Osteochondritis Dissecans (OCD)

Osteochondritis dissecans (OCD) is a condition in which a segment of subchondral bone — and its overlying articular cartilage — undergoes avascular necrosis, potentially leading to separation and fragmentation of the osteochondral fragment from the parent bone. It predominantly affects the knee (most common site), the elbow (capitellum — in throwing athletes), and the talus. In the knee, the lateral aspect of the medial femoral condyle is the most common site (approximately 70% of cases). Early diagnosis in a skeletally immature patient offers the best chance of healing; displaced fragments in the adult represent a significant articular cartilage problem requiring osteochondral restoration.

• Epidemiology: incidence approximately 15–30 per 100,000; male predominance (3:1) in most series; bilateral in 20–30% of cases; two age groups — juvenile OCD (open physis, better prognosis for healing) and adult OCD (closed physis, poor healing potential); the lateral aspect of the medial femoral condyle is the most commonly affected site in the knee (the `classic` OCD site); the lateral femoral condyle and the patella are less commonly affected; the weightbearing zone of the femoral condyle has the worst prognosis for spontaneous healing
• Aetiology: incompletely understood; repetitive microtrauma (most widely accepted theory — repetitive impact loading of the subchondral bone leads to fatigue fractures and vascular disruption); ischaemia; genetic predisposition; anomalous ossification centre development
• The OCDlesion consists of a segment of subchondral bone with overlying cartilage that has become partially or completely separated from the surrounding normal bone; the cartilage may remain intact (stable, better prognosis) or may be breached (unstable, joint fluid infiltrates underneath the fragment, separating it further)

• MRI sign of instability: high T2 signal completely surrounding the fragment on MRI (the `halo sign`) indicates fluid has tracked underneath the fragment, undercutting it — this is the most important MRI sign of instability; a high signal line at only one margin may represent fibrous tissue (not fluid) and does not necessarily indicate instability; circumferential high signal = unstable = surgical treatment indicated even in juvenile OCD

• History: activity-related knee pain, often vague and poorly localised; mechanical symptoms (locking, catching) if the fragment is loose or partially detached; effusion; the history may span months to years before diagnosis; young athletes (gymnastics, football, basketball) are most commonly affected
• Wilson sign: a clinical test for OCD of the medial femoral condyle; with the knee flexed to 90°, internally rotate the tibia and slowly extend the knee; pain reproduced between 30° and 60° of extension (the tibial spine impinges on the OCD lesion); relieved by external rotation of the tibia; sensitivity approximately 60–80%; not highly specific; used mainly in younger patients
• Effusion; tenderness over the lesion (palpable if the lesion is at the classic medial condyle site); loose body may be palpable in the joint

• Plain radiographs: AP, lateral, tunnel (notch) view; the tunnel view (PA with knee in 45° flexion) is essential — it profiles the posterior femoral condyle where most OCD lesions arise and may be missed on a standard AP; OCD appears as a well-demarcated subchondral lucency with a surrounding sclerotic rim on the condylar surface; loose bodies visible if displaced; the tunnel view should be included in the assessment of any young patient with unexplained knee pain
• MRI: gold standard for assessing the extent, stability, and cartilage status of an OCD lesion; T2-weighted sequences assess fluid undermining (circumferential high signal = unstable); T1 characterises the subchondral bone changes; gadolinium-enhanced MRI (dynamic contrast) shows vascular enhancement within the fragment in healing juvenile OCD (good prognostic sign) vs lack of enhancement (avascular, poor healing potential)
• CT scan: excellent for characterising the bony anatomy — fragment size, depth, and subchondral bone loss; useful for surgical planning (OATS plug sizing, screw fixation planning); can measure the crater volume

