Abstract
INTRODUCTION
Total knee arthroplasty (TKA) is one of the most successful and beneficial treatments for osteoarthritic knees. We have developed posterior-stabilized (PS) total knee prosthesis for Asian patients, especially Japanese patients, and have used it since November, 2010. The component was designed based on the CT images of osteoarthritic knees, aiming to achieve deep flexion and stability. The purpose of this study was to analyze in- vivo kinematics of this new prosthesis.
METHODS
We analyzed a total of 28 knees implanted with PS TKAs: Fourteen knees with the new PS prosthesis (group A), and the other fourteen knees with a popular PS prosthesis as a control group (group B). Preoperative data of both groups were not significantly difference. Flat-panel radiographic knee images were recorded during five static knee postures including full extension standing, lunge at 90° and maximum flexion, and kneeling at 90° and maximum flexion. The three-dimensional position and orientation of the implant components were determined using model-based shape matching techniques. The results of this shape-matching process have standard errors of approximately 0.5° to 1.0° for rotations and 0.5 to 1.0 mm for translations in the sagittal plane. Unpaired t-tests were used for statistical analysis and probability values less than 0.05 were considered significant.
RESULTS
The maximum implant flexion angles tended to be greater in group A than group B (Fig. 1a), averaging 117±8° and 109 ± 14° in lunge (p=0.054), and 119 ± 7° and 110 ± 14° in kneeling (p=0.061), in Groups A and B respectively. Femoral external rotation was significantly smaller in group A than group B (Fig. 1b), averaging 7±7° and 10±5° mm in lunge (p<0.05), and 6±7° and 8±3° in kneeling (p<0.05). The medial condylar AP translations were greater in group A than group B (Fig. 1c), averaging −7±3mm and −3±3mm in lunge (p<0.05), and −6±2mm and −4±3mm in kneeling (p<0.05), respectively. Lateral condylar AP translations were not significantly different between the two groups except at 90° kneeling (Fig. 1d).
DISCUSSION
Both implants are designed to achieve deep flexion, preventing edge loading until 155° flexion. Post/cam engagement occurs at 75° flexion in both system and the implant shapes are similar. Positive correlations have been reported between increasing femoral posterior translation and greater maximum knee flexion. The cam/post design of the new implant is configured to provide approximately 8mm of posterior femoral translation at 120° flexion which, on average, was slightly greater than observed in the control knees. Slightly greater lunge and keeling flexion in knees with the new design may be a manifestation of this greater posterior femoral translation. Previous studies have failed to demonstrate axial rotation as a predictor of greater flexion. Likewise, our data do not show a relationship between axial rotation and maximum flexion. The new TKA designed for Asian knees appears to perform comparably to a traditional high-flexion PS design, but may show some improvement in functional flexion due to greater posterior femoral translation.
