IMPROVED IOL POWER CALCULATION FOR PSEUDOPHAKIC EYES UNDERGOING LATE IOL EXCHANGE; A FORMULA THAT BACK-CALCULATES THE PROPORTIONS OF THE EYE IN ITS PHAKIC STATE.

To improve the IOL power calculation in pseudophakic eyes undergoing IOL exchange when the power of the implanted IOL is unknown.

IOL exchanges are becoming more common and often the power of the implanted IOL is unknown. Calculating the optimal power of the new IOL is difficult, as fourth generation formulae require the eye’s phakic anterior chamber depth (pACD). Without a pACD, surgeons are forced to resort to 2-variable formulae such as SRK-T and Holladay 1. To facilitate the use of modern IOL formulae, we developed a formula that back-calculates what the phakic lens thickness and pACD of the eye is likely to have been.

Biometry data was collected from eyes presenting to a private cataract clinic. The relationship between crystalline lens thickness and age was modelled. Following routine cataract surgery, biometry was repeated to refine how the position of an IOL in the capsular bag can be predicted as a proportion of the pre-operative lens thickness (a refinement of Olsen’s C-constant). Using this data, a calculator was built that uses the patient’s age and pACD to estimate what the proportions of the eye are likely to have been when phakic. A retrospective case series was completed to compare the refractive outcomes of this method against the alternative approach of using a 2-variable formula.

Crystalline lens thickness was modelled against age from an IOLMaster 700 (Carl Zeiss Meditec) biometry dataset of 3490 eyes. Olsen’s C-constant was refined for hydrophobic C-loop haptic IOLs and hydrophilic plate-haptic IOLs including assessment of biometric covariates. The model for crystalline lens thickness and our refined Olsen’s C-constant were combined into a calculator that estimates the patient’s preoperative pACD from their age and pseudophakic ACD, then runs fourth-generation IOL formulae. This calculator was tested in a single-surgeon case series of 15 consecutive IOL exchanges, which demonstrated the superiority of this approach compared to using the SRK-T formula.

We present a novel approach to IOL power calculation in pseudophakic eyes undergoing IOL exchange when the power of the implanted IOL is unknown. Our calculator demonstrated superior refractive outcomes to the SRK-T formula in a single-surgeon consecutive case series of IOL exchanges.