Investigators have developed a new formula based on angle-to-angle (ATA) depth to predict postoperative anterior chamber depth (ACD) and potentially improve the accuracy of IOL power selection.
Because ATA depth remains unchanged after phacoemulsification and IOL implantation, it might serve as an independent preoperative parameter for predicting postoperative ACD. Using anterior segment OCT (AS-OCT), the authors define ATA depth as the perpendicular distance between the posterior corneal surface and the intersection point of a line joining both angle recesses on the cross-sectional horizontal image with the corneal vertex.
Study design
To assess the formula validity, a cohort of 304 eyes (276 patients) with acrylic IOLs was equally divided into a training set and a validation set.
In the training set, postoperative ACD measured 1 month after surgery was analyzed via multiple linear regression analysis with 5 preoperatively measured variables: ATA depth, ATA width, preoperative ACD measured with AS-OCT, axial length (AL), and corneal power. Using the results, the authors developed a regression formula that uses 3 preoperative parameters—ATA depth, preoperative ACD, and AL—for predicting postoperative ACD.
In the validation set data, agreement was measured between actual and predicted postoperative ACD using the new formula and the Sanders-Retzlaff-Kraff theoretic (SRK/T) and Haigis formulas.
Outcomes
The ATA depth formula had the highest standard partial regression coefficient value with measured postoperative ACD on AS-OCT at 0.71. By comparison, the SRK/T coefficient was 0.36 and the Haigis coefficient was 0.55. The postoperative ACD absolute prediction error was also smallest with the new formula (P<0.0001).
Limitations
The ACD used in this analysis was derived from OCT and not biometry, so the measurement is not directly comparable. Additionally, the study examines a relatively small number of eyes and does not quantify differences in eyes with more normal biometry and those at the extremes. Future studies should compare this new formula with other fourth-generation formulas that use anterior chamber depth.