Scleral biomechanics control ocular growth in myopic eyes

The biomechanics of the sclera control eye growth in myopia, according to Neville McBrien, PhD, of theDepartment of Optometry and Vision Sciences, University of Melbourne, Melbourne, Australia.

With increased axial elongation in myopia, the sclera becomes thinner, especially at the posterior pole wherethe sclera, which is connective tissue composed predominately of extracellular matrix (collagen type 1), isbiomechanically weaker in myopic eyes compared with emmetropic eyes due to markedly decreased collagensynthesis, he explained.

Experimental studies showed that when scleral strips were subjected to stretching in one direction, the elasticproperties and the scleral "creep rate" increase markedly in eyes developing myopia, and conversely,significantly decrease in eyes recovering from myopia.

There is a contractile mechanism, however, at work in the eye as shown in an in vivo tree-shrew experiment inwhich IOP was increased to about 100 mm Hg. After an immediate increase in axial length, IOP that was elevatedfor 1 hour resulted in a gradual decrease in the axial length. The role of myofibroblasts as a mediator ofbiomechanical resistance to axial elongation has been demonstrated recently. The contractile response of scleralfibroblasts in primary culture is increased markedly by the addition of all isoforms of transforming growthfactor-beta. This increased contractile response is associated with the increased presence of alpha-smoothmuscle actin, a marker for myofibroblasts.

"When we look at the future of this research, we need to determine the contribution of myofibroblasts on scleralmechanical properties by looking at the pharmacokinetics involved in the myofibroblast contraction, utilizethree-dimensional culture systems to define quantitatively the contributions of the extracellular matrix andstress in cell phenotype, and develop matrix- and cell-based therapies for strengthening the scleral tissue asanother approach to therapy," Dr. McBrien said.