Undetected Intraoperative Pelvic Movement Can Lead to Inaccurate Acetabular Cup Component Placement during Total Hip Arthroplasty: A Mathematical Simulation Estimating Change in Cup Position

AbstractIntraoperative patient movement is a common but underappreciated phenomenon in total hip arthroplasty (THA). Such movement can significantly affect the accuracy with which the acetabular cup component is implanted. To evaluate the effect of intraoperative movement on cup position, we performed a study using mathematical modeling to simulate intraoperative movement. Mathematical simulations were used to simulate pelvic movements during THA. Pelvic axial rotation, tilt, and obliquity were simulated, and the resulting changes in intended cup position were calculated. The rate of change of inclination and anteversion per degree of pelvic movement was calculated, establishing a ratio relating cup angle sensitivity to pelvic movement. These sensitivities were used to construct nomograms showing the per-degree effect of pelvic movement on cup position. The effect of pelvic movement on cup position was multifactorial and dependent on the intended cup orientation. For a cup intended to be inserted at 15° anteversion and 40° inclination, each degree of pelvic rotation induced changes of 0.64° and –0.20°, respectively. For this same cup orientation, each degree of pelvic tilt induced changes of 0.77° for anteversion and 0.17° for inclination. Pelvic obliquity was associated with a 1:1 ratio with inclination, with each degree of obliquity inducing 1° of change in inclination. Anteversion was unaffected by changes in pelvic obliquity. This study demonstrates the consequences of undetected pelvic movement on cup position, including the increased risk of the acetabular component being placed in an orientation that could increase wear or the likelihood of impingement and dislocation.