Reconstructing ocular surfaces by Purkinje images using an exact ray approach: estimating IOL decenter and tilt.

ABSTRACT

Recently, an alternate method for recovering the shape of the anterior surface of an intraocular lens (IOL) (by Purkinje images), and capable of returning a Zernike polynomial representation of that surface, was proposed (Ophthal. Physiol Opt., 29, 2009, 80-91). However, in moving toward a clinically applicable method, it is important to estimate parameters such as surface radius, lens tilt and decenter. Previously, radius of curvature (for the anterior surface of an IOL) was estimated by finding the best-fit sphere. A methodology is presented here to recover lens tilt and decenter using this alternate method. The theory is developed and then tested in simulation. An IOL is added to the Navarro eye, and then recovered by assuming (i) the same 'full eye', and (ii) a reduced two surface version (the 'simplified' eye). A number of decenter and tilt settings are tested, from which root mean squared (RMS) surface errors, best-fit spheres, tilt and decenter values are estimated. Mis-measurement of the axial position of the posterior surface of the IOL is also simulated. The full eye model produces low RMS surface and radii of curvature errors, that increase linearly with axial shift. The error behaviour is not greatly affected by changes in IOL decenter and/or tilt. The simplified eye (n = 1.32 for aqueous) with n = 1.4760 (at lambda = 880 nm) for the IOL, fits the 'full eye' results most consistently. Lens decenter is consistently estimated to within 0.015 mm (for a 1 mm decenter), and tilt <0.15 degrees (for a 5 degree tilt).