Patellar height assessment in total knee arthroplasty: a new method

We described in 1981 a method to evaluate patellar height in normal and symptomatic knees on sagittal X-ray view. This index is a frequently used method, yet it is not suitable after a total knee arthroplasty (TKA).

The original method measures the distance between the distal margin of the articular surface of the patella (point A) and the anterosuperior angle of the tibial plateau (point T), then the length of the patellar articular surface (AP). The index is AT/AP ratio (normal values range from 0.8 to 1.2). After TKA, the T landmark is no longer available, so we must define a new T’ landmark. This point is situated at the intersection between the line perpendicular to the tibial posterior cortex elevated at the tip of the fibular head and the tibial anterior cortex. This remarkable landmark can be identified before and after TKA, with a new relative index AT’/AP ratio. This modified method allows the comparison of patella height before and after TKA.

We have used this modified index with the collaboration of several authors during the testing of different models of TKA, with an accurate reproducibility. Repeatability (usually called intra-observer reliability) was good, with intra-class correlation coefficients (ICCs) between 0.58 and 0.75 among the observers. Reproducibility (usually called inter-observer reliability) was also considered as good, with ICC ranging from 0.64 to 0.72.

Patella height measurement has to be assessed with the original method (AT/AP) to detect patella infera that could influence the surgical approach. The correlation between original and modified indexes has to be assessed. The modification of patella height after TKA could be evaluated through the modified index and compared with functional results.