Anatomic Variation of the Knee Extensor Mechanism: The Effect on Tibiofemoral Joint Load and Joint Kinematics

Abstract

Abstract Background: Variation to extensor mechanism features of patella thickness, patella height, and tuberosity AP (anterior-posterior) and SI (superior-inferior) position will effect knee kinematics and tibiofemoral (TF) joint loads. The variations primarily relate to alterations of the patellar ligament angle, relative to the long axis of the tibia. There are two objectives in this study, first, determine the desired setting of each feature within the analog configuration of the Kansas Knee Simulator (KKS) to better match measured in-vitro TF-AP loads. Second, to describe the correlations between the features, quadriceps load, TF kinematics and patellofemoral (PF) kinematics Methods: To determine the desired analog configuration in the KKS, a custom instrumented tibia tray (ITT) was assembled with total knee replacement components from DePuy Synthes (Attune Primary, Size 8). The ITT measures TF joint load while simulating a dynamic walk and squat motion in the KKS. A design of experiments (DOE) was established, using the Taguchi analysis method to minimize the calculated RMS error. Each feature was varied between three levels. All experiments were conducted in a random order in three separate trials. To determine kinematic correlations between features, a different simulated walk was applied to 18 fresh frozen cadaver knees (age 59.3 ± 13.2 years old, 25.1 ± 6.4 BMI) in the KKS. Data points were taken at every 5% of the cycle for quadriceps load, and TF and PF kinematics. The data were normalized to create an 18x151 correlation matrix for principle component (PC) analysis. Subsequent PCs were included for analysis until 80% total explained variation was reached. The resulting coefficient matrix (“loadings”) was used to show the correlation between variables for each PC. Results: Signal-to-noise ratios for each feature were used to set the desired analog configuration at a patella thickness of 28.3-mm, patella height of 44.4-mm, and a tuberosity position of 29.3-mm (SI) and 40.6-mm (AP). The desired configuration reduced the calculated RMS error from 71.95-lbs in the original configuration to 4.76-lbs. The most sensitive factors affecting TF-AP load are patella thickness and tuberosity position. The PC analysis found the first 5 PCs accounted for over 80% of the variation. PC-1 explained 33.4% of the total variation and shows a correlation of patella height to changes in a majority of kinematic descriptions included in this analysis. The remaining PCs showed correlations to specific kinematics and anatomic measures at decreasing amounts of explained variation. Conclusion: Patella thickness and tuberosity position, and the corresponding patella ligament angle, have the greatest effect on TF-AP load. These measures are associated with minimal explained variation and correlated to limited kinematic changes. Patella height had minimal effect on TF-AP load, but is associated with the majority of explained variation and correlated to changes in quadriceps load, tibia rotation, and patella spin, tilt, and ML translation. TF-AP load is most affected by variation in patella thickness and tuberosity position, and kinematic changes are most affected by variation in patella height. Understanding the effects of variation in the three extensor mechanism features will guide more informed conclusions in future research, prosthetic and surgical tool development, and decisions regarding medical intervention.