Effects of acetabular component orientation on dislocation propensity for small-head-size total hip arthroplasty.
OBJECTIVE
Examine the role of surgical orientation of the acetabular cup on posterior dislocation propensity for small-head-size total hip arthroplasty.


METHODS
A finite element model of a widely used total hip arthroplasty system was examined for peak resisting moment and range-of-motion prior to impingement, as well as prior to onset of posterior dislocation. Acetabular component surgical orientation was varied.


BACKGROUND
Dislocation is a leading cause of total hip replacement failure, with an incidence between 2% and 11%. Clinical registries imply acetabular component orientation to be a leading predictor of dislocation. The finite element method permits this complex kinetic behavior to be addressed systematically.


METHODS
Twenty-five combinations of cup abduction (five angles) and anteversion (five angles) were studied, with the resultant resisting moment about the cup center being tracked in each case. Key events were benchmarked, and a novel dislocation resistance index was developed for multi-factor comparison.


RESULTS
Increasing tilt and/or anteversion resulted in a monotonically increasing range-of-motion prior to impingement, as well as increased peak resisting moment. Range of motion was more sensitive to tilt, while peak resisting moment was more sensitive to anteversion. Peak resisting moment for 22-mm head size was nearly 25% less than that for a 26-mm head.


CONCLUSIONS
Increased cup tilt and anteversion discourage posterior dislocations of small-head-size components.


CONCLUSIONS
Pre-existing soft tissue compromise and untoward patient motions/postures are largely beyond surgeon control. However, other factors being equal, especially for small-head-size components, many posterior dislocations that would otherwise occur might be prevented by suitable tilt and anteversion of the acetabular component.
Nadzadi
Mark E
ME
Department of Orthopaedic Surgery, Orthopaedic Biomechanics Laboratory, University of Iowa, 2181 Westlawn Building, Iowa City, IA 52242, USA.
Pedersen
Douglas R
DR
Callaghan
John J
JJ
Brown
Thomas D
TD
D000077:Acetabulum; D019644:Arthroplasty, Replacement, Hip; D001696:Biomechanical Phenomena; D020342:Finite Element Analysis; D006617:Hip Dislocation / Q000503:physiopathology* / Q000517:prevention & control; D006622:Hip Prosthesis; D006801:Humans; D011237:Predictive Value of Tests; D011474:Prosthesis Design; D011475:Prosthesis Failure; D016059:Range of Motion, Articular