, Volume 472, Issue 2, pp 529-542
Date: 12 Nov 2013

Stability and Trunnion Wear Potential in Large-diameter Metal-on-Metal Total Hips: A Finite Element Analysis

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Topic
Hip

Abstract

Background

Large-diameter femoral heads for metal-on-metal THA hold theoretical advantages of joint stability and low bearing surface wear. However, recent reports have indicated an unacceptably high rate of wear-associated failure with large-diameter bearings, possibly due in part to increased wear at the trunnion interface. Thus, the deleterious consequences of using large heads may outweigh their theoretical advantages.

Questions/purposes

We investigated (1) to what extent femoral head size influenced stability in THA for several dislocation-prone motions; and the biomechanics of wear at the trunnion interface by considering the relationship between (2) wear potential and head size and (3) wear potential and other factors, including cup orientation, type of hip motion, and assembly/impaction load.

Methods

Computational simulations were executed using a previously validated nonlinear contact finite element model. Stability was determined at 36 cup orientations for five distinct dislocation challenges. Wear at the trunnion interface was calculated for three separate cup orientations subjected to gait, stooping, and sit-to-stand motions. Seven head diameters were investigated: 32 to 56 mm, in 4-mm increments.

Results

Stability improved with increased diameter, although diminishing benefit was seen for sizes of greater than 40 mm. By contrast, contact stress and computed wear at the trunnion interface all increased unabatedly with increasing head size. Increased impaction forces resulted in only small decreases in trunnion wear generation.

Conclusions

These data suggest that the theoretical advantages of large-diameter femoral heads have a limit. Diameters of greater than 40 mm demonstrated only modest improvement in terms of joint stability yet incurred substantial increase in wear potential at the trunnion.

Clinical Relevance

Our model has potential to help investigators and designers of hip implants to better understand the optimization of trunnion design for long-term durability.

The institutions of one or more of the authors have received, during the study period, funding from the NIH (Grants AR46601 and AR53553) (TDB), the Veterans Administration (JJC, TDB), and the National Center for Research Resources (Grant UL1 RR024979) (JME).
One of the authors (JJC) certifies that he or she, or a member of his or her immediate family, has received or may receive payments or benefits, during the study period, an amount of more than USD 1,000,001, from DePuy Orthopaedics, Inc (Warsaw, IN, USA). One of the authors (TDB) certifies that he or she, or a member of his or her immediate family, has received or may receive payments or benefits, during the study period, an amount of USD 10,000 to USD 100,000, from Smith & Nephew, Inc (Memphis, TN, USA).
All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research editors and board members are on file with the publication and can be viewed on request.
The study was performed at the University of Iowa, Iowa City, IA, USA.