides a better picture of bony detail. Because of this, CT is generally recommended in patients with knee trauma, since it can show fractures too subtle for radiography.

CURRENT USES OF MRI TO EVALUATE ACUTE KNEE PAIN

As mentioned above, MRI is useful in evaluating suspected meniscal and ligamentous injuries.

Patients with meniscal injury may report a history of twisting injury while bearing weight. Symptoms may include locking or catching, with loss of motion related to a mechanical block. Ligamentous injury may be due to a direct blow or forceful stress while the patient is bearing weight. In tearing of the anterior cruciate ligament, the patient may report having heard a pop at the time of injury, followed by swelling. Valgus and varus stresses may lead to collateral ligament injury. Often, more than one injury coexists: more than a third of meniscal tears are associated with anterior cruciate ligament injury.¹²

Figure 3 shows how T2-weighted MRI was used to evaluate for suspected meniscal injury in our 47-year-old female patient with left knee pain after a motor vehicle accident.

Figure 4 shows how sagittal T2-weighted MRI was used in a 35-year-old man with intermittent locking of the right knee following a snowboarding injury. In this patient, MRI was able to uncover coexisting injuries.

Still a matter of debate

MRI’s role in the diagnosis of knee pain is still a contentious issue.

Advantages of MRI are that it is noninvasive, it does not use ionizing radiation, it gives multiplanar images, and it provides images of soft-tissue structures, which other imaging methods cannot.¹² It is a well-proven and widely accepted test. Its sensitivity for detecting meniscal and cruciate ligament injury ranges from 75% to 88%,¹ and it can help in the evaluation of other injuries for which radiography is not useful, including synovitis, bone bruise, stress or insufficiency fracture, osteochondral defects, and osteonecrosis.

In addition, several studies show that using MRI to establish the diagnosis in acute knee pain can mean that 22% to 42% fewer arthroscopic procedures need to be performed.^4–8 Authors of a prospective double-blind study⁸ recommended that MRI be used in patients with acute knee injury when the findings of the clinical history and examination by orthopedic surgeons prove equivocal.⁸ MRI evaluation is especially desirable for young, active patients who wish to resume activity as soon as possible.

A routine MRI examination consists of T1- and T2-weighted images in three planes, although the number of sequences and planes varies from hospital to hospital. The use of gadolinium contrast is indicated only when osteomyelitis, septic arthritis, or a mass is suspected.

Disadvantages of MRI include its cost: Medicare reimbursement for knee MRI is around $400, compared with $200 for knee CT and $50 for knee radiography with four views. Also, while studies have shown MRI to have a high sensitivity and specificity in the diagnosis of acute knee injury, some have reported a high false-positive rate for the detection of meniscal tear.¹³,¹⁴ MRI has also been shown to have a lower sensitivity than arthroscopy for lesions of the articular cartilage.¹³ Furthermore, MRI has been shown to reveal cartilage lesions, osteophytes, and meniscal abnormalities in asymptomatic study volunteers with no history of pain, trauma or knee disease.¹⁴ Therefore, findings on MRI must closely correlate with findings on the history and physical examination.

 

Additional indications for knee MRI

Cartilage can be assessed on routine MRI sequences of the knee. Since closed MRI systems have more powerful magnets than open systems, closed MRI systems provide greater anatomic detail.

MRI can identify other lesions, such as spontaneous osteonecrosis of the knee, usually seen in elderly women who may present with sudden knee pain. In such patients, MRI findings of focal replacement of the bone marrow and surrounding edema are specific for osteonecrosis.

Opinions vary as to whether bone marrow edema is always associated with pain. Sequential MRI studies have shown persistence of bone marrow edema for 2 years in patients with degenerative arthritis whose symptoms have waned. Bone marrow edema may be associated with pain but may be absent or inconsequential in the presence of pain.

