Exploring the Mechanisms of Head-Trunnion Mechanics in Modular Hip Prostheses: The Influence of Material and Structural Stiffness

Research output: Chapter or section in a book/report/conference proceedingChapter in a published conference proceeding


The majority of primary total hip arthroplasty procedures performed throughout the world use modular junctions, such as the trunnion-head interface; however, the failure of these press-fit junctions is currently a key issue that is still not fully understood. Previous experimental testing within our group found that material choice and head size can substantially influence the strength of the initial press-fit connection. The aim of the study was to attempt to predict the differences in taper connection strength for different implant materials and geometries using analytical and numerical approaches.

Titanium alloy trunnions and two diameters (28 and 36 mm) of cobalt chrome and ceramic heads were considered in this study. Analytical models developed by Fessler and Fricker (1989) were extended by MacLeod et al. (2016) to incorporate the influence of radial deformation and head size using the Lamé approximation for thin-walled cylinders. Nonlinear axisymmetric finite element models were developed which explicitly modelled the profile of the trunnion surface (20 µm ridge height), metal plasticity and included frictional effects at the taper interface. For each of the different approaches, the connection strength, defined as the longitudinal pull-off force required to break the taper assembly, was evaluated and compared to the experimental result.

Neither analytical approach was able to adequately capture the experimentally measured differences in connection strength between the head sizes and materials. The finite element simulations, however, accurately predicted the differences (up to 20%) in connection strength for the two head sizes. The simulation results also highlighted significant local deformation occurring at the top and bottom of the taper interface, which the analytical models were unable to predict. This indicates that local deformation of the head and taper, particularly along the longitudinal axis, is a key variable responsible for the differences between head sizes and materials.
Original languageEnglish
Title of host publicationBritish Orthopaedic Research Society Conference 2018 - BORS 2018
Publication statusPublished - 2018


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