Optic nerve head (ONH) biomechanics has been hypothesized to play an important role in the development and progression of glaucoma, but it is not well understood. The dearth of available data is due to the technical challenges involved in the measurement of ONH tissue mechanical properties (stiffness) and the complexity of the ONH and scleral geometry. Further complicating the study of ONH biomechanics is the biologic variability in the load-bearing structure, which includes geometry (scleral thickness, neural canal shape and size, laminar pore size and beam thickness, etc.), and tissue stiffness, which may change with age, pathology, extracellular matrix (ECM) composition, and connective tissue remodeling.
Midgett, Nguyen and coworkers quantified the pressure-induced deformation of the lamina cribrosa in eight human eyes from six donors using second harmonic image generation (SHG) microscopy. Mechanical strain was estimated using digital volume correlation analysis of the image volumes taken at pressures from five to 45 mmHg. Results suggest that older age was associated with lower strain, indicating a stiffer lamina with age, although this result is limited by the small sample size (n = 6) and cross-sectional nature of the study, and so must be confirmed. Sectoral analysis also suggested that laminar strains were highest in the temporal and inferior quadrants, which is important as these regions are most associated with focal glaucoma damage. Finally, tensile strains in the plane of the sclera/lamina were much higher than the shear and compressive strains, which is important when one considers the possible mechanisms of biomechanical damage to the axons in the laminar region.
In-plane tension is the dominant strain component in the lamina
As the authors acknowledge, these results should be viewed with caution due to the small sample size, and the assumptions and approximations inherent to the strain calculation methods. In addition, SHG imaging requires imaging the lamina from the posterior side, necessitating dissecting away the retrolaminar optic nerve to expose the laminar structure. This is difficult, and some of the laminar structure may have been cut away in this process, thereby altering its mechanical response. In addition, experimental evidence in dogs and pigs indicates that retrolaminar tissue pressure in the optic nerve never falls below ~4 mmHg due to pial tension, which contributes to the translaminar pressure difference in vivo. Removal of the optic nerve for imaging also removes this back pressure, which is likely to alter the measured laminar biomechanics somewhat. In spite of these limitations, this paper is important in providing evidence that in-plane tension is the dominant strain component in the lamina.
Fortunately, recent advances in OCT and other imaging technologies are improving, which may lead to more comprehensive clinical assessments of ONH biomechanical behavior in the near future.