Because fluid-sensitive T2-weighted MRI is exquisitely sensitive for mobile water protons (ie, in bone marrow edema), it is important that a cause for the edema-like signal be sought on the MRI scan, since this finding is nonspecific and may be associated with articular disease, trauma, osteonecrosis, infection, or bone tumors. Additionally, clinicians need to be aware that the findings on MRI depend on the quality of the study, and are influenced by technical factors such as magnet strength, imaging planes, and use of surface coils.

MRI should be used in patients in whom surgical treatment, ie, arthroscopy, is being considered. As discussed above, several studies have shown that a significant number of unnecessary arthroscopies may be prevented when preceded by an MRI examination.

Other indications include cases in which clinical findings are equivocal in the setting of acute injury, in competitive athletes in whom an immediate diagnosis and treatment is required, and in patients who present a high surgical risk. MRI should not be routinely used to diagnose the painful or injured knee,¹³ and if the skilled physical examination does not indicate findings of ligamentous or meniscal injury, conservative therapy should be prescribed.¹ MRI is also not useful and offers little for patients in whom changes of degenerative joint disease are evident on radiographs.

Figure 5 shows the use of MRI in the evaluation of a 45-year-old man with left knee pain after a motorcycle accident.

ULTRASONOGRAPHY HAS ONLY A LIMITED ROLE

Ultrasonography does not play a major role in the evaluation of acute knee pain in the United States, in part because the accuracy of the results depend much on the technical skills and experience of the operator.

Ultrasonography can be useful in evaluating for rupture of the quadriceps and patellar tendon, or to assess a repaired tendon after surgery,¹⁵ and it is a quick and reliable way to determine the presence of joint effusion and popliteal cyst. It is also used to guide needle placement for joint aspiration and injection.

Radiography plays a key role in the initial evaluation of acute knee pain in adults. Yet conflicting studies and the absence of clear guidelines may leave the primary care physician uncertain as to which imaging test to order—ie, whether radiography is sufficient, and when computed tomography (CT) or magnetic resonance imaging (MRI) is needed. This article reviews the indications for radiologic examination of the knee and discusses indications for cross-sectional imaging studies. Imaging in oncology patients is not discussed here.

ACUTE KNEE PAIN: A TYPICAL SCENARIO

A 47-year-old woman presents to the emergency department with left knee pain after a motor vehicle accident that occurred the day before. The car she was driving hit a tree, and she hit her knee on the dashboard. She was wearing a seatbelt at the time of the accident. She says she was unable to walk immediately after the accident because of knee pain.

The initial examination in the emergency room reveals swelling and pain throughout the range of motion. The anterior drawer test and the Lachman test are negative (see below).

Initial radiographs (Figure 1) reveal no acute fracture or effusion, but focal ossification adjacent to the proximal medial femoral condyle may indicate a past injury to the medial collateral ligament.

The patient is discharged home with a knee immobilizer, pain medication, and crutches, with instructions for a follow-up visit in the orthopedics clinic.

Five days later, she returns to the emergency department complaining of continuing knee pain. She says the knee gives way when she puts weight on it. The physical findings are unchanged, and she is discharged home with a follow-up appointment with orthopedics in 3 days.

At the follow-up visit, she complains of persistent knee pain in the medial aspect of the knee joint. Physical examination is difficult because of pain and swelling, and it reveals mild joint effusion with no gross instability. She has pain on the medial side with valgus stress, but there appears to be a hard end point. There is no posterior sag, and the Lachman test is negative.

Based on the physical examination and the patient’s complaints, she receives a diagnosis of medial collateral ligament strain and injury. She is given a hinged brace and is instructed to undergo a physical rehabilitation program.

Three weeks after the initial evaluation, she returns to the orthopedics clinic with continuing knee problems. Mild knee effusion persists, but she has less pain and swelling, allowing a more complete examination. The examination reveals less limitation of range of motion and a hint of positivity on the Lachman test. The knee is diffusely tender, and the pain seems out of proportion with the maneuvers used during the examination. She requests more pain medication. You suspect internal derangement of the knee. Which imaging test should you order to further evaluate this patient?

