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Patellar instability

Case contributed by Praveen Jha
Diagnosis certain

Presentation

Patient presented with recurrent patellar dislocation.

Patient Data

Age: 30 years
Gender: Female
mri

Lateral patellar subluxation is seen, with medial patellofemoral ligament tear. Moderate knee joint effusion is seen with associated patella alta. The demonstrated images show (in order):

Sagittal T2 shows patellar height ratio normal value should be 1. Patella alta seen in this case.

Axial STIR shows trochlear facet asymmetry calculated as 10/24 x 100 = 41%Normal value is 60-70%. Less than 40 degree is trochlear dysplasia. This case is borderline trochlear dysplasia.

Axial STIR shows lateral trochlear inclination. Normal value 20-25 degree & lt 11 degree is considered trochlear dysplasia.

Axial T1 shows lateral patellofemoral angle. Angle opening laterally is considered normal. In this case, it is opening medially.

Axial T2 shows 'Q' angle & lt;15 mm is normal,15-20 mm is borderline. Distance of more than 20 mm is significant, as seen in this case, and suggests medial patellofemoral subluxation. 

Axial STIR shows trochlear depth. It is calculated as (55.6+50.9/2) - 48.9 = 4.0 mm. Normal depth is 12 mm. Less than 3 mm is considered dysplasia. This case shows borderline trochlear dysplasia.

Case Discussion

Patellar instability with lateral patellar subluxation was observed operatively. Medial patellofemoral ligament (MPFL) repair was done. However, trochlear dysplasia was not considered severe enough by orthopaedician, for surgical correction.

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