Secondary Synovial Osteochondromatosis of the knee with complicated Baker’s cyst

A 35-year-old male complained of gradually progressive pain and swelling of the right knee since one year. He was a known hypertensive on treatment since three years. There was no trauma or surgery in the past. Local examination did not show any signs of inflammation. Mild tenderness and restriction of movements was present.

Antero posterior and lateral radiographs of the knee revealed osteophytes and reduction of tibiofemoral & patello femoral joint space. Multiple intra articular, small, peripherally calcified loose bodies were present.

Magnetic resonance imaging (MRI) of the knee revealed degenerative changes. Medial patellar facet showed Grade II chondromalacia. There was thinning of cartilage in the medial patello femoral joint with loss of cartilage along medial femoral condyle and associated marrow edema. Multiple loose bodies were seen in the joint.

Posterior horn of medial meniscus showed a complex tear reaching upto its articular surface. There was medial extrusion of medial meniscus by ~ 5.2mm. There was a cystic lesion measuring ~ 6x 3.6x1.7 cm, showing multiple thin septae and loose bodies, along the medial head of the gastrocnemius muscle. It was seen to insinuate between the medial head of the gastrocnemius and semimembranosus tendons, in a typical position for a Baker’s cyst. Mild knee joint effusion with diffuse synovial thickening and soft tissue edema were present.  A diagnosis of secondary synovial osteochondromatosis with complicated Baker’s cyst was made and the patient was managed conservatively as he declined surgery.

Osteochondromatosis is a benign neoplastic process with hyaline cartilaginous nodules in the subsynovial tissue of a joint, tendon sheath or bursa. It can be of two types – primary or secondary. In the primary type, there is synovial hyperplasia and osteochondral metaplasia leading to chondromatosis or osteochondromatosis. It affects patients in the third to fifth decades of life and is more common in males. Clinical features are pain, swelling and restricted range of motion. Most commonly, there is monoarticular involvement of large joints such as knee, elbow and shoulder. Polyarticular or extra articular involvement is less frequent and rarely, a synovial sheath or bursa may be affected. [1] In the secondary type, there are features of arthritis. Secondary chondromatosis results from mechanical injury to the hyaline cartilage and is related to osteochondral fractures, neuropathic arthropathy, osteoarthritis or prior infective or inflammatory arthritis. Radiographs show multiple intra articular loose bodies with extrinsic erosion in upto half of the cases. Ultrasound reveals a heterogeneous mass with hyperechoic foci representing chondral fragments within the joint, bursa or tendon sheath. These foci may show posterior acoustic shadowing. CT shows hypoattenuating areas of synovial thickening with signs of degenerative joint disease. A smaller number of nodules showing lamellated calcification are seen, usually larger in size compared to the primary form. On MRI, the fragments appear low to intermediate signal intensity on T1 weighted images and hyperintense on T2 images with hypointense calcifications. [2]

A differential for this condition is Pigmented villonodular synovitis which may be localized or diffuse type. Synovial thickening will show T2 hypointense areas with blooming on gradient echo images due to hemosiderin. [3] Another differential is rheumatoid arthritis which classically shows marginal erosions, periarticular osteopenia, tenosynovitis, joint space narrowing and subluxation. [4] Other differentials are lipoma arborescens, tumoral calcinosis and synovial chondrosarcoma. Villous lipomatous proliferation of the synovium is seen in lipoma arborescens. Frond-like fatty areas similar to subcutaneous fat are seen on MRI which supress on fat-saturated images.[3] Tumoral calcinosis typically presents as amorphous, cystic and multilobulated calcification in periarticular distribution. Hyperphosphatemia is usually present. Synovial chondrosarcoma often demonstrates cortical destruction with marrow invasion, extra-articular extension and possible metastases. The incidence of malignant transformation in chondromatosis is 1-5 %. [5]

Treatment of choice is surgical resection and in few cases synovectomy is also necessary to avoid recurrence. Arthroscopic treatment is emerging and can reduce patient morbidity. [2]