A SYSTEMATIC AND COST-EFFECTIVE APPROACH IS NEEDED

The case presented above represents a typical scenario for the presentation of acute knee pain and illustrates the diagnostic challenges.

Knee pain is a common reason for emergency room visits, and it accounts for approximately 1.9 million visits to primary care clinics annually.¹ In the emergency department, most patients undergo plain radiography to assess for fracture, yet approximately 92% of radiographic studies do not show a fracture.² Clearly, the evaluation of knee pain requires a systematic, accurate, and cost-effective approach.

Key elements of the physical examination

In acute knee pain, accurate diagnosis begins with a detailed history and physical examination.

The anterior drawer test is done to evaluate the anterior cruciate ligament. With the relaxed knee flexed to approximately 80° and the foot stabilized in a neutral position, the examiner grasps the proximal tibia in a firm yet gentle grip, and then applies anterior force, noting the degree of anterior displacement compared with the other knee.

The Lachman test, a variation of the anterior drawer test, is more definitive for the anterior cruciate ligament and is carried out with the knee in 15° of flexion and external rotation, in order to relax the iliotibial band. The upper hand grasps the distal thigh, and the lower hand, with the thumb on the tibial tubercle, pulls the tibia forward. The degree of anterior motion in millimeters is noted and compared with that on the other side, and the end point is graded as “soft” or “hard.” An end point is considered hard when a ligament abruptly halts the motion of the bone being tested against the stabilized bone. An end point is considered soft when the ligament is disrupted and the restraints are the more elastic secondary stabilizers.

Debate continues

Some authors contend that in skilled hands a thorough history, physical examination, and radiographic examination are sufficient to diagnose trauma-related intra-articular knee disorders.³ Others contend that MRI plays a key role in the initial evaluation. A number of studies^4–8 have shown that using MRI in the initial evaluation not only identifies key lesions, but also may eliminate the need for an invasive diagnostic procedure (ie, arthroscopy).

For example, MRI can reveal fracture, stress fracture, insufficiency fracture, and transient patellar dislocation—conditions that may satisfactorily explain knee symptoms.

 

PLAIN RADIOGRAPHY STILL THE FIRST STEP IN KNEE EVALUATION

Radiography is the first step in the evaluation of knee pain. It is quick and inexpensive and can yield many diagnostic clues. It can readily reveal fractures, osteochondral defects, bony lesions, joint effusions, joint space narrowing, and bone misalignment.

In patients with knee trauma, supine anteroposterior and cross-table lateral radiographic images are generally obtained. In patients whose knee pain is not due to trauma, standing projections are done, as well as dedicated projection of the patellofemoral articulation. A standing series is most helpful for evaluating joint space and alignment.

Applying the Ottawa rules

When a patient presents to the emergency room with acute knee pain, the immediate concern is whether he or she has a fracture. The Ottawa knee rules⁹ for when to order radiography in adults with knee pain are highly sensitive for detecting a clinically important fracture. If any one of the five Ottawa criteria applies—ie, the patient is age 55 or older, has tenderness at the head of the fibula, has patellar tenderness, is unable to flex the knee to 90°, or is unable to bear weight—then radiography is indicated.

While studies have validated the ability of the Ottawa rules to detect important fractures in acute knee injury,²,¹⁰ fracture is the cause of only a small percentage of knee complaints in the primary care setting. More common causes include osteoarthritis, meniscal injury, ligamental injury, and crystal arthropathy, and these account for approximately half of all diagnoses. Sprain and strain account for most of the rest of knee injuries.¹

Acute exacerbations of osteoarthritis

Osteoarthritis is a chronic problem, yet it is not unusual for a patient to present to the primary care physician with an acute exacerbation of joint pain. The clinical hallmarks include age over 50, stiffness lasting less than 30 minutes, bony enlargement and tenderness, and crepitus. The radiographic hallmarks, according to the Kellgren-Lawrence grading scale, are joint space narrowing, osteophytes, subchondral cysts, and sclerosis. These radiographic findings correlate well with clinical findings in these patients.¹¹

Situations in which radiography is less helpful

In some cases the radiographic findings may not explain the patient’s clinical signs and symptoms. For example, in suspected crystalline and septic arthritis, the clinical presentation may include warmth, erythema, and effusion. Arthrocentesis would be indicated in such a patient. Indeed, in the case of suspected pseudogout, chondrocalcinosis may be radiographically evident. However, it is also present in many patients without symptoms or with osteoarthritis, so radiographic evidence does not provide a definite diagnosis.

