Elsevier

Journal of Biomechanics

Volume 43, Issue 7, 7 May 2010, Pages 1310-1315
Journal of Biomechanics

Material and surface factors influencing backside fretting wear in total knee replacement tibial components

https://doi.org/10.1016/j.jbiomech.2010.01.015Get rights and content

Abstract

Retrieval studies have shown that the interface between the ultra-high molecular weight polyethylene insert and metal tibial tray of fixed-bearing total knee replacement components can be a source of substantial amounts of wear debris due to fretting micromotion. We assessed fretting wear of polyethylene against metal as a function of metal surface finish, alloy, and micromotion amplitude, using a three-station pin-on-disc fretting wear simulator. Overall, the greatest reduction in polyethylene wear was achieved by highly polishing the metal surface. For example, highly polished titanium alloy surfaces produced nearly 20 times less polyethylene wear compared with blasted titanium alloy, whereas, decreasing the micromotion amplitude from 200 to 50 μm produced approximately four times less polyethylene wear for the same blasted titanium alloy surface. Although the effect of the metal alloy was much smaller than the effect of metal surface roughness or the micromotion amplitude, CoCr discs produced slightly greater polyethylene fretting wear than titanium alloy discs under each condition. The results are essential in design and manufacturing decisions related to fixed-bearing total knee replacements.

Introduction

Particulate wear debris from implants is largely recognized as a major cause of bone resorption and osteolysis, representing one of the leading reasons of clinical failure of total joint replacements. Recently, there has been increasing awareness that, in addition to articulating surfaces, non-articulating surfaces, such as implant fixation interfaces and modular component junctions, can generate substantial amounts of debris as a result of micromotion and fretting wear. In total knee replacements, fretting between the backside of the ultra-high molecular weight polyethylene insert and the tibial tray of fixed bearing tibial components is documented to generate significant amounts of polyethylene debris. Although difficult to quantify, some studies have estimated polyethylene from the backside of the tibial tray alone to wear an average of 100–138 mm3 per year (Conditt et al., 2005; Li et al., 2002). These investigators have further noted that such rates were comparable to runaway wear from the articular surfaces of severely wearing polyethylene acetabular components in total hip replacements, and capable of originating clinically significant levels of bone resorption.

The importance of fretting in knee replacements has led to redesigning locking mechanisms, reducing micromotion. However, the relative influences of metal surface roughness, alloy, and micromotion on fretting wear have not been systematically studied. Rather, most studies have used retrieved implants, scoring damage on the inferior surface (Engh et al., 2001; Harman et al., 2007; Rao et al., 2002; Surace et al., 2002; Wasielewski et al., 1997), measuring polyethylene extrusion from inferior insert surfaces into tibial tray screw holes (Conditt et al., 2005), or measuring decreases in depths of manufacturer’s stamped markings on the inferior polyethylene surface (Crowninshield et al., 2006). Although these studies provide valuable information, they are indirect retrospective measurements.

Variables affecting backside interface micromotion and the resultant wear (Conditt et al., 2004b) include metal surface finish, alloy, implant design, and locking mechanism. Retrieval studies have not isolated or quantified individual influences of variables on backside wear, as they cannot systematically control independent variables. Wear simulator studies inherently include the complexities of implant design, locking mechanism, and other variables, again, making it difficult to isolate the effects of individual variables, or even to separate backside wear from articular surface wear without additional indirect measurements (Muratoglu et al., 2007).

In this study, we used a crossing-path fretting wear simulator to measure the relative effects of metal surface finish, alloy, and micromotion amplitude, representative of those on the backside of tibial components, on fretting wear behavior of polyethylene against metal, under simulated physiological conditions.

Section snippets

Simulator

The three-station simulator generated fretting wear by creating small tangential micromotion between pins and discs. Mounted in an MTS 812 servohydraulic load frame (MTS Corporation, Minneapolis, MN), it consisted of three pin-on-disc assemblies each in a chamber of 90% bovine serum (Fig. 1). Discs were mounted on the base plate, while each pin was mounted at the end of an L-shaped structure, 165 mm from its corresponding vertical rod.

Cyclic axial load was applied through a central ball joint.

Results

By dimensional measurements using the CMM, polyethylene wear was between 0 and 1.63 mm3/million cycles (Table 1). In general, volumetric wear rates calculated from height loss measurements were somewhat smaller than volumetric wear rates obtained by gravimetric measurements, but the trends were generally the same (Table 1). Since gravimetric measurements directly measured the total magnitude of weight loss due to wear, in the analysis, gravimetric data were used.

All else being equal, blasted

Discussion

The present study used an in vitro fretting simulator to model loads and motions at the tibial insert/metal tray interface of modular total knee replacements with fixed bearings, and to produce wear mechanisms observed clinically at this interface. This simulator provided a few key features that were critical to achieve this goal. First, the simulator produced micromotion amplitudes in the range of many clinically stable and well-functioning components. Second, crossing-path motion was applied,

Conflict of interest statement

This study was supported by Depuy Orthopaedics, Inc., Warsaw, IN, USA and the Los Angeles Orthopaedic Hospital Foundation, Los Angeles, CA, USA. Sarah Aust is an employee of Depuy Orthopaedics. The other authors have no potential conflicts to disclose. The sponsor was not involved in the decision to submit the work for publication.

Acknowledgement

This study was supported by Depuy Orthopaedics, Inc., Warsaw, IN, and Orthopaedic Hospital Foundation, Los Angeles, CA.

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