A Rapidly Calibratable Parametric Hip Capsule Model for Evaluating Capsular Restraint in Total Hip Arthroplasty

The passive restraining function of the hip capsule plays a critical role in maintaining joint stability after total hip arthroplasty (THA). However, restoring this function remains challenging due to the complex biomechanics of the capsule and the lack of efficient quantitative tools for implant parameter adjustment. This study developed a rapidly calibratable numerical parametric hip capsule model to characterize passive capsular mechanics and to quantitatively evaluate implant geometric and positioning parameters.

A nonlinear spring-based model was constructed by dividing the capsule into 12 sectors with 25 parameters and calibrated using experimentally measured torque-rotation data from cadaveric hips. Model accuracy was validated through comparison with finite element (FE) simulations and experimental measurements across multiple motions. The calibrated framework was applied to nine hip specimens to assess the influence of femoral head size, neck length increment, and femoral stem anteversion on capsular restraint after THA.

The model achieved efficient calibration (0.74 h per specimen; < 1 s per iteration) and demonstrated strong agreement with FE simulations, with an overall torque root-mean-square error (RMSE) of 0.21 ± 0.02 Nm. Rotational RMSEs relative to experiments ranged from 5.2° to 9.4°. Parametric analyses indicated that femoral head size alone was insufficient to restore capsular restraint, whereas coordinated adjustment of neck length and stem anteversion significantly improved stability and reduced rotational hypermobility.

This study presents a computationally efficient and accurate hip capsule modeling framework that enables rapid calibration and quantitative evaluation of capsular mechanics, providing a practical tool to support personalized THA planning and postoperative stability assessment.