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TECHNICAL PAPERS: Joint/Whole Body

Wear in the Prosthetic Shoulder: Association With Design Parameters

[+] Author and Article Information
Andrew R. Hopkins

Biomechanics Section, Mechanical Engineering Department, Imperial College London, London, SW7 2AZ, UK

Ulrich N. Hansen1

Biomechanics Section, Department of Mechanical Engineering, Imperial College London, Room 637, Mechanical Engineering Building, South Kensington Campus, London SW7 2AZ, UKu.hansen@imperial.ac.uk

Andrew A. Amis

Biomechanics Section, Mechanical Engineering Department, Imperial College London, London SW7 2AZ, UK; Musculoskeletal Surgery Department, Imperial College London, London SW7 2AZ, UK

Lucy Knight, Mark Taylor

Bioengineering Science Research Group, School of Engineering Sciences, University of Southampton, Highfield Campus, Southampton SO17 1BJ, UK

Ofer Levy, Stephen A. Copeland

The Reading Shoulder Surgery Unit, Royal Berkshire Hospital, Reading RG1 602, UK

1

Corresponding author.

J Biomech Eng 129(2), 223-230 (Oct 04, 2006) (8 pages) doi:10.1115/1.2486060 History: Received May 25, 2005; Revised October 04, 2006

Total replacement of the glenohumeral joint provides an effective means for treating a variety of pathologies of the shoulder. However, several studies indicate that the procedure has not yet been entirely optimized. Loosening of the glenoid component remains the most likely cause of implant failure, and generally this is believed to stem from either mechanical failure of the fixation in response to high tensile stresses, or through osteolysis of the surrounding bone stock in response to particulate wear debris. Many computational studies have considered the potential for the former, although only few have attempted to tackle the latter. Using finite-element analysis an investigation, taking into account contact pressures as well as glenohumeral kinematics, has thus been conducted, to assess the potential for polyethylene wear within the artificial shoulder. The relationships between three different aspects of glenohumeral design and the potential for wear have been considered, these being conformity, polyethylene thickness, and fixation type. The results of the current study indicate that the use of conforming designs are likely to produce slightly elevated amounts of wear debris particles when compared with less conforming joints, but that the latter would be more likely to cause material failure of the polyethylene. The volume of wear debris predicted was highly influenced by the rate of loading, however qualitatively it was found that wear predictions were not influenced by the use of different polyethylene thicknesses nor fixation type while the depth of wearing was. With the thinnest polyethylene designs (2mm) the maximum depth of the wear scar was seen to be upwards of 20% higher with a metal-backed fixation as opposed to a cemented design. In all-polyethylene designs peak polymethyl methacrylate tensile stresses were seen to reduce with increasing polyethylene thickness. Irrespective of the rate of loading of the shoulder joint, the current study indicates that it is possible to optimize glenoid component design against abrasive wear through the use of high conformity designs, possessing a polyethylene thickness of at least 6mm.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

Three retrieved glenoids: (left) keeled all-polyethylene glenoid worn to the point of becoming transparent; (middle) metal-backed glenoid worn through to the metal; and (right) severely worn all-polyethylene glenoid causing weakening and ultimately fracture of the glenoid component

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Figure 2

Finite-element model of the ANATOMICA glenoid

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Figure 3

FE model of the post-operative scapula used in the wear analyses

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Figure 4

The complete FE model of the shoulder with rigid humeral head

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Figure 5

The predicted effect of glenohumeral radial mismatch on peak contact pressures for all-polyethylene glenoids during 0–180deg abduction

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Figure 6

Effect of glenohumeral radial mismatch on contact pressures during unloaded abduction from 0deg (left) to 180deg (right). Scale indicates magnitude of contact pressures.

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Figure 7

Effect of conformity on surface wear of polyethylene liner. Scale indicates the depth of wear.

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Figure 8

Peak wear depth versus radial mismatch

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Figure 9

Volumetric wear rate versus radial mismatch

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Figure 10

Effect of liner thickness on peak contact pressures observed for cemented all-polyethylene glenoids

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Figure 11

Effect of liner thickness on peak contact pressures observed for metal-backed glenoids

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Figure 12

Linear wear generated over 1 million cycles of loading versus polyethylene thickness

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