A 20-year-old man with a history of juvenile idiopathic arthritis (JIA) presented with chronic knee pain and swelling refractory to disease-modifying antirheumatic drugs, intra-articular injections, oral steroids, and nonsteroidal anti-inflammatory drugs. The patient denied catching, popping or instability, although he did have limited range of motion that he attributed to swelling. He denied a history of prior injury or trauma. Radiographs were ordered to evaluate for underlying osseous pathology (Figure 1A). Subsequently, an MRI of the knees with contrast was ordered to evaluate for internal derangement and synovitis (Figure 1B-D).
Synovitis with frond-like synovial proliferation
Pigmented villonodular synovitis
Although intra-articular lesions are uncommonly encountered in clinical practice, imaging plays a key role in the identification, diagnosis, and management of these lesions. Aside from cartilaginous and osseous loose bodies arising from cartilage damage, common intra-articular pathologic processes include primary synovial (osteo)chondromatosis and pigmented villonodular synovitis (PVNS). Lipoma arborescens is rare. The clinical history coupled with imaging features and disease distribution often allow for a definitive diagnosis.
Lipoma arborescens is a rare entity characterized by fatty proliferation within subsynovial tissue. It can be seen in adults and children with no sex predilection.1 It is surmised that chronic synovial irritation predisposes to the development of lipoma arborescens because it is most frequently seen in patients with chronic joint pathology, including rheumatoid arthritis, osteoarthritis, or trauma.1 Lipoma arborescens is typically asymptomatic in mild cases; however, patients can develop intermittent effusions with synovial thickening and synovitis, which can become symptomatic.
Lipoma arborescens most commonly occurs within the suprapatellar recess of the knee joint with a characteristic frond-like appearance.2 The lesion follows typical MR signal of fat, including high signal on T1 and low signal on fat-suppressed images. Although the fat does not typically enhance, the overlying synovium often enhances avidly, particularly in cases of florid synovitis. Similar findings are seen on CT, where lipoma arborescens measures fat density. Calcifications and associated hemorrhage are rare.3
Although this entity is typically asymptomatic, synovectomy can be performed for symptomatic cases.3 There is a risk of recurrence, particularly with incomplete resection. This patient had a synovectomy with significant improvement in symptoms.
Primary synovial (osteo)chondromatosis is a rare, benign, neoplastic entity characterized by nodular synovial growth. It affects the large joints, most commonly the knee.4 Although it can affect a wide age range, it is seen more frequently in men 30-60 years of age.4 The degree of ossification of the nodular synovial growths is variable.
The diagnosis can typically be made on radiographs, particularly when ossified. If the bodies are ossified on the radiograph, this helps distinguish synovial (osteo)chondromatosis from PVNS. Numerous nodules are typically round and similar in size and may appear lamellated or have a target sign. MRI is more sensitive, with both ossified and cartilaginous nodules visible. Joint effusions and synovial thickening are commonly seen. The synovial thickening is best seen on postcontrast T1 fat-suppressed images.
Malignant transformation of primary synovial chondromatosis to chondrosarcoma is rare, with the mean time to transformation approximately 20 years. Synovial chondrosarcoma may be difficult to differentiate based on imaging but often presents as a “snowstorm” appearance.5 Intra-articular primary synovial chondromatosis predisposes to early osteoarthritis.
Treatment of synovial (osteo)chondromatosis is primarily surgical with removal of the chondral bodies. Performing an associated synovectomy is controversial.
Lastly, primary synovial (osteo)chondromatosis should not be confused with secondary (osteo)chondromatosis. Secondary (osteo)chondromatosis has chondral or osteochondral loose bodies within the joint from pieces of cartilage that have been displaced from the underlying articular cartilage, most commonly due to underlying osteoarthritis. Secondary (osteo)chondromatosis can be distinguished from primary (osteo)chondromatosis on imaging in that the secondary form has fewer joint space bodies of variable size.4
Intra-articular PVNS is a rare benign hypertrophic synovial proliferation affecting individuals in the third through fifth decades of life. Patients often present with a palpable mass and chronic pain. In addition, they may have significant loss of function. Malignant transformation is rare.6
Radiographs can be normal, although nonspecific findings such as a joint effusion and soft-tissue swelling are often seen.6 Visible calcifications within PVNS are rare. Extrinsic mechanical erosions of the bone are more common and more likely to be seen in small, constrained joints. MR imaging demonstrates a synovial-based mass that may have magnetic susceptibility artifacts due to the hemosiderin content.6 If the lesion has a fair amount of hemosiderin deposition, one will see low signal on T2-weighted and especially gradient echo MRI images at the areas of hemosiderin deposition. This low signal, if present, can help distinguish PVNS from primary synovial (osteo)chondromotosis. Erosions, joint effusion, and synovial thickening can also be seen on MRI. MRI provides additional details on intra-articular and exta-articular extent for surgical planning purposes.
Although surgical resection is ideal, a combination of pharmacotherapy and radiation therapy can also be used, particularly with diffuse involvement. In cases of untreated intra-articular PVNS, the lesions can cause articular cartilage destruction and development of secondary osteoarthritis.
Intra-articular synovial lesions are uncommonly encountered in routine clinical practice. When present, imaging is essential for the diagnosis and management, as key imaging characteristics can often provide a definitive diagnosis, precluding unnecessary biopsy. Imaging findings that are most helpful in differentiating causes of synovitis with synovial proliferation include frond-like fatty proliferation in the subsynovial space with avid synovial enhancement in the setting of lipoma arborescens; the presence of multiple, small, uniform-sized, ossified or chondroid bodies within the joint in primary synovial (osteo)chondromatosis; and MRI evidence of low-signal intensity on T2-weighted and gradient echo sequences (blooming from magnetic susceptibility artifacts) in PVNS. There are times when the lesions are not definitely pathognomonic on imaging (especially synovial chondromatosis and PVNS) and biopsy may be indicated to confirm the diagnosis.
Shah R, Shelat N, Bennett DL. Synovitis with Synovial Proliferation. J Am Osteopath Coll Radiol. 2016;5(4):20-22.