Patella alta

Case contributed by Weiling Tan

Presentation

Right knee recurrent patella dislocation, anterior knee pain. No trauma.

Patient Data

Age: 25 years
Gender: Male

AP, lateral, skyline

x_ray

Bilateral patella alta noted. The Caton-Dechamps index measures 1.9 on the right and 18 on the left. 

Caton-Deschamps index

Photo

The Caton-Deschamps index for the evaluation of patellar height is the ratio of AT:AP and has a normal value range of 0.6–1.2. Values less than 0.6 are indicative of patella infera and values greater than 1.2 are indicative of patella alta 3.

Case Discussion

Patella alta is a condition also known as high riding patella and associated with patellar instability. Such patients commonly present with anterior knee pain or recurrent patellar dislocation.

This condition is usually assessed on the lateral knee radiographs, with flexion of about +/- 30 degrees. This is because this position helps to relax the surrounding structures including the posterior capsule. 

It is important to understand the movement of the patella at various knee flexions. At the complete extension of the knee, the patella rests on the supra patellar fat pat and moves inferiorly with increased knee flexion. At 20-30 degrees flexion of the knee, the patella lies in the shallow part of the intercondylar groove with the inferior pole in contact with the femur.

However, with patella alta, the patella only engages in the trochlear groove at high knee flexion. Therefore, performing the knee lateral radiograph with the knee in 20-30 degree flexion is important to ensure accurate measurement of the various ratios. 

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