Commentary & Perspective

Commentary & Perspective on
"Posterior Slope of the Tibial Implant and the Outcome of Unicompartmental Knee Arthroplasty"
by Philippe Hernigou, MD, and Gerard Deschamps, MD

Commentary & Perspective by
Richard D. Scott, MD*,
Department of Orthopaedics, Brigham and Women's Hospital and New England Baptist Hospital, Boston, Massachusetts

This report contains useful information for those who perform unicompartmental knee arthroplasty. While the ideal candidate for unicompartmental knee arthroplasty is thought to be a patient with an intact anterior cruciate ligament¹, it remains controversial as to whether an absent or deficient anterior cruciate ligament is a contraindication to the procedure.

This study contains forty-nine knees with "damaged" (thirty-one) or absent (eighteen) anterior cruciate ligaments and reports the fate of those knees at a mean follow-up of sixteen years. The sixteen-year survivorship of most of these knees refutes the contention that an absent anterior cruciate ligament is an absolute contraindication to unicompartmental knee arthroplasty.

In addition, the effect of the posterior tibial slope of the tibial component on outcome is studied in patients with both normal and abnormal anterior cruciate ligaments. This also supplies very important information regarding proper surgical technique for this increasingly popular procedure.

Ninety-nine knees were entered into this retrospective study. At a mean follow-up of sixteen years, seventy-seven knees had not undergone revision. Standing lateral radiographs showed a linear relationship between anterior displacement of the tibia on the femur and the posterior slope of the tibial component. With increasing posterior tibial slope, there was increasing dysfunction of the anterior cruciate ligament with anterior subluxation of the tibia on the femur.

There were five late ruptures of intraoperatively "normal" anterior cruciate ligaments in knees with >13° of posterior slope of the tibial component. Among eighteen knees with an absent anterior cruciate ligament at the time of the arthroplasty, the eleven knees in which the implant survived all had a posterior slope that was <5°, whereas the failures all had a posterior slope of >8°.

This information helps to establish some technical guidelines for the surgeon who performs a unicompartmental knee arthroplasty. Whether the anterior cruciate ligament is intact or deficient, one should avoid excessive posterior slope of the tibial component of a sagittally nonconforming unicompartmental prosthesis to avoid late onset anteroposterior instability.

In this commentator's opinion, certain other criteria should be met before proceeding with unicompartmental knee arthroplasty in a patient with a deficient anterior cruciate ligament. The normal tibial wear pattern is anterior and peripheral in the early stages of osteoarthritis in the varus knee². As the anterior cruciate ligament becomes deficient, the wear pattern moves posteriorly. Unicompartmental knee arthroplasty is contraindicated, therefore, if the wear pattern of medial-compartment arthrosis has moved to the posterior third of the tibia.

In addition, when an anterior cruciate ligament deficiency affects the dynamics of the knee, an increasing lateral tibial thrust occurs, resulting in the so-called "kissing lesion" that consists of an osteophyte and erosion on the medial aspect of the lateral femoral condyle³. A large "kissing lesion" implies that mediolateral instability is appreciable and that stability probably will not be restored to the knee by the performance of unicompartmental arthroplasty.

One important aspect of this study was not sufficiently reported by the authors. They noted that seventy-four of the ninety-nine unicompartmental knee arthroplasties were medial-compartment replacements but failed to separate out the twenty-five lateral arthroplasties from the seventy-four medial ones. The normal wear pattern in lateral compartment disease is posterior for both the femur and the tibia. The adverse effects of a deficient anterior cruciate ligament and excessive posterior tibial slope should be accentuated in a lateral arthroplasty compared with a medial one. It would be of great interest to learn the results of the group that had lateral unicompartmental arthroplasty.

*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

1. Kozinn SC, Scott R. Unicondylar knee arthroplasty. J Bone Joint Surg Am. 1989;71:145-50.
2. White SH, Ludkowski PF, Goodfellow JW. Anteromedial osteoarthritis of the knee. J Bone Joint Surg Br. 1991;73:582-6.
3. Scott RD, Santore RF. Unicondylar unicompartmental replacement for osteoarthritis of the knee. J Bone Joint Surg Am. 1981;63:536-44.

Copyright © 2004 by the The Journal of Bone and Joint Surgery, Inc.