While radiography may not always identify the cause of knee pain, it is useful in excluding serious problems such as fractures, advanced degenerative changes, and neoplasms, and it may help direct further management. Radiography is not useful in the evaluation of the cruciate and collateral ligaments, the menisci, and the hyaline cartilage of the knee and may fail to show an insufficiency or stress fracture. To evaluate these structures and associated soft tissues, MRI is preferable.

COMPUTED TOMOGRAPHY IN ACUTE KNEE PAIN

CT is the imaging method of choice when patients have knee trauma but radiographs are negative for fracture. CT can detect and help analyze fracture better, and it can better define fractures seen on conventional radiographs (Figure 2). CT is fast, the procedure lasting only a few minutes. It costs less than MRI and provides a better picture of bony detail. Because of this, CT is generally recommended in patients with knee trauma, since it can show fractures too subtle for radiography.

CURRENT USES OF MRI TO EVALUATE ACUTE KNEE PAIN

As mentioned above, MRI is useful in evaluating suspected meniscal and ligamentous injuries.

Patients with meniscal injury may report a history of twisting injury while bearing weight. Symptoms may include locking or catching, with loss of motion related to a mechanical block. Ligamentous injury may be due to a direct blow or forceful stress while the patient is bearing weight. In tearing of the anterior cruciate ligament, the patient may report having heard a pop at the time of injury, followed by swelling. Valgus and varus stresses may lead to collateral ligament injury. Often, more than one injury coexists: more than a third of meniscal tears are associated with anterior cruciate ligament injury.¹²

Figure 3 shows how T2-weighted MRI was used to evaluate for suspected meniscal injury in our 47-year-old female patient with left knee pain after a motor vehicle accident.

Figure 4 shows how sagittal T2-weighted MRI was used in a 35-year-old man with intermittent locking of the right knee following a snowboarding injury. In this patient, MRI was able to uncover coexisting injuries.

Still a matter of debate

MRI’s role in the diagnosis of knee pain is still a contentious issue.

Advantages of MRI are that it is noninvasive, it does not use ionizing radiation, it gives multiplanar images, and it provides images of soft-tissue structures, which other imaging methods cannot.¹² It is a well-proven and widely accepted test. Its sensitivity for detecting meniscal and cruciate ligament injury ranges from 75% to 88%,¹ and it can help in the evaluation of other injuries for which radiography is not useful, including synovitis, bone bruise, stress or insufficiency fracture, osteochondral defects, and osteonecrosis.

In addition, several studies show that using MRI to establish the diagnosis in acute knee pain can mean that 22% to 42% fewer arthroscopic procedures need to be performed.^4–8 Authors of a prospective double-blind study⁸ recommended that MRI be used in patients with acute knee injury when the findings of the clinical history and examination by orthopedic surgeons prove equivocal.⁸ MRI evaluation is especially desirable for young, active patients who wish to resume activity as soon as possible.

A routine MRI examination consists of T1- and T2-weighted images in three planes, although the number of sequences and planes varies from hospital to hospital. The use of gadolinium contrast is indicated only when osteomyelitis, septic arthritis, or a mass is suspected.

Disadvantages of MRI include its cost: Medicare reimbursement for knee MRI is around $400, compared with $200 for knee CT and $50 for knee radiography with four views. Also, while studies have shown MRI to have a high sensitivity and specificity in the diagnosis of acute knee injury, some have reported a high false-positive rate for the detection of meniscal tear.¹³,¹⁴ MRI has also been shown to have a lower sensitivity than arthroscopy for lesions of the articular cartilage.¹³ Furthermore, MRI has been shown to reveal cartilage lesions, osteophytes, and meniscal abnormalities in asymptomatic study volunteers with no history of pain, trauma or knee disease.¹⁴ Therefore, findings on MRI must closely correlate with findings on the history and physical examination.

