Bone and Soft-Tissue Disorders
= OSTEOCHONDRITIS DISSECANS = OSTEOCHONDRAL FRACTURE
= fragmentation + possible separation of a portion of the articular surface
Etiology:
- subchondral fatigue fracture as a result of shearing, rotatory / tangentially aligned impaction forces / repetitive microtrauma
- ? autosomal dominant trait associated with short stature, endocrine dysfunction, Scheuermann disease, Osgood-Schlatter disease, tibia vara, carpal tunnel syndrome
Age: adolescence; M >F
- asymptomatic / vague complaints
- clicking, locking, limitation of motion
- swelling, pain aggravated by movement
Location:
- knee: medial (in 10% lateral) femoral condyle close to fossa intercondylaris; bilateral in 20–30%
- humeral head
- capitellum of elbow
- talus
- purely cartilaginous fragment unrecognized on plain film
- fracture line parallels joint surface
- mouse = osteochondrotic fragment
Location: posterior region of knee joint, olecranon fossa, axillary / subscapular recess of glenohumeral joint - mouse bed = sclerosed pit in articular surface
- soft-tissue swelling, joint effusion
MR:
- focus of abnormal signal in subarticular marrow
- defect in overlying cartilage
- loose bodies of heterogeneous low-SI in coronoid and olecranon fossa outlined by hyperintense joint fluid on T2WI
DDx: spontaneous osteonecrosis, neuroarthropathy, degenerative joint disease, synovial osteochondromatosis
Juvenile Osteochondritis Dissecans of Knee 
= disease manifestation before physeal closure
Etiology: repetitive trauma
Age: 10–15 years; M÷F= 3÷1 to 4÷1
- pain, clicking / catching of joint, loss of function ← intra-articular loose bodies
Site:
- medial femoral condyle: posterolateral aspect (51%), weight-bearing portion (19%)
- lateral femoral condyle: medial aspect (7%)
- signs of lesion instability (= high-grade lesion):
- line of fluid signal intensity on T2WI between fragment + parent bone surrounded by low-intensity rim
- multiple / >5 mm cysts near lesion
- fracture in articular cartilage
- fluid-filled osteochondritis dissecans lesion
Osteochondritis Dissecans of Capitellum
Age: adolescent boys 13–16 years of age
Cause: overhead throwing activity / gymnastics
- dull poorly localized pain in elbow
- limited extension + locking of elbow
- lucent defect within capitellum
- flattening of anterior border of capitellum