Wear of a Novel Ceramic on Metal Bearings for Hip Prosthesis

New Applications for Ceramics 197

Wear of a Novel Ceramic on Metal Bearings for Hip Prosthesis

J. Fisher, P, Firkins, J. L. Tipper, R. Farrar and E. Ingham

Introduction

Concerns about polyethylene wear debris induced osteolysis has led to renewed interest in alternative bearing couples such as ceramic on ceramic and metal on metal. Conventionally, bearing couples in artificial joints, as well in engineering systems, are comprised of different materials. They frequently have a harder surface and a sacrificial softer surface that wears, for example metal on polyethylene in artificial joints. In alternative hard bearing couples, "like on like"

material combinations have been used. While harder materials typically wear less, it is not common practice in tribological systems to design "like on like"

bearing couples, as adhesive friction and wear can be high, and both bearing surfaces have the potential for wear. To date, hard bearing couples in artificial hip joints have not been studied as dissimilar bearing materials.

Metal on metal bearing couples are attractive from a design perspective, as the material toughness and hardness allows design flexibility, while delivering low wear of the order of one cubic millimetre per year. While the wear volume is low, and at least ten fold lower than cross linked polyethylene, the wear particles are very small, (circa 10 nm), providing a large surface area for metal ion release [1].

Elevated metal ion levels are a clinical concern for patients with metal on metal bearings [2]. Ceramic femoral heads have been extensively used for over thirty years in artificial hip joints and offer a reliable alternative to metallic alloy femoral heads. In this study alumina ceramic femoral heads were paired with metallic acetabular cups producing a novel differential hardness hard bearing couple.

We report the wear performance of this novel differential hardness ceramic on metal bearing couple [3] and compare it with conventional metal on metal bearings.

Methods

Size 28 mm diameter bearing couples were selected for this study. Standard metal on metal bearings, comprising of cobalt chrome alloy femoral heads and acetabular cups, were used as controls. Biolox Forte ceramic femoral heads were articulated against cobalt chrome acetabular cups to produce a differential ceramic on metal bearing couple. At least three couples of each type were studied in the Leeds physiological hip joint simulator, under both standard walking cycle conditions and microseperation conditions [4,5]. Wear tests were carried out to five million cycles in 25% (viv) new born calf serum and wear measurements were carried out every million cycles. Wear was determined gravimetrically, metallic ion levels in the lubricant were determined by atomic absorption spectroscopy and wear particles were isolated and characterised by TEM [1].

Wear surfaces were analysed by 2D surface profilometry and SEM.

1 9 8 SESSION 6.3

Results

The overall volumetric wear rates of the metal on metal and ceramic on metal bearings after five million cycles are shov^n in Table 1.

Bearing

Ceramic on metal Metal on metal

Overall wear rate;

mmVmillion cycles 0.01

1.6

Cobalt ion levels ppm

0.5 18

Table 1:

Volumetric wear rates of metal on metal and ceramic on metal bearings

The metal on metal bearings showed a higher initial bedding in wear rate and then the steady state wear rate reduced to 1.25 mmV million cycles, providing an overall wear rate of 1.6 mmVmillion cycles. In contrast the ceramic on metal wear rate was over 100 fold lower at 0.01 mmVmillion cycles. The reduction in metallic wear was reflected in a similar level of reduction in the metal Ion concentration in the lubricant. While the cobalt ion levels for the metal on metal bearing were 18 ppm, the levels of less than one ppm for the ceramic on metal hips were close to the resolution of the measuring system used and similar to the ion levels found with metal on polyethylene and ceramic on ceramic hip prostheses in this hip simulator system. An additional set of simulator studies was carried out with size 36mm Biolox Delta ceramic heads on cobalt chrome acetabular cups. The wear of the ceramic on metal was less than 0.01 mmVmillion cycles, approximately 100 fold less than the wear of the 36mm diameter metal on metal bearing. Under microseperation simulator conditions, the wear of both metal on metal and ceramic on metal increased, but the ceramic on metal bearing had a substantially lower wear rate. Wear particles from both bearing types was similar in size in the 10 to 30 nm size range. No wear or damage was detected on the harder ceramic femoral heads.

Discussion

Differential hardness hard on hard bearings have not been previously studied.

Although a reduction in wear was postulated for ceramic on metal, the magnitude of the reduction, 100 fold was surprising. In addition to the benefit of differential hardness and only one wearing surface, ceramic on metal may also have benefited from reduction in the chemical and corrosive wear found in metal on metal. The ceramic on metal bearing has now entered clinical studies and short term studies of metal ion levels In the patients will allow us to determine if the wear reduction found in vitro is reflected in clinical performance.

Acknowledgments

This work was supported by EPSRC and DePuy.

New Applications for Ceramics 199

References

1. Firkins P. J., Tipper J. L., Saadatzadeh M. R., Inghann E., Stone M. H., Farrar R., Fisher J.

Quantitative analysis of wear and wear debris from metal-on-metal hip prostheses tested in a physiological hip joint simulator. Bio-Medical Materials and Engineering 11, 143-157

(2001).

2. Ingham E., Fisher J. Biological reactions to wear debris in total joint replacement. Proc Instn Mech Engrs J Engineering in Medicine 214 H, 21 - 37 (2000).

3. Firkins P. J., Tipper J. L., Ingham E., Stone M. H., Farrar R., Fisher J. A novel low wearing differential hardness, ceramic-on-metal hip joint prostheses. Journal of Biomechanics 34,

1291-1298 (2001).

4. Firkins P. J., Tipper J. L., Ingham E., Stone M. H., Farrar R., Fisher J. Influence of simulator kinematics on the wear of metal-on-metal hip prostheses. Proc Instn Mech Engrs J Engineering in Medicine 215 H, 119-121 (2001).

5. Williams S., Butterfield M., Stewart T., Ingham E., Stone M. H., Fisher J. Wear and deformation of ceramic-on-polyethylene total hip replacements with joint laxity and swing phase microseporation. Proc Instn Mech Engrs J. Engineering in Medicine 217 H, J.

147-153 (2003).