Patellofemoral arthroplasty (PFA) has a long record of use in the treatment of isolated patellofemoral arthritis, with outcomes influenced by patient selection, surgical technique, and trochlear implant design.

The trochlear components have evolved from inlay-style to onlay-style designs, which have reduced the incidence of patellar instability.

Inlay-design trochlear prostheses are inset within the native trochlea, flush with the surrounding articular cartilage. The component rotation is therefore influenced by the native trochlear inclination, which tends to be internally rotated relative to the anteroposterior and transepicondylar axes of the femur, accounting for the high incidence of patellar instability with inlay-design components.

Onlay-design trochlear components are implanted perpendicular to the anteroposterior axis of the femur, resecting the anterior trochlear surface flush with the anterior femoral cortex and positioning the implant irrespective of the native trochlear inclination, which is the number one reason for the significant improvement in patellar tracking with onlay-style trochlear implants.

Minimizing the risk of patellar instability with onlay-design PFAs has enhanced mid-term and long-term results and leaves progressive tibiofemoral arthritis as the primary failure mechanism beyond 10 to 15 years.

Revision PFA to an onlay-design is reasonable to consider in the situation of a failed inlay-style trochlear prosthesis, if no tibiofemoral arthritis is present. Otherwise, revision to total knee arthroplasty can yield predictable results.