 

Additional indications for knee MRI

Cartilage can be assessed on routine MRI sequences of the knee. Since closed MRI systems have more powerful magnets than open systems, closed MRI systems provide greater anatomic detail.

MRI can identify other lesions, such as spontaneous osteonecrosis of the knee, usually seen in elderly women who may present with sudden knee pain. In such patients, MRI findings of focal replacement of the bone marrow and surrounding edema are specific for osteonecrosis.

Opinions vary as to whether bone marrow edema is always associated with pain. Sequential MRI studies have shown persistence of bone marrow edema for 2 years in patients with degenerative arthritis whose symptoms have waned. Bone marrow edema may be associated with pain but may be absent or inconsequential in the presence of pain.

Because fluid-sensitive T2-weighted MRI is exquisitely sensitive for mobile water protons (ie, in bone marrow edema), it is important that a cause for the edema-like signal be sought on the MRI scan, since this finding is nonspecific and may be associated with articular disease, trauma, osteonecrosis, infection, or bone tumors. Additionally, clinicians need to be aware that the findings on MRI depend on the quality of the study, and are influenced by technical factors such as magnet strength, imaging planes, and use of surface coils.

MRI should be used in patients in whom surgical treatment, ie, arthroscopy, is being considered. As discussed above, several studies have shown that a significant number of unnecessary arthroscopies may be prevented when preceded by an MRI examination.

Other indications include cases in which clinical findings are equivocal in the setting of acute injury, in competitive athletes in whom an immediate diagnosis and treatment is required, and in patients who present a high surgical risk. MRI should not be routinely used to diagnose the painful or injured knee,¹³ and if the skilled physical examination does not indicate findings of ligamentous or meniscal injury, conservative therapy should be prescribed.¹ MRI is also not useful and offers little for patients in whom changes of degenerative joint disease are evident on radiographs.

Figure 5 shows the use of MRI in the evaluation of a 45-year-old man with left knee pain after a motorcycle accident.

ULTRASONOGRAPHY HAS ONLY A LIMITED ROLE

Ultrasonography does not play a major role in the evaluation of acute knee pain in the United States, in part because the accuracy of the results depend much on the technical skills and experience of the operator.

Ultrasonography can be useful in evaluating for rupture of the quadriceps and patellar tendon, or to assess a repaired tendon after surgery,¹⁵ and it is a quick and reliable way to determine the presence of joint effusion and popliteal cyst. It is also used to guide needle placement for joint aspiration and injection.

Radiography plays a key role in the initial evaluation of acute knee pain in adults. Yet conflicting studies and the absence of clear guidelines may leave the primary care physician uncertain as to which imaging test to order—ie, whether radiography is sufficient, and when computed tomography (CT) or magnetic resonance imaging (MRI) is needed. This article reviews the indications for radiologic examination of the knee and discusses indications for cross-sectional imaging studies. Imaging in oncology patients is not discussed here.

ACUTE KNEE PAIN: A TYPICAL SCENARIO

A 47-year-old woman presents to the emergency department with left knee pain after a motor vehicle accident that occurred the day before. The car she was driving hit a tree, and she hit her knee on the dashboard. She was wearing a seatbelt at the time of the accident. She says she was unable to walk immediately after the accident because of knee pain.

The initial examination in the emergency room reveals swelling and pain throughout the range of motion. The anterior drawer test and the Lachman test are negative (see below).

Initial radiographs (Figure 1) reveal no acute fracture or effusion, but focal ossification adjacent to the proximal medial femoral condyle may indicate a past injury to the medial collateral ligament.

The patient is discharged home with a knee immobilizer, pain medication, and crutches, with instructions for a follow-up visit in the orthopedics clinic.

