Simulator Study of MOM using Steep-cup Flexion - A Clinically Relevant Incorporation of Intermittent Edge-loading

Authors

  • Ian Clarke Orthopedic Research Department of Orthopedics Loma Linda University Medical Center, Rehabilitation Building 11406 Loma Linda Drive Loma Linda University, Loma Linda, CA 92354 US
  • Julia Shelton Dept. Mechanical Eng. Queen Mary University, London UK
  • John Bowsher Anterior Spine Devices Branch, FDA , Washington, DC
  • Christina Savisaar Orthopedic Joints Devices Branch, FDA, Washington DC
  • Thomas Donaldson Donaldson Arthritis Research Foundation 900 E. Washington Street, Suite 200 Colton, CA 92324 US

DOI:

https://doi.org/10.15438/rr.7.3.190

Keywords:

hip arthroplasty, MOM bearings, edge loading, simulator, wear

Abstract

Background: Adverse-wear phenomenon in metal-on-metal (MOM) arthroplasty has been attributed to “edge-loading” of the CoCr cups. Simulator studies of steeply-inclined cups run in the ‘Anatomic-cup’ model represented many variations in design and test parameters with no coherent rationale. We created an algorithm to synthesize MOM test parameters and noted that wear areas typically averaged only 10-15% of cup surface. In contrast, retrievals showed wear areas extending to 60% of cup surface. We hypothesized that MOM wear studies run in the orbital hip simulator with the ‘Inverted-cup’ model would, (i) differentiate normal-loading versus edge-loading, (ii) demonstrate cup wear areas x3.8-times larger than on femoral heads, cover 30% of cup surface, and (iii) double the wear-rates measured in prior Anatomic-cup study.
Methods: Edge-loading occurs when the cup rim is allowed to truncate the habitual wear area that provides optimal tribological conditions. A MOM algorithm was developed to synthesize relevant test parameters. The 60mm MOM bearings donated for this study were run in an orbital hip simulator using the Inverted-cup model. Tests #1 and #2 to one million cycles (1-Mc) duration assessed wear at peak cup inclinations 40° and 50°. Test #3 evaluated edge-loading with peak cup inclinations achieving 70° (5-Mc duration).

Results: Wear areas in Inverted-cups averaged 1663mm2 in tests #1 and 2, were fully contained within cup rims, and covered 30% of cup surface as predicted by algorithm. Test-3 with 70° cup inclination produced the predicted edge-loading with volumetric wear-rates averaging 2mm3/Mc, approximately 5-fold greater wear than prior Anatomic-cup study.

Discussion and Conclusions: Simulator studies of steep-cup mechanisms necessitate production of clinically-relevant wear-patterns such that the biomechanical and tribological functionality is respected. As an aid to steeply-inclined cup analyses, the MOM algorithm allowed integration of confounding test parameters. The algorithm successfully differentiated between “normal” and “edge loaded” cups and the MOM wear areas were as predicted for three cup inclinations. Also as predicted, wear-patterns in Inverted-cup model exactly reversed those of the Anatomic-cup model. Even with only intermittent edge-loading, Test-3 produced 5-fold greater wear than our prior Anatomic study.

Clinical Significance: The Inverted-cup simulator model successfully mobilized the cup to produce larger wear areas that were more representative of those in-vivo and therefore reproduced more realistic test conditions for studies of edge-loaded cups.

Author Biographies

Ian Clarke, Orthopedic Research Department of Orthopedics Loma Linda University Medical Center, Rehabilitation Building 11406 Loma Linda Drive Loma Linda University, Loma Linda, CA 92354 US

BSc, PhD

Julia Shelton, Dept. Mechanical Eng. Queen Mary University, London UK

Professor

Christina Savisaar, Orthopedic Joints Devices Branch, FDA, Washington DC

PhD, DFA

Thomas Donaldson, Donaldson Arthritis Research Foundation 900 E. Washington Street, Suite 200 Colton, CA 92324 US

MD

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Published

2017-10-01

How to Cite

Clarke, I., Shelton, J., Bowsher, J., Savisaar, C., & Donaldson, T. (2017). Simulator Study of MOM using Steep-cup Flexion - A Clinically Relevant Incorporation of Intermittent Edge-loading. Reconstructive Review, 7(3). https://doi.org/10.15438/rr.7.3.190

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Section

Original Article