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Editors Selection IGR 8-4

Anatomical Structures: Posterior Sclera

Comment by Crawford Downs on:

66616 Three-Dimensional Strains in Human Posterior Sclera Using Ultrasound Speckle Tracking, Pavlatos E; Perez BC; Morris HJ et al., Journal of Biomechanical Engineering, 2016; 138:

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Scleral biomechanics likely plays an important role in the development and progression of glaucoma, but the contribution of scleral structural stiffness to the disease is not well understood. Experimental and numerical studies indicate that the peripapillary sclera is influential in determining ONH biomechanics because it defines the mechanical boundary condition for the lamina cribrosa at the scleral canal. Pavlatos, Liu, and colleagues report measurements of 3D peripapillary scleral strains in the nasal quadrant of intact human donor eyes. They measured through-thickness scleral strain in nine human eyes from donors ranging from 24-72 years old using their novel scanning ultrasound speckle tracking method coupled with posterior globe inflation at pressures from 10 to 19 mmHg. Key findings were that: (1) tensile strain was primarily oriented in-plane with the sclera and perpendicular to the scleral canal, and strains were much lower in the circumpapillary direction, which agrees with previous findings showing that scleral canal expansion is resisted by the circumpapillary ring of collagen fibrils in the peripapillary sclera; (2) through-thickness compressive strains and in-plane tensile strains were similar in magnitude, indicating that the sclera is nearly incompressible and validating a key assumption used in many biomechanical models; (3) there are substantial scleral strain concentrations associated with the penetrating vasculature and Circle of Zinn-Haller in the sclera. Increased scleral strains associated with blood vessels is the most interesting finding of the study, as individual variations in vascular morphology and the vessels' proximity to the scleral canal have the potential to drive large variations in laminar biomechanics. However, the study used human donor eyes in which these vessels were not pressurized with blood, so additional in vivo validation of this interesting result will be required. Testing of additional quadrants in more eyes will provide additional insight into quadrant-dependent and age-related changes in scleral strain.

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