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Editors Selection IGR 9-4

Basic research: Scleral stiffness

Comment by Ross Ethier on:

11739 Viscoelastic material properties of the peripapillary sclera in normal and early-glaucoma monkey eyes, Downs JC; Suh JK; Thomas KA et al., Investigative Ophthalmology and Visual Science, 2005; 46: 540-546

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Downs et al. (10) build on previous work, using a monkey model to investigate how biomechanical effects might play a role in glaucomatous optic neuropathy. Here the focus is characterization of the biomechanical changes that occur in the peripapillary sclera of eyes subjected to elevated IOP. Downs et al. used laser ablation of the trabecular meshwork in monkeys to induce IOP elevations (up to 44 mmHg) that were sustained for four to nine weeks.

Stiffness of the peripapillary sclera in early glaucoma eyes was increased compared to peripapillary scleral stiffness in control eyes

This study is important for several reasons. First, it unequivocally demonstrates that the sclera can adaptively respond to mechanical (pressure) loading, and that this adaptation occurs remarkably quickly, i.e., within weeks. We know similar adaptation occurs in many other load-bearing tissues in the body, such as arterial walls and bones, where adaptation 'toughens' and hence mechanically protects the loaded tissue; presumably a similar protective response underlies the observations in this study. An interesting corollary is that there must be mechano-sensitive cells in the posterior pole of the eye, likely in the sclera itself. Second, this adaptation likely protects more than just the sclera: because of mechanical coupling between the peripapillary sclera and the lamina cribrosa, scleral stiffening would be expected to reduce the stretching and deformation of optic nerve head tissues due to elevated IOP. This 'mechanoprotective' adaptation might thus be an important factor in preserving optic nerve function. It will be fascinating to see how this adaptation progresses in eyes subjected to longer-term IOP elevation, as well as to try to understand the mechanobiology of this response in monkey and human eyes.

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