Determining specified intraocular lens powers when silicone oil is to be used in the vitreous chamber.

Purpose: To apply a theoretical approach to determining how specified intraocular lens (IOL) powers should change when vitreous oil substitution is combined with IOL implantation.

Setting: University laboratory, private Ophthalmological practice.

Design: Theoretical raytracing.

Methods: Raytracing was done backwards from the retina with equi-convex 20 diopters (D) and 25 D IOLs, of refractive index 1.5332, to the object side of the anterior IOL surface. The 1.336 vitreous index was replaced with a high index 1.405 silicone oil. Raytracing was repeated with increase in specified power, that power as if 1.336 index was still surrounding the IOL, so that the object reduced vergence on the anterior side of the lens matched that of the original IOL power. This was done for a range of lens shapes from plano-convex (front surface flat), through equi-convex, to plano-convex (back surface flat), and for a range of axial lengths. The true power, the power with 1.336 index on the object side and silicone oil on the image side, was also determined.

Results: Replacing vitreous by silicone oil increases the necessary specified IOL power. This increase varies from approximately 14% for flat back surfaces, to 40% for equi-convex lenses, to 80% for flat front surface IOLs. True powers increase by about 15% across the range of IOL shapes. In terms of percentages, effects of changing the original IOL power and the axial length are small.

Conclusions: When silicone oil is to remain in an eye after cataract surgery, biconvex IOLs require much higher specified powers than convex-plano IOLs.