New Developments in the Basic Science of Ceramics and Other Alternative Bearings 21
Comparison of the Functional Biological Activity and Osteolytic Potential of Ceramic on Ceramic and Cross Linked Polyethylene Bearings in the Hip
J. Fisher, A. Galvin, J. Tipper, T. Stewart, M. Stone and E. Ingham
Introduction
There is considerable Interest in the use of the ceramic on ceramic bearing couple in hip replacement as an alternative to conventional or cross linked polyethylene bearings. Ceramic on ceramic bearings are particularly attractive for younger and more active patients, who hove the potential to generate more wear and wear debris, and are at risk of earlier failure due to osteolysis with polyethylene bearings. Highly cross linked polyethylene acetabular cups hove been introduced into clinical practice in recent years, and some laboratory studies with these new materials have shown extremely low wear. In some cases it has not been possible to measure the wear of highly cross-linked polyethylene, due to moisture absorption artefacts. Extremely low wear has also been recorded in simulator studies and in vivo for ceramic on ceramic bearings. In this case accurate measurements of low wear can be made.
When selecting bearings for young and active patients, there is a need to understand the relative functional osteolytic potential of the bearing couples. The osteolytic potential of the bearing couple is not only dependent on the wear volume, but also on the biological activity of the wear particles. In this paper we summarise our recent research on the wear, wear debris and functional biological activity of conventional polyethylene, highly cross linked polyethylene and Biolox Forte alumina ceramic on ceramic bearing couples.
Methods
The wear of the bearing couples was investigated in the Leeds physiological hip joint simulator in the anatomical position [1,2]. Prostheses (n >3) of each type of prosthesis was tested to five million cycles under standard walking cycle conditions in 25% (v/v) new born calf serum. In addition, ceramic on ceramic bearings were studied under microseparotion conditions, which have been shown to represent more clinically relevant stripe wear [3]. The wear rate was measured every one million cycles. The wear particles were isolated and characterised using SEM and TEM [4,5]. The biological activity of the wear particles and functional biological activity and osteolytic potential was predicted using the methods described by Fisher et al [6]. The biological reactivity of the wear particles was also confirmed by direct cell culture with macrophages and determination of osteolytic cytokines [7,8].
Four different types of polyethylene acetabular cups were studied articulating against polished metallic femoral heads and these were compared to Biolox Forte ceramic on ceramic bearings. All bearing couples were size 28 mm diameter. Polyethylene acetabular cups were manufactured from two different
22 SESSION 1.2
resins, GUR1020 and GUR1050 which were sterilised with low levels of gamma irradiation (4 and 2.5 MRad respectively). In addition GUR 1050 acetabular cups that were highly cross linked with 10 MRad irradiation and re melted were studied.
Ceramic acetabular cups were also sterilised with gamma irradiation.
Results
The volumetric wear rates, specific biological activity per unit volume of wear (SBA), and the functional biological activity (FBA) or osteolytic potential for all four bearing combinations are shown in (Table 1). The volumetric wear rate was four times lower for the highly cross linked polyethylene compared to the conventional polyethylene sterilised with a low dose of irradiation. Under standard conditions the wear of the ceramic on ceramic bearing was reduced by more than one hundred times compared to the highly cross linked polyethylene material. Under more severe microseparation conditions the wear of the ceramic on ceramic bearing was six times lower than the highly cross linked polyethylene.
Bearing type
GUR 1020 polyethylene; 4 MRad GVF*
GUR 1050 polyethylene; 2.5 MRad Highly cross linked GUR 1050 polyethylene; 10 MRad
Biolox Forte ceramic on ceramic Biolox Forte ceramic on ceramic (microseparation)
Wear Volume [mmVlO* cycles]
35 45 9
0.04 1.5
SBA
0.49 0.93 0.92
0.2
FBA
17 40 8
0.3
Table 1:
Wear rate, Specific biological activity (SBA) and functional biological activity (FBA) for the different bearing materials.
*GVF; Gannnna vacuum foil
The wear particles generated by the ditterent bearing couples hod different specific biological activities, associated with differences in the volumetric concentrations in different size ranges. This was confirmed with direct cell culture studies [7]. In particular the GUR 1050 resin produced a greater proportion of the wear volume as sub micron size particles and higher levels of biological reactivity.
Similarly, the highly cross linked polyethylene wear particles also showed high levels of biological activity associated with most of the volume of the particles being sub micron in size. There were insufficient wear particles generated from the ceramic on ceramic bearing under standard conditions to determine their biological reactivity. Under microseparation conditions, the ceramic on ceramic bearings produced particles with a bimodal size distribution and these particles had the lowest specific biological reactivity.
Combining the wear volume and specific biological activity allowed the prediction of the functional biological activity and osteolytic potential.
Differences between the conventional polyethylenes were found, with the less
New Developments in the Basic Science of Ceramics a n d Other Alternative Bearings 2 3
Ireactive GUR1020 resin having the lowest osteolytic potential. The highly cross inked polyethylene showed a four fold reduction in wear rate, but only a two fold reduction in osteolytic potential compared to the conventional GUR1020 resin material. The ceramic on ceramic bearing had a fifty fold reduction in osteolytic potential compared to the highly cross linked polyethylene, due to a reduction in both wear volume and reactivity of the wear particles.
Discussion
In this study we hove determined finite wear rates for highly cross linked polyethylene in a hip joint simulator, which has allowed direct quantitative comparison with the wear of ceramic on ceramic bearings. A substantial reduction in wear was found with the ceramic on ceramic bearings. Analysis of the wear particles and direct cell culture with macrophages revealed major differences in the reactivity of the wear particles as measured by the release of osteolytic cytokines. Higher molecular weight polyethylene resin and highly cross linked polyethylene were found to produce smaller and more reactive particles compared to the lower molecular weight GUR1020 polyethylene. Ceramic on ceramic bearings generated particles with the lowest level of reactivity due to their, size and shape and chemical composition. The predicted osteolytic potential (FBA) showed the considerable advantage that can be achieved by the clinical use of ceramic on ceramic bearings.
Aclcnowledgments
This work was supported by EPSRC, ARC, DePuy international and Stryker Howmedica.
References
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2. Endo M., Tipper J. L., Barton D. C , Stone M. H., Ingham E., Fisher J. Comparison of wear, w e a r debris a n d functional biological activity of moderately crosslinked a n d non-crosslinked polyethylene in hip prostheses. Proc. Instn. M e c h . Engrs. 216 H, 111-122 (2002).
3. Stewart I , Tipper J. L., Streicher R., Ingham E., Fisher J., Long-term w e a r of HIPed alumina o n alumina bearings for THR under microseparation conditions. J Materials Science: Materials in Medicine 12, 1053-1056 (2001).
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7. Ingram J. H., Stone M., Fisher J., Ingham E. The influence of molecular weight, crosslinking and counterface roughness on TNF-alpha production by macrophages in response to ultra high molecular weight polyethylene particles. Biomateriols, 25, 3511-3522 (2004).
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