The Biomechanics of the Mouse Sclera: Effects of Strain, Age, and Glaucoma
Experimental glaucoma was induced by injecting a thick slurry (viscoelastic solution) into the front of the eye (anterior chamber) to block the aqueous outflow channels. This caused an immediate and sustained increase in the intraocular pressure (IOP), a lengthening or pushing back (called permanent axial elongation) of the eyes, and damage to the axons of the retinal ganglion (optic nerve) cells. By 6 weeks, the eyes of all experimental animals suffered nerve cell axonal loss (called optic nerve hypoplasia) at the pressure points, were statistically larger in axial length and diameters, and exhibited a thinner peripapillary sclera (i.e. the sclera was thinner in the area surrounding the optic nerve head). However, the degree of structural changes and axon loss varied greatly among mouse types. For example, Aca23 mice suffered less axon loss (only 0.57+/-17%) while their healthy controls showed much more axon loss (21+/-31%). Inflation test results showed that the sclera of all animals had a different pressure-strain response, and all the experimental glaucoma eyes developed a stiffer pressure-strain response than the contralateral (opposite eye) controls. The degree of stiffening also varied with the type of mouse. Comparing the collagen 8A2 mutants to their wild type, the eyes with a stiffer pressure-strain response at baseline developed less axonal damage.
The team is currently developing a method to calculate the regionally varying stress response of the sclera from the imposed pressure and measured displacement to determine material stiffness (of the stress-strain response). This will allow them to determine whether the glaucoma-induced stiffening response is caused primarily by changes in anatomical dimensions or material properties of the sclera. A fundamental understanding of the relationship between scleral biomechanics and the severity of glaucoma damage could one day lead to the development of non-invasive screening procedures to identify individuals at higher risk for glaucoma. In addition, Dr. Nguyen’s discoveries could lead to new drug therapies to slow the progression of the disease by altering the mechanical behavior of the sclera.
Nguyen C, Cone FE, Nguyen TD, Coudrillier B, Pease ME, Steinhart MR, Oglesby EN, Jefferys JL, Quigley HA. Invest Ophthalmol Vis Sci. 2013 Mar 11;54(3):1767-80. doi: 10.1167/iovs.12-10952.
First published on: April 1, 2010
Last modified on: April 1, 2013