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Polymer-on-Metal or Metal-on-Polymer Total Disc Arthroplasty: Does It Make a Difference?

Moghadas, Parshia M. PhD; Shepherd, Duncan E. T. PhD, FI Mech E; Hukins, David W. L. DSc, FRSE; Mahomed, Aziza PhD, MI Mech E

doi: 10.1097/BRS.0b013e318257fdd3

Study Design. Mechanical testing of total disc arthroplasty (TDA).

Objective. To compare the friction between a polymer socket-on-metal ball and metal socket-on-polymer ball TDA.

Summary of Background Data. A degenerate intervertebral disc can be replaced by TDA. The most common designs have a ball and socket articulation; the contact between the surfaces leads to friction. Friction needs to be minimized to prevent loosening and wear. One of the common material combinations in disc arthroplasty devices is the articulation of a metal socket on polymer ball. However, the combination of a polymer socket on metal ball (which is used in hip arthroplasty) has not been investigated for TDA.

Methods. TDA models with either a polymer socket/metal ball or a metal socket/polymer ball were manufactured with ball radii of 10 and 14 mm, each with a radial clearance of 0.35 mm. Samples were tested using a spine simulator with a lubricant of diluted newborn calf serum. Each sample was subjected to an axial load of 1200 N; motions of flexion-extension, lateral bending, and axial rotation were then applied at frequencies of 0.25 to 2 Hz. Frictional torque was measured to compare the performance of the TDAs.

Results. The frictional torque was found to be significantly higher for a disc with a metal socket/polymer ball than for a disc with a polymer socket/metal ball for both 10 and 14 mm radii in axial rotation, lateral bend, and extension. The frictional torque in flexion (0°–6°) was not found to be significantly different between the 2 different material combinations. However, when the flexion motion was reduced to 0° to 2°, frictional torque in the metal socket/polymer ball was found to be significantly higher than the polymer socket/metal ball.

Conclusion. TDA with a combination of a polymer socket/metal ball has lower friction than the conventional TDA with metal socket/polymer ball. This conclusion has implications in the design of TDA.

The combination of metal socket on polymer ball is widely used for ball-and-socket total disc arthroplasty, but the reverse combination has never been used. This study showed that the reverse material combination creates less friction.

From the School of Mechanical Engineering, University of Birmingham, Edgbaston, Birmingham, United Kingdom.

Address correspondence and reprint requests to Parshia M. Moghadas, PhD, School of Mechanical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom; E-mail:

Acknowledgment date: November 9, 2011. Revision date: February 13, 2012. Acceptance date: March 19, 2012.

The device(s)/drug(s) that is/are the subject of this manuscript is/are not intended for human use.

This work was supported by the Engineering and Physical Sciences Research Council [grant number EP/F014562/1].

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

© 2012 Lippincott Williams & Wilkins, Inc.