Proximal tibial strain in medial unicompartmental knee replacements

A biomechanical study of implant design

C. E. H. Scott, M. J. Eaton, R. W. Nutton, F. A. Wade, P. Pankaj, S. L. Evans

Abstract

As many as 25% to 40% of unicompartmental knee replacement (UKR) revisions are performed for pain, a possible cause of which is proximal tibial strain. The aim of this study was to examine the effect of UKR implant design and material on cortical and cancellous proximal tibial strain in a synthetic bone model. Composite Sawbone tibiae were implanted with cemented UKR components of different designs, either all-polyethylene or metal-backed. The tibiae were subsequently loaded in 500 N increments to 2500 N, unloading between increments. Cortical surface strain was measured using a digital image correlation technique. Cancellous damage was measured using acoustic emission, an engineering technique that detects sonic waves (‘hits’) produced when damage occurs in material.

Anteromedial cortical surface strain showed significant differences between implants at 1500 N and 2500 N in the proximal 10 mm only (p < 0.001), with relative strain shielding in metal-backed implants. Acoustic emission showed significant differences in cancellous bone damage between implants at all loads (p = 0.001). All-polyethylene implants displayed 16.6 times the total number of cumulative acoustic emission hits as controls. All-polyethylene implants also displayed more hits than controls at all loads (p < 0.001), more than metal-backed implants at loads ≥ 1500 N (p < 0.001), and greater acoustic emission activity on unloading than controls (p = 0.01), reflecting a lack of implant stiffness. All-polyethylene implants were associated with a significant increase in damage at the microscopic level compared with metal-backed implants, even at low loads. All-polyethylene implants should be used with caution in patients who are likely to impose large loads across their knee joint.

Cite this article: Bone Joint J 2013;95-B:1339–47.

Footnotes

  • This research was supported by grants from the British Association for Surgery of the Knee and Joint Action. Implants were provided by Biomet UK and DePuy Synthes who had no other involvement with the study.

    The authors would like to thank Professor H. Simpson and Mr C. Howie for the use of the Edinburgh Orthopaedic Engineering Centre to implant all of the tibiae; Mr M. Moran and Mr G. Dall for the cement; and Dr R. Wallace, A. MacLeod and N. Conlisk for Orthopaedic Engineering support in Edinburgh.

    No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.

    This article was primary edited by D. Rowley and first-proof edited by G. Scott.

  • Received January 15, 2013.
  • Accepted June 4, 2013.
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