ReviewBiomechanics and wound healing in the cornea☆
Introduction
The structural and reparative properties of the cornea are essential to its function as a resilient, yet transparent, barrier to intraocular injury. Because the cornea is also the scaffold for the major refractive surface of the eye, any mechanical or biological response to injury will also influence optical performance. Consequently, the same mechanisms responsible for preserving ocular integrity can undermine the goals of achieving predictable and stable visual outcomes after keratorefractive surgery.
Even in an era of high-precision treatment algorithms, discrepancies between intended and realized visual outcomes are common. The shape-subtraction model of photokeratectomy that forms the basis of LASIK and PRK ablation routines (Munnerlyn et al., 1988) assumes a biologically and biomechanically inert cornea (Roberts, 2000) and does not account for non-idealities in the laser-tissue interaction. While empirical modifications to algorithms and major advances in laser delivery platforms have improved the statistical predictability of LASIK and PRK, the ability to anticipate confounding biological responses at the level of the individual patient remains limited. In some cases, a predisposition to mechanical instability or abnormal regulation of healing can lead to serious complications such as keratectasia or loss of corneal transparency (severe haze). The goal of research in this setting is to improve outcomes and reduce complications by discerning details of the biomechanical and wound healing pathways, identifying measurable predictors of individual responses and developing therapeutic models for controlling or compensating for these factors.
In this review, we highlight selected basic and practical considerations in corneal biomechanics and wound healing specific to the setting of photoablative corneal surgery, which accounts for the vast majority of all refractive surgery done today. These processes are approached temporally to distinguish between immediate biomechanical effects, later wound healing effects and ongoing biomechanical-wound healing interactions that help create a new steady state. Ultimately, biomechanical and wound healing responses are linked in time and space and are described separately only for the sake of clarity.
Section snippets
Corneal biomechanics
It is evident from incisional refractive surgery that the cornea is not mechanically inert. The role of biomechanics is therefore important to consider in routine LASIK or surface ablation procedures and in special cases where the biomechanical status of the cornea is abnormal (for example, after any previous refractive surgery or after penetrating keratoplasty). Biomechanical changes can manifest clinically as immediate corneal shape changes, shape instability over time and increased
Wound healing
The immediate postoperative refractive results of LASIK or surface ablation are dominated by the programmed ablation zone geometry, the laser-tissue interaction and perioperative biomechanical responses. Healing ensues immediately, however, and the cornea's optical properties are further modified. Biological diversity in this response is the norm, even in genetically similar individuals or contralateral eyes of the same patient. As such, it is a major factor in refractive overcorrection,
The interface of wound healing and biomechanics
The cornea undergoes significant structural and biological alterations after refractive surgery, and the relationship between these processes has been studied in postmortem tissue. A histopathological study in LASIK flaps of organ donors found a relationship between wound maturity and resistance to flap distraction (lifting) forces (Schmack et al., 2005). Flap cohesive strength was maximal at the flap margins, was associated with hypercellular fibrotic scars and increased as a function of
Acknowledgements
We are grateful to Cynthia Roberts, Ph.D. and Todd Doehring, Ph.D. for their contributions to Fig. 2, Fig. 3, respectively, and to Rajiv Mohan, Ph.D, Renato Ambrosio, Jr., MD, and Marcelo Netto, MD for work in generating components of Fig. 4.
Supported in part by US Public Health Service grants EY010056 (SEW) and EY015638 (SEW) from the National Eye Institute and HD049091 (WJD) from the National Institute of Child Health and Human Development, Multidisciplinary Clinical Research Career
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Proprietary interest statement: the authors have no proprietary or financial interest in relation to this manuscript.