In some challenging retinal surgery cases, it may become necessary to place silicone oil in the vitreous cavity. When cataract surgery is required in these eyes, the silicone oil poses a major challenge to proper IOL Calculation in measurement and also IOL power selection.
Measurement in Silicone Oil
The Ultrasound waves are slowed down in speed in the silicone due to the difference in material density. This change in speed makes the Ultrasound A-Scans virtually impossible and clinically inadequate.
IOLMaster, however, with the use of partial coherence interferometry, is able to accurately measure the proper axial length of the eye. When measuring the eye, the setting must be selected for "silicone eye" mode and the proper density adjustments are automatically done. This will provide the axial length. That number can now be used to do the IOL calculation, with considerations of the next section.
Alternatively, if IOLMaster is not available, one may do an A-Scan intraoperatively once the silicone oil has been removed. This has been shown to have good predictability as compared to pre-operative IOLMaster measurements. (El-Baha and Hemeida, Retina 2009:29(1):64-68)
IOL Power Calculation in Silicone Oil
Here is the primary decision that needs to be considered, is the silicone oil going to remain or be removed?
If the silicone oil is going to be removed, then the standard formulas may be used to achieve the desired calculation.
If the silicone oil will remain in the eye for extended period after the cataract surgery with IOL implantation, then the IOL power needs to be adjusted up on average 3-4 diopters. This is because the silicone oil results in a hyperopic shift. For example if one has measured and planned on a +20.0 D lens, for silicone oil adjustment, one may choose a +24.0 D lens in the eye that will continue to retain the silicone oil. (Grinbaum et. al., JCRS, 1996: 22(6):726-9).
Another point to consider is also, when the removal of silicone oil is considered, one may still do the combined cataract/IOL/Silicone removal surgery with a topical anesthesia. Silicone oil removal may be done through the phaco incision, through the pupil, through a planned posterior capsular capsulorhexis.