Hip stress distribution - Predictor of dislocation in hip arthroplasties. A retrospective study of 149 arthroplasties

Abstract

Dislocation after hip arthroplasty is still a major concern. Recent study of the volumetric wear of the cup has suggested that stresses studied in a one-legged stance model could predispose arthroplasty dislocation. The aim of this work was to study whether biomechanical parameters of contact stress distribution in total hip arthroplasty during a neutral hip position can predict a higher possibility of the arthroplasty dislocating. Biomechanical parameters were determined using 3-dimensional mathematical models of the one-legged stance within the HIPSTRESS method. Geometrical parameters were measured from standard anteroposterior X-ray images of the pelvis and proximal femora. Fifty-five patients subjected to total hip arthroplasty that later suffered dislocation of the head and, for comparison, ninety-four total hip arthroplasties that were functional at least 10 years after the implantation, were included in the study. Arthroplasties that suffered dislocation had on average a 6% higher resultant hip force than the control group (p = 0.004), 11% higher peak stress on the load-bearing area (p = 0.001) and a 50% more laterally positioned stress pole (p = 0.026), all parameters being less favorable in the group of unstable arthroplasties. There was no statistically significant difference in the gradient index or in the functional angle of the weight bearing. Our study showed that arthroplasties that show a tendency to push the head out of the cup in the representative body position—the one-legged stance—are prone to dislocation. An unfavorable resultant hip force, peak stress on the load bearing and laterally positioned stress pole are predictors of arthroplasty dislocation.

Arthroplasty for femoral neck fractures is at risk for under restoration of lateral femoral offset

“…7,38,39 Restoration of femoral offset is potentially the most important factor for dislocation in THA in the setting of a well-positioned acetabular component. 22,43,44 If 3 mm of femoral offset is added to the preoperative position, there is a reduction in the rate of hip instability, with an odds ratio of 0.94. 22 In our study, NAS under-restored lateral femoral by 3 mm or more over 60% of the time by a mean of 4.8 ± 6.0 mm during THA for a femoral neck fracture.…”

“…22 In our study, NAS under-restored lateral femoral by 3 mm or more over 60% of the time by a mean of 4.8 ± 6.0 mm during THA for a femoral neck fracture. There are 2 biomechanical consequences to under-restoring lateral femoral offset: (1) a reduction in the abductor moment leading to an inadequate joint reactive force 22,43,44 ; and (2) impingement of the femoral component on the acetabular component (intra-prosthetic) or other soft tissues (extra-articular) earlier as a result of reduced clearance. 45 When lateral femoral offset is restored, or slightly increased, it provides optimal impingement free range of motion.…”