Does mobile-bearing knee arthroplasty motion change with activity?
Introduction
Mobile-bearing total knee arthroplasty (TKA) has been a popular knee implant design choice since the introduction of the LCS design (DePuy Orthopaedics, Warsaw, IN) in 1977. Posterior cruciate ligament (PCL) sacrificing, rotating platform (RP) and PCL-retaining meniscal bearing (MB) variants both were designed to minimally constrain knee kinematics while minimizing fixation interface stress and polyethylene wear [1], [2], [3]. A similar surgical technique is followed to implant either design variant [4]. Sacrifice of the PCL necessitates the use of a translationally constrained prosthesis with a single polyethylene bearing rotating in the transverse plane without constraint [5]. The PCL-retaining meniscal bearing knee prosthesis incorporates separate medial and lateral mobile polyethylene bearings sliding independently in circularly arced keyways running antero-postriorly (A/P) in the metal tibial component. This design allows unrestrained A/P translation and axial rotation of the femur relative to the tibia, limited only by the periarticular tissues. The LCS femoral component has an anatomical articulating surface, and the radii of curvature decrease posteriorly. The LCS femoral and tibial components are fully conforming in the sagittal plane from full extension to 30° flexion, and less conforming for greater flexion due to the decreasing radii of curvature of the femoral posterior condyles.
There have been many reports of satisfactory clinical results [2], [5], [6], [7], [8], [9], [10] and knee kinematics of mobile bearing TKA [11], [12], [13], [14], [15], [16]. Most fluoroscopic studies of TKA kinematics have focused primarily on comparisons of different TKA designs in subjects performing a single activity. As patients perform more than one activity of daily living, and knee kinematics are controlled by both implant geometry and soft tissue tension, it is critically important to understand how dynamic conditions affect in vivo kinematics of mobile bearing prostheses. This study seeks to address two specific questions regarding the in vivo kinematics of mobile bearing knee prostheses: First, do mobile-bearing knee arthroplasty motions change with dynamic weight-bearing activity? Second, are the kinematics different between meniscal bearing and rotating platform variants of a common mobile-bearing TKA design?
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Materials and methods
The kinematics of 20 knees (10 patients) with well-functioning bilateral TKA were analyzed during non-weight-bearing knee extension, weight-bearing knee extension, and during a weight-bearing stair-step activity. All patients were Japanese, and were operated by one experienced surgeon (Y.I.), using the same knee replacement system in two configurations; the LCS™ RP prosthesis and LCS™ meniscal-bearing prosthesis (DePuy, Warsaw, IN). Between the years 2002 and 2007, the senior surgeon at our
Results
Tibial internal rotation in RP Knees averaged 3.5° (SD 1.3°) during non-weight-bearing activity, 3.4°(SD 2.6°) during weight-bearing activity and 2.9° (SD 1.7°) during the step activity (Fig. 1A). Tibial internal rotation in MB Knees averaged 4.0° (SD 2.4°) during non-weight-bearing activity, 3.3°(SD 1.5°) during weight-bearing activity and 3.7° (SD 1.8°) during the step activity (Fig. 2A). There were no significant pair-wise differences in tibial rotation for any of the three activities (p >
Discussion
Knee motions have a direct impact on patient function [19], [20], [21], [22] and implant wear [11], [23], [24]. Therefore, it is critically important to understand kinematics of knees after TKA. Most previous fluoroscopic analyses have focused primarily on the influence of implant design (e.g. mobile vs. fixed) [11], [12], [13], [14], [25], [26] and the presence of the posterior cruciate ligament (retaining vs. substituting) [21], [25], [27] during individual motions. It is necessary to observe
Conflict of interest
The authors declare no conflict of interest.
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