Five days later, she returns to the emergency department complaining of continuing knee pain. She says the knee gives way when she puts weight on it. The physical findings are unchanged, and she is discharged home with a follow-up appointment with orthopedics in 3 days.

At the follow-up visit, she complains of persistent knee pain in the medial aspect of the knee joint. Physical examination is difficult because of pain and swelling, and it reveals mild joint effusion with no gross instability. She has pain on the medial side with valgus stress, but there appears to be a hard end point. There is no posterior sag, and the Lachman test is negative.

Based on the physical examination and the patient’s complaints, she receives a diagnosis of medial collateral ligament strain and injury. She is given a hinged brace and is instructed to undergo a physical rehabilitation program.

Three weeks after the initial evaluation, she returns to the orthopedics clinic with continuing knee problems. Mild knee effusion persists, but she has less pain and swelling, allowing a more complete examination. The examination reveals less limitation of range of motion and a hint of positivity on the Lachman test. The knee is diffusely tender, and the pain seems out of proportion with the maneuvers used during the examination. She requests more pain medication. You suspect internal derangement of the knee. Which imaging test should you order to further evaluate this patient?

A SYSTEMATIC AND COST-EFFECTIVE APPROACH IS NEEDED

The case presented above represents a typical scenario for the presentation of acute knee pain and illustrates the diagnostic challenges.

Knee pain is a common reason for emergency room visits, and it accounts for approximately 1.9 million visits to primary care clinics annually.¹ In the emergency department, most patients undergo plain radiography to assess for fracture, yet approximately 92% of radiographic studies do not show a fracture.² Clearly, the evaluation of knee pain requires a systematic, accurate, and cost-effective approach.

Key elements of the physical examination

In acute knee pain, accurate diagnosis begins with a detailed history and physical examination.

The anterior drawer test is done to evaluate the anterior cruciate ligament. With the relaxed knee flexed to approximately 80° and the foot stabilized in a neutral position, the examiner grasps the proximal tibia in a firm yet gentle grip, and then applies anterior force, noting the degree of anterior displacement compared with the other knee.

The Lachman test, a variation of the anterior drawer test, is more definitive for the anterior cruciate ligament and is carried out with the knee in 15° of flexion and external rotation, in order to relax the iliotibial band. The upper hand grasps the distal thigh, and the lower hand, with the thumb on the tibial tubercle, pulls the tibia forward. The degree of anterior motion in millimeters is noted and compared with that on the other side, and the end point is graded as “soft” or “hard.” An end point is considered hard when a ligament abruptly halts the motion of the bone being tested against the stabilized bone. An end point is considered soft when the ligament is disrupted and the restraints are the more elastic secondary stabilizers.

Debate continues

Some authors contend that in skilled hands a thorough history, physical examination, and radiographic examination are sufficient to diagnose trauma-related intra-articular knee disorders.³ Others contend that MRI plays a key role in the initial evaluation. A number of studies^4–8 have shown that using MRI in the initial evaluation not only identifies key lesions, but also may eliminate the need for an invasive diagnostic procedure (ie, arthroscopy).

For example, MRI can reveal fracture, stress fracture, insufficiency fracture, and transient patellar dislocation—conditions that may satisfactorily explain knee symptoms.

 

PLAIN RADIOGRAPHY STILL THE FIRST STEP IN KNEE EVALUATION

Radiography is the first step in the evaluation of knee pain. It is quick and inexpensive and can yield many diagnostic clues. It can readily reveal fractures, osteochondral defects, bony lesions, joint effusions, joint space narrowing, and bone misalignment.

In patients with knee trauma, supine anteroposterior and cross-table lateral radiographic images are generally obtained. In patients whose knee pain is not due to trauma, standing projections are done, as well as dedicated projection of the patellofemoral articulation. A standing series is most helpful for evaluating joint space and alignment.

