Preliminary optimization of cup-implant orientation in total-hip arthroplasty using a parametric predictive analysis of lower-limb dynamics influenced by spine stiffness

The traditional Lewinnek safe zone used for Total-Hip Arthroplasty (THA) surgery has been found to be inadequate, as dissatisfaction rates have risen after this surgery. It is evident that spinopelvic parameters and spine stiffness, factors that have been overlooked previously, must be taken into account for optimal surgical outcomes. In this paper, a novel predictive dynamic modeling approach was proposed to address this issue. This approach involved the development of a multibody model of a human that contained nonlinear spinal elements, which was validated by comparing it to literature in-vitro experiments and conducting a motion-capture experiment. To simulate human sit-to-stand motion, this model was employed with an optimal control approach based on trajectory optimization. Human joint angles were extracted from conducted simulations of different scenarios: normal, fused, and stiff spines. It was found that spine stiffness had a significant effect on lower-limb motion and the risk of implant impingement. Different scenarios of spine stiffness were examined, such as different levels of spinal fusion or an anatomically stiff spine. The optimal acetabular-cup orientation was calculated based on implant-impingement criteria using predicted motions for different spinal-condition scenarios, and the results compared to the clinically recommended orientation values for the same categories of patients. Our preliminary optimization suggests increasing the anteversion-cup angle from \(23 ^{\circ }\) (normal spine) to \(29 ^{\circ }\) for an anatomically stiff spine. For fused spines, the angle should fall within the range of 27–38^∘, depending on the level of fusion. This research is the first of its kind to examine spine flexibility in different scenarios and its impact on lower-limb motion. The findings of this paper could help improve THA surgical planning and reduce the risk of hip impingement or dislocation after THA.