Introduction: Prosthesis Design

The mobile-bearing unicondylar knee arthroplasty design comprises a spherical metal component and a flat tibial baseplate with a fully congruent mobile polyethylene bearing. The aim of the device when first conceived in 1976 was to protect against polyethylene wear and component failure while retaining a high range of motion. The principles of the design were to provide an implant constrained only by the restored natural tensions of the ligaments and muscles. A fully congruent bearing allowing maximal contact area between the prosthesis and bearing reduces polyethylene wear and, to enable this congruity without constraining the prosthesis, the bearing is fully mobile. The upper surface of the mobile meniscal polyethylene bearing is spherically concave, matching the spherical surface of the prosthetic femur. The undersurface of the meniscal bearing is flat, lying on the tibial baseplate. This configuration of a spherical femur, flat tibia, and fully unconstrained and mobile meniscal bearing enables physiologic tension to be maintained in the retained anterior cruciate and medial collateral ligaments over the entire range of knee flexion. The meniscal bearing allows sliding and rolling with mainly a compressive stress applied to the underlying bone, decreasing shear stress at the implant-bone interface, which is an important factor in component loosening.

The implant most commonly used is the Oxford unicompartmental knee replacement.¹ This design was originally used as a bicompartmental device but was first used as a unicompartmental device in 1982 (Phase 1). Since then two further design phases have been introduced, retaining the design features while improving the instrumentation and the method of implantation. In Phase 1 the femoral surface was prepared with cutting blocks to remove angular cuts of bone to fit the nonarticular surface of the femoral component. However, this did not allow accurate balance of the flexion-extension gap at implantation, and the Phase 2 prosthesis was introduced in 1987 and sought to improve this. The femoral component was changed to a spherically concave inner surface and the femoral bone surface was prepared with a mill rotating around a spigot. Incremental 1-mm milling of the distal femoral surface allows careful balancing of the knee ligaments throughout flexion (Fig. 18–1). The Phase 3 design, introduced in 1998, provided new instruments and an increased range of components to facilitate the implantation through a short incision. The aim was to gain the short-term advantages associated with this less invasive incision² while maintaining the longer term survival, by maintaining the original design features of the fully congruent articulation (Fig. 18–2). Although the Oxford mobile-bearing unicompartmental knee was the first of its kind, there are several other mobile-bearing knees based on a similar philosophy. The AMC knee (Uniglide) utilizes a fully mobile bearing restoring the natural tension within the knee; however, there is a difference in the shape of the femoral component. The radius of the femoral component is constant up to 45° of flexion but decreases toward the posterior portion of the condyle.

Figure 18–1 The Phase 1 and Phase 2 Oxford Partial Knee Replacement, showing the progression from distal femoral cuts in the Phase 1 to spherical distal femoral surface in Phase 2.

Figure 18–2 (A) Intraoperative photograph showing the minimally invasive incision possible with the Phase 3 Oxford Partial Knee Replacement. (B) Postoperative radiograph of the Phase 3 Oxford Partial Knee Replacement. Note the skin clips in place to show the incision.

We have reviewed the long-term results of the prostheses mentioned above. There is substantial literature regarding the Oxford Knee and more limited literature about the other prosthesis. Long-term outcome is an important factor since impressive early results of an implant may dramatically change with longer term follow-up. For example, in the PCA fixed-bearing unicompartmental prosthesis, 90% of patients were reported as having a satisfactory functional result at 2 years,³ yet the cumulative survival at 3.7 years was only 77%.⁴ Our aim is to review the long-term results, at 10 years and greater, of mobile-bearing unicompartmental replacements.

Measuring Outcome: Explanation of Study Types

Assessing Survivorship

The measurement of success in arthroplasty surgery is important to provide information not only for the surgeon selecting the type of surgery and individual prosthesis to use but also when counseling patients regarding expected outcome from an intervention. The most commonly used measure of success is the survival of the implant (time to revision), but it is also important to assess the functional outcome of the arthroplasty in surviving patients, since this provides more complete information regarding the success of the patients’ treatment than pure survival analysis.⁵ Modern survival analysis will define a failure end point and provide an assessment of how many patients failed and how long after the operation this failure occurred. It is therefore important to understand the exact nature of the defined end point in each study as some authors use removal of the implants as the failure end point, whereas others use further surgery of any type. The difference between the two end points may affect the reported survival. Survival analysis may be performed using the life table method or Kaplan-Meier method. Whichever is employed, there are a number of features that must be understood about the reported data. Survival figures are cumulative, which allows a prediction of the expected failure rate in the long term, reducing the need for large numbers of prostheses to have reached the long-term follow-up point. However, the number at risk at each time point must be known since, if this number reduces to less than 15, it becomes difficult to interpret the data. Loss to follow-up is also important, and studies should present worst- and best-case scenarios.