Biomechanics of the cornea in keratoconus and other corneal ectasias

Abstract

Purpose. The relationship between structure and function of the cornea is discussed. Altered biomechanical properties of the corneal stroma are responsible for the development of primary and secondary ectasia. Material and Methods. A systematic literature review of the biochemical components of the corneal stroma and their im- portance for the biomechanics of the cornea was performed. The physical basis of the theory of elasticity and necessary extensions in terms of corneal nonlinearity and inhomoge- neity are discussed. The different biomechanical conditions leading to primary ectasia (e. g. keratoconus) and secondary ectasia after Lasik surgery are compared. Results. Primary ectasias are the result of altered biochemical properties of the stromal collagens. A different arrangement of collagen fibres in the stroma is another possible cause of primary ectasia. Post-Lasik ectasia is caused by the decou- pling of the flap and the residual stroma. The wall tension within the residual posterior stroma increases by a factor of 2 after Lasik. The thinned posterior stroma is not able to permanently withstand the intraocular pressure. Conclusion. The collagens of the stroma are decisive for the structure and thus also for the function of the cornea as an imaging element. Measurement results on the biomechanics of the cornea obtained with air-puff procedures provide only limited information on the biomechanics of the cornea and on the understanding of the aetiology of keratoconus, since here the mechanical loads to deform the cornea only act only for a few milliseconds, while the deforming force of the intraocular pressure acts permanently. Keywords Keratoconus, Post-Lasik Ectasia, Biomechanics, Collagen, Theory of Elasticity