title: Scleral biomechanics in the aging monkey eye.
abstract: OBJECTIVE
  To investigate the age-related differences in the inhomogeneous, anisotropic, nonlinear biomechanical properties of posterior sclera from old (22.9 +/- 5.3 years) and young (1.5 +/- 0.7 years) rhesus monkeys.
  
  
  METHODS
  The posterior scleral shell of each eye was mounted on a custom-built pressurization apparatus, then intraocular pressure (IOP) was elevated from 5 to 45 mm Hg while the 3D displacements of the scleral surface were measured with speckle interferometry. Each scleral shell's geometry was digitally reconstructed from data generated by a 3-D digitizer (topography) and 20-MHz ultrasound (thickness). An inverse finite element (FE) method incorporating a fiber-reinforced constitutive model was used to extract a unique set of biomechanical properties for each eye. Displacements, thickness, stress, strain, tangent modulus, structural stiffness, and preferred collagen fiber orientation were mapped for each posterior sclera.
  
  
  RESULTS
  The model yielded 3-D deformations of posterior sclera that matched well with those observed experimentally. The posterior sclera exhibited inhomogeneous, anisotropic, nonlinear mechanical behavior. The sclera was significantly thinner (P = 0.038) and tangent modulus and structural stiffness were significantly higher in old monkeys (P < 0.0001). On average, scleral collagen fibers were circumferentially oriented around the optic nerve head (ONH). No difference was found in the preferred collagen fiber orientation and fiber concentration factor between age groups.
  
  
  CONCLUSIONS
  Posterior sclera of old monkeys is significantly stiffer than that of young monkeys and is therefore subject to higher stresses but lower strains at all levels of IOP. Age-related stiffening of the sclera may significantly influence ONH biomechanics and potentially contribute to age-related susceptibility to glaucomatous vision loss.
authors.affiliation: Ocular Biomechanics Laboratory, Devers Eye Institute, Portland, Oregon 97232, USA.
authors.forename: Michaël J A
authors.initials: MJ
authors.lastname: Girard
authors.forename: J-K Francis
authors.initials: JK
authors.lastname: Suh
authors.forename: Michael
authors.initials: M
authors.lastname: Bottlang
authors.forename: Claude F
authors.initials: CF
authors.lastname: Burgoyne
authors.forename: J Crawford
authors.initials: JC
authors.lastname: Downs
mesh_terms: D000375:Aging / Q000502:physiology*; D000818:Animals; D016880:Anisotropy; D001696:Biomechanical Phenomena; D003094:Collagen / Q000378:metabolism; D004548:Elasticity; D005260:Female; D021621:Imaging, Three-Dimensional; D007429:Intraocular Pressure / Q000502:physiology; D008253:Macaca mulatta / Q000502:physiology*; D008297:Male; D012590:Sclera / Q000502:physiology*
chemical_list: D003094:Collagen