Importance of pin geometry on pin-on-plate wear ...

Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine http://pih.sagepub.com/

Importance of pin geometry on pin-on-plate wear testing of hard-on-hard bearing materials for artificial hip joints A A Besong, Z M Jin and J Fisher Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine 2001 215: 605 DOI: 10.1243/0954411011536127 The online version of this article can be found at: http://pih.sagepub.com/content/215/6/605

Published by: http://www.sagepublications.com

On behalf of:

Institution of Mechanical Engineers

Additional services and information for Proceedings of the Institution of Mechanical Engineers, Part H: Journal of Engineering in Medicine can be found at: Email Alerts: http://pih.sagepub.com/cgi/alerts Subscriptions: http://pih.sagepub.com/subscriptions Reprints: http://www.sagepub.com/journalsReprints.nav Permissions: http://www.sagepub.com/journalsPermissions.nav Citations: http://pih.sagepub.com/content/215/6/605.refs.html

>> Version of Record - Jun 1, 2001 What is This?

Downloaded from pih.sagepub.com by guest on January 7, 2012

605

Technical Note

Importance of pin geometry on pin-on-plate wear testing of hard-on-hard bearing materials for articial hip joints A A Besong1, Z M Jin1* and J Fisher2 1Department of Mechanical and Medical Engineering, University of Bradford, UK 2School of Mechanical Engineering, University of Leeds, UK

Abstract: The contact mechanics between the pin and the plate used in simple wear screening tests were investigated in this study. Both soft-on-hard, such as ultra-high molecular weight polyethylene ( UHMWPE )-on-metal or UHMWPE-on-ceramic, and hard-on-hard, such as metal-on-metal, bearing couples were considered. The eVect of the pin geometry and the misalignment between the pin and the plate were investigated on the predicted contact pressure distribution at the bearing surfaces using the nite element method. It was demonstrated that in the case of soft-on-hard bearing couples, neither the geometrical discontinuity of the pin surface nor the misalignment could cause a signicant increase in the contact stress. However, for hard-on-hard combinations, even with a very small misalignment of 0.5° between the pin and the plate, the geometrical discontinuity could lead to a more than tenfold increase in the predicted contact stress. This elevated contact stress may lead to a large scatter in the wear data and, even more importantly, structural damage of the bearing surfaces. Keywords: pin-on-plate, metal-on-metal, hip implants, contact mechanics, pin geometry

1

INTRODUCTION

The continuous eVort to improve the long-term service life of total joint replacements has maintained interest in new potential materials for bearing surfaces. A new material or material combination has to undergo stringent wear testing before being adopted as t for use in manufacturing prosthetic joints. Simple conguration wear testing has been used extensively as an ideal method for screening novel materials, compared with existing ones for use in joint replacements, prior to more complex joint simulator tests. At a fractional cost of a full simulator wear test, the reciprocating pin-on-plate apparatus has enabled better control of the individual tribological variables, leading to a better understanding of how these tribological factors independently inuence the wear mechanisms [1–12]. These simple tests also provide important information about the relative wear performance of bearing materials. However, the practical issue The MS was received on 29 May 2001 and was accepted after revision for publication on 24 July 2001. * Corresponding author: Department of Mechanical and Medical Engineering, University of Bradford, Bradford BD7 1DP, UK. H03001 © IMechE 2001

to face has been how to relate what is measured in such simple sliding tests to how the material behaves in the real situation. Simple unidirectional reciprocating pinon-plate tests have, for many decades, been used in screening bearing materials. In recent years, multidirectional motions have been introduced, in order to address the in vivo kinematic situation more realistically, at least in the hip joint [13]. The ultra-high molecular weight polyethylene ( UHMWPE )-on-metal wear rates measured from these multidirectional machines have been shown to be similar to those observed in vivo [13] and wear particles thereof are known to be clinically relevant [14], suggesting that similar basic wear mechanisms occur in vivo and in vitro. In metal-on-metal bearings, however, there is not suYcient information to assert whether the wear rate and the metallic wear particles from the pin-on-plate conguration tests are similar to those experienced in vivo. Despite the signicant development and extensive use of simple pin-on-plate tests, there have been relatively few studies of the eVect of the pin geometry on measured wear rates. Various surface proles of the pin have been used, including spherical and at-ended cylindrical,


Proc Instn Mech Engrs Vol 215 Part H

606

A A BESONG, Z M JIN AND J FISHER

conical or spherical forms as shown in Fig. 1. There are clear diVerences in the contact stress between the pin and plate conguration, and the conforming convex-onconcave prosthetic hip joint. Although it is possible, to some extent, to adjust the load applied in the pin-onplate test to match the overall magnitude of the contact stress experienced in the joint, it would be very diYcult to simulate the actual contact pressure distribution. This can become a serious problem when hard-on-hard bearing couples are tested, and extremely large contact stress, many times that experienced in vivo, can be developed in the test components. The geometrical discontinuity on the sliding surface of the pin and the misalignment between the pin and the plate may serve as a stress raiser within the tribological system. The aim of this study was to investigate the eVect of misalignment between the pin and the plate on contact stress levels in the current design of pin-on-plate tests for metal-on-metal bearings. The polyethylene-on-metal material combination was also considered as a reference case.

