Quantification of Scleral Biomechanics and Collagen Fiber Alignment

Abstract

The stiffness of the sclera is important in several ocular disorders, and there is hence a need to quantify the biomechanical properties of this tissue. Here, we present two methods for measuring the stiffness of scleral ocular tissues: ocular compliance testing and digital image correlation strain mapping. In tandem with these approaches, we provide two methods to spatially quantify the anisotropic alignment of collagen fibers making up the sclera, using second harmonic generation microscopy and small-angle light scattering. Together, these approaches allow specimen-specific measurement of tissue stiffness and collagen alignment, which are key factors in determining how the eye responds to mechanical loads.

Measurement of Ocular Compliance Using iPerfusion

“…Although the applied volume approach is valid in principle, practical application requires isolating the response of the eye from that of the measurement system and accounting for the pressure-dependence of ocular compliance itself. As described by Campbell et al (2018) the pressure response following a bolus injection depends critically on the dynamics of the external fluidic system that is coupled to the eye for the purpose of measuring ocular compliance. Any fluidic system will have its own intrinsic hydraulic resistances and compliances and will thereby alter the measured response to a step change in applied flow or pressure.…”

“…Furthermore, with the notable exception of Campbell et al (2018), few studies have accounted for the considerable pressure-dependence of ocular compliance. As ϕ can decrease 3-fold or more between 8 and 20 mmHg, not accounting for this pressure-dependence can lead to significant inaccuracies in the assessment of ocular compliance.…”

“…This complication is particularly important when using the applied volume approach with smaller eyes (e.g., mice, rats, or tree shrews) when a relatively small injected volume could lead to significant pressure spike, at which point ϕ may change several-fold in a very short period of time. Resolving ϕ and its corresponding pressure P ϕ [see Campbell et al (2018) for details] becomes more difficult as the magnitude of the pressure spike increases, although this effect could be minimised by reducing the injected volume. Similarly, values of ϕ should always be reported with their corresponding value of P ϕ .…”

“…There are two approaches to measure ocular compliance: (i) by causing a change in ocular volume and measuring the resulting change in IOP (the applied volume approach ), or (ii) by causing a change in IOP and measuring the resulting change in ocular volume (the applied pressure approach ). We previously described the applied volume approach (Campbell et al, 2018). Here, we focus on the applied pressure approach, which has the advantage of a faster temporal response compared to the applied volume approach and thus increases experimental throughput.…”

“…This approach is similar to measurement of K in humans and eyes from larger species (Pallikaris et al, 2005). However, as highlighted recently (Campbell et al, 2018), the dynamic response of the measurement system when coupled to the eye, and the pressure dependence of ocular compliance itself, make it difficult to correctly measure ocular compliance, particularly in eyes of smaller animals, which are more amenable for research. How to accurately and robustly measure ocular compliance therefore remains an open question.…”