Anterior femoral offset is a flawed measurement of patellofemoral overstuffing

Patellofemoral joint (PFJ) issues after total knee arthroplasty (TKA) are becoming a topic of interest once again. Due to the complex three-dimensional shape of the trochlea, various two-dimensional proxy measurements on plain X-rays have been described. One of these measurements is the anterior femoral offset (AFO). It is the distance between the anterior femoral cortex and the trochlea's most anterior point (MAP) on a true lateral X-ray after TKA. Because the relationship between the trochlea's MAP and its groove, which is the primary joint surface articulating with the patella, is unknown, the purpose of this study was to measure the distance between the MAP and the trochlear groove.

After femoral component implantation, the surgeon identified the trochlea's MAP and the trochlear groove in ten consecutive TKAs and measured their distance. Measurements were performed with a ruler on a true lateral photograph and with a radiographic marker on a lateral radiograph in four different knee flexion angles, according to a previously published protocol.

The trochlear groove had a mean distance from the MAP between 2.09 ± 0.15 and 5.50 ± 0.17 mm, depending on the position. In no case is the trochlear groove visible on a true lateral view.

On a true lateral X-ray, the trochlea's MAP omits the trochlear groove. Because the patella primarily articulates with the trochlear groove and because the relationship between the MAP and the trochlear groove is variable depending on the knee flexion angle, any conclusions regarding overstuffing based on a postoperative lateral knee X-ray are flawed.