Analysis and evaluation of a biomedical polycarbonate urethane tested in an in vitro study and an ovine arthroplasty model. Part II: in vivo investigation
Introduction
The long-term survival of total hip replacement [1] can be influenced by a wide range of factors including the nature of the implant used, surgical technique [2], [3], [4], [5], the condition of the host bone, post-operative care, and the age, health and activity level of the patient. In conventional acetabular components designed for total hip arthroplasty, one of the most frequent failure mechanisms has been wear particle mediated osteolysis [6], [7], [8], [9]. Acetabular component failure is a common reason for revision surgery [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. A prosthesis incorporating a compliant layer with greater resistance to wear and in vivo degradation should significantly reduce the level of wear particle mediated osteolysis and thereby improve the long-term stability.
Polyurethane (PU) elastomers have a unique combination of toughness, durability and flexibility, biocompatibility and biostability that makes them suitable materials for use in a diverse range of implantable medical devices. Our in vitro studies assessed the resistance of Corethane 80A to the main degradation mechanisms observed in PUs: hydrolysis, environmental stress cracking (ESC), metal ion oxidation (MIO) and calcification.
Corethane 80A, a commercially available polycarbonate urethane (PCU), presently branded as Bionate (produced under licence by Polymer Technology Group, Berkeley, CA), exhibited excellent resistance to these degradation mechanisms, implying that the material could provide a more robust bearing layer in a prototype compliant layer acetabular cup [1].
In this second part of the two papers on material, we report a series of tests on samples of Corethane 80A retrieved from an in vivo trial using a fully functioning total hip arthoplasty (THA) with a Corethane 80A compliant layer implanted into sheep, and make comparisons with a series of in vitro cups stored at 37°C in dry and PBS environments.
The sheep proximal femur, although a different shape, is a reasonable morphological analogue to the human femur, making the ovine model increasingly popular for orthopaedic conditions [24], [25]. Sheep tolerate the procedure well and bear weight early in the post-operative period, making this model more appropriate than canine models [26], [27], [28].
This paper reports on the biostability of the PU material and forms part of a research programme designed to evaluate a soft layer bearing from its theoretical design through to a practical demonstration of the technology in an in vivo model of hip arthroplasty. This research is being reported from design conception through material testing to a practical application of a novel acetabular cup design implanted into an ovine hip arthroplasty model for 4 years. The overall objective was to develop a non-polyethylene bearing system that performs in a way analogous to natural articular cartilage by limiting the wear debris production by almost frictionless articulation and thus extending the useful lifetime of artificial joints [29], [30], [31].
Section snippets
Experimental materials and procedures
The PU cup used in the trial comprises two layers: the softer inner bearing layer, which is made from Corethane 80A, and an outer shell made from a harder grade PU, Corethane 75D (Fig. 1). The prototype cemented THA joint (acetabular and femoral components) is a scaled-down design suitable for sheep based on Howmedica's Exeter Hip System.
The cups were manufactured in a two-part injection moulding process [32], with the Corethane 75D shell moulded first followed by the soft compliant bearing
Macroscopic observations
No significant physical or chemical degradation was observed after 3 years in vivo (Fig. 4). Host variables such as weight, age, mobility and implant duration produced little difference and there was remarkable consistency in the condition of the cups retrieved.
At retrieval, observations of fractures in the Corethane 75D shell were rare, occurring on only one occasion. Fracture of the hard shell however, did not cause the compliant layer (Corethane 80A) to fail nor result in loosening of the
Conclusions
The real-time and accelerated in vitro tests reported in Part I examined the main degradation mechanisms (hydrolysis, ESC, MIO and calcification) observed in biomedical PUs [1]. Of the four materials tested, Corethane 80A, was shown to be most suited to long-term implantation. The PUs with a polycarbonate soft segment (Corethane 80A and ChronoFlex 80A) were the more biostable materials. The work on in vivo retrieved samples of Corethane 80A reported here validate the in vitro studies,
Acknowledgements
This work was supported by a CASE Award from the EPSRC and Stryker Howmedica Osteonics.
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