A forty-year-old male patient presented with a gradually progressive swelling on the outer aspect of the right ankle which he first noticed two years before. It was associated with pain, aggravated on walking and relieved on rest. The skin overlying the swelling appeared normal and there was no scar or sinus. There was no trauma to the site in the past. Distal neurovascular examination was unremarkable.
On external examination the patient showed tumefaction over the posterolateral aspect of the right ankle. Radiographic examination showed a well-defined soft tissue swelling with partly calcified nodules adjacent to the lateral malleolus. There were no obvious bony erosions. The possibility of synovial chondromatosis was considered and cross-sectional imaging was planned for better evaluation of the extent of the lesion, any bone erosions or degenerative changes and to rule out malignancy.
Plain computed tomography (CT) showed a lobulated soft tissue density lesion with osteochondral loose bodies. Fibula appeared normal with no erosion or periosteal reaction. There were no features of arthritis in ankle joint. A non-contrast magnetic resonance (MR) image shows a predominantly T2 hyperintense lesion measuring ~ 5x5x2.2 cm adjacent to the lateral malleolus of fibula. Few fluid levels were seen within the lesion. On T1-weighted images there were few interspersed fragments which were isointense to bone. The peroneal tendons were seen separately and were normal.
Surgery was planned to remove the loose bodies, relieve pain and to prevent early osteoarthritis. Intraoperatively, a lobulated synovial lined mass was excised which showed multiple nodules within it. Histopathology showed cartilaginous nodules surrounded by synovium and few areas of ossification. There were no features of atypia or malignancy.
Synovial osteochondromatosis is of two forms- primary and secondary. The primary form is a benign condition where there is proliferation of multiple cartilaginous nodules in the synovial membrane of joints (intraarticular type), bursae or tendon sheaths (extraarticular type). Areas of ossification can eventually form within the lesion. Its usual age of occurrence is in the third to fifth decades of life. In the secondary form, osteoarthritis is an associated finding and this form occurs in older individuals.  It is usually monoarticular with the knee being the most affected joint. Other joints that may be involved include hip, shoulder and elbow.  This case of extraarticular chondromatosis seemed to be arising from the tenosynovial membrane or possibly from an adventitial bursa which can develop over the peroneal tubercle. Such an adventitial bursa may be progressively symptomatic when inflamed.
Patients can present with pain, swelling and restriction of motion of the affected joint. Plain radiographs show multiple intraarticular calcifications of similar size and shape with ‘ring and arcs’ type of mineralisation representing cartilage. Adjacent bony erosions may be seen which will be better depicted on CT. MR imaging usually shows low to intermediate signal intensity on T1 and high signal intensity on T2 images with hypointense calcifications. It shows the extent of the lesion within the joint better than CT and the visualisation of tendons aids in better surgical planning. Peripheral and septal enhancement may be seen on contrast study.  Giant cell tumour of the tendon sheath can be differentiated not only with the different MRI signal intensity but also because it usually does not have calcifications. Extraarticular chondroma is not related to the tendon sheath or bursa nor does it have a synovial lining. It is more common in joints of hands and feet. Marrow invasion or erosion of bone would indicate conversion to chondrosarcoma which is rare.
The treatment would be a complete synovectomy with removal of cartilaginous loose bodies to avoid recurrence in the future.  The surgical planning might depend on the joint involved, for example the hip has a tighter capsule than other joints leading to more compression and elevation of intraarticular pressure.  Open surgery was the treatment of choice earlier while more recently arthroscopic approach has been more popular. Recurrence rates have been reported to be ranging from 7 percent to 23 percent. 
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