Biometry of the silicone oil-filled eye

Purpose The advent of silicone oil tamponade has resulted in improved succe ss rates in complicated retinal detachment surgery. Its use, however, can i nduce cataract formation in phakic eyes. In selected patients, removal of s ilicone oil can be combined with phacoemulsification of the cataract and in traocular lens (IOL) implantation, thus avoiding a further operation. This, however, poses a problem when trying to decide the power of IOL to be used , since the echographically measured axial length (AL) of an eye is greater in the presence of silicone oil. We performed ultrasound examination in th e presence of silicone oil of viscosity 1300 centistokes, in order to deter mine whether the measured AL varied from the true AL by a constant factor. Methods The ALs of 7 phakic eyes were measured by A-mode echography, with a nd without silicone oil of viscosity 1300 centistokes in the posterior segm ent. The retina was attached in all cases. The control group consisted of 6 phakic eyes with attached retinae undergoing vitrectomy without the use of silicone oil. The ALs in the control group were measured before and after vitrectomy. Results The mean ratio of true AL to measured AL in the presence of silicon e oil was 0.71 (standard deviation 0.01; range 0.70-0.73; median 0.71) in t he 7 eyes in this study. In the control group, vitrectomy appeared to have no significant effect on AL. Conclusions We have established a constant which corrects for the apparent increase in AL induced by silicone oil of viscosity 1300 centistokes. This conversion factor, when used in existing biometry formulae (SRK/T), allows estimation of the power of IOL required in eyes undergoing combined catarac t extraction, removal of silicone oil and lens implantation.