Ex vivo testing of intact eye globes under inflation conditions to determine regional variation of mechanical stiffness

Abstract

BackgroundThe eye globe exhibits significant regional variation of mechanical behaviour. The aim of this present study is to develop a new experimental technique for testing intact eye globes in a form that is representative of in vivo conditions, and therefore suitable for determining the material properties of the complete outer ocular tunic.MethodsA test rig has been developed to provide closed-loop control of either applied intra-ocular pressure or resulting apical displacement; measurement of displacements across the external surface of the eye globe using high-resolution digital cameras and digital image correlation software; prevention of rigid-body motion and protection of the ocular surface from environmental drying. The method has been demonstrated on one human and one porcine eye globe, which were cyclically loaded. Finite element models based on specimen specific tomography, free from rotational symmetry, were used along with experimental pressure-displacement data in an inverse analysis process to derive the mechanical properties of tissue in different regions of the eye’s outer tunic.ResultsThe test method enabled monitoring of mechanical response to intraocular pressure variation across the surface of the eye globe. For the two eyes tested, the method showed a gradual change in the sclera’s stiffness from a maximum at the limbus to a minimum at the posterior pole, while in the cornea the stiffness was highest at the centre and lowest in the peripheral zone. Further, for both the sclera and cornea, the load–displacement behaviour did not vary significantly between loading cycles.ConclusionsThe first methodology capable of mechanically testing intact eye globes, with applied loads and boundary conditions that closely represent in vivo conditions is introduced. The method enables determination of the regional variation in mechanical behaviour across the ocular surface.

The hierarchical response of human corneal collagen to load

“…The paint speckles optimised the DIC process and were discrete enough not to provide another surface layer to the specimens. It has been shown in a separate study that the addition of such a speckle pattern to specimens does affect their mechanical response [25] .…”

“…The healthy intact cornea deforms almost perfectly elastically under quasi-static pressure testing [24] , [25] , suggesting there is little reorientation or interfibrillar/interlamellar sliding, which require significant energy expenditure [26] . The majority of deformation due to increases in IOP occur in the periphery [27] with the stiffer central and paracentral region [28] , [29] maintaining approximately the same shape.…”

Biomechanical Simulation of Stress Concentration and Intraocular Pressure in Corneas Subjected to Myopic Refractive Surgical Procedures

“…Other ex vivo measurements with a numerical-experimental protocol, which include biaxial tension and bending experiments, should be used to calibrate corneal mechanical properties individually 14 , 15 . Furthermore, measuring the entire shape of the cornea 16 , 17 instead of the apical displacements, with an in vivo biomechanical analyser is indispensable for this purpose 18 , because the limbus stiffness is 47.3% lower than that of the central cornea.…”

Whole-globe biomechanics using high-field MRI

“…A common technique in the study of the ONH is to clamp the equatorial sclera and inflate the posterior half of the globe (Coudrillier et al, 2013; Coudrillier et al, 2015). It is worried that this clamping may create an artificial constraint on the sclera and that these tests do not truly capture the in situ environment (Whitford et al, 2016). We found almost no change in equatorial diameter with IOP in sheep eyes.…”