Applying the Ottawa rules

When a patient presents to the emergency room with acute knee pain, the immediate concern is whether he or she has a fracture. The Ottawa knee rules⁹ for when to order radiography in adults with knee pain are highly sensitive for detecting a clinically important fracture. If any one of the five Ottawa criteria applies—ie, the patient is age 55 or older, has tenderness at the head of the fibula, has patellar tenderness, is unable to flex the knee to 90°, or is unable to bear weight—then radiography is indicated.

While studies have validated the ability of the Ottawa rules to detect important fractures in acute knee injury,²,¹⁰ fracture is the cause of only a small percentage of knee complaints in the primary care setting. More common causes include osteoarthritis, meniscal injury, ligamental injury, and crystal arthropathy, and these account for approximately half of all diagnoses. Sprain and strain account for most of the rest of knee injuries.¹

Acute exacerbations of osteoarthritis

Osteoarthritis is a chronic problem, yet it is not unusual for a patient to present to the primary care physician with an acute exacerbation of joint pain. The clinical hallmarks include age over 50, stiffness lasting less than 30 minutes, bony enlargement and tenderness, and crepitus. The radiographic hallmarks, according to the Kellgren-Lawrence grading scale, are joint space narrowing, osteophytes, subchondral cysts, and sclerosis. These radiographic findings correlate well with clinical findings in these patients.¹¹

Situations in which radiography is less helpful

In some cases the radiographic findings may not explain the patient’s clinical signs and symptoms. For example, in suspected crystalline and septic arthritis, the clinical presentation may include warmth, erythema, and effusion. Arthrocentesis would be indicated in such a patient. Indeed, in the case of suspected pseudogout, chondrocalcinosis may be radiographically evident. However, it is also present in many patients without symptoms or with osteoarthritis, so radiographic evidence does not provide a definite diagnosis.

While radiography may not always identify the cause of knee pain, it is useful in excluding serious problems such as fractures, advanced degenerative changes, and neoplasms, and it may help direct further management. Radiography is not useful in the evaluation of the cruciate and collateral ligaments, the menisci, and the hyaline cartilage of the knee and may fail to show an insufficiency or stress fracture. To evaluate these structures and associated soft tissues, MRI is preferable.

COMPUTED TOMOGRAPHY IN ACUTE KNEE PAIN

CT is the imaging method of choice when patients have knee trauma but radiographs are negative for fracture. CT can detect and help analyze fracture better, and it can better define fractures seen on conventional radiographs (Figure 2). CT is fast, the procedure lasting only a few minutes. It costs less than MRI and provides a better picture of bony detail. Because of this, CT is generally recommended in patients with knee trauma, since it can show fractures too subtle for radiography.

CURRENT USES OF MRI TO EVALUATE ACUTE KNEE PAIN

As mentioned above, MRI is useful in evaluating suspected meniscal and ligamentous injuries.

Patients with meniscal injury may report a history of twisting injury while bearing weight. Symptoms may include locking or catching, with loss of motion related to a mechanical block. Ligamentous injury may be due to a direct blow or forceful stress while the patient is bearing weight. In tearing of the anterior cruciate ligament, the patient may report having heard a pop at the time of injury, followed by swelling. Valgus and varus stresses may lead to collateral ligament injury. Often, more than one injury coexists: more than a third of meniscal tears are associated with anterior cruciate ligament injury.¹²

Figure 3 shows how T2-weighted MRI was used to evaluate for suspected meniscal injury in our 47-year-old female patient with left knee pain after a motor vehicle accident.

Figure 4 shows how sagittal T2-weighted MRI was used in a 35-year-old man with intermittent locking of the right knee following a snowboarding injury. In this patient, MRI was able to uncover coexisting injuries.

Still a matter of debate

MRI’s role in the diagnosis of knee pain is still a contentious issue.

Advantages of MRI are that it is noninvasive, it does not use ionizing radiation, it gives multiplanar images, and it provides images of soft-tissue structures, which other imaging methods cannot.¹² It is a well-proven and widely accepted test. Its sensitivity for detecting meniscal and cruciate ligament injury ranges from 75% to 88%,¹ and it can help in the evaluation of other injuries for which radiography is not useful, including synovitis, bone bruise, stress or insufficiency fracture, osteochondral defects, and osteonecrosis.

