Quantification of Self-Polishing from Ex Vivo 36mm Metal-on-Metal Total Hip Prostheses +

1Joyce, T J;

1Grigg, H;

12Langton, D J;

2Nargol, A V N

+1Newcastle University, Newcastle upon Tyne, UK,

2University Hospital of North Tees, Stockton on Tees, UK

t.j.joyce@ncl.ac.uk

INTRODUCTION:

Most total hip replacement operations are very successful procedures.

Typically a hip prosthesis consists of a cobalt chrome femoral head

which articulates within a polymeric acetabular cup. The majority of hip

prostheses fail due to aseptic loosening, which is considered to be the

body’s reaction to polyethylene wear debris. Therefore one way to

remove this problem is to replace the polyethylene with a harder

material such as cobalt chrome, to give a metal-on-metal (MoM)

articulation. Clinical results for such MoM hip prostheses are generally

positive and similar encouraging results are also reported from in vitro

studies [1]. In these laboratory tests ‘self polishing’ of the articulating

surfaces of test prostheses has been reported [2]. Self polishing

describes a beneficial process in which surfaces become smoother

through the process of articulating against each other. Therefore self-

polishing can also be tied in with a ‘bedding in’ period. Self-polishing

has been claimed from explanted hip prostheses but these assertions are

based on qualitative rather than quantitative data [3-5].

METHODS:

Two MoM total hip prostheses, consisting of femoral head and

acetabular cup, of 36mm nominal articulating diameter, were obtained at

revision surgery (figure 1). The explants were examined using a ZYGO

NewView 5000 non-contacting profilometer. This device allowed

measurements of surface roughness to be obtained and these were taken

at a range of positions on the explanted heads and cups. A minimum of

165 readings were taken per component. Roughness was measured in

terms of Ra (roughness average) and Rsk (skewness). In addition the

spherical diameters of each head and cup were measured using a

Mitutoyo 544 co-ordinate measuring machine. From these

measurements, together with knowing the material properties of cobalt

chrome and typical operating conditions for a total hip replacement the

minimum effective film thickness and thus the lambda ratio was

calculated for the total hip replacement when new, based on the

‘unworn’ area, and for the ‘worn’ area using the following equations [6]: 21.0

2*

65.0

*

min 80.2

−

=

xxxRE

w

RE

u

R

h η

( ) ( )[ ] 2/12

2

2

1

min

aaRR

h

+=λ

In the above equations, hmin is the minimum effective film thickness (m),

Rx is the effective radius (m), η is the lubricant viscosity (Pa s), u is the

entraining velocity (m/s), E* is the equivalent elastic modulus (Pa), w is

the load (N), λ is the lambda ratio, and Ra1 and Ra2 are the roughness

values of the femoral head and acetabular cup.

Fig. 1: The femoral head (left) and acetabular cup (right) from one of the

explanted total hip replacements.

RESULTS: The roughness data indicted that, in the ‘worn’ (figure 2) areas, Ra

values were reduced compared with ‘unworn’ (figure 3) parts of the

articulating surfaces of the explants. Typical values were 0.006µm Ra

and 0.020µm Ra respectively. In addition the Rsk values were negative

in the ‘worn’ areas compared with positive in the ‘unworn’ areas. Using

the measured data allowed lambda ratios of 0.9 (boundary lubrication)

for the ‘unworn’ area, and 3.0 (mild mixed lubrication) in the ‘worn’

area to be calculated.

Fig. 2: Typical topography of ‘worn’ region of explant. Note lack of

localized peaks, multi-directional scratches and Ra value of 0.003µm.

Fig. 3: Typical topography of ‘unworn’ region of explant. Note

localized peaks, lack of scratches and Ra value of 0.015µm.

DISCUSSION:

To the authors’ best knowledge this is the first time that quantitative

data related to the surface topography of explanted MoM total hip

replacements has been offered which supports the concept of self-

polishing in vivo. While other papers have offered this opinion, Milosev

et al [4] only offered Ra data for femoral components, and both Milosev

et al [4] and Reinisch et al [5] reported on components that had high

(0.1-0.15µm and 0.07µm respectively) Ra values in both ‘worn’ and

‘unworn’ areas. In addition the skewness values reported here show that

the surface has changed from one with a majority of peaks (positive

Rsk) to one with a majority of ‘valleys’ (negative Rsk). The lambda

ratios also indicate that these prostheses, when ‘worn’ would have

operated in the benign mild mixed lubrication regime, a useful

improvement on the lower initial lambda ratios.

REFERENCES:

[1] Rieker, C.B., Schon, R. and Kottig, P., “Development and

validation of a second-generation metal-on-metal bearing:

Laboratory studies and analysis of retrievals”, J Arthroplasty, 19

(8, Supp 1), 2004, 5-11.

[2] Firkins, P.J., Tipper, J.L., Ingham, E., Stone, M.H., Farrar, R. and

Fisher, J., “Influence of simulator kinematics on the wear of

metal-on-metal hip prostheses”, J Engng in Med, 215, 2001, 119-

121.

[3] Sieber, H., Rieker, C. and Kottig, P., “Analysis of 118 second-

generation metal-on-metal retrieved hip implants”, J Bone Jt Surg,

81-B, 1999, 46-50.

[4] Milosev, I., Trebse, R., Kovac, S., Cor, A. and Pisot, V.,

“Survivorship and Retrieval Analysis of Sikomet metal-on-metal

total hip replacements at a mean of seven years”, J Bone Jt Surg,

88(6), 2006, 1173-1182.

[5] Reinisch, G., Judmann, K.P., Lhotka, C., Lintner, F. and

Zweymuller, K.A., “Retrieval study of uncemented metal-metal

hip prostheses revised for early loosening”, Biomaterials, 24,

2003, 1081-1091.

[6] Jin, Z.M., Stone, M., Ingham, E. and Fisher, J., “Biotribology”,

Current Orthopaedics, 20(1), 2006, 32-40.

Poster No. 2276 • 56th Annual Meeting of the Orthopaedic Research Society