The impact of biomechanical properties of the corneoscleral shell on eye hydrodynamics (an experimental study)

It is hypothesized that impaired biomechanical properties of the sclera around the optic disc and, largely, in the corneoscleral shell may play an essential role in the pathogenesis of primary open-angle glaucoma (POAG). The paper’s objective is to study experimentally the impact of elastic scleral properties on intraocular fluid (IF) outflow. We determined the mechanical characteristics and the crosslinking level of the sclera as well as hydrodynamic parameters in three groups of experimental animals: 1) intact eyes of young (2 months of age) and old (2 years) rabbits; 2) eyes treated in vivo by threose, which increases crosslinking of scleral collagen; 3a) eyes of young and old rabbits after they were treated by collalysin, a proteolytic enzyme, 3b) eyes of rabbits pre-treated by treose and subsequently treated by collalysin. The hydrodynamic parameters were measured using Glautest 60. Differential scanning calorimetry was used to reveal the crosslinking level of scleral collagen. The biomechanical parameters of sclera were determined by Autograph AGS-H. The normal age-related increase of scleral stiffness (Young’s modulus) from 23.1±4.2 MPa to 41.4±6.3 MPa (p<0.05) is followed by a moderate growth of cross links and a somewhat reduced IF outflow. At the same time, an increase of scleral stiffness (to 65.4±6.0 MPa) caused by threose, i.e. a pathological growth of crosslinking, is accompanied by a certain intraocular pressure increase and a significant impairment of IF outflow. Collalysin-treated old rabbits revealed a fall in scleral stiffness (to 27.9±4.9 MPa) and an improved IF hydrodynamics, whereas young animals showed only a slight change in these parameters. The sclera treated by threose and containing excessive cross links also became less stiff (43.4±4.5 MPa) and improved IF hydrodynamics after collalysin treatment. It may be assumed that the biomechanical properties of sclera affect the eye hydrodynamics: IF outflow is deteriorating with the increase of scleral stiffness that is caused by excessively generated cross links in its collagen structures, which may be a risk factor for POAG. Proteolytic therapy with collalysin helps reduce the amount of these links, make the sclera less stiff and improve hydrodynamic parameters of the eye.Key words: sclera, intraocular fluid, collagen, crosslinking, elasticity modulus.