In addition, several studies show that using MRI to establish the diagnosis in acute knee pain can mean that 22% to 42% fewer arthroscopic procedures need to be performed.^4–8 Authors of a prospective double-blind study⁸ recommended that MRI be used in patients with acute knee injury when the findings of the clinical history and examination by orthopedic surgeons prove equivocal.⁸ MRI evaluation is especially desirable for young, active patients who wish to resume activity as soon as possible.

A routine MRI examination consists of T1- and T2-weighted images in three planes, although the number of sequences and planes varies from hospital to hospital. The use of gadolinium contrast is indicated only when osteomyelitis, septic arthritis, or a mass is suspected.

Disadvantages of MRI include its cost: Medicare reimbursement for knee MRI is around $400, compared with $200 for knee CT and $50 for knee radiography with four views. Also, while studies have shown MRI to have a high sensitivity and specificity in the diagnosis of acute knee injury, some have reported a high false-positive rate for the detection of meniscal tear.¹³,¹⁴ MRI has also been shown to have a lower sensitivity than arthroscopy for lesions of the articular cartilage.¹³ Furthermore, MRI has been shown to reveal cartilage lesions, osteophytes, and meniscal abnormalities in asymptomatic study volunteers with no history of pain, trauma or knee disease.¹⁴ Therefore, findings on MRI must closely correlate with findings on the history and physical examination.

 

Additional indications for knee MRI

Cartilage can be assessed on routine MRI sequences of the knee. Since closed MRI systems have more powerful magnets than open systems, closed MRI systems provide greater anatomic detail.

MRI can identify other lesions, such as spontaneous osteonecrosis of the knee, usually seen in elderly women who may present with sudden knee pain. In such patients, MRI findings of focal replacement of the bone marrow and surrounding edema are specific for osteonecrosis.

Opinions vary as to whether bone marrow edema is always associated with pain. Sequential MRI studies have shown persistence of bone marrow edema for 2 years in patients with degenerative arthritis whose symptoms have waned. Bone marrow edema may be associated with pain but may be absent or inconsequential in the presence of pain.

Because fluid-sensitive T2-weighted MRI is exquisitely sensitive for mobile water protons (ie, in bone marrow edema), it is important that a cause for the edema-like signal be sought on the MRI scan, since this finding is nonspecific and may be associated with articular disease, trauma, osteonecrosis, infection, or bone tumors. Additionally, clinicians need to be aware that the findings on MRI depend on the quality of the study, and are influenced by technical factors such as magnet strength, imaging planes, and use of surface coils.

MRI should be used in patients in whom surgical treatment, ie, arthroscopy, is being considered. As discussed above, several studies have shown that a significant number of unnecessary arthroscopies may be prevented when preceded by an MRI examination.

Other indications include cases in which clinical findings are equivocal in the setting of acute injury, in competitive athletes in whom an immediate diagnosis and treatment is required, and in patients who present a high surgical risk. MRI should not be routinely used to diagnose the painful or injured knee,¹³ and if the skilled physical examination does not indicate findings of ligamentous or meniscal injury, conservative therapy should be prescribed.¹ MRI is also not useful and offers little for patients in whom changes of degenerative joint disease are evident on radiographs.

Figure 5 shows the use of MRI in the evaluation of a 45-year-old man with left knee pain after a motorcycle accident.

ULTRASONOGRAPHY HAS ONLY A LIMITED ROLE

Ultrasonography does not play a major role in the evaluation of acute knee pain in the United States, in part because the accuracy of the results depend much on the technical skills and experience of the operator.

Ultrasonography can be useful in evaluating for rupture of the quadriceps and patellar tendon, or to assess a repaired tendon after surgery,¹⁵ and it is a quick and reliable way to determine the presence of joint effusion and popliteal cyst. It is also used to guide needle placement for joint aspiration and injection.