Bearing Surfaces in Total Knee Replacement

Despite positive results over the past 20 years for total knee arthroplasty (TKA), ensuring long-term survivorship and low revision rates has been a goal that is being achieved through improvements in prosthetic bearing surfaces and joint kinematics. The five manuscripts in this special section will highlight several new bearing surfaces and designs in TKA, as well as revisit previous technologies and their present role.

A limitation to the overall successes of TKA is the 5 to 20% dissatisfaction rates despite improved outcomes and reduced pain. This dissatisfaction might be a result of the disruption of the natural knee joint kinematics. The principle behind mobile-bearing/rotating platform TKAs is that they create a rotational articular surface, which decreases the rotational forces across the knee. White et al highlighted kinematic and in vitro studies of these bearings that have demonstrated their benefits compared with fixed bearings, although there are conflicting data concerning their clinical benefits. Varadarajan et al focused on a new innovative design, the biomimetic knee, which could provide a more “natural” feeling and function to the knee joint. These novel implants are reversed engineered from magnetic resonance imaging and biplanar fluoroscopy to provide a three-dimensional representation of the patient's knee through the full range of motion. Currently, computational results have shown improved kinematics over standard implant designs; however, we are awaiting in vivo analyses to determine a definitive conclusion.

Implant survivorship of TKA over the past 20 years has greatly improved; however, polyethylene wear and eventual aseptic loosening are still major causes of revision. Chakravarty et al highlighted several new developments in the processing and production of polyethylene inserts which have markedly improved wear rates. Of note, the development of sequentially irradiated/annealed and vitamin E polyethylene has been of substantial interest. Sequentially, irradiated/annealed polyethylene has been demonstrated to retain the mechanical properties of highly cross-linked polyethylene with a substantial reduction in free radicals and lower wear rates. Similarly, vitamin E has been added to the polyethylene processing, which is thought to reduce oxidation and subsequent free radicals; however, there are still questions of strength and fatigue resistance. Oxidized zirconium knee arthroplasties have been growing in popularity since 2003. These prostheses have been purported to provide the durability of a metal component, while providing a reduced coefficient of friction of a ceramic surface without the fragility. Schuttler et al reported on several studies demonstrating that oxidized zirconium bearing surfaces have led to decreased wear rates in retrieval analyses. Doran et al revisited the use of all-polyethylene tibial components with the advances in prosthesis designs and biomaterials, as well as through advances in surgical technique, many of the shortcomings of these components have been addressed and can be used in all patient populations with a few exceptions.

Since the development of the original TKA, we have made great advances upon current prosthesis designs to improve clinical outcomes, patient satisfaction, and implant survivorship. Currently, much of the developments described in this issue are in the early stages of clinical use. Although quite promising, long-term outcome studies are needed to further define their place in the knee arthroplasty field.