Temporal Evolution and Stationary Point Mechanisms of Pelvic Characteristic Angles Throughout the Sit-to-Stand Cycle

Prosthetic impingement and dislocation remain prevalent complications following total hip arthroplasty (THA). The "safe zone" concept guiding acetabular component placement is predicated on static body positions, while neglecting the kinematic changes in pelvic angles throughout the stand-to-sit (STS) transition cycle. This static-centric approach constitutes a primary contributing factor to THA-related complications. Consequently, investigating the kinematic regularity of pelvic motion across the entire STS cycle is of particular significance for optimizing surgical strategies and improving postoperative outcomes.

Dynamic pelvic angles are analyzed in 10 healthy individuals performing STS using optical motion capture combined with biodynamics simulations. The skeletal model is scaled to each subject, and inverse kinematics is used to compute motion trajectories, from which the time-varying characteristics of sacral slope (SS), pelvic incidence (PI), and pelvic tilt (PT) angles are extracted.

The skeletal model of the STS movement cycle is simulated, revealing the nonlinear variations in pelvic characteristic angles. Notably, the variation curves of SS and PT angles both exhibit distinct stationary points during the critical phase of the movement cycle, with the two parameters showing opposite changing trends. Specifically, PT angle decreases sharply to a trough within the 44.48%-53.19% interval of the movement cycle and then gradually rebounds from this stationary point. In contrast, SS angle increases first and then decreases, with its stationary point highly coinciding with that of PT. At the trough stationary point of PT or SS, body center of gravity (COG) trajectory exhibits a strong correlation with PT or SS kinematics and synchronous transitions in displacement velocity and direction, highlighting the crucial role of pelvic posture adjustment in body COG regulation.

A biomechanical framework for investigating the kinematic characteristics of pelvic angles during the STS task is proposed. In healthy individuals, the kinematic patterns of pelvic motion during STS demonstrate prominent nonlinear characteristics, accompanied by distinct stationary points in the pelvic characteristic angles around the mid-STS phase. The current acetabular safe zone is insufficient to account for dynamic variations in pelvic orientation, and the identified critical stationary points of pelvic angles during the mid-STS phase may provide theoretical support for preoperative planning and postoperative rehabilitation in patients undergoing THA.