Instability and dislocation remain leading indications for revision of primary Total Hip Arthroplasty (THA). Many studies have addressed the links between implant design and propensity for dislocation, however a comprehensive review of the ability of modern THA constructs to protect against joint instability is needed.

Accordingly, the objective of this study is to provide objective data about THA risks to be considered in the treatment algorithm to protect against adverse joint loading conditions and joint instability. Adverse loading conditions were assessed in a population of activities of daily living using data from telemetric hip implant representation in an FE simulation which included probabilistic representation of clinical implant position variation. Separately, dislocation was assessed using a series of strength-calibrated joint contact and muscle forces for a variety of THA implant designs in subject-specific musculoskeletal models of patients performing activities consistent with posterior and anterior THA dislocation. The resistive moment at the point of dislocation, as well as the overall dislocation rate per construct, provide relevant measures of resistance of the THA construct to dislocation.

Based on a range of acetabular implant alignments reported clinically, variation in cup anteversion/retroversion had the largest impact on liner peak loading location of any degree of freedom throughout the prescribed activities. Cup inclination also showed a relationship to response liner loading;​ stem variation in either longitudinal plane was not strongly correlated to edge loading propensity across activities.

Increased cup anteversion and inclination reduced the occurrence of posterior hip dislocation (82% vs 48% for anteversion and 86% vs 34% for inclination with neutral liners), however increased the occurrence of anterior hip dislocation (13% vs 94% and 39% vs 70%). Increased hip abduction and internal rotation reduced the occurrence of dislocation during posterior activities (79% vs 43% and 76% vs 50% for neutral liners respectively), but increase the occurrence of dislocation in anterior dislocation activities (45% vs 69% and 46% vs 67% for neutral liners respectively). Use of a lipped liner did increase the resistive moment at posterior dislocation, by an average of 5.2 Nm, and the flexion angle at dislocation by 1.4 degrees compared to a neutral liner. There was a reduction of similar magnitude in resistance to anterior dislocation. In each instance, a lipped liner with a posterior-inferior lip position reduced the occurrence of posterior dislocation, but increased the occurrence of anterior dislocation.

Considering implant geometry, head offset had a large impact on the resulting resistive moment of the THA construct, with a sensitivity of approximately 3.8 Nm/mm of additional offset. Increasing head diameter increased resistive moment from 21 Nm to 27 Nm, a sensitivity of 0.75 Nm/mm of head diameter. Three lipped liners were considered with increasing jump distance (JD), which is a linear measure of distance a head must translate to dislocate. These designs corresponded to 23 Nm, 25 Nm, and 31 Nm resistive moments, respectively, a sensitivity of approximately 2.8 Nm/mm of additional jump distance. A dual-mobility acetabular design resulted in a resistive moment of 30 Nm.

Tradeoffs between acetabular component position, design, and patient activity and the relative clinical risk of adverse implant loading as well as anterior and posterior dislocation must be considered and weighted accordingly. A quantitative understanding of tradeoffs in the dislocation risk inherent to THA construct options is valuable in supporting surgical decision making. Computer modeling provides a framework for meaningful design assessments which can be transferred and have meaningful input to clinical practice.