Image quality in polypseudophakia for extremely short eyes

Aim-To evaluate the image quality produced by polypseudophakia used for str ongly hypermetropic and nanophthalmic eyes. Methods-Primary aberration theory and ray tracing analysis were used to cal culate the optimum lens shapes and power distribution between the two intra ocular lenses for two example eyes: one a strongly hypermetropic eye, the o ther a nanophthalmic eye. Spherical aberration and oblique astigmatism were considered. Modulation transfer function (MTF) curves were computed using commercial optical design software (Sigma 2100, Kidger Optics Ltd) to asses s axial image quality, and the sagittal and tangential image surfaces were computed to study image quality across the field. Results-A significant improvement in the axial MTF was found for the eyes w ith double implants. However, results indicate that this may be realised as a better contrast sensitivity in the low to mid spatial frequency range ra ther than as a better Snellen acuity. The optimum lens shapes for minimum s pherical aberration (best axial image quality) were approximately convex-pi ano for both lenses with the convex surface facing the cornea. Conversely, the optimum lens shapes for zero oblique astigmatism were strongly meniscus with the anterior surface concave. Correction of oblique astigmatism was o nly achieved with a loss in axial performance. Conclusions-Optimum estimated visual acuity exceeds 6/5 in both the hyperme tropic and the nanophthalmic eyes studied (pupil size of 4 mm) with polypse udophakic correction. These results can be attained using convex-piano or b iconvex lenses with the most convex surface facing the cornea. If the poste rior surface of the posterior intraocular lens is convex, as is commonly us ed to help prevent migration of lens epithelial cells causing posterior cap sular opacification (PCO), then it is still possible to achieve 6/4.5 in th e hypermetropic eye and 6/5.3 in the nanophthalmic eye provided the anterio r intraocular lens has an approximately convex-piano shape with the convex surface anterior. It was therefore concluded that consideration of optical image quality does not demand that additional intraocular lens shapes need to be manufactured for polypseudophakic correction of extremely short eyes and that implanting the posterior intraocular lens in the conventional orie ntation to help prevent PCO does not necessarily limit estimated visual acu ity.