Abstract
INTRODUCTION
Ceramic-on-ceramic hip resurfacing offers a bone conserving treatment for more active patients without the potential metal ion risks associated with resurfacing devices. The Biolox Delta ceramic material has over 15 years of clinical history with low wear and good biocompatibility but has been limited previously in total hip replacement to 48mm diameter bearings [1]. Further increasing the diameter for resurfacing bearings and removing the metal shell to allow for direct fixation of the ceramic cup may increase the wear of this material and increase the risk of fracture.
METHODS
Eighteen implants (ReCerf™, MatOrtho, UK; Figure1) were wear tested; six were ⊘40mm (small) and twelve ⊘64mm (large). All small and six large implants were tested under ISO 14242 standard conditions for 5 million cycles (mc) at 30° inclination (45° clinically). The six remaining large implants were tested under microseparation conditions in which rim contact was initiated during heel strike of the gait cycle for 5mc. Cups were orientated at 45° inclination (60° clinically) to allow for separation of the head and cup with a reduced 50N swing phase load and a spring load applied to induce a 0.5mm medial-superior translation of the cup. Wear was determined gravimetrically at 0.5mc, 1mc and every mc after.
RESULTS
Wear was low in both standard and microseparation tests, less than 1mm3cumulatively over 5mc (Figure 2). Standard conditions showed a run-in wear phase over the first mc followed by negligible wear in both diameters. The run-in wear significantly increased from 0.2mm3/mc in the 40mm diameter bearings to 0.5mm3/mc with the larger diameter implants. Under microseparation conditions, there was low wear over the first mc, increasing to 0.28mm3/mc between 1–3mc. The wear rate reduced to 0.11mm3/mc from 3=5mc. Stripe wear was evidenced on the microseparated components. There were no incidences of fracture or squeaking.
DISCUSSION
Biolox Delta is known for its low wear rates but published results have only reported testing up to ⊘36mm [2]. Increasing the diameter to 64mm showed increased wear compared to smaller diameters but this was only significant over the first mc suggesting similar performance long term. Microseparation testing of these large sized bearings doubled the cumulative wear produced over 5mc but wear measured was still much lower than other bearing combinations. Wear of metal-on-metal resurfacing implants under these high angle, microseparation conditions has been reported up to 10.5mm3/mc [3], significantly higher than any wear rate reported in the current study. Despite the 3mm wall thickness, no fracture of the cup occurred but stripe wear was observed in the ceramic components.
SIGNIFICANCE
Biolox Delta ceramic is appropriate for use in larger diameters without excessive wear or damage to the bearings. The improved biocompatibility of the material may allow for hip resurfacing to be offered to more patients than currently available.
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