Cataract/Anterior Segment, Cornea/External Disease, Refractive Mgmt/Intervention
A retrospective evaluation of available IOL formulas for eyes with keratoconus found that keratoconus-specific formulas showed more predictive accuracy than non-keratoconus formulas, especially when using total keratometry vs standard keratometry measures. Some non-keratoconus formulas performed well in patients with milder ectasia.
Study Design
This retrospective cohort study compared 13 formulas for IOL calculations: 4 classic formulas (Hoffer Q, Holladay 1 [H1], Haigis, and SRK/T), 1 modified classic formula (H1-EKR), 5 newer formulas (Barrett Universal II [BU2], Cooke K6, EVO 2.0, Kane, and Pearl-DGS), and 3 keratoconus-specific formulas (Kane KCN, BU2 KCN: M-PCA, and BU2 KCN: P-PCA). Eighty-seven eyes (67 patients) with keratoconus underwent a comprehensive preoperative exam including visual acuity, fundoscopy, and slit-lamp assessment as well as biometric measurements obtained using the Zeiss IOLMaster 700 SS-OCT (e.g., K, total keratometry, axial length). All patients underwent standard cataract surgery with monofocal IOL implantation and were evaluated with each formula to compare the accuracy of the predictions.
Outcomes
Keratoconus-specific formulas were superior to non-keratoconus formulas overall and were most accurate in eyes with severe keratoconus (defined by the authors as K >50 D). In patients with non-severe keratoconus (<50 D), EVO 2.0 (a non-keratoconus adjusted formula) also performed well. Evaluating posterior corneal astigmatism via total keratometry led to improvements in predictability in all formulas compared to using standard keratometry values.
Limitations
In this study, various IOL platforms were used and there was a lack of standardization in surgical technique. This may lead to refractive misses unrelated to the accuracy of the IOL calculations. Additionally, the included patients showed a variety of disease states but were arbitrarily divided into severe and non-severe keratoconus using a cutoff value of 50 D.
Clinical Significance
Performing cataract surgery in eyes with keratoconus presents a unique challenge. The assumptions of corneal power and effective lens position found in standard formulas are less applicable in patients with corneal ectasia, so alternative formulas are needed. This study highlights the benefits of utilizing keratoconus-specific formulas (i.e., Barret True K KCN, Kane KCN) and further suggests that total keratometry, which incorporates posterior cornea measures (via SS-OCT), is superior to traditional keratometry.
Financial Disclosures: Dr. Daniel Choi discloses financial relationships with Glaukos (Lecture Fees/Speakers Bureau); Kala Pharmaceuticals (Consultant/Advisor).