Elsevier

Biomaterials

Volume 26, Issue 6, February 2005, Pages 633-643
Biomaterials

Analysis and evaluation of a biomedical polycarbonate urethane tested in an in vitro study and an ovine arthroplasty model. Part II: in vivo investigation

https://doi.org/10.1016/j.biomaterials.2004.02.064Get rights and content

Abstract

The polyurethane (PU) elastomer Corethane 80A (Corvita) is being considered as the acetabular bearing material in a novel total replacement hip joint. Its biostability was investigated in vitro (Analysis and evaluation of a biomedical polycarbonate urethane tested in an in vitro study and an ovine arthroplasty model. Part I: material selection and evaluation, Biomaterials, in press) together with three other commercially available biomedical PUs: Pellethane 2363-80A (DOW Chemical), a polyhexamethylene oxide based PU, PHMO-PU (CSIRO, not supplied as a commercial product) and ChronoFlex AL-80A (CardioTech). From the in vitro studies, Corethane 80A displayed the best overall resistance to hydrolysis, ESC, MIO and calcification, followed by ChronoFlex 80A and PHMO-PU, with Pellethane 80A being the least stable. Building on the in vitro investigation, the follow-up in vivo study (reported here) assessed Corethane 80A as the bearing layer in a prototype compliant layer acetabular cup, in a fully functioning ovine total hip arthoplasty (THA) model. PU degradation in the retrieved cups was analysed using a range of analytical and physical-testing methods including mechanical testing, differential scanning calorimetry, Fourier transform infrared spectroscopy and environmental scanning electron microscopy. The Corethane 80A functioned well in the THA model, with the bearing surfaces of the retrieved hip cups showing no significant evidence of biodegradation or wear damage after 3 years in vivo. The findings in this study provide compelling evidence for the biostability and effectiveness of acetabular cups incorporating a Corethane 80A compliant bearing layer.

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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