Comparison of the hydrolytic degradation and deformation properties of a PLLA-lauric acid based family of biomaterials.

Addition of lauric acid to PLLA results in a significantly increased rate of hydrolytic degradation, with the time-to-loss of tensile strength directly related to the concentration of lauric acid. In this study, the hydrolytic degradation profiles of four materials were studied: amorphous PLLA, amorphous PLLA containing 1.8 wt % lauric acid, amorphous PLLA containing 4.5 wt % lauric acid, and pre-crystallized PLLA containing 1.8 wt % lauric acid. Hydrolytic degradation was monitored through mass profiles, molecular weight profiles, crystallinity and the development of mechanical properties and deformation mechanisms (through simultaneous small-angle X-ray scattering and tensile testing), and a "phase diagram" of properties suggested. The key factor in determining the development of properties was found to be the time at which crystallization occurred in relation to the loss of molecular weight, with the two factors most affecting this being the lauric acid content and the pre-degradation annealing treatment.

The effect of crystallinity on the deformation mechanism and bulk mechanical properties of PLLA.

Poly (l-lactide) is a widely studied biomaterial, currently approved for use in a range of medical devices, however, most in vitro studies have so far focussed upon either the bulk properties during degradation and/or deformation, or on the microstructure of the unloaded material during degradation. This study aimed to combine these approaches through the technique of simultaneous small-angle X-ray scattering and tensile testing at various stages of degradation up to 8 months, on material with a range of induced microstructures. Results showed that the amorphous material deformed by crazing in the dry, wet and degraded states, however, the mechanism by which the craze developed changed significantly on hydration. Despite this difference, there was little change in the bulk mechanical properties. Crystalline materials deformed through crystal-mediated deformation, with contributions from both cavitation and fibrillated shear, but surprisingly, differences in the length scales within the spherulitic structure caused by annealing at different temperatures had very little effect on the mechanism of deformation, though differences were seen in the bulk properties. Furthermore, hydration had little effect on the crystalline materials, though degradation over 8 months resulted in loss of mechanical properties for samples produced at higher annealing temperatures. In conclusion, the introduction of crystallinity had a huge effect on both bulk and microscopic properties of PLLA, but the spherulitic structure of the crystalline material affected the bulk properties significantly more than it did the micromechanism of deformation.

Analysis and evaluation of a biomedical polycarbonate urethane tested in an in vitro study and an ovine arthroplasty model. Part I: materials selection and evaluation.

The polyurethane elastomer (PU) Corethane 80A (Corvita) is being considered as the acetabular bearing material in a novel total replacement hip joint. The biostability of Corethane 80A was investigated in vitro (this work) and in vivo (reported separately) in a fully functioning ovine total hip arthroplasty (THA) model, with the PU as the bearing layer in a prototype compliant layer acetabular cup. The 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. The performance of the polycarbonate PU Corethane 80A was assessed alongside three other commercially available biomedical PUs: polyether PUs Pellethane 2363-80A (DOW Chemical) and PHMO-PU (CSIRO, not supplied as a commercial material) as well as polycarbonate PU ChronoFlex AL-80A (CardioTech). Chemical and structural variables that affect the properties of the materials were analysed with particular attention to the nature of the material's hard and soft segments. PU degradation was probed using a range of analytical tools and physical-testing methods, including mechanical testing, differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) and environmental scanning microscopy (ESEM). Corethane 80A displayed the best overall resistance to hydrolysis, ESC, MIO and calcification, followed by ChronoFlex 80A and PHMO-PU. Pellethane 80A was the least stable. This study provides compelling evidence for the biostability and effectiveness of Corethane 80A and points to its suitability for use as a compliant bearing layer in hip arthroplasty, and possibly also other joints.

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

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.