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Matthew A. Reilly; Hydrostatic Skeleton Model of Accommodation. Invest. Ophthalmol. Vis. Sci. 2012;53(14):1351.
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© ARVO (1962-2015); The Authors (2016-present)
Estimate the residual stresses in the lens capsule as a function of age.
A hydrostatic skeleton one in which an enclosed incompressible fluid acts as a skeletal element and is acted on by its surrounding prestressed muscular membrane which serves as the agonist muscle. The hydrostatic pressure of the fluid induces residual stress in the membrane. The shape of the hydroskeleton is determined by the interaction of external forces, fluid pressure, and membrane properties. The geometric model of accommodation (Reilly and Ravi, Vision Res. 2010) was recast as a hydrostatic skeleton model of accommodation, enabling an estimation of the residual stresses in the capsule during accommodation. The previously-derived area dilation-based force calculation (Ravi and Reilly, ARVO 2010) was augmented by the inclusion of a residual stress term. This allowed an estimate of residual stress by determining the modulus at which the model-predicted lens/capsule dilation force was equal to the experimental stretching force (Manns et al., IOVS 2007) corrected for the presence of connective tissues.
Inclusion of the residual strain in the stretching force formulation allowed reconciliation of the geometric model of accommodation with measurements in the literature. Residual stress within the lens capsule increases with age, implying an increase in internal pressure. This increased internal pressure results from lens growth. During accommodation, the capsule relaxes to recover its natural force balance between residual membrane stresses and pressure exerted by the lens fiber cells.
Inability of the capsule to recover to a sufficiently accommodated state is the root of presbyopia. Age-related residual stress increases in the lens capsule are driven by natural lens growth. Due to the intrinsic constitutive behavior of the lens capsule (i.e. concave upward; Burd, Biomech Model Mechanobiol. 2009), increased residual stress necessarily increases the force required to deform the capsule. This perspective also resolves the apparent physiological anomaly of muscle contractility resulting in decreased tension by recasting the capsule as a transparent muscle proxy. This finding may also have implications for lens refilling as a treatment for presbyopia.
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