Investigation into the calcification and aging of intraocular lenses

Ghatora, Baljit (2010) Investigation into the calcification and aging of intraocular lenses. (PhD thesis), Kingston University, .


Poly (methyl methacrylate) (PMMA) has been the preferred biocompatible biomaterial for use in intraocular lenses (IOLs) since 1949, when its optical benefits were discovered by Harold Ridley. Calcification of intraocular lenses is an area that is not addressed adequately in literature, but is a problem that requires repeat surgery, which is not usually an option in third world countries. Calcification is a setback to IOLs that needs to be further studied so that possible solutions can be found. The effects of sterilisation and incubation in an extreme calcific environment, namely a metastable calcium salt solution at pH 7.4 at 37[degrees]C, on the properties of gamma-irradiated and ethylene-oxide-sterilised medical grade PMMA, was analysed over a period of nine weeks. From these initial studies, energy dispersive X-ray analysis identified deposits containing calcium, chlorine and potassium as soon as one day after incubation. It was proposed that calcification occurred as a result of diffusion of water into the PMMA, which then attracted calcium-containing deposits to accumulate on the surface. Subsequently, as gamma irradiation is the milder and preferred form of sterilisation, this was further studied after incubation in a stimulated aqueous humpur solution, whereby it was discovered that the diffusion coefficient 2.87 x 10[sup]-12m[sup]s[sup[-1] for the gamma sterilised PMMA compared to the un-sterilised PMMA diffusion coefficient of 2.42 x 10[sup]-12m[sup]2s[sup]-1. The approximatley 18% faster diffusion rate within the gamma sterilised PMMA was attributed to the chain scission that had occurred during the sterilisation process. Through the use of triple detection gel permeation chromatography, comparisons of the Mark-Houwink value identified the structural breakdown of the PMMA as a result of the sterilisation process. Surface modification of PMMA via fluorination was carried out using three different approaches to reduce diffusion and, in turn, calcification. The surface modified PMMA after immersion into the simulated aqueous humour solution was examined for surface deposits using analytical techniques. The chemically modified samples showed a promising reduction in calcium-containing deposits when analysed by scanning electron microscopy. However the drawback in the method used for chemical fluorination resulted in the PMMA surface not being evenly fluorinated. Therefore an improved method of applying direct gas fluorination to the PMMA was carried out which produced more consistent recults. This method was better at reducing both calcification and diffusion into the PMMA. PMMA was also doped with a per-fluorinated acridine and cast into a thin film to provide the best reduction of diffusion and hindrance of calcific deposits.

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