Calculations of actual corneal astigmatism using total corneal refractive power before and after myopic keratorefractive surgery

Abstract

PurposeTo calculate actual corneal astigmatism using the total corneal refractive astigmatism for the 4-mm apex zone of the Pentacam (TCRP4astig) and keratometric astigmatism (Kastig) before and after photorefractive keratectomy or laser in situ keratomileusisMethodsUncomplicated 56 eyes after more than 6 months from the surgery were recruited by chart review. Various corneal astigmatisms were measured using the Pentacam and autokeratometer before and after surgery. Three eyes were excluded and 53 eyes of 38 subjects with with-the-rule astigmatism (WTR) were finally included. The astigmatisms were investigated using polar value analysis. When TCRP4astig was set as an actual astigmatism, the efficacy of arithmetic or coefficient adjustment of Kastig was evaluated using bivariate analysis.ResultsThe difference between the simulated keratometer astigmatism of the Pentacam (SimKastig) and Kastig was strongly correlated with the difference between TCRP4astig and Kastig. TCRP4astig was different from Kastig in magnitude rather than meridian before and after surgery; the preoperative difference was due to the posterior cornea only; however, the postoperative difference was observed in both anterior and posterior parts. For arithmetic adjustment, 0.28 D and 0.27 D were subtracted from the preoperative and postoperative magnitudes of Kastig, respectively. For coefficient adjustment, the preoperative and postoperative magnitudes of Kastig were multiplied by 0.80 and 0.66, respectively. By arithmetic or coefficient adjustment, the difference between TCRP4astig and adjusted Kastig would be less than 0.75 D in magnitude for 95% of cases.ConclusionsKastig was successfully adjusted to TCPR4astig before and after myopic keratorefractive surgery in cases of WTR. For use of TCRP4astig directly, SimKastig and Kastig should be matched.

Prediction of postoperative refractive astigmatism before toric intraocular lens implantation

“…The correction of astigmatism with toric IOLs is based on the assumption that all astigmatism originates from the cornea and lens. However, it is known that the difference between postoperative refractive and corneal astigmatism is not nil [ 1 , 2 ]. Postoperative refractive astigmatism is thought to include lens capsule (and/or IOL) tilt and decentration, and unknown ocular components in addition to corneal astigmatism.…”

Comparison between the change in total corneal astigmatism and actual change in refractive astigmatism in transepithelial photorefractive keratectomy (tPRK), laser in situ keratomileusis (LASIK) and femtosecond laser assisted laser in situ keratomileusis (FsLASIK)

“…This is normally considered when calculating the total corneal refractive power. [7][8][9][28][29][30] The difference between the TCA values calculated using the standard thick and thin lens formulae is less than 0.07DC for a notional corneal thickness of 500µ, refractive index of 1.376 and astigmatic power of −4.00DC. This small difference may be important under certain circumstances, but it is negligible within the context of the current study.…”

Surgically induced astigmatism in canines following sutured dorsonasal vs dorsotemporal clear corneal incisions

“…TCRP K1 and TCRP K2 with their respective meridians were used for SIA calculations from 3 mm and 4 mm zones. For SIA using both TCRP3 and TCRP4, significant differences were found between right and left eyes, with a dorsonasal incision creating more SIA than a dorsotemporal incision (Table ).…”

“…In this study, TCRP at the 3 mm and 4 mm zones was evaluated with repeated measures and used in SIA calculations. While the 3 mm zone is supposed to most closely match the standard zone of keratometric analysis, several human studies have evaluated both areas .…”