Optimising the X-ray Visibility of Polyethylene Biomaterials

: (Alternative Format Thesis)

Abstract

In the UK there are more than 200,000 joint replacements performed each year, and the majority of the implanted devices have an ultra-high molecular weight polyethylene bearing. Polyethylene has limited X-ray attenuation which means that on a standard radiograph the polyethylene bearing component of a joint replacement cannot be seen. Consequently, indirect methods have been developed to assess the condition of the polyethylene part based on the surrounding metallic component positioning; but these indirect methods cannot be used for devices designed without a metallic backing, and can cause misdiagnosis as a result of inaccuracies. In this thesis, a novel polyethylene with increased radiopacity has been created through the diffusion of an iodised oil-based contrast fluid (Lipiodol Ultra Fluid), and its suitability for the long-term medical application explored.

Prior to commencing the experiments, the key challenges in the development of a new polyethylene were identified through a literature review. The review revealed the radiopaque polyethylene was an original idea, but that the following criteria must be met; the temperature of the treatment needed to be carefully controlled to ensure material properties were maintained, the radiopacity needed to be sufficient to be clinically useful, the polyethylene needed to be manufacturable and commercially viable, and any degradation of the material in vivo needed to be minimised.

The influence of Lipiodol treatment on the tensile, physical and chemical properties of the polymer was investigated using infrared spectroscopy, differential scanning calorimetry and tensile testing. The optimal treatment condition of 105C was identified from this experimental work, as polyethylene samples treated at this temperature had a significantly higher radiopacity (approximately 600 HU) and had tensile properties comparable with untreated polyethylene.

Once the treatment conditions had been chosen, the X-ray visibility of the radiopaque polyethylene under a clinical set-up was examined. The feasibility of using radiopaque polyethylene for model-based Roentgen Stereophotogrammetric Analysis (RSA) was investigated, and the precision and accuracy of the results reported for rotational or translational movements. The radiopaque polyethylene was found to have an X-ray attenuation sufficiently high to be visualised in standard radiographs, and it was possible to perform model-based RSA with a precision less than 0.06 mm for translational movements and more than 95% accuracy. The quality of the surface models was the main source of error, and this resulted from the radiopaque treatment causing swelling of the samples.

To better understand the parameters of the diffusion of the iodised oil into polyethylene, a practical finite-element model was developed for the prediction of the diffusion profile of Lipiodol into polyethylene. The model applied Fickian diffusion theory in three-dimensions and was able to accurately predict the diffusion pro le within the polyethylene to within 80% accuracy. It was concluded that the diffusion behaviour of oil within polyethylene is not truly Fickian, as the experiments indicated that the diffusion coefficient of the semi-crystalline polymer varied with temperature. Nevertheless, the model was sufficiently accurate for use as a design aid for manufacturing planning of radiopaque polyethylene parts.

The final study investigated the influence of Lipiodol treatment on the chemical stability of polyethylene and quantified; oxidation after accelerated ageing, changes to mechanical properties after ageing, and leaching of the oil from of polyethylene under physiological conditions. Oil infused samples (with both Lipiodol and Vitamin E) showed higher tensile yield, ultimate strength, elongation at failure and oxidative stability compared with untreated polyethylene. Leaching of the Lipiodol did occur but stabilised after 2 weeks. The accelerated ageing reduced the radiopacity, and after 4 weeks the radiopacity had reduced by 54%, but the radiopaque polyethylene was still clearly visible on CT scans.

This research has examined the influence that the infusion of an oil-based contrast agent has on the properties of ultra-high molecular weight polyethylene. The feasibility of the material for clinical application for radiography-based techniques such as RSA, has been demonstrated, and accelerated ageing results indicate the chemical stability of the material is comparative to Vitamin E polyethylene.

Date of Award	17 Feb 2021
Original language	English
Awarding Institution	University of Bath
Supervisor	Elise Pegg (Supervisor) & Richie Gill (Supervisor)