Personalized Corneal Biomechanics

Ariza Gracia, Miguel Angel; Flecha-Lescún, Julio; Rodríguez Matas, José Félix; Calvo Calzada, Begoña (2019). Personalized Corneal Biomechanics. In: Doweidar, Mohamed H. (ed.) Advances in Biomechanics and Tissue Regeneration (pp. 3-20). Elsevier 10.1016/B978-0-12-816390-0.00001-7

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Degradation or loss of vision greatly influences the quality of life. The shape and optical properties of the cornea are determined by the mechanical balance between intraocular pressure (IOP) and the internal stresses of corneal tissue. Interventions such as refractive surgery and pathologies such as Keratoconus can alter this mechanical balance and compromise visual acuity. Refractive surgeries are applied to change the curvature of the corneal surface and to modify its optical power. Despite the surgical breakthroughs over the last decades (radial keratotomy, photorefractive keratotomy, or laser in situ keratomileusis), the unpredictability of the surgical outcomes remains. This unpredictability is manifested inside effects that can lead to unexpected results in visual acuity after an intervention. Sometimes, undercorrection (≈11.9%) or overcorrection (≈4.2%) may occur and a second “enhancement” procedure is required. Different clinical devices are used in the clinic to characterize the geometrical, biological, and mechanical characteristics of the eyeball and, eventually, to assess the propriety of a treatment. For example, the geometry of the cornea is readily available by means of corneal topography, and the eyeball's IOP can be measured either with contact (Goldmann) or with noncontact tonometers (ORA, CorVis), which are also able to provide a set of dynamic biomarkers that aims to provide better insight on the quality of the corneal tissue (stroma). Unfortunately, many factors can affect the readings of these devices and the measured response of the cornea is, generally, the overall response of the system. Thus, the mechanical response of the cornea cannot be uniquely tethered to a closed set of origins. Ex vivo mechanical tests in combination with in silico models can be the key to solve, or at least partially alleviate, the mysteries of corneal biomechanics. In this chapter, we fly over those devices commonly used in the clinic to characterize corneal biomechanics, show how experimental mechanical tests and numerical models are coupled to determine the mechanical properties of the corneal tissue, and, finally, present how different mechanical and optical outcomes of personalized refractive interventions (e.g., relaxing incisions) can be simulated solely using clinical data, providing that the mechanical and geometrical characterization of the eyeball has been properly carried out.

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