2

MATERIALS AND METHODS

2.1 The pin-on-plate reciprocator

3

The multidirectional pin-on-plate test considered by Tipper et al. [15] was analysed in this study. Figure 2a shows a simple schematic diagram of the set-up used. A at-ended spherical pin, representing either polyethylene or cobalt–chrome, was loaded against a cobalt–chrome plate as shown in Fig. 2b. The radius of the spherical surface of the pin was 100 mm, and the diameter of the at face was 3 mm. A typical axial load of 80 N was applied to the pin, giving a nominal contact pressure of 11.3 MPa between the pin and the plate. The lubricant used in the experiment was 25 per cent bovine serum.

2.2 Finite element modelling The contact mechanics between the pin and the plate were analysed using the nite element method (ABAQUS 5.8). A three-dimensional model of the pinon-plate set-up was developed, based on the dimensions of the pin used in the experimental study, and a small, but suYcient, portion of the plate as shown in Fig. 3. Both the pin and the plate were represented by threedimensional eight-noded brick elements and the surface-

Fig. 1 Proc Instn Mech Engrs Vol 215 Part H

based contact technique was used to model the contact between the pin and the plate. A coeYcient of friction of 0.1 was assumed at the contact interface between cobalt–chrome alloys in the presence of bovine serum. The plate was modelled as cobalt–chrome alloy, while the pin was modelled as either polyethylene or cobalt– chrome alloy. All the materials considered in the nite element model were assumed to be linear elastic and therefore, only the elastic modulus and Poisson’s ratio were required. Table 1 shows the values of both the elastic modulus and the Poisson’s ratio for both cobalt– chrome (CoCr) and polyethylene adopted in the present nite element simulation. The pin was loaded at the top surface and the axis of the pin was assumed to move only in the vertical direction. The base of the metallic plate was assumed to be fully constrained. The plate was assumed to be either perpendicular to the axis of the pin to represent the ideal contact condition (perfectly aligned) or slightly tilted to investigate the eVect of misalignment. The mesh density of the nite element model was systematically increased by sequentially halving the element size, until an optimum was reached. RESULTS AND DISCUSSION

3.1 Perfectly aligned condition The contact stresses between the pin and the plate when perfectly aligned were obtained from the nite element analysis for both the UHMWPE-on-CoCr and CoCron-CoCr material combinations as shown in Fig. 4. Under this ideal condition, the contact stress distribution was symmetrical about the axis of the pin. The load used in the nite element model was 80 N, according to the experimental set-up, to give a nominal stress distribution of 11.3 MPa, while the actual prediction from the present nite element model was about 5 MPa within the majority of the contact zone for both bearing couples. This diVerence was caused by the large stress concentration towards the edge of the contact region, owing to the geometrical discontinuity on the pin surface from the spherical to the at surface. It is generally very diYcult to predict accurately the magnitude of this stress concentration, since both the mesh density and the actual detailed pin geometry at the discontinuity point can have a profound eVect. In any case, it would be expected that the original pin geometry around the discontinuity

Various surface proles of the pin used for pin-on-plate conguration tests Downloaded from pih.sagepub.com by guest on January 7, 2012

H03001 © IMechE 2001

IMPORTANCE OF PIN GEOMETRY ON PIN-ON-PLATE WEAR TESTING

607

(a)

(b)

Fig. 2

(a) Schematic representation of the pin-on-plate conguration and (b) schematic diagram of the crosssection of the pin and plate. (From Tipper et al. [15])

region would be modied rapidly either owing to wear or plastic deformation of the pin.

3.2 Misalignment condition The misalignment between the pin and the plate caused a signicant increase in the predicted contact pressure distribution as shown in Figs 5 and 6, particularly for the cobalt–chrome pin. It is noted that the contact stress distribution became asymmetric, with a signicant increase in magnitude around the discontinuity region. Even at a misalignment of as small as 0.5°, the predicted peak contact pressure exceeded 300 MPa, about 15 times that under the perfectly aligned condition. On the other hand, the increase in the contact pressure for the polyethylene pin owing to the misalignment was relatively small, about 25 per cent. It is also interesting to note that as the misalignment increased further, the maximum contact pressure decreased for the cobalt–chrome pin. This was because of the spherical surface of the pin coming into the contact as shown in Fig. 6. H03001 © IMechE 2001

