Trochlear dysplasia with prior ACL reconstruction
A young dancer with full thickness ACL tear, repaired 18 months ago, complaints of persistent discomfort and give-way sensation on movement of knee.
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Post arthroscopic case of ACL reconstruction shows screw fixation at lateral femoral and medial tibial condyle with no loosening and minimal adjoining bone marrow edema. ACL is of intermediate signal intensity and appears parallel to the blumensaats line.
The patello femoral articulation is incongruent, with lateral patellar instability. Findings evident on axial sections are:
- Thickened medial patellar retinaculum and tibiofemoral ligaments.
- Dysplastic trochlear fossa
- Flattened and asymmetrical medial facet
- Abnormal medial-lateral facet ratio <40% MT/LT
- Mildly reduced troclear depth 2.3mm
The lateral trochlear inclination angle is normal. There is not obvious ventral prominence either. No obvious features of articular cartilage edema/erosion or medial patellofemoral ligament (MPFL)scarring/tear/avulsion are seen.
Hence it was concluded that the give-way sensation was attributable to the transient lateral patellar instability due to coexistent mild degree of femoral trochlear dysplasia and not a post arthroscopic consequence.
Femoral trochlear dysplasia is a well known entity, many signs and measurements are used to categorise and characterise the degree of dysplasia, on xrays and MRI.
MRI is the best modality to evaluate the signs, measurements and features of soft tissue and bony injury in acute or chronic patellar instability consequent to trochlear dysplasia. One should however be careful to measure them 3 cm above the tibiofemoral joint space to avoid eroneous calculations.
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