Biomechanics of Ophthalmic Crosslinking

Abstract

Crosslinking involves the formation of bonds between polymer chains, such as proteins. In biological tissues, these bonds tend to stiffen the tissue, making it more resistant to mechanical degradation and deformation. In ophthalmology, the crosslinking phenomenon is being increasingly harnessed and explored as a treatment strategy for treating corneal ectasias, keratitis, degenerative myopia, and glaucoma. This review surveys the multitude of exogenous crosslinking strategies reported in the literature, both "light" (involving light energy) and "dark" (involving non-photic chemical processes), and explores their mechanisms, cytotoxicity, and stage of translational development. The spectrum of ophthalmic applications described in the literature is then discussed, with particular attention to proposed therapeutic mechanisms in the cornea and sclera. The mechanical effects of crosslinking are then discussed in the context of their proposed site and scale of action. Biomechanical characterization of the crosslinking effect is needed to more thoroughly address knowledge gaps in this area, and a review of reported methods for biomechanical characterization is presented with an attempt to assess the sensitivity of each method to crosslinking-mediated changes using data from the experimental and clinical literature. Biomechanical measurement methods differ in spatial resolution, mechanical sensitivity, suitability for detecting crosslinking subtypes, and translational readiness and are central to the effort to understand the mechanistic link between crosslinking methods and clinical outcomes of candidate therapies. Data on differences in the biomechanical effect of different crosslinking protocols and their correspondence to clinical outcomes are reviewed, and strategies for leveraging measurement advances predicting clinical outcomes of crosslinking procedures are discussed. Advancing the understanding of ophthalmic crosslinking, its biomechanical underpinnings, and its applications supports the development of next-generation crosslinking procedures that optimize therapeutic effect while reducing complications.

Figure 1.

Overview of ophthalmic crosslinking for disease stabilization. Crosslinked regions of tissue are shown in green. Dotted lines indicate the progression of disease if crosslinking had not been applied. (Top) In corneal crosslinking (CXL) for keratoconus stabilization, the stiffening of the cornea prevents the progression of corneal steepening (dotted line). (Middle) In scleral crosslinking (SXL) for myopia stabilization, the stiffening of the sclera prevents further axial elongation of the globe (dotted line). (Bottom) In scleral crosslinking for glaucoma stabilization, the stiffening of the peripapillary sclera reduces strain on the lamina cribrosa and prevents further distention of the lamina cribrosa (dotted line).

Figure 2.

Overview of metrics used to quantify mechanical changes due to crosslinking the cornea or sclera. (Top) A sample which is wholly elastic will immediately deform when a load is added or removed. A sample which is viscous will continue to deform over time if a load is present. A sample which is viscoelastic, such as the cornea, will have both a viscous and elastic component in its deformation response to load. (Middle) Many different mechanical moduli are reported in ocular biomechanics literature. Young's modulus, also known as the uniaxial elastic modulus, is the resistance to deformation from a uniaxial load. Shear modulus is resistance to deformation due to a shear load. Bulk modulus, also known as the volumetric elastic modulus, is the resistance to deformation given a volumetric compression. The tangent modulus is the instantaneous slope of the stress-strain curve at a given load (stress) when the curve is no longer linear (if the stress-strain curve is linear, tangent modulus is the same as Young's modulus). Dynamic viscosity and shear viscosity are the time-dependent (rate-dependent) equivalents of Young's modulus and shear modulus, respectively. Acoustic velocity is the propagation speed of a pressure wave and is dependent on the material's bulk modulus, shear modulus, and density. Shear wave velocity is the propagation speed of a shear wave, and is dependent on shear modulus and density. (Bottom) Methods of assessing even smaller-scale mechanics include: adhesion force, which is the force required to retract an atomic force microscopy cantilever embedded in the sample; temporal decorrelation, as measured by DLS or OCT, is a measure of the quasi-Brownian displacements which result from random thermal energy within the sample; bond strength can be measured by the time required for the sample to be digested by enzymes.

Figure 3.

Crosslinks within the collagen hierarchical microstructure (Referencing figures and text^,–81) From right to left: Chemical crosslinks can be formed between residues in the primary collagen molecule, between collagen molecules in the tropocollagen triple helix, between tropocollagen in the microfibril, and within or between components of the ECM which surround the collagen fibrils (which are mostly proteoglycans^82,83). Interlamellar crosslinks are not formed by chemical crosslinking. Enzymatic crosslinks are generally formed between tropocollagen in the microfibril or can be formed among the proteoglycans surrounding the fibrils.