4

DISCUSSION

The simple pin-on-plate test method has been used extensively in many studies reported in the literature and has been shown to be appropriate for UHMWPE-on-hard (metal or ceramic) combinations. The wear rates obtained from these simple screening tests are generally consistent with those from simulators or clinically retrieved prostheses. This is partially owing to the relatively low variability in the contact stress between the pin and the plate associated with either the geometrical discontinuity or the misalignment as shown in this study. However, the at-ended geometry adopted for the polyethylene wear pin may not be suitable when hard-onhard bearing couples are tested. Visual and scanning electron microscopy observations of metallic pin and plate components have revealed asymmetric wear scars, presumably resulting from small misalignments within the system [15]. It has been shown from the present nite element modelling that even a small angular misalignment, when combined with the geometrical discontinuity of the pin surface, could increase the stress level between



608

Fig. 3

Table 1


Cross-sectional view of the nite element mesh for the pin and the plate

Bulk elastic material properties for UHMWPE and cobalt–chrome alloys used for the nite element modelling

Cobalt–chrome alloy UHMWPE

Fig. 4

Elastic modulus (GPa)

Poisson’s ratio

210 1.0

0.25 0.3

a metal pin and a metal plate by as much as 15 times, and this can potentially introduce large variability in wear rates measured from these tests. This study supports the need for robust and precisionengineered systems when testing and comparing hardon-hard bearing couples such as metal-on-metal using the simple pin-on-plate conguration. Better still, the pin surface should be designed without geometrical discontinuity in the contact region, and should provide a relatively constant contact pressure between the pin and the plate during the test. This can be very diYcult to achieve. For example, it has been shown both experimentally and theoretically in previous studies that an original spherical pin could be worn rapidly at the initial stage of the testing, and the worn prole of the pin was not too dissimilar to a at-ended spherical pin considered in this study [15, 16 ]. Therefore, even for a spherical pin, a signicant increase in the contact stress can be expected owing to misalignment when the pin is worn and reassembled for testing again. This may not be a serious problem as far as the increased stress owing to the geometrical discontinuity and the misalignment does not cause excessive surface damage. However, when the metal pin is coated with a diVerent material, the increased stress may well be large enough to cause structural damage of the coating and the use of a spherical geometry may be more appropriate [17].

5

CONCLUSION

The importance of the pin geometry has been investigated in this study, with particular reference to the use of the pin-on-plate conguration for simple wear screening testing. Both UHMWPE-on-metal and metal-on-

Contact stress distributions between the pin and the plate under the perfectly aligned condition for both UHMWPE-on-metal and metal-on-metal combinations



H03001 © IMechE 2001

IMPORTANCE OF PIN GEOMETRY ON PIN-ON-PLATE WEAR TESTING

Fig. 5

EVect of misalignment (angular inclination) between the pin and the plate on the predicted peak contact pressure for both UHMWPE-on-metal and metal-on-metal combinations

Fig. 6

Typical contact pressure distributions between the metal pin and the metal plate under various degrees of misalignment

metal bearing couples have been considered. The contact stress between the pin and the plate has been predicted using the nite element method, in particular to examine the eVect owing to the geometrical discontinuity on the pin surface and the misalignment between the pin and the plate. It has been shown that for the UHMWPE-onmetal combination, the increase in the contact stress between the polymeric pin and the metallic plate is modest owing to these factors, thus providing a theoretical justication for the at-ended conical polymeric pins H03001 © IMechE 2001

609

extensively used in previous studies. However, the geometrical discontinuity of the pin surface, combined with the misalignment between the pin and the plate can cause a signicant increase in the contact stress, when a metalon-metal bearing couple is considered. It has been shown that even a slight misalignment of 0.5° can cause a 15 times increase in the contact stress between a at-ended spherical cobalt–chrome pin and a cobalt–chrome plate. This may increase the variability of the wear rates measured, but may not be a serious problem providing that



