Experimental evaluation of the viscoelasticity of porcine vitreous

This study aims to estimate the material properties of the porcine vitreous whiletesting it in close to its natural physiological conditions. Eighteen porcineeyes were tested within 48 h post-mortem. A custom-built computer-controlledtest rig was designed to support, load and monitor the behaviour of eye globeswhile being subjected to dynamic rotation cycles mimicking saccade eye movement.Specimens were glued to the base of a container, surrounded by gelatin, frozenand cut in half to expose the vitreous. After thawing, the container wassubjected to concentric dynamic rotations of up to 5°, 10° or15°, while taking 50 MP photos of the specimen every 2 ms. The imageswere analysed by a digital image correlation algorithm to trace the movement ofmarked points on the vitreous surface with different radii from the centre ofthe posterior chamber. The initial camera image was used in building afinite-element model of the test set-up, which was used in an inverse analysisexercise to estimate the material properties of the vitreous. Angulardisplacements of the monitored points were up to 3.3°, 4.1° and3.9° in response to eye rotations of 5°, 10° and15°, respectively. With the experimental relationships between eyerotation and angular displacements used as target behaviour, the inverseanalysis exercise estimated the initial shear modulus, the long-term shearmodulus and the viscoelastic decay constant of the porcine vitreous as 2.10± 0.15 Pa, 0.50 ± 0.04 Pa and 1.20 ± 0.09s−1 , respectively. Consideration of the viscoelasticity ofthe vitreous was essential to represent its experimental behaviour. Testing thevitreous in close to its normal physiological conditions produced estimations ofthe initial shear modulus and long-term shear modulus that were, respectively,smaller and larger than reported values (Zimberlinet al . 2010Soft Matter 6, 3632–3635. (doi:10.1039/b925407b ), Liuet al . 2013J. Biomech. 46, 1321–7. (doi:10.1016/j.jbiomech.2013.02.006) , Rossietal . 2011Invest. Ophthalmol. Vis. Sci. 52,3994–4002. (doi:10.1167/iovs.10-6477 )).