The effect of vitreous humor in attenuating IOP-transient-induced strain of the posterior sclera

Abstract

Purpose : To compare the effect of the human vitreous humor on the scleral biomechanics during intraocular pressure (IOP) transients at different IOP magnitudes, baselines, and frequencies. Analyzing the vitreous-sclera biomechanical behavior could contribute to elucidate IOP-related mechanisms in eye disorders like glaucoma and myopia, and vitreous replacement.

Methods : We measured the strain response in a human donor sclera coat (65 yo) while varying IOP across physiologic ranges and frequencies of IOP fluctuation. IOP was controlled by a custom ocular anterior chamber infusion device (Bose Electroforce 5500) and measured by an in-line piezoelectric sensor (Millar Mikro-Tip). We compared scleral biomechanics and IOP measurements in two conditions of the vitreous chamber: i) with the natural vitreous; ii) and after replacing the vitreous with balance salt solution. Strain maps around the optical nerve head (ONH) were measured by a three-dimensional optical method (3D-Digital Image Correlation). Scleral strain was measured during IOP cycles (Fig.1) by changing baseline (bIOP =10, 15, 20 mmHg), range (ΔIOP = 2.5, 5, 10 mmHg), and frequency (f = 0.5, 1.0, 2.0 Hz).

Results : Statistical comparison (Fig.2) between vitreous and saline conditions revealed that the vitreous significantly (p<0.001) attenuated the strain at the maximum IOP (ε_peak) during cycling IOP variations at the different frequency, baseline, and range, while replacing the vitreous with saline exposed the sclera to higher strain. Also, multiple linear regression analysis showed significant dependence (p<0.05) of ε_peak from bIOP, ΔIOP, and f.

Conclusions : Natural vitreous significantly attenuates scleral strain induced during physiologic transient fluctuations in IOP. Loss or age-related liquefaction of vitreous may remove this damping function and increase IOP-related stress induced on sclera and ONH.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.

 

A representative case of the scleral strain over time obtained for the two conditions of the vitreous chamber during 4-cyclic IOP variations at 2 Hz (ΔIOP= 10mmHg, bIOP=10mmHg).

 

Statistical comparison by linear mixed-effect models of ε_peak (average over 4 points of the strain responses, Fig.2) over ΔIOP (A, B, C) and over frequency (D, E, F) obtained at different IOP baselines, for i) natural vitreous,and ii) saline. The effect of the natural vitreous is highly significant in attenuating ε_peak compared to saline in all IOP profiles. (**): p <0.001.