Causes / typical injury mechanism: Synovial chondromatosis (SC) is a benign proliferation of hyaline cartilage in the subsynovial connective tissue of joints, tendon sheaths, and bursae, forming nodules that can enlarge and ossify. The underlying mechanism of synovial chondromatosis is not known.

Classic history and presentation:

• SC has a gradual, nonspecific onset of symptoms, where the patient describes pain, recurrent effusions, crepitus, and locking of the affected joint.
• Some patients may be entirely asymptomatic.
• Individuals may have a large effusion, apparent joint deformity, palpable mass, joint-line tenderness, mechanical pain, and muscle atrophy. Instability and effects on neurovascular structures are less commonly reported.
• The most commonly affected joints are the knees, hips, shoulders, elbows, ankles, and wrists, in order of descending frequency.
• Less commonly, the fingers, acromioclavicular, temporomandibular, proximal tibiofibular, as well as intervertebral facet joints and tenosynovial locations are affected.
• It is bilateral in 10% of cases.
• It is polyarticular in 5% of cases.

Prevalence:

• Primary SC has an incidence of 1 in 100 000.
• Age – Typically seen in patients aged 30-60 years.
  • Primary SC: Typical patient is aged 30-40 years.
  • Secondary SC: Typical patient is aged 50-60 years.
• Sex / gender – 2:1 ratio men to women (ranges from 1.8 to 3 times more common in men compared to women).

Risk factors:

• Rare familial association (related to type 2 collagen abnormalities, such as Wagner-Stickler syndrome)
• Aberrations to chromosome 6
• Fibronectin (FN1) and/or activin receptor 2A (ACVR2A) genetic rearrangement

Pathophysiology: SC originates as metaplasia of hyaline cartilage foci in the subsynovial connective tissue of joints, tendon sheaths, or bursae, most commonly at the transitional zone between the synovium and articular cartilage. The cartilaginous foci begin as sessile nodules that have the potential to grow, become pedunculated, and detach to form loose bodies.

SC occurs in 3 stages:

• Stage I (early) – Active, inflammatory intrasynovial process without loose bodies.
• Stage II (intermediate) – Active synovial proliferation with transitional loose bodies.
• Stage III (late) – Minimal inflammation and minimal synovial disease with multiple loose bodies.

Intra-articular loose bodies can continue to grow from the nutrients within the synovial fluid and could reattach to the synovium or be reabsorbed. There is potential for extra-articular involvement if the loose bodies break out of the joint capsule or form primarily on the tendon sheath or bursa. About two-thirds of the cartilaginous foci undergo endochondral ossification, while the rest are nonossified.

Grade / classification system (if any):

• Primary SC –
  • Rare.
  • Idiopathic; occurs in an otherwise normal joint.
  • Loose bodies are uniform, small, round, and more numerous.
• Secondary SC –
  • More common.
  • Occurs in the setting of joint destruction and synovitis (ie, osteoarthritis, single traumatic injury, repetitive trauma, osteochondritis dissecans, Charcot arthropathy, osteonecrosis, infection).
  • Loose bodies are variable in size and fewer than in primary SC.