Biomechanical impact of the sclera on corneal deformation response to an air-puff: a finite-element study

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Purpose: Glaucoma is the leading cause of irreversible blindness worldwide and results from the death of retinal ganglion cells [1-8]. Altered mechanical properties of the sclera – the white part of the eye – have been implicated in the pathogenesis of glaucoma [1-5, 7]. To our knowledge, a method for the clinical evaluation of scleral properties has yet to be devised. We have sought to address this deficiency using a commercially-available non-contact tonometer and an inverse finite-element analysis. Methods: Human donor eyes were obtained from a local eye bank and mounted in a rigid fixture. One eye from each pair had the sclera undergo a stiffening treatment, with the contralateral eye serving as a control. A CorVis ST non-contact tonometer was used to load the eye at several levels of intraocular pressure (IOP) and quantify the resulting deformation behavior. The CorVis ST utilizes a high-speed camera and image analysis software to output dynamic deformation response parameters of the cornea – the clear part of the eye – during air-puff loading. An axisymmetric model of the whole eye was created using the average dimensions for the adult human eye. The ocular tissues were modeled as hyperelastic and nearly incompressible, with material properties based on literature values [10-12]. The eye is naturally under tension while loaded by intraocular pressure [8, 9]. Therefore, the unloaded state of the eye was first estimated and assumed to represent a stress-free state. The IOP was then applied to induced the residually stressed state. Finally, the eye was loaded by simulating the air-puff generated by the CorVis ST non-contact tonometer. The intraocular pressure and scleral material properties were varied between simulations to determine their influence on the cornea's deformation response during air-puff loading. The maximum rearward displacement of the corneal apex was recorded for each IOP-mechanical property pairing for comparison with experimental findings. Results and Discussion: For each value of IOP tested, it was shown that increasing the ratio of scleral to corneal stiffness resulted in decreasing maximum apical displacement of the cornea. The model demonstrated that the stiffer the sclera was, the higher the apparent stiffness of the whole eye – normally interpreted as reflecting only the stiffness of the cornea. Additionally, the model showed that increasing the IOP while keeping all other factors constant resulted in decreasing maximum apical displacement, which is consistent with literature reports. The trends shown in the finite-element analysis were also observed in the experimental results from human donor eyes. Implications: The finite element model presented in this study demonstrates that scleral material properties have an important impact on the biomechanical deformation response of the cornea in air-puff induced deformation. Namely, the stiffer the sclera, the greater will be the limitation on corneal deformation. This may have important clinical implications. Often in the clinic, the observed biomechanical deformation response of the cornea is attributed solely to the material properties of the cornea. However, it is clear in the finite-element analysis that the maximum apical displacement of the same cornea varies significantly with varying scleral properties. This suggests that that the observed biomechanical response to an air puff may be used to deduce the IOP as well as the mechanical properties of the cornea and sclera. Obtaining these additional parameters from a current clinical diagnostic modality will lead to improved diagnostic capabilities for glaucoma.


Engineering: 2nd Place (The Ohio State University Edward F. Hayes Graduate Research Forum)


cornea, sclera, finite element, CorVis