Primary synovial osteochondromatosis
Synovial osteochondromatosis is a benign condition of idiopathic causes, usually it is monoarticular and predominantly in males in their 3rd to 5th decade.
In this condition, there is a proliferation and metaplastic transformation of the synovium resulting in formation of multiple cartilaginous nodules. Clinically it present with joint pain, swelling, and limitation of motion.
At the beginning the active phase shows synovial proliferation and formation of intrasynovial cartilaginous nodules and progress to the final phase in which inactive synovial disease and nodules are present, these nodules may break off into the joint space. These nodules may contain only cartilage (synovial chondromatosis), cartilage and bone (osteochondomatosis), or mature bone with fatty marrow. they derive their nutrition from the synovial fluid.
When calcified bodies are present, the radiographic findings consist of multiple intra-articular calcified nodules of uniform in size may be associated with a mild joint effusion, marginal erosion, and late secondary degenerative joint disease.
On MRI the appearances are variable, non-mineralized lesions are formed of cartilage and give cartilage signal intensity (that is isointense relative to muscle on T1-weighted images and hyperintense on T2-weighted images ). When the nodules contain calcification, small areas of low signal intensity are observed with all pulse sequences. when mature bone forms and fatty marrow display low signal intensity of cortical bone peripherally and high signal intensity of bone marrow fat centrally on T1-weighted images.
One thing to note when diagnosing primary synovial osteochondromatosis is that the joint itself is essentially normal at the beginning, degenerative changes only happen at the late stages.