Primary Synovial Osteochondromatosis

= benign self-limiting monoarticular disorder characterized by proliferation + metaplastic transformation of synovium with formation of multiple intrasynovial cartilaginous / osteocartilaginous nodules with tendency for detachment and migration within joint space

Cause: hyperplastic synovium with cartilage metaplasia (foci <2–3 cm); loose body may remain free floating / form conglomerate with other loose bodies into large mass / reattach to synovium with either reabsorption or continued growth

Histo: foci of hyaline cartilage with mineralized chondroid matrix beneath synovial surface + within subsynovial connective tissue; hypercellularity + nuclear atypia may be confused with malignancy

Composition of cartilaginous bodies:

• cartilage alone / cartilage + bone / mature bone + fatty bone marrow

Age: 3^rd–5^th decade; M÷F = 2÷1 – 4÷1

Phase:

1. early phase = synovial proliferation → formation of intrasynovial cartilaginous nodules
2. late phase = inactive synovial disease with persistent nodules that may break off into joint cavity

• slow-growing soft-tissue mass within joint
• progressive joint pain for several years
• limitation of motion / locking ± hemorrhagic joint effusion

Location: knee (most common in >50%, in 10% bilateral) elbow >hip >shoulder >ankle >wrist; usually monoarticular, occasionally bilateral

Sites: joint / tendon sheath / ganglion / bursa / periarticular

• PATHOGNOMONIC multiple calcified / ossified loose bodies in a single joint (bony shell of remodeled lamellar bone is rare) in a rings-and-arcs morphology
• varying degrees of bone mineralization (25–30% of chondromas show NO radiopacity)
• characteristically uniform size of nodules that may vary between a few mm and several cm
• marginal pressure erosion of adjacent bone in joints with tight capsule (eg, hip)
• widening of joint space (from accumulation of loose bodies)
• joint effusion uncommon
• NO osteoporosis

CT:

• multiple calcified / ossified intraarticular bodies
• intraarticular soft-tissue mass of near-water attenuation containing multiple small calcifications

MR:

• homogeneous lobulated intraarticular mass isointense to muscle on T1WI + hyperintense to muscle on T2WI ± osteochondral bodies

DDx: large effusion, soft-tissue tumor

• osteochondral bodies:
  • multiple foci of low signal intensity ← calcifications
  • peripheral contrast enhancement of chondral lesions
  • intraarticular bodies with central area of high SI on T1WI = with fatty marrow

Cx:

1. Long-standing disease → secondary degenerative arthritis ← chronic mechanical irritation + destruction of articular cartilage by loose bodies
2. Malignant dedifferentiation to synovial chondrosarcoma (in <5%)

Rx: surgical synovectomy with removal of loose bodies (recurrence is common)

DDx:

1. Synovial sarcoma, chondrosarcoma
2. Osteochondral fracture (history of trauma), osteochondritis dissecans, osteonecrosis
3. Secondary osteochondromatosis
4. Pigmented villonodular synovitis, synovial hemangioma, lipoma arborescens