Getting it Right by Analyzing Failures and Biomechanical Models   |    
Modes of Implant Failure After Hip Resurfacing: Morphological and Wear Analysis of 267 Retrieval Specimens
Michael M. Morlock, PhD1; Nick Bishop, Dipl-Ing1; Jozef Zustin, MD2; Michael Hahn, Dipl-Ing2; Wolfgang Rüther, MD2; Michael Amling, MD2
1 Biomechanics Section, TUHH Hamburg University of Technology, Denickestrasse 15, 21073 Hamburg, Germany. E-mail address for M.M. Morlock: morlock@tuhh.de
2 University Medical Center Hamburg-Eppendorf, Martinistrasse 52, D-20246 Hamburg, Germany
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Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants of less than $10,000 from DePuy, Zimmer, Smith and Nephew, Biomet, and Corin. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

The Journal of Bone and Joint Surgery, Inc.
J Bone Joint Surg Am, 2008 Aug 01;90(Supplement 3):89-95. doi: 10.2106/JBJS.H.00621
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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° compared with 50°). 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.

Clinical Relevance: Femoral neck fractures are the main reason for early hip resurfacing failure. Insufficient cup fixation and positioning can cause later failure and thus, patients at risk of increased wear should be carefully monitored in order to prevent excessive exposure to metal ions. Appropriate education is required to reduce the length of the surgeon learning curve.

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    Accreditation Statement
    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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