• Non-operative management for stable juvenile OCD (ICRS I–II, open physis): non-weight-bearing or protected weight-bearing (crutches, brace) for 6–12 weeks; restriction from high-impact activities for 3–6 months; the juvenile skeleton has significant healing potential — healing rates of 60–70% for stable lesions managed non-operatively; serial MRI at 3-month intervals to confirm healing; a positive prognostic sign is decreasing oedema and reducing signal at the fragment margin; failure to show healing at 3–6 months warrants surgical intervention
• Retrograde drilling: for stable juvenile OCD lesions not healing with non-operative treatment; a guidewire or drill is passed through the intact articular cartilage into the sclerotic base of the OCD lesion (retrograde = from the joint surface away from the articular cartilage, or through a transpatellar route avoiding the articular cartilage); drilling creates channels through the sclerotic bone, disrupting the avascular zone and promoting revascularisation and healing; the cartilage surface must be intact for this to be appropriate
• Fragment fixation (for unstable in situ fragment, ICRS Grade III): arthroscopic fixation of the fragment using headless compression screws (Herbert screws, Acutrak screws) or bioabsorbable pins; the fragment is debrided, the base is freshened (to promote healing), and bone graft is added before fixation; the cartilage surface is preserved; screws must be countersunk below the articular surface; fixation is the preferred treatment for ICRS Grade III (unstable but fragment in situ) regardless of skeletal maturity — preserves the cartilage surface; healing rates of 80–90% with rigid fixation
• Loose body removal and crater management (ICRS Grade IV): the loose body is removed arthroscopically; the crater is then managed based on size — small craters (<1 cm²) can be managed with microfracture; medium craters (1–4 cm²) are best treated with OATS; large craters (>4 cm²) may require ACI or fresh osteochondral allograft; if the fragment is large and viable, refixation should be attempted before considering crater restoration

• OCD of the capitellum: affects young overhead throwing athletes (baseball pitchers, gymnasts, racket sport players); the anterolateral capitellum is the most common site; compressive loading of the radiocapitellar joint during valgus stress (throwing) + the tenuous blood supply of the capitellum (single vessel entering posteriorly) explains the predilection; presentation — lateral elbow pain, restricted extension, locking (loose body); treated non-operatively in stable juvenile OCD; unstable or displaced fragments require arthroscopic debridement, loose body removal, and drilling or grafting of the crater; outcomes are less predictable than knee OCD; significant residual loss of extension is common; prevention is key in young throwers — pitch count limits and early diagnosis

• Prognostic factors for healing in juvenile OCD: open physis (best predictor); small lesion size (<2 cm²); lesion in a non-weightbearing zone; no mechanical symptoms; good MRI signal at the fragment margin; contrast enhancement on gadolinium MRI (vascularity of the fragment); versus closed physis, large lesions (>3 cm²), weightbearing zone involvement, and mechanical symptoms (locking) = poor prognosis for non-operative healing; early surgery should be considered in lesions with multiple poor prognostic features even if technically ICRS Grade I–II
• Fragment viability assessment before fixation: not all displaced fragments are suitable for refixation; factors indicating a viable fragment — large size (>50% of the crater); reasonable cartilage quality (no significant fibrillation or delamination); bone on the base of the fragment (not a pure cartilage flap); absence of extensive cystic degeneration within the fragment; a fragment that is purely cartilaginous with no bone cannot be biologically fixed and should be debrided; the decision for fixation vs removal + crater restoration is made at arthroscopy
• Tunnel view (notch view) X-ray: the PA view with the knee in 45° flexion (tunnel or notch view) profiles the posterior articular surfaces of the femoral condyles — the most common OCD site; the classic OCD lesion at the lateral aspect of the medial femoral condyle is often missed on a standard AP view; the tunnel view should be considered a mandatory view in any young patient with unexplained knee pain; failure to request this view results in delayed diagnosis

• OCD knee: lateral aspect of medial femoral condyle (70% of cases); juvenile (open physis = healing potential); adult (closed physis = poor healing); bilateral 20–30%
• MRI sign of instability: circumferential high T2 signal (`halo sign`) completely surrounding the fragment = fluid undercutting = UNSTABLE = surgical treatment indicated
• Wilson sign: internally rotate tibia, extend knee; pain at 30–60° = OCD medial condyle; relieved by external rotation; sensitivity 60–80%
• Tunnel (notch) view X-ray: MANDATORY for OCD assessment; PA with knee 45° flexion; profiles posterior condyles where OCD most commonly occurs; often missed on standard AP
• Non-operative for stable juvenile OCD (ICRS I–II): 6–12 weeks non-weight-bearing; serial MRI; 60–70% healing rate; fail to heal at 3–6 months = surgery
• Retrograde drilling: for stable non-healing OCD with intact cartilage; disrupts sclerotic base; promotes revascularisation; cartilage surface NOT breached
• Fragment fixation (ICRS Grade III): headless screws (Herbert/Acutrak); debride base + bone graft + fixation; 80–90% healing; preserves articular cartilage surface
• Grade IV (loose body): remove fragment; address crater by size — microfracture (<1 cm²), OATS (1–4 cm²), ACI/allograft (>4 cm²)
• OCD capitellum: lateral elbow pain in young throwers; anterolateral capitellum; tenuous single posterior blood supply; restricted extension; arthroscopic debridement for unstable/displaced
• Good prognostic factors: open physis + small lesion + non-weightbearing zone + no mechanical symptoms + gadolinium contrast enhancement