Implant/Construct Interactions in the Biomechanics of Total Hip Dislocation

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DESCRIPTION (provided by applicant): Dislocation remains second only to aseptic loosing as a leading cause of failure in total hip arthroplasty (THA). To complement the limited capabilities for mechanistic investigation afforded by clinical registries or by laboratory experimental studies, an anatomically and kinetically realistic three-dimensional nonlinear finite element (FE) model of THA dislocation has been developed. The formulation builds on substantial previous FE work, to now also include implant interaction(s) with the surrounding anatomic bony surfaces and with the hip joint capsule. The pre-processing sequence is structured for efficient parametric variation of individual implant design parameters and of surgical positioning of the components. The capsule is materially represented in terms of a heterogeneous hyperelastic continuum sheath whose anatomy and mechanical properties are taken from earlier cadaver testing. Input kinematic and kinetic sequences are taken from earlier motion studies of (non-THA-implanted) subjects performing dislocation-prone challenge maneuvers. Corroborative pilot work experimentally with a servo-hydraulic loading system and a novel trans-pelvic THA implantation protocol has reproduced the computations in terms of impingement/dislocation behavior, including capsule compromise effects, of comparable quantitative magnitude. Pilot work computationally has shown that the stable range of motion and the moment developed to resist dislocation depend strongly upon (1) the overall degree of capsule mechanical degradation, (2) focal defects either of internal capsule substance continuity (e.g., incisions) or of external attachment integrity (e.g., detachment from bony insertions), and (3) technical specifics of surgical repair. Four hypotheses, with corresponding specific aims, are structured as an analytical framework for parametric study of the interactions between capsular integrity, dislocation motion challenge, implant design, and component surgical placement. Relevance: THA dislocation rates vary by about an order of magnitude (approximately 1% to 10%), depending on surgical approach, implant design, component placement/orientation, and mechanical competence of the capsule. The problem is particularly vexing for revision surgery (" 8% incidence), a growing proportion of many surgeons' practices and in which capsule compromise is commonplace. Surgeon awareness and interest in this area has increased greatly in recent years, with various capsule repair/reinforcement procedures showing promise, and with many new options arising in terms of implant design and control of component placement. Realistic finite element analysis is an ideal vehicle for systematically assessing the complex interactions of the many factors influencing dislocation propensity, toward the goal of reducing this troublesome complication.