Commentary & Perspective on
"Patellar Impingement Following Unicompartmental Arthroplasty"
by P. Hernigou, MD, and G. Deschamps, MD

Commentary & Perspective by
Richard Scott, MD*,
Department of Orthopedic Surgery, Brigham and Women's Hospital, New England Baptist Hospital, Boston, MA

After unicompartmental knee arthroplasty, patellar complications are not frequently reported and are not a major cause of failure. Rather, most long-term studies report the most frequent modes of failure to be component wear, component loosening, and degeneration of the opposite compartment^1-8.

This study by Hernigou and Deschamps is the first to focus on the radiologic and clinical morbidity related to the patellofemoral compartment after unicompartmental knee arthroplasty at a follow-up of ten to twenty years. Although the authors reported some interesting observations, readers must be cautioned to avoid extrapolating these as "generic" findings since some are most likely related to use of a specific prosthetic design and surgical technique.

There are five predisposing factors in patellar impingement following unicompartmental knee arthroplasty.

The first is prosthetic design. The leading edges of the femoral component (i.e., the anterior and lateral edges in a medial unicompartmental arthroplasty) must be smooth and tapered to avoid any offset between the prosthesis and the trochlear articular cartilage.

The second factor is component size. Too large a component will make recession of its leading edges very difficult, allowing patellar impingement. The prosthetic system therefore should offer a spectrum of sizes.

The third factor is surgical technique. Proper technique creates a recess in the trochlear articular cartilage to accommodate the leading edges of the component. Intramedullary or extramedullary alignment jigs can be used to place the femoral component in proper flexion.

A fourth factor may be the natural orientation of the patella. Thus, a patient with patellar alta can achieve a greater degree of knee flexion before the point of potential impingement is reached. The authors of this study mentioned that they measured the Insall-Salvati index, but they did not report on those findings in the Results or in the Discussion.

A fifth factor, discussed in the article by Hernigou and Deschamps, consists of preoperative and postoperative joint congruence and patellar tilt. It is intuitive that a patella that tilts towards the resurfaced compartment is more likely to impinge whereas one that tilts toward the opposite compartment would promote secondary articular cartilage degeneration. It would be interesting to correlate the tilt observed on immediate preoperative and postoperative patellar radiographs with subsequent impingement or osteoarthritis. Should valgus knees with lateral patellar tilt automatically undergo a lateral release, with or without a medial advancement?

Factors that may predispose a patient who has undergone unicompartmental knee arthroplasty to patellar osteoarthritis, other than patellar tilt, include the disease process and the intraoperative findings. Chondrocalcinosis is a common finding in elderly women who have a valgus deformity and osteoarthritis in the lateral compartment. Thus, we might ask whether chondrocalcinosis is a contraindication to unicompartmental knee arthroplasty. Also, are asymptomatic focal areas of eburnated patellar bone a contraindication?

In summary, patellofemoral impingement and secondary osteoarthritis can occur in the second decade after unicompartmental knee arthroplasty in the medial or the lateral compartment. These complications may be responsible for some patellofemoral disability but are rarely a reason for revision. Their occurrence can be attributed to multiple factors including prosthetic design, surgical technique, and intrinsic patient characteristics.

*The author did not receive grants or outside funding in support of his research or preparation of this manuscript. He did not receive payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the author is affiliated or associated.

References

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