The design of every post-surgical knee arthroplasty study begins with the question “How soon after surgery should we assess the patients?”. The consensus, based primarily upon clinical rating systems, is that patients' scores reach a plateau roughly one year after surgery, and that observations performed at that time should be indicative of the long-term behavior of the joint. This is satisfactory for long-term studies of clinical performance. However, when new devices are introduced there is a need to determine as quickly as possible if the device performs as designed. Waiting a year or more after surgery to characterize a device's performance may place additional patients at risk of receiving an inferior design, or may delay widespread availability of a superior design. The goal of this study was to assess knee arthroplasty patients at 6–12 weeks, 6 months and 1 year after surgery to determine if their tibiofemoral kinematics changed during functional activities.

A total of 13 patients (7 female) were recruited from an ongoing clinical study to participate in this IRB-approved sub-study. All subjects received fixed-bearing, cemented, posterior-cruciate-retaining total knee arthroplasty of the same design from a single surgeon. Subjects averaged 69 years, 169cm tall, and 28 BMI. Subjects were studied at 6–12 weeks, at 6 months and at 12 months post-surgery, when they showed an average clinical flexion of 106°, 113° and 115°, respectively. Subjects' knees were observed using pulsed-flat-panel-fluoroscopy during three activities: lunging to maximum flexion with their foot placed on a 20cm step, kneeling to maximum flexion on a padded bench, and step-up/down on a 20cm step without progression of the contralateral limb. Model-image registration was used to register 3D geometric models of the implants with their radiographic projections based upon measured projection parameters. 3D knee kinematics were derived from the registered models, including joint angles and the antero-posterior translation of the medial and lateral condyles relative to the tibial baseplate.

There were no statistically significant changes in knee kinematics between the 6–12 week and 6 month, and 6-month and 12-month visits during the kneel and lunge activities (Table 1). Similarly, there were no pair-wise differences in tibial rotation or condylar translation during the dynamic step activity at any flexion angle (Figure 1).

Traditional thinking suggests studies of knee mechanics should be performed at least one year after surgery to make observations that are predictive of long-term joint function. In three different functional activities, we could not demonstrate significant changes in knee kinematics between 6–12 weeks and 6 months, nor between 6 months and 12 months. If these results can be confirmed in a larger subject cohort, and for a range of TKA designs, then functional follow-up studies of novel knee arthroplasty designs might be justified as early as 6–12 weeks after surgery, making it possible to accelerate confirmation devices are performing in patients as designed.

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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.