Review of the Mechanical Behaviour of Cornea and Its Implications on Surgery

Biological soft tissue are of special interest to the researchers as they exhibit unique anisotropic behaviour and are leading contributors to the quality of outcome of a surgical procedure. In this paper attempts have been made to review the strategy adopted by various researchers for conducting experiments in order to understand the mechanical behaviour of cornea and their use in ophthalmology. Studies have been conducted for identifying ultra-structure of cornea and their contribution on mechanical properties of the cornea. The other area focused by the researchers is on the deformation of cornea under stress conditions. This analysis is based on the assumption of treating cornea as a homogenous membrane. In the present work various approaches undergone by the researchers have been reviewed in a great detail. As an outcome of this literature review a numerical model has selected to be looked upon in the future since no purposefully research was conducted as of now. Finally it is proposed that by using the structural analysis of cornea a surgical procedure could be generated on the basis of quality of vision developed after the surgery. 1.0 INTRODUCTION There has been a tremendous growth in medication and surgical techniques being followed by the people of medicine. Most of the practices adopted by the surgeons can be explained and evaluated by the engineering principles. In the field of ophthalmology various researchers conducted researches related to the outcome of surgery[14], comparisons of various techniques of surgery[15] and combination of various types of surgery has been done. To evaluate, explain and analyse these procedures engineering principles are extensively being used. Kurl A Buzard [3] in his articles raised the question Biomechanics of the Cornea, Who needs it? Specifically speaking in the field of ophthalmology to quantify the response of the cornea after a surgical procedure, mechanical properties of cornea and the effect of these properties on the accuracy of diagnostic tools has been an area of great concern. Till now, most of the research was focused on surgical methods for reducing the post keratoplasty astigmatism and myopia i.e. keratotomy or LASIK. Under this project attempts shall be made to initiate the process of improving the method and development of the tools, which will produce useful information for controlling and reducing post keratoplasty corneal aberrations like astigmatism and myopia. This will also generate better understanding of the behaviour of cornea and its response to surgery, and will also give useful tips for improving the surgery and hence will give direction to researchers for achieving better visual rehabilitation with minimum medication and corrective surgical procedures to generate higher satisfaction level among the patients. To control and rectify the astigmatism evaluation of various refractive surgeries, suturing techniques, models for contact lens fitting etc. had been done from time to time. It has also been observed that even when efforts are made to control the refractive surgery procedures no attempts has been made to evaluate the process of penetrating keratoplasty using numerical or simulation models. This work focuses on the review of various models as proposed by the researchers and comparisons of the assumptions and their influence on the performance of the models. As a conclusion of this the intentions are to select the model best suited for analysis or simulation of post penetrating keratoplasty astigmatism. 2.0 REVIEW OF LITERATURE It was utmost important to control the process as the quality of outcome was measured in terms of uncorrected visual acuity. The objectives are improvement in the quality of life, lesser surgery and recovery time after refractive surgeries. The constituents and behaviour of cornea was understood by experiments and models were constructed to validate the behaviour. In these experiments attempts were made to understand the constituents of the cornea and their contribution to the biomechanical behaviour of the cornea. Also El shiekh and Anderson[9] in their work compared the various methods of calculating the physical properties of cornea. 2.1 Evaluation of structure of cornea Meek et al[17] in 1987 using x ray diffraction study first discussed about the anisotropy by presenting the orthogonal distribution of collagen fibrils in the corneal stroma i.e. inferior superior and nasal lateral followed by Daxer and Fratzl [8] and was discovered by Newton and Meek[19] that these collagen form a annulus in the limbus of the cornea. It was also observed by others that collagen fibrils are the stress bearing components in the cornea and all other structures and membranes like Bowmans membrane does not contribute significantly in bearing the stress implied on the cornea by various factors within a certain limit. In 2003 Boote et al[6] using X ray diffraction study proposed that collagen fibrils are more closely packed in peripapillary cornea and the loss in transparency is compensated by the reduction in thickness of the cornea in this region, they concluded that it has potential implications in transparency and refractive index of the cornea. Yashika and Ushiki [31] 1991 has quantified the three dimensional organization of collagen fibrils, this was important from point of view that it was understood later that collagen fibrils are the stress bearing members of the cornea and therefore it behaves anisotropically. Richhariya et al, International Journal of Advanced Engineering Research and Studies E-ISSN2249–8974 IJAERS/Vol. II/ Issue II/Jan.-March.,2013/ 2.2 Understanding the behaviour of cornea To explain the behaviour of cornea various numerical models and simulations of mechanical behavior of cornea and eyeball had been proposed by researchers for evaluation of surgical incisions and recovery of