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Open Access Technical Note

A biomechanical assessment of modular and monoblock revision hip implants using FE analysis and strain gage measurements

Habiba Bougherara1, Rad Zdero12*, Suraj Shah2, Milan Miric1, Marcello Papini1, Paul Zalzal3 and Emil H Schemitsch24

Author Affiliations

1 Department of Mechanical and Industrial Engineering, Ryerson University, Toronto, ON, M5B-2K3, Canada

2 Martin Orthopaedic Biomechanics Laboratory, St. Michael's Hospital, Toronto, ON, M5B-1W8, Canada

3 Department of Surgery, McMaster University, Hamilton, ON, L8S-4L8, Canada

4 Department of Surgery, University of Toronto, Toronto, ON, M5G-1L5, Canada

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Journal of Orthopaedic Surgery and Research 2010, 5:34  doi:10.1186/1749-799X-5-34

Published: 12 May 2010

Abstract

Background

The bone loss associated with revision surgery or pathology has been the impetus for developing modular revision total hip prostheses. Few studies have assessed these modular implants quantitatively from a mechanical standpoint.

Methods

Three-dimensional finite element (FE) models were developed to mimic a hip implant alone (Construct A) and a hip implant-femur configuration (Construct B). Bonded contact was assumed for all interfaces to simulate long-term bony ongrowth and stability. The hip implants modeled were a Modular stem having two interlocking parts (Zimmer Modular Revision Hip System, Zimmer, Warsaw, IN, USA) and a Monoblock stem made from a single piece of material (Stryker Restoration HA Hip System, Stryker, Mahwah, NJ, USA). Axial loads of 700 and 2000 N were applied to Construct A and 2000 N to Construct B models. Stiffness, strain, and stress were computed. Mechanical tests using axial loads were used for Construct A to validate the FE model. Strain gages were placed along the medial and lateral side of the hip implants at 8 locations to measure axial strain distribution.

Results

There was approximately a 3% average difference between FE and experimental strains for Construct A at all locations for the Modular implant and in the proximal region for the Monoblock implant. FE results for Construct B showed that both implants carried the majority (Modular, 76%; Monoblock, 66%) of the 2000 N load relative to the femur. FE analysis and experiments demonstrated that the Modular implant was 3 to 4.5 times mechanically stiffer than the Monoblock due primarily to geometric differences.

Conclusions

This study provides mechanical characteristics of revision hip implants at sub-clinical axial loads as an initial predictor of potential failure.