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Robert Wilkes, Matthew Reilly; Estimation of Ciliary Muscle Forces Required to Induce Corneal Deformation. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4047.
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Changes in the shape of the cornea (+0.4 D) during accommodation have been reported [Pierscionek, et al., Eye, 2001]. Contraction of the ciliary muscle may drive this change through tendon forces centripetally applied to the sclera through the ciliary body. We estimated this force using inverse finite element analysis (IFEA). This technique offers a method for estimating ciliary muscle forces.
Inverse Finite Element Analysis (IFEA) has been used extensively to estimate mechanical parameters of tissue in vivo, particularly where access to the tissue would require disruption of its physiological state . The premise of the IFEA method is to create a computer model of the tissue by incorporating known parameters such as geometry, force, pressure, or stiffness and then iteratively solving the model for the unknown parameters that produce a physiologically realistic result. We created a structural model of the eye in silico that includes the relevant force-transmitting components: sclera, cornea, aqueous, vitreous, supraciliaris, ciliary body, ciliary muscle, zonules, and lens. Tissue geometry [cites] and mechanical properties were obtained from the literature [cites]. Pressurization of the aqueous and vitreous, and pretension in the cornea, sclera, ciliary body, zonules, and lens were incrementally introduced in the model until a balance of forces was indicated by minimal displacement from the “textbook” geometry. Centripetal (radially inward) force was then incrementally applied to the ciliary muscle to simulate its circumferential contraction. The power of the anterior corneal surface was then computed.
A net force of 0.23 Newtons brought the corneal distortion to an equivalent 0.4 D power increase during simulated accommodation representing focal distance change from 6 m to 11 cm. This compares favorably with 0.10 N zonular force obtained for IFEA of the isolated lens for full accommodation (Burd, et al., Vis Res 2002).
We have produced a first estimate of the ciliary muscle force during full accommodation. A radial force of 0.23 N produced a +0.4 D power change due to collateral distortion of the anterior cornea curvature from forces induced in the coats of the eye by ciliary muscle contraction.
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