A recent study by Feucht et al, published in the KSSTA Knee Journal, compared the Arthrosurface HemiCAP® Wave implant, which is based on an "inlay" Arthroplasty design, versus an "onlay" design implant for isolated patellofemoral disease. While both implant groups showed similar improvements in function and pain scores, none of the patients in the "inlay" group showed progression of osteoarthritis (OA). However, 53% of the patients in the "onlay" group, showed progression of OA in other parts of the knee, namely the femoral-tibial compartments. Progression of OA is the number one reason for revising a patellofemoral implant to a Total Knee.
Steven Ek, President and CEO of Arthrosurface® commented, "Over 20 years ago, the orthopedic community largely abandoned patellofemoral joint arthroplasty. A relatively high number of these first generation implants, which were simply mounted on top of the existing arthritic joint surfaces, were revised early to total knees due to the progression of OA. In 2006, after reviewing the clinical history of PF arthroplasty, Arthrosurface® took a completely different approach in the design of its PF implants. They were specifically created to be recessed and set into the arthritic joint surface, preserving the native biomechanics of the knee and replicating the patient's existing joint surface. This novel concept became known as an "inlay" implant. I believe our patient's remarkable success stories and their unmatched return to high-demand activities reignited interest in treating isolated PFJ disease within the orthopedic community. Many of the large orthopedic companies have since re-introduced an updated version of their PF devices, but they are all still "onlay" designs. I truly believe that they have missed the point. This publication identifies the profound clinical outcome differences in tibiofemoral OA progression between "onlay" and "inlay" devices. We are very happy that this study has validated our design team's insight. Our hope is that younger surgeons who have begun treating patients using PF implants will fully appreciate the clinical history of PF arthroplasty devices in the context of this study."
While it is not known why over half of the patients with the "onlay" devices developed progressive OA in the other knee compartments, the authors of the study stated that, "one hypothesis is that the more anatomic approach of the inlay design better reproduces the complex kinematics of the patella-femoral joint. Soft tissue irritation due to over-stuffing of the patella-femoral joint with an "onlay" component may cause persistent synovitis, which is a well-known risk factor for the development and progression of OA due to secretion of pro-inflammatory cytokines."
Matthias Schurhoff MD, VP of Clinical Affairs at Arthrosurface, commented, "Historically, problems related to poor implant design are seen early on, usually within the first few years after implantation. This study highlights a concern that 'onlay' implants for isolated PF disease may disturb normal knee kinematics leading to overloading and disease progression in other parts of the joint. 'Inlay' arthroplasty may offer younger patients a better long term option, especially if they want to continue to stay active."