Abstract
INTRODUCTION
Total knee arthroplasty (TKA) is a very successful procedure with good clinical outcomes. However, the effects of obesity on TKA outcomes remain controversial and inconclusive. The objective of this study was to quantify the biomechanical effects of simulated obesity on Cruciate Retaining (CR) and Posterior Stabilized (PS) TKA in human cadaveric knees. We hypothesized that biomechanical characteristics of CR TKA will be less dependent on simulated obesity compared to PS TKA.
METHODS
Eight cadaveric knees (4 male, 4 female) average age 68.4 years (range, 40–86 years) underwent TKA and were tested using a custom knee testing system. Specifically, Cruciate Retaining (CR) and Posterior Stabilized (PS) Lospa Knee System (Corentec Inc.) were implanted and tested sequentially using internal control experimental design. The muscle loading was determined based on the physiological cross-sectional area ratio of the quadriceps and hamstring muscles. The ratios were then applied to CDC data representing the average male height and used to simulate a BMI of 25, 30, and 35 at knee flexion angles (KFA) of 15, 30, 45, 60, 75, and 90 degrees. Patellofemoral and tibiofemoral joint contact areas and pressures were measured using the K-scan sensor system (Tekscan Inc, South Boston, MA). Contact area, force, pressure and peak contact pressure were obtained and analyzed for each specimen. Knee kinematics were quantified using a Microscribe 3DLX digitizer (Revware Inc, Raleigh, North Carolina). Repeated measure analysis of variance with a Tukey post hoc test was used to compare loading conditions. Comparisons between the CR and PS TKA groups were made with a paired t-test. The significance level was set at 0.05.
RESULTS
At higher KFAs, the tibia position in the CR knees was more anterior relative to the femur than in the PS design (p<0.5). The greatest effect was seen at 90 KFA and a BMI of 25 with the tibia being 4.7 mm further anterior in the CR knees compared to the PS knees. The PS knees showed a greater anterior translation with increasing BMI at higher KFAs (Fig 1). Tibial rotation was unaffected by an increase in BMI through all flexion angles, regardless of implant type. The CR knees had greater tibiofemoral forces at higher KFA compared to the PS for all BMI levels. From 15 to 60 KFA, the lateral tibiofemoral pressures in the CR knee demonstrated greater increases as the BMI increased, however, this was only statistically significant at 60 KFA.
At higher KFA, there was a significantly higher patellofemoral force in the PS knee compared to the CR knee. There was a significant increase in patellofemoral contact area for PS knees vs. CR knees. The CR knee showed significantly greater percent change in peak pressures with an increase in BMI from 25 to 35 in mid flexion (Fig 2).
CONCLUSION
Tibia kinematics with CR TKA were less dependent to increased muscle loading simulating increased BMI compared to PS TKA; however patellofemoral and tibiofemoral contact pressure showed larger changes with increased BMI for the CR TKA compared to the PS TKA.
