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Commentary & Perspective | ||||||||
Commentary & Perspective on Unicompartmental designs have been more frequently used in Europe than in the United States, but unicompartmental knee replacement is now gaining prominence as a standard reconstructive option in the United States. The article by Argenson et al., from Marseille, France, provides intermediate clinical results of and survivorship data for a contemporary unicompartmental implant. Over time, unicompartmental knee designs have changed, as have the surgical techniques utilized to implant them and the selection criteria for patients who receive them. The "modern implant design" used in this study consists of a curved femoral component that articulates with a flat modular tibial component. Both components are cemented. The instrumentation facilitates accurate bone resection, tibiofemoral alignment, and tracking. The clinical results and patient satisfaction reported in this series are both good; most knees received high clinical scores (147 knees [92%] of 160 had Hospital for Special Surgery knee scores of 85 to 100 points, and eight knees [5%] had scores of 70 to 84 points), and, at the most recent follow-up, the average arc of active flexion was 128° postoperatively. The ten-year survivorship was 94% with revision for any reason or radiographic loosening as the end point. This report raises several important points in regard to design, patient selection, and surgical technique. Argenson et al. found no component loosening or impending loosening with use of this design and its instrumentation. Other reports1,2 in the literature have shown a consistent percentage of component loosening, more frequently on the tibial side. In a review of 100 knees treated with the Robert-Brigham unicondylar knee replacement, Kozinn and Scott reported that of the thirteen revisions, nine were performed because of loosening of one or both components1. In addition, Emerson et al. found a higher rate of component failure, especially for the tibial component, in a fixed-bearing design compared with a mobile-bearing design2. Thus, we can surmise that the improved component survivorship reported by Argenson et al. is a result of better component tracking, less edge-loading of the implant, and better centralization of the femoral component with respect to the tibial component. Patient selection continues to be key to the long-term success of unicompartmental arthroplasty. Three of the five revisions in this study by Argenson et al. were performed because of progression of osteoarthritis in the patellofemoral joint (two knees) and in the lateral tibiofemoral compartment (one knee). Patellofemoral changes were not included in the selection criteria for unicompartmental arthroplasty at the time of this study. However, the two patients who had a revision because of patellar arthritis had had "extensive erosions of the patellar cartilage at the time of the unicompartmental arthroplasty." The authors concluded that the adequacy of the patellofemoral articulation must be evaluated prior to or at the time of surgery. This reviewer shares that conclusion. Minor fibrillation or blistering of the cartilage and small full-thickness cartilage defects on the medial facet of the patella or trochlea does not preclude a successful unicompartmental arthroplasty. More extensive degeneration or maltracking of the patella is probably a contraindication to unicompartmental arthroplasty. The cartilage in the opposite tibiofemoral compartment must be healthy. In the series reported by Argenson et al., the one knee that was revised to a total knee arthroplasty had progressive osteoarthritis in the lateral tibiofemoral compartment. This reviewer has not found minor joint-line tenderness to be a concern, but joint-space narrowing demonstrated on standing radiographs or, better yet, on stress radiographs, as recommended by Argenson et al., indicates that a revision to a total knee arthroplasty should be considered. This reviewer has not found arthroscopy necessary to make this assessment, but Romanowski and Repicci3 have recommended a limited arthroscopic evaluation of the opposite compartment. The optimal tibiofemoral alignment following unicompartmental arthroplasty has yet to be determined. All orthopaedic surgeons agree that extremes of overcorrection and undercorrection are undesirable. Undercorrection will potentially overload the implant, and overcorrection will promote progression of osteoarthritis in the opposite compartment. Argenson et al. recommend moderate undercorrection of postoperative varus deformity for a medial replacement. In the group that had a medial unicompartmental arthroplasty, the average preoperative deformity of 9° of varus was corrected to an average of 5° of varus postoperatively, which places the weight-bearing axis through the central part of the medial compartment. The lateral unicompartmental arthroplasties were corrected from an average of 8° of valgus preoperatively to an average of 4° of valgus postoperatively, which is probably closer to the optimal alignment. There were many more medial unicompartmental arthroplasties (145) than lateral unicompartmental arthroplasties (fifteen) in their series. Two medial unicompartmental arthroplasties were revised because of polyethylene wear. These numbers are too small to permit a conclusion regarding the optimal alignment of medial or lateral unicondylar arthroplasty. No soft-tissue releases were done in this study, which may predispose the prosthesis to some undercorrection. This reviewer believes that the postoperative alignment of the medial implants should be closer to the physiological valgus alignment of 5°3. Logic suggests that this alignment would optimize the load across the knee and thus should promote the longest period of survivorship. *The author did not receive grants or outside funding in support of the research or preparation of this manuscript. The author received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Biomet). 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. | ||||||||
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