Mobile Versus Fixed Bearings in Total Knee Arthroplasty

Why Consider a Mobile-Bearing Alternative?

Fixed-bearing knees, whether cruciate retaining or substituting, have a good track record at 10 to 15 years of follow-up. My own 10-year experience with fixed bearings demonstrates 100% survivorship of femoral components, tibial components, and all-polyethylene patellar components. Reoperations were necessary in 5% of cases with metal-backed patellar components and 2% of cases without the patella resurfaced; 4% of flat modular tibial inserts wore to a point at which they needed to be exchanged by 10 years after surgery. Metal-backed patellar components are no longer used in most systems.

Controversy persists regarding the need for universal patellar resurfacing. A 2% failure rate of unresurfaced patella at 10 years still makes nonresurfacing a viable option for selected patients in both fixed-bearing and mobile-bearing articulations.

The 4% incidence of tibial insert wear requiring reoperation is the problem that in theory might be addressed by mobile-bearing articulations. These were designed to allow high conformity between metal and plastic to minimize stresses and lower the potential incidence of wear. Stress on the polyethylene is related to conformity and can be related mathematically to the difference in the radius of curvature between one articulation and the other. The greater the difference, the greater is the stress, and the smaller the difference, the less the stress on the polyethylene. Round-on-round and flat-on-flat articulations therefore would represent low-stress articulations, whereas round-on-flat articulations could create high stresses. By the mid-1990s, most knee system designers had abandoned round-on-flat articulations in favor of those with more top-side conformity.

It must be remembered, however, that many factors other than conformity affect polyethylene wear ( Box 15.1 ). These include the method of fabrication of the polyethylene—that is, whether it is compression molded or bar extruded. The resin used also makes a difference, as does quality control. Other factors include the surface preparation, the thickness, the effect of gamma radiation on oxidation, dynamic forces such as sliding and shearing that are imparted by the individual patient, and the opposing surface (the femoral side) and undersurface (the back side).

Back-side wear has received a lot of attention. Retrievals from virtually all modular total knee arthroplasty systems have been shown to have a variable pattern and extent of back-side wear. My experience with modular tibial inserts began in 1984. It was extremely rare to encounter back-side wear from inserts implanted from the mid-1980s to the early 1990s. It became more frequent among many total knee systems in the mid-1990s. Most likely, multiple factors were involved; they might include the resins used, methods of polyethylene preparation, and sterilization methods, especially gamma radiation in the presence of oxygen. An important contributory factor also must be the increased conformity of the top side of the inserts that were then in use. With round-on-flat inserts, forces were dissipated at the top surface of the insert before they were transferred through the polyethylene to the insert–tray interface ( Fig. 15.1 ). On the other hand, conforming inserts would transfer this stress directly to the back side. It is probably not a coincidence that I have some patients with bilateral knee replacements with a conforming insert on one side and a flat insert on the other who suffer back-side wear and osteolysis on the conforming side and more benign top-side wear on the flat side.

Herein lies the first of several potential advantages of rotating-platform mobile-bearing knees. This type of articulation can provide high conformity on the top side to minimize surface wear without the adverse effects of constraint. Back-side wear is addressed by allowing the undersurface of the insert to move freely as determined by the dynamic forces across the knee on a flat-on-flat surface that provides the least amount of stress to the polyethylene. In addition to being a flat-on-flat articulation, the rotating-platform insert articulates on a highly polished chrome-cobalt surface. It also mandates unidirectional rather than multidirectional wear on both the top side and the back side. Unidirectional wear is known to be favorable to the longevity of polyethylene.

A second important advantage of a rotating-platform mobile bearing is the ability of the prosthetic articulation to accommodate to malrotation between the femur and the tibia that is created at operation by the surgeon or occurs postoperatively during functional activities. This malrotation is what imparts torsional stresses through the conforming insert to the back side in fixed bearings. In a rotating-platform knee, the surgeon can choose optimal placement of the tibial tray on the proximal tibial bone and allow the insert to accommodate to the femur in all degrees of flexion ( Fig. 15.2 ).

Yet another advantage of mobile bearings is maintenance of high contact area in high flexion. During high flexion in both the normal and replaced knee, the lateral condyle of the femur generally rolls posteriorly on the tibial plateau. In fixed bearings, this high rollback cannot occur in the presence of conforming sagittal topography. It can occur with round-on-flat articulations, but this can lead to catastrophic late posterior polyethylene wear. A mobile-bearing articulation allows posterior translation of the femur on the tibia to occur in high flexion while maintaining high conformity at the top-side articulation ( Fig. 15.3 ).

Mobile bearings without stops can subluxate, impinging on soft tissues, and even possibly dislocate. Bearings with stops, however, might be subject to wear because of repetitive impingement on the stop. To prevent impingement, most mobile-bearing inserts are slightly smaller than equivalent fixed-bearing inserts ( Fig. 15.4 ). This allows a certain amount of rotation or translation to occur on top of the tray before soft tissue impingement can occur.

Mobile-bearing knees also are somewhat more sensitive to certain technical considerations, whereas they are forgiving to others, such as malrotation. For rotating-platform knees, the most significant technical complication is “spinout” of the bearing (dislocation), usually a result of flexion gap asymmetry. As a consequence of flexion gap asymmetry with or without posterior cruciate tightness, a mobile bearing could fail early with spinout. Fixed-bearing knees, however, may be able to tolerate flexion gap asymmetry for a much longer period, failing late as a result of progressive instability problems or polyethylene wear. The surgeon is implicated for the early mobile bearing failure, whereas the natural course of events may exonerate the surgeon in a fixed-bearing knee.