Modes of implant failure after hip resurfacing: morphological and wear analysis of 267 retrieval specimens.
BACKGROUND
Resurfacing of the hip joint is experiencing a revival due to improvements in materials, design, and manufacturing techniques. Despite good midterm outcomes, the high early rate of failure and concerns about metal debris require a detailed morphological and wear analysis of retrieved resurfacing implants in order to understand failure mechanisms.


METHODS
A worldwide collection of hip resurfacing revision devices was initiated, and 267 components were received. Devices were analyzed by patient demographics, radiographic positioning, and wear, as well as morphologically and histologically. Specimens were grouped into four different failure types. They were also stratified into rim-loaded or non-rim-loaded groups. Failures were also assessed by surgeon learning-curve effects.


RESULTS
Time to failure was significantly different between the four revision-type groups: Specimens with fractures involving the implant rim were most common (46%) and failed earliest after surgery (mean of ninety-nine days), followed by fractures inside the femoral head (20%, 262 days) and loose cups (9%, 423 days). Revisions not due to fractures or cup loosening (25%) occurred at a mean of 722 days after surgery. Rim-loaded implants exhibited an average twenty-one to twenty-sevenfold higher wear rate than implants without rim-loading. Rim-loaded implants also showed a steeper mean cup inclination than their non-rim-loaded counterparts (59 degrees compared with 50 degrees ). Most failures occurred during the learning curve of the surgeon (the first fifty to 100 implantations).


CONCLUSIONS
Failures on the femoral side usually occur within the first nine months after surgery and appear to be most directly related to the implantation technique or patient selection. Later failures are observed mainly due to acetabular problems, either due to dramatically increased wear or poor cup anchorage. Improper cup anteversion may be similar to or more important than cup inclination in producing excessive wear.
Morlock
Michael M
MM
Biomechanics Section, TUHH Hamburg University of Technology, Denickestrasse 15, 21073 Hamburg, Germany. morlock@tuhh.de
Bishop
Nick
N
Zustin
Jozef
J
Hahn
Michael
M
Rüther
Wolfgang
W
Amling
Michael
M
D001696:Biomechanical Phenomena; D005260:Female; D006622:Hip Prosthesis; D006801:Humans; D008297:Male; D011474:Prosthesis Design; D011475:Prosthesis Failure; D012086:Reoperation