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Anterior chamber angle opening during corneoscleral indentation: the mechanism of whole eye globe deformation and the importance of the limbus.

PURPOSE
To determine how mechanical interaction among iris, cornea, limbus, sclera, and IOP contribute to angle opening during indentation of the cornea or sclera.


METHODS
A finite-element model of the globe was developed. The model consisted of three elastic isotropic segments--iris, cornea, and sclera--and a two-component anisotropic segment representing the limbus. The model was tested against published in vitro experiments and then applied to angle opening during indentation in vivo. Indentation of the central cornea with a cotton bud, indentation with a small or large eyecup during ultrasound biomicroscopy, indentation with a gonioscopy lens, and scleral indentation during goniosynechialysis were modeled.


RESULTS
The anisotropic limbus model matched published data better than any isotropic model. Simulation of all clinical cases gave results in agreement with published observations. The model predicted angle opening during indentation by a cotton bud or small eyecup but angle narrowing when the sclera was indented by a large eyecup. The model of indentation gonioscopy showed narrowing of the angle on the indentation side and opening of the angle on the opposite side. Nonuniform opening of the angle was predicted when the scleral surface was indented.


CONCLUSIONS
The two-component model of the stiff fibers embedded in a soft matrix captured the mechanical properties of the complex limbal region effectively. The success of this model suggests that, at least in part, corneoscleral mechanics drive angle opening rather than aqueous humor pressurization.