Abstract
Introduction
Alignment of the acetabular cup and femoral components directly affects hip joint loading and potential for impingement and dislocation following total hip arthroplasty (THA) [1]. Changes to the lines of action and moment generating capabilities of the muscles as a result of component position may influence overall patient function. The objectives of this study were to assess the effect of component placement on hip joint contact forces (JCFs) and muscle forces during a high demand step down task and to identify important alignment parameters using a probabilistic approach.
Methods
Three patients following THA (2 M: 28.3±2.8 BMI; 1 F: 25.7 BMI) performed lower extremity maximum isometric strength tests and a step down task as part of a larger IRB-approved study. Patient-specific musculoskeletal models were created by scaling a model with detailed hip musculature [2] to patient segment dimensions and mass. For each model, muscle maximum isometric strengths were optimized to minimize differences between model-predicted and measured preoperative maximum isometric joint torques at the hip and knee.
Baseline simulations used patient-specific models with corresponding measured kinematics and ground reaction forces to predict hip JCFs and muscle forces using static optimization. To assess the combined effects of stem and cup position and orientation, a 1000 trial Monte Carlo simulation was performed with input variability in each degree of freedom based on the ±1 SD range in component placement relative to native geometry reported by Tsai et al. [3] (Figure 1). Maximum confidence bounds (1–99%) were predicted for the hip JCF magnitude and muscle forces for three prime muscles involved in the task (gluteus medius, gluteus minimus and psoas). HJC confidence bounds were compared to Orthoload measurements from telemetric implants from 6 patients performing the step down task. Sensitivity of hip JCF and muscle force outputs was quantified by Pearson Product-Moment correlation between the input parameter and the value of each output averaged across four points in the cycle.
Results
Variation in the placement of the stem and cup produced an average maximum confidence bound (1–99%) in hip JCF of 277.7±91.1N and forces of 259.4±58.3N in the gluteus medius for all three patients (Figure 2). Sensitivity to cup and stem placement varied among the three patients; however, in general, hip JCFs were more sensitive to the position of the stem than the cup (Figure 3). Hip JCF was most sensitive to stem anteversion (0.64±0.10) and the superior/inferior stem position (0.42±0.19).
Discussion
Variation in stem anteversion and medial/lateral cup position contributed the largest amount of variability in hip JCF and muscle forces during a step down task. The probabilistic analysis characterized bounds for output parameters, considering interactions between alignment parameters. Alignments that avoid increases in JCF and muscle loading during high demand tasks may lead to earlier recovery of function, by reducing muscle fatigue and the need to develop compensatory movement patterns.
Acknowledgements
This research was supported in part by DePuy-Synthes.
