CIM Case: Knee OCD - Skeletally Immature

Clinical Scenario

Patient: 12-year-old male Presentation: 6-month history of anterior knee pain worsened by activity, occasional locking and giving way Relevant history: Active sportsman playing football and basketball, no previous knee injuries, pain started insidiously, now limiting sport, occasional swelling after activity Examination findings:

• Mild effusion
• Tenderness over medial femoral condyle (palpable with knee flexed 90°)
• Wilson test positive (pain with internal rotation during extension 90° to 30°, relieved by external rotation)
• Full range of motion
• No ligamentous laxity
• No meniscal signs
• Normal alignment
• Neurovascularly intact

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Investigations Provided

Laboratory Results

Imaging

Image 1: AP, Lateral, and Tunnel View (Notch View) Knee Radiographs

Radiological features:

• Well-circumscribed lucent lesion on lateral aspect of medial femoral condyle
• Lesion size approximately 15mm × 12mm
• Sclerotic margin visible
• No loose body in joint
• Physes open (skeletally immature)
• No evidence of fragmentation on X-ray
• Classic location for OCD

Image 2: MRI Knee

MRI findings:

• 15mm lesion on lateral aspect of medial femoral condyle (classic location)
• T2-weighted: High signal rim behind fragment (fluid signal)
• Subchondral cysts visible
• Articular cartilage disrupted (not intact)
• Fragment appears in situ but unstable
• No displaced loose body
• Minimal joint effusion

Questions & Model Answers

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Key Teaching Points

Pattern Recognition

This pattern suggests Juvenile OCD:

• Adolescent with activity-related knee pain
• Pain localised to medial femoral condyle
• Positive Wilson test (specific but not sensitive)
• Effusion with activity
• X-ray: Lesion on lateral aspect of medial femoral condyle
• Open physes

Classic OCD Location:

Wilson Test:

• Patient extends knee from 90° to 30° with tibia internally rotated
• Pain reproduced as tibial spine impinges on OCD lesion
• Relief with external rotation
• Specific but not sensitive (many false negatives)

Critical Management Points

1. Stability determines management - not age alone
2. T2 rim sign on MRI = unstable - needs surgery regardless of age
3. Juvenile OCD has better prognosis - but unstable still needs surgery
4. Save native fragment if possible - bioabsorbable fixation preferred
5. Prepare bed to bleeding bone - key to healing
6. Compliance is critical - set expectations with family
7. Criteria-based return to sport - not calendar-based

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Common Examiner Follow-ups

Q: "What is the Wilson test and how does it work?"

The Wilson test is specific for medial femoral condyle OCD:

• Mechanism: The tibial eminence impinges on the lateral aspect of the medial femoral condyle during knee extension with internal rotation
• Positive test: Pain at 30° flexion (where OCD lesion is contacted) that resolves with external rotation
• Sensitivity: Low (30-50%)
• Specificity: High (80-90%)

A negative Wilson test does NOT exclude OCD. The test is useful when positive but cannot be used to rule out the condition.

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Q: "What if the fragment falls into the joint during fixation?"

Intraoperative fragment displacement:

1. Don't panic - this is manageable
2. Retrieve the fragment (loose body grasper)
3. Assess quality - can it still be salvaged?
4. If yes:
   • Debride both surfaces
   • Bone graft the bed
   • Fix in place
5. If no (fragmented, poor cartilage):
   • Remove fragment
   • Prepare bed
   • Microfracture if small
   • OATS if large
   • Consider staged ACI if very large

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Q: "What is retroarticular drilling?"

Retroarticular drilling is a technique to promote healing of OCD without violating the articular cartilage:

Technique:

• Done under fluoroscopic guidance
• Drill from outside the joint (extraarticular) into the subchondral bone of the OCD lesion
• Penetrate the sclerotic zone without breaching the cartilage surface
• Creates channels for vascular ingrowth from healthy bone
• Used for stable lesions with intact cartilage but concerning MRI appearance

Advantages:

• Preserves articular surface completely
• Promotes biological healing
• Minimally invasive

Disadvantages:

• Requires fluoroscopy
• May not be sufficient for unstable lesions

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Q: "When would you use OATS versus ACI?"

General Approach:

• Small defects (<1.5cm): Microfracture
• Medium defects (1.5-4cm): OATS
• Large defects (>4cm): ACI or OATS mosaicplasty
• Very large defects: ACI preferred (no donor site issue)

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Related Topics

• Patellar Instability
• Meniscal Tears
• ACL Injuries in Children
• Tibial Spine Fractures
• Knee Arthroscopy
• Cartilage Restoration