Hip Joint Loading in Subjects With Total Hip Replacements

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Abstract

This project introduces an analytical parametric model that is ideally suited to study both the variation in hip forces due to variations in muscle force distribution, and variations in kinematics and kinetics during dynamic activities on the hip forces. After an analysis of these sources of variability in hip forces, abnormalities in hip joint loading were identified in subjects with THRs as compared to normal subjects. Finally the joint geometries that have been previously associated with implant loosening and their effect on the hip joint forces were considered.

The model calculates a parametric range of muscle activation levels such that the internal moments produced by the muscles balance the external moments obtained from gait analysis. A Hill-based muscle model (Delp et al., 1990) is used to calculate the resultant muscle forces that correspond to the activation levels. The contact force, which is the vector sum of the muscle forces and the intersegmental joint forces obtained from gait analysis, is then calculated. The orientation angles of the contact force in the frontal and transverse planes are computed from the force components. The twisting moment about the long axis of the femur or implant (implant twisting moment) and the bending moment about the neck of the femur or implant are also calculated. The contact forces for both walking and stair climbing are very similar to those obtained from instrumented prostheses and other models.

Next sources of intrasubject (due to uncertainty in muscle force distribution) and intersubject (due to differences in gait patterns between subjects) variability were considered. The hypothesis that intersubject variability was significantly greater than intrasubject variability was confirmed. Whether variability was represented by the width of the 95% confidence interval or the coefficient of variation, intersubject variabiUty was larger than intrasubject variability by an order of magnitude.

Regarding intrasubject variability, the relative influences of hip muscle groups on the overall contact force and implant twisting moment were calculated. While hip abductors are most influential in determining the second peak contact force, hip flexors are most important in early stance and should not be neglected. Hip flexors and abductors were equally influential in determining the implant twisting moment.

Total hip patients do not walk or climb stairs normally after surgery, the key gait adaptation being a decreased hip adduction moment and a decreased internal rotation moment during walking and decreased adduction and external rotation moments during stair climbing. As much as 37% of the variation in postoperative gait parameters was determined by the preoperative gait parameters. There were some relationships between joint positioning and postoperative THR gait, however there were no consistent or outstanding patterns observed. Next kinetic determinants of hip contact forces and twisting moments were examined.

During walking, sagittal plane moments were the best predictors of hip forces during walking, followed by the adduction moment and internal or external rotation moments. The external rotation moment was most highly correlated with the peak twisting moment during both activities.

The next objective was to determine whether there were differences in hip forces between subjects with THRs and normal subjects. Even though there were gait adaptations in subjects with THRs, for example the reduced hip adduction moment during walking and stair climbing, the hip forces were not significantly lower than those in the normal subject group. It is likely that this is attributable to the fact that the sagittal plane moments, which were not reduced compared to normal, were more strongly correlated to the peak contact forces than the adduction moment. The implant twisting moment was most highly correlated to the external rotation moment, which was also not significantly less than normal after surgery.

The final hypotheses tested were that an increased vertical or horizontal joint center position, a decreased offset, or a decreased abductor muscle moment arm would be related to increased hip forces. The hypotheses were based on predictions from in vitro (cadaver and model-based) studies as well as results demonstrating an increased incidence of loosening or decreased muscle strength with these joint positions. In this group of THR subjects, increased sagittal plane joint center position was related to increased implant twisting moment and a decreased offset was associated with an increased second contact force peak during walking. These findings are in general agreement with the hypotheses. There was no relationship between the horizontal joint center position and the force parameters, and the relationship observed between the abductor moment arm length and the contact force was in the opposite direction as that predicted by the hypothesis. The hypothesis most directly contradicted, that pertaining to the abductor moment arm length, was based on reasoning proposed by in vitro (model and cadaver-based) studies rather than chnical outcome studies. This fact points to the need for models to more fully incorporate the idea that the hip loading environment may be more strongly governed by postoperative gait patterns than the simple mathematical relationships between geometry and hip forces and/or muscle forces.

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