Patellar instability - recurrent lateral patellar subluxation
History of recurrent patellar dislocations.
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MR findings related to the patellofemoral dislocation associated injuries: There is evidence of small bone contusion seen involving the medial facet of the patellar articular surface. A bone contusion also noted in the lateral femoral condyle. There is also evidence of patellofemoral joint effusion along with thickening and increased signal intensity of the medial patellar retinaculum as well as medial patellofemoral ligament. The vastus medialis oblique muscles however appears to be intact and normal. Lateral patellar retinaculum also appears to have mild increased signal intensity. The patellofemoral joint cartilage is however normal. A small globular appearing hyperintense structure measuring approximately 5.3mm is noted in the patellofemoral joint on its superolateral aspect- this is likely to represent displaced osteochondral fragment. Tibiofemoral joint effusion also noted.
Findings related to chronic patellar instability:
- Thickening an edema of the medial patellar retinaculum and patellofemoral ligament is evident with evidence of significant patellofemoral joint effusion. Partial tear/sprain of the medial patellar retinaculum is noted at the site of patellar attachment while the retinaculum fibers at the femoral attachment appear to be intact. The tear is leading to mildly laxed appearance of the patellar retinaculum.
- The femoral trochlear sulcus appears to be relatively flat with asymmetrical appearance of the medial and lateral trochlear facet. The femoral trochlear sulcus angle is approximately 155° (measured at the articular cartilage) which is increased indicating shallow femoral trochlea with trochlear dysplasia (Type-C dysplasia).
- The lateral trochlear inclination angle is only approximately 4° (normal greater than 11°) which is also indicative of trochlear dysplasia.
- Trochlear facet asymmetry is noted with altered facet ratio and increased size of the lateral trochlear facet.
- The insall salvati index (patellar height ratio) is 129- which is just within +2SD range.
- The distance between the tibial tubercle to the deepest point of trochlear groove is approximately 19.7mm which is mildly increased (normal is 15 mm and > 20mm is highly indicative of laterally placed tibial tubercle).
These above features suggest that patellar instability is predominantly due to dysplastic and shallow of femoral trochlea with asymmetric appearance of the femoral trochlear facet (Type-C trochlear dysplasia). The other contributing factor to patellar instability may include relatively lateral location of the tibial tubercle in relation with the deepest point of the femoral trochlear groove.
The insall-salvati index is just within normal range and patellar alta is unlikely. The injury pattern described above is highly indicative of recent/recurrent lateral patellar dislocation with resulting sprain/injury of the patellar retinaculum and medial patellofemoral ligament which as a result appears to be laxed - this factor may be contributing to the chronic patellar instability