Background: The mobile-bearing feature of the Oxford unicompartmental knee replacement has the potential to optimize polyethylene wear, thereby leading to longer-term function of the implant. The function of the bearing requires intact soft tissues, with the ligaments being balanced throughout the range of motion intraoperatively through bone resection only. Final limb alignment is determined by the restored soft-tissue tension. The purposes of this study were to determine the limb alignment achieved in the absence of ligament release and to investigate the interplay of failure mode, survivorship, and limb alignment.
Methods: Fifty-five knees in fifty-one patients with medial compartment osteoarthritis had a unicompartmental replacement with an Oxford prosthesis. Evaluation included Knee Society clinical scores, radiographic evaluation, survivorship analysis, and modes of failure. The average duration of clinical follow-up was 11.8 years. Only two patients (three knees) were lost to follow-up.
Results: The mean postoperative Knee Society knee score and function score at the latest follow-up evaluation were 75 and 90 points, respectively. The overall alignment of the knee was restored to neutral, averaging 5.6° of valgus alignment. Forty-seven of the fifty-five knees had the mechanical axis crossing the central 50% of the tibial plateau. Seven knees had revision surgery, and six of them required conversion to a total knee prosthesis. The main reason for revision was the progression of arthritis in the lateral compartment, which occurred in four knees at an average of 10.2 years postoperatively. These four knees had not been overcorrected into excessive valgus at the time of the original surgery, and we found no correlation, with the numbers studied, between alignment and bearing size. Survivorship analysis showed that the rate of survival at ten years was 85% with failure for any reason as the end point, 90% with progression of lateral compartment arthritis as the end point, and 96.3% with component loosening as the end point.
Conclusions: With this unicompartmental knee arthroplasty, the mechanical limb alignment resulting from balancing the knee ligaments, accomplished without releasing them, was consistently through the center of the knee. Progression of arthritis in the lateral compartment was the most common reason for late failure in this series and was not related to the initial postoperative alignment.
Level of Evidence: Therapeutic Level IV. See Instructions to Authors for a complete description of levels of evidence.