610


the elevated stress level is not suYcient to cause excessive surface damage. However, when testing coated bearing surfaces, the increased contact stress may lead to structural failures of the coating and use of a spherical pin without geometrical discontinuity may be more appropriate. ACKNOWLEDGEMENTS This work was supported as part of the SUREHIP MEDLINK Collaborative Project with funding from the EPSRC and DTI. SUREHIP is a collaborative project between the Universities of Leeds and Bradford, Firth Rixson Superalloys Limited, Ionbond UK Limited, and DePuy International, a Johnson and Johnson Company. REFERENCES 1 McKellop, H., Clarke, I., Markolf, K. and Amstutz, H. Friction and wear properties of polymer, metal and ceramic prosthetic joint materials evaluated on a multi-channel screening device. J. Biomed. Mater. Res., 1981, 15, 619. 2 Dowson, D., El-Hady Diab, M. M., Gillis, B. J. and Atkinson, J. R. Inuence of counterface topography on the wear of ultra high molecular weight polyethylene under wet and dry conditions. In Polymer Wear and Its Control (Ed. L.-H. Lee), 1985, pp. 171–187 (American Chemical Society, Washington, DC ). 3 Weightman, B. and Light, D. A comparison of RCH1000 and Hi-Fax 1900 ultra-high molecular weight polyethylenes. Biomaterials, 1985, 6, 177–183. 4 Saikko, V. Wear and friction properties of prosthetic joint materials evaluated on a reciprocating pin-on-plate apparatus. Wear, 1993, 166, 169–178. 5 Derbyshire, B., Fisher, J., Dowson, D., Hardaker, C. S. and Brummitt, K. Wear of UHMWPE sliding against untreated, titanium nitride-coated and ‘Hardcor’-treated stainless steel counterfaces. Wear, February 1995, 181–183(1), 258–262. 6 Fisher, J., Firkins, P., Reeves, E. A., Hailey, J. L. and Isaac, G. H. The inuence of scratches to metallic counterfaces on the wear of ultra-high molecular weight polyethylene. Proc. Instn Mech. Engrs, Part H, Journal of Engineering in Medicine, 1995, 209(H4), 263–264. 7 He, Y. J., Winnubst, A. J. A., Schipper, D. J., Bakker, P. M. V., Burggraaf, A. J. and Verweij, H. Friction and wear behaviour of ceramic-hardened steel couples under reciprocating sliding motion. Wear, April 1995, 184(1), 33–43.


8 Hailey, J. L., Ingham, E., Stone, M., Wroblewski, B. M. and Fisher J. Ultra-high molecular weight polyethylene wear debris generated in vivo and in laboratory tests; the inuence of counterface roughness. Proc. Instn Mech. Engrs, Part H, Journal of Engineering in Medicine, 1996, 210(H1), 3–10. 9 Besong, A. A., Tipper, J. L., Ingham, E., Stone, M. H., Wroblewski, B. M. and Fisher J. Quantitative comparison of wear debris from UHMWPE that has and has not been sterilised by gamma irradiation. J. Bone Jt Surg., 1998, 80-B(2), 340–344. 10 Ward, L. P., Subramanian, C., StraVord, K. N. and Wilks, T. P. Sliding wear studies of selected nitride coatings and their potential for long-term use in orthopaedic applications. Proc. Instn Mech. Engrs, Part H, Journal of Engineering in Medicine, 1998, 212(H4), 303–315. 11 Endo, M. M., Barbour, P. S., Barton, D. C., Wroblewski, B. M., Fisher, J., Tipper, J. L., Ingham, E. and Stone, M. H. A comparison of the wear and debris generation of GUR 1120 (compression moulded) and GUR 4150 (ram extruded) ultra high molecular weight polyethylene. Biomed. Mater. Engng, 1999, 9, 113–124. 12 Jones, V. C., Barton, D. C., Fitzpatrick, D. P., Auger, D. D., Stone, M. H. and Fisher, J. An experimental model of tibia counterface polyethylene wear in mobile bearing knees: the inuence of design and kinematics. Biomed. Mater. Engng, 1999, 9, 189–196. 13 Saikko, V. A multidirectional motion pin-on-disk wear test method for prosthetic joint materials. J. Biomed. Mater. Res., 1998, 41, 58–64. 14 Matthews, J. B., Besong, A. A., Green, T. R., Stone, M. H., Wroblewski, B. M., Fisher, J. and Ingham, E. Evaluation of the response of primary human peripheral blood mononuclear phagocytes to challenge with in vitro generated clinically relevant UHMWPE particles of known size and dose. J. Biomed. Mater. Res., 2000, 52(2), 296–307. 15 Tipper, J. L., Firkins, P. J., Ingham, E. and Fisher, J. Quantitative analysis of the wear and wear debris from low and high carbon content cobalt–chrome alloys used in metal on metal total hip replacement. J. Mater. Sci.: Mater. Medicine, 1999, 10, 353–362. 16 Jin, Z. M., Firkins, P., Farrar, R. and Fisher, J. Analysis and modelling of wear of cobalt–chrome alloys in a pinon-plate test for a metal-on-metal total hip replacement. Proc. Instn Mech. Engrs, Part H, Journal of Engineering in Medicine, 2000, 214(H6), 559–568. 17 Williams, S., Tipper, J., Eileen, I., Stone, M. and Fisher, J. In vitro analysis of the wear, wear debris and biological activity of surface engineered coatings for use in metal-onmetal total hip replacements. Proc. Instn Mech. Engrs, Part H, Journal of Engineering in Medicine (submitted).


H03001 © IMechE 2001