Abstract
Total knee arthroplasty (TKA) is an exceptionally successful and robust treatment for disabling knee disease, but many efforts continue to improve patient postoperative satisfaction and performance. One approach to improving performance is to restore TKA motions closer to those in healthy knees. Based upon an idealized model of knee motions, it is possible to design tibiofemoral articulating surfaces to promote natural kinematics and force transfer (Fiedler et al., Acta Bioeng Biomech, 2011). Such an asymmetric design is expected to promote rollback in stance phase that continues through deeply flexed activities. The purpose of this study is twofold: (1) To determine if a TKA designed on a theoretical basis achieves the proposed motions in vivo, and (2) To track postoperative kinematic patterns with examinations at 6–12 weeks, 6 months and one year postoperatively. This paper reports results of the initial cohort that has completed 6–12 week and 6-month examinations.
Eight patients, including 3 females, with unilateral TKA for varus osteoarthritis provided written informed consent prior to beginning the study. Patients averaged 66±9 years, 168±14cm, and 28±3 BMI. Patients performed three weightbearing activities observed using pulsed x-ray flat-panel imaging at 30Hz: stepping up from flexion to extension on a 20cm step, lunging to maximum flexion with the foot placed on a 20 cm step, and kneeling to maximum flexion with the shin placed on a padded support. Three-dimensional knee kinematics were quantified using model-image registration to determine flexion, tibial internal rotation, anteroposterior movement of the femoral condyles (relative to the tibial AP center) and average center of rotation (CoR) in the transverse plane.
During the maximum-flexion lunge and kneeling activities subjects exhibited average knee flexion of 104°–110° and tibial internal rotation of 2°–6° (Table 1). At 6–12 weeks, the medial/lateral condyles were at −3mm/−8mm and −1mm/−6mm during maximum flexion lunge and kneeling, respectively. During the stair activity from 0° to 70° flexion, there were small tibial internal rotations (1°/5°) and anterior medial (2mm/5mm) and lateral (3mm/3mm) condylar translations at both time points (Figure 1). The average CoRs for the stair activity were medial +18% and +5% for the 6–12 week and 6-month exams, respectively.
It has long been assumed knee kinematics change during a patient's first one or two postoperative years. In our early post-op cohort, changes in weight-bearing kinematics over the first 6 postoperative months are small. In maximal flexion activities, patients exhibited flexion similar to similar cohorts studied at least one year post-op (Clin Orthop, 410:131–138, 2003). Similarly, kinematics during the weight-bearing step activity were similar in pattern and magnitude to those previously reported for posterior cruciate-retaining (CR) TKA at least one year post-op (Clin Orthop, 426:187–193, 2004). The average CoRs were medial for the stair activity, which is normal for healthy knees but uncommon for CR TKA. Early post-op results with an asymmetric CR TKA implant intended to promote physiologic motion show flexion and stair kinematics similar to many successful CR designs at longer follow-up. The medial CoR indicates closer-to-physiologic motion than commonly is observed in CR TKA.
