Investigators assessed the accuracy of 5 IOL formulas for calculating refractive outcomes following the triple procedure of Descemet membrane endothelial keratoplasty (DMEK), phacoemulsification, and IOL implantation. The results of the investigation support a proposal to modify corneal power calculation to minimize the postoperative hyperopic surprise and to improve accuracy of optical biometry measurements of corneal power.
Study design
This retrospective study of 86 eyes involved a post-hoc analysis of the refractive outcomes of triple procedures in patients with cataracts and Fuchs Endothelial Cell Dystrophy. The Hoffer Q, Holladay I, SRK/T, Barrett II, and Haigis formulas were compared (using ULIB [User Group for Laser Interference Biometry] constants) before and after a corneal power adjustment was performed using a thick lens equation that was developed by Borasio et al. in 2006 for corneas that are status post-keratorefractive surgery. The mean absolute error was calculated by subtracting the predicted spherical equivalent refraction from the actual postoperative refraction. The adjusted corneal power, which was derived from anterior radius of curvature, posterior radius of curvature, and central corneal thickness, was subtracted from the mean absolute error to ascertain how the adjustment would affect the predicted outcome. The effect of the adjusted corneal power is to decrease the corneal power, which the authors postulate is overestimated by optical biometers in the setting of corneal edema.
Outcomes
Mean axial length was 24.00 mm (21.83–27.83); average Kmean was 43.8 (39.65–47.69 D). All formulas except the Haigis formula resulted in hyperopic error (Hoffer Q = +0.66 D, Holladay I = +0.59 D, SRK/T = +0.51 D, Barrett II = +0.90 D, Haigis = –0.10 D). The corneal power adjustment resulted in significantly lower mean absolute error for all but the Haigis formula (Hoffer Q: decreased from 1.02 to 0.82 D; Holladay I decreased from 0.97 to 0.85 D; SRK/T decreased from 0.93 to 0.85 D; and Barrett II decreased from 1.16 to 0.9 D); the Haigis formula did not decrease because the unadjusted mean error was already myopic and reducing the corneal power with the adjustment only worsens the myopia.
Limitations
This study is limited by its retrospective design and small sample size. Use of multiple surgeons, biometers, and IOL models prevented optimization of formula constants.
Clinical significance
It is common practice to choose a myopic target of between –0.5 to –1.0 D, depending on surgeon preference, to reduce the risk of “hyperopic surprise” in DMEK triples. This study proposes a mathematical adjustment based on patients’ corneal parameters that may more accurately guide lens choices in DMEK triples. It also reinforces the known hyperopic outcomes that result from choosing emmetropic lens targets in this setting and proposes the Haigis formula as being a potential outlier among the pervading formulas because it seems to yield slightly myopic outcomes, even without an adjustment to the corneal power.