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Foreword - Rotating-platform Knees: Demonstrating Advancements in Total Knee Arthroplasty

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The presence of congenital anatomic abnormalities or previous traumatic knee injuries often results in premature arthritic change in the knee joint. This results in the presentation of patients who require total knee arthroplasty (TKA) at a young age. Typically, these subjects have increased activity requirements and expect their implant to survive three or more decades. To meet the needs of these patients, future TKA design must improve functional performance and reduce articular bearing surface wear while maintaining the excellent long-term fixation typically obtained in properly aligned and balanced TKAs currently in use.

The use of mobile-bearing designs offer the potential of improved performance by creating knee kinematic patterns that more closely mimic those of the normal knee.¹ As well, newer designs of knee replacement systems offer reduced polyethylene wear due to increased implant conformity and contact area with resulting reductions in polyethylene stresses. ^2-5

Currently, regulations of the Food and Drug Administration restrict the sale of mobile-bearing knee systems to two orthopedic manufacturing companies in the United States. However, a review of international markets demonstrates growing worldwide interest in the concept of mobile-bearing TKA, reflected by the presence of many mobile-bearing TKA designs being marketed and sold by multiple international implant manufacturers. These designs differ in condylar geometry and bearing motion patterns, and each must be analyzed to evaluate the likelihood of prolonged satisfactory performance that is typical of traditional rotating platform TKA designs in use today.

Future research in mobile bearing knees will focus on the analysis of condylar articular surface geometry and on stabilizing cam post mechanisms, with the overarching goal of creating implants that provide kinematic patterns similar to normal, nonimplanted knee joints. Further study is required to analyze the long-term effects of wear on the inferior surface of the mobile polyethylene bearing, particularly in designs that permit both rotation and anteroposterior translation (and therefore multidirectional motion patterns) to occur on the inferior aspect of the polyethylene bearing. Efforts to design implant mechanisms that allow conversion of a previously implanted fixed-bearing TKA to a mobile-bearing construct, or conversely, revising a mobile-bearing TKA to a fixed-bearing design without tibial component removal, are desirable. Lastly, increased clinical use of mobile-bearing TKA designs in revision TKA is likely. Problems often associated with revision TKA using fixed-bearing implants, such as aseptic loosening and failure due to damage to constraining mechanisms, may be lessened with use of mobile-bearing TKA because of the reduced stresses transmitted to the fixation interface and constraining devices (posts, hinge bolts, etc).

The following collection of manuscripts provides the reader with an update on current clinical and basic science information on the use of mobile-bearing TKA.

References

1. Ranawat CS, Komistek RD, Rodriguez JA, et al. In vivo kinematics for fixed and mobile-bearing posterior stabilized knee prostheses. Clin Orthop Relat Res. 2004; 418:184-90.
2. Buechel FF Sr. Long-term follow-up after mobile-bearing total knee replacement. Clin Orthop Relat Res. 2002; 404:40-50.
3. McEwen HMJ, Barnett PI, Bell CJ, et al. The influence of design, materials and kinematics on the in vitro wear of total knee replacements. J Biomech. 2005; 38:357-365.
4. Otto JK, Callaghan JJ, Brown TD. Gait cycle finite element comparison of rotating-platform total knee designs. Clin Orthop Relat Res. 2003; 410:181-188.
5. Greenwald AS, Heim CS. Mobile-bearing knee systems: ultra-high molecular weight polyethylene wear and design issues. In: Pellegrini V, ed. Rosemont, Ill: Instructional course lectures, American Academy of Orthopaedic Surgeons, 2005; 43:195-206.

Author

Dr Dennis is from the Department of Biomedical Engineering, University of Tennessee, Knoxville, Tenn, Oak Ridge National Laboratory/University of Tennessee Center for Musculoskeletal Research, Knoxville, Tenn, and Rocky Mountain Musculoskeletal Research Laboratory, Denver, Colo.