Hard-on-hard total hip impingement causes extreme contact stress concentrations.
BACKGROUND
Impingement events, in addition to their role immediately proximate to frank dislocation, hold the potential to damage new-generation hard-on-hard bearings as a result of the relatively unforgiving nature of the materials and designs. Because of the higher stiffness and tighter design tolerances of metal-on-metal and ceramic implants, surgical positioning plausibly has become even more important.


OBJECTIVE
We asked (1) whether, and under what cup orientation conditions, hard-on-hard impingements might challenge implant material failure strength; and (2) whether particle generation propensity at impingement and egress sites would show similar dependence on cup orientation.


METHODS
Realistic computational simulations were enabled by multistage finite element analyses, addressing both global construct motion and loading, and focal stress concentrations at neck impingement and rim egress sites. The global model, validated by a cadaveric simulation in a servohydraulic hip simulator, included both hardware components and advanced anisotropic capsule characterization. Parametric computational runs explored the effect of cup orientation for both ceramic-on-ceramic and metal-on-metal bearing couples for two distinct motion sequences associated with dislocation.


RESULTS
Stress concentrations from impingement increased nearly linearly with increased cup tilt and with cup anteversion. In some situations, peak values of stress approached or exceeded 1 GPa, levels challenging the yield strength of cobalt-chromium implants, and potentially the fracture strength of ceramics. The tendency for impingement events to generate debris, indexed in terms of a new scraping severity metric, showed orientation dependences similar to that for bulk material failure.


CONCLUSIONS
Damage propensity arising from impingement events in hard total hip bearings is highly orientation-dependent.
Department of Orthopaedics and Rehabilitation, University of Iowa, Iowa City, IA 52242, USA.
Jacob M
JM
Elkins
Megan K
MK
O'Brien
Nicholas J
NJ
Stroud
Douglas R
DR
Pedersen
John J
JJ
Callaghan
Thomas D
TD
Brown
D019644:Arthroplasty, Replacement, Hip / Q000009:adverse effects*; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D020342:Finite Element Analysis; D006617:Hip Dislocation / Q000209:etiology* / Q000473:pathology / Q000503:physiopathology; D006621:Hip Joint / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D006622:Hip Prosthesis; D006801:Humans; D011474:Prosthesis Design; D011475:Prosthesis Failure; D013314:Stress, Mechanical