Purpose: To establish whether toric intraocular lens (IOL) calculation on the basis of anterior corneal measurements alone, in corneas with oblique anterior astigmatism, will result in a systematic overcorrection or undercorrection of postoperative refractive astigmatism, and to establish whether adjustment of oblique anterior corneal astigmatism to allow for the likely effect of posterior corneal astigmatism is needed in such eyes.
Setting: The Queen Elizabeth Hospital, Department of Ophthalmology, Adelaide, South Australia.
Design: Retrospective case series.
Methods: Data were collected from consecutive eyes with oblique anterior corneal astigmatism undergoing cataract surgery or refractive lens exchange. Toric IOL calculations were made using unadjusted anterior corneal astigmatism measurements and an astigmatically neutral technique. By comparison of targeted vs achieved astigmatic correction using vector analysis, the astigmatic power prediction error and angle of error were derived.
Results: In 50 eyes, the mean postoperatively measured refractive astigmatism was 0.42 diopters (D) (SD 0.33 D), the mean targeted refractive astigmatism was 0.15 (SD 0.18), and the vector-derived mean absolute prediction error was 0.23 D (SD 0.21). All 50 eyes (100%) fell at or within ±1.00 D of the target and 45 eyes (90%) at or within ±0.50 D. The geometric mean astigmatism correction index was 0.98, and the mean absolute angle of error was 8.63 degrees (SD 8.55).
Conclusions: There is no systematic error attributable to posterior corneal astigmatism in toric IOL calculation and no need for adjustment of anteriorly measured corneal keratometric values for toric IOL calculation in eyes with oblique anterior corneal astigmatism. To do so may introduce an error in astigmatic outcome.