Osteochondritis Dissecans of the Knee

Osteochondritis dissecans (OCD) is an acquired idiopathic lesion of subchondral bone that can affect the overlying articular cartilage. It ranges from softening of the cartilage to complete detachment of a bony fragment (loose body). The knee is the most common location (75% of all OCD).

Etiology

Cause uncertain (multifactorial):

• Repetitive microtrauma: Most accepted theory (active adolescents)
• Vascular insufficiency: Ischemia of subchondral bone
• Genetic factors: Familial clustering reported
• Ossification: Aberrant ossification centre

Epidemiology

• Incidence: 15-30 per 100,000
• Age: Juvenile (open physis) vs Adult (closed physis)
• Gender: Male greater than Female (2:1)
• Bilateral: 20-30% of cases (always xray other knee)

Anatomy

Classic Location:

• Medial Femoral Condyle (MFC): 75% of cases
• Specific Site: Lateral aspect of the MFC (lateral wall of intercondylar notch)
• Other Sites: Lateral femoral condyle (20%), Patella (5%), Trochlea (less than 1%)

Why Lateral MFC?

• Impingement of the Tibial Spine or PCL against the lateral aspect of the MFC during knee motion
• Occurs near the insertion of the PCL

Pathophysiology

1. Subchondral Bone Event: Ischemia or stress fracture of subchondral bone
2. Overlying Cartilage: Initially intact (stable)
3. Separation: Bone fragment separates from parent bone
4. Instability: Cartilage breach allows fluid ingress (unstable)
5. Loose Body: Fragment detaches completely

The global-practice section names the ROCK group (Research in OsteoChondritis of the Knee) and its arthroscopic, probe-based classification — a modern, reproducible system worth knowing alongside the older Guhl/ICRS and Cahill schemes.

• The principle. ROCK divides lesions at arthroscopy (by probing) into two families — immobile (stable) and mobile (unstable) — because mobility on probing, not appearance alone, drives the drilling-versus-fixation decision.
• Immobile (stable) lesions — drilled if they fail non-operative care:
  • Cue ball — intact, normal-looking cartilage, no demarcation.
  • Shadow — a subtle demarcation visible through intact cartilage.
  • Wrinkle in the rug — a raised fissure or buckle of the cartilage surface, still stable.
• Mobile (unstable) lesions — need fixation or restoration:
  • Locked door — a demarcated lesion that cannot be hinged open on probing.
  • Trapdoor — a fragment that hinges open on probing.
  • Crater — a detached fragment/loose body leaving an empty crater.
• Why it matters. It gives a shared vocabulary that correlates with intra-operative stability better than MRI alone (recall the poor juvenile MRI specificity) and maps directly to treatment: immobile → drilling; mobile → fixation of a salvageable fragment, or restoration of an unsalvageable crater.

History

• Pain: Vague, poorly localized knee pain
• Activity-related: Worse with impact/sports
• Swelling: Intermittent effusion, especially after activity
• Mechanical symptoms: Catching, locking (suggests unstable flap or loose body)
• Onset: Insidious, durable

Physical Examination

Inspection:

• Quadriceps atrophy (if chronic)
• Effusion (mild to moderate)
• Gait: Antalgic, tibial external rotation (to avoid impingement)

Palpation:

• Wilson's Sign: Specific but low sensitivity
• Tenderness: Over the classical site (medial femoral condyle) with knee flexed 90°

Rehabilitation Protocol

• Weight Bearing:

  • Drilling: Immediate WBAT (stable)
  • Fixation: Touch-down for 6 weeks (protect repair)
  • Grafting: Non-weight bearing 6 weeks

• Range of Motion:

  • Early passive ROM (prevents stiffness)
  • CPM often used for cartilage repair

• Return to Sport:

  • Conservative: 3-6 months (MRI healing)
  • Surgical: 6-9 months (bone union on CT/MRI)
  • Criteria: No pain, full ROM, radiographic healing

Prognostic Factors

• Age: Strongest predictor. Open physis = Good. Closed = Poor.
• Size: Smaller lesions (less than 2cm) heal better.
• Location: Classic MFC heals better than Patella/Trochlea.
• Stability: Stable heals better than unstable.

Success Rates

• Juvenile Conservative: 50-70% healing rate.
• Drilling: 80-90% success in failed conservative juvenile.
• Fixation: 80-90% success if bone quality good.
• Excision: Poor long-term results (OA risk), good short-term pain relief.

The topic states that stable juvenile lesions heal roughly 50-70% of the time and that size is a key prognostic factor — but which lesions actually fail deserves spelling out, because it sets the surgical threshold and surveillance intensity.

• Normalise the size, do not just measure it. Lesion size is best expressed relative to the femoral-condyle width (a normalised ratio) rather than as an absolute millimetre value, because a given lesion is more significant in a smaller condyle. Larger normalised lesions are less likely to heal.
• The one-in-three rule. With a standardised non-operative protocol, roughly two-thirds of stable juvenile lesions heal by six months and about one-third fail — so a stable lesion justifies a non-operative trial but does not guarantee healing.
• Predictors of failure (which lower your threshold). Beyond a larger normalised size, an effusion (swelling) and mechanical symptoms (giving-way, catching, locking) at presentation predict failure to heal; notably, chronological age alone is a weaker predictor than physeal status and these lesion features.
• Practical use. A small, quiet, stable lesion in an open-physis knee can be watched with serial imaging and activity restriction; a large lesion with an effusion or mechanical symptoms should be counselled as high-risk for failing conservative care and moved earlier toward drilling.

Global Epidemiology

• Incidence roughly 15-30 per 100,000 children and adolescents; rising with year-round single-sport participation worldwide.
• Peak age 10-15 years (juvenile, open physis); a smaller adult cohort presents with closed physes.
• Male predominance approximately 2:1; bilateral in 20-30% (image the contralateral knee).
• Classic site is the lateral aspect of the medial femoral condyle; lateral femoral condyle lesions associate with discoid meniscus.

Side-by-Side Guidance

Registry and Restoration Data

• OCD is a leading indication for cartilage restoration registries (e.g. German KnorpelRegister DGOU, and ICRS-aligned datasets) tracking microfracture, OAT/mosaicplasty, ACI/MACI and osteochondral allograft outcomes.
• Registry signals mirror the literature: fragment preservation and osteochondral restoration outperform excision for long-term joint survival.

High- vs Limited-Resource Practice