Posterior corneal astigmatism calculated in eyes requiring a range-of-toric IOL powers

Session Details

Session Title: Pseudophakic IOLs: Toric

Session Date/Time: Tuesday 10/10/2017 | 08:30-10:30

Paper Time: 09:18

Venue: Room 4.4

First Author: : B.LaHood AUSTRALIA

Co Author(s): :    M. Goggin                    

Abstract Details

Purpose:

Despite its importance in cataract and refractive surgery, little is known about the posterior cornea. Corneal power calculations have historically been based on anterior corneal measurements alone. Modern lens formulae and calculators are incorporating adjustments for posterior corneal astigmatism. There is no current gold standard for measuring posterior corneal astigmatism. Our aim was to calculate posterior corneal astigmatism from refraction, IOL and anterior corneal data in pseudophakic eyes. These calculated values will assist in toric IOL calculation and set a benchmark for attempts to measure posterior corneal astigmatism directly with devices currently in development.

Setting:

The Ophthalmology department of The Queen Elizabeth Hospital, a teaching hospital in Adelaide, South Australia

Methods:

367 consecutive eyes underwent phacoemulsification surgery with implantation of a Zeiss AT TORBI709MP toric intraocular lens (IOL). Six weeks post-operatively, posterior corneal astigmatism was calculated by taking the subjective refractive astigmatism and vector subtracting the IOL astigmatism (both calculated to the corneal plane) and then vector subtracting from this the measured post-operative anterior corneal astigmatism. Statistical analysis was then performed to assess the variation in posterior corneal astigmatism and its associations with other measured variables.

Results:

The majority of eyes (64.25%) had vertically steep posterior corneas with oblique (21.79%) and horizontal (13.97%) less common. Vertical predominance was consistent across all age groups. In anterior “with-the-rule” (WTR) eyes, the posterior cornea was steep vertically in 52.94%. In 77.11% of “against-the-rule” (ATR) eyes the posterior cornea was steep vertically. Mean ± Standard Deviation (SD) posterior corneal astigmatism was 0.87D ± 0.51D (median 0.78D). There was a small but statistically significant (p = 0.024) increase in average posterior corneal astigmatism power as anterior corneal astigmatism power increased (Pearson r = 0.12).

Conclusions:

Using this calculation, posterior corneal astigmatism appears to have a greater average power than previous estimates. The orientation of posterior corneal astigmatism was found to vary more than expected though still vertical steepness predominates, especially in ATR eyes. The magnitude of posterior corneal astigmatism appears to increase, but only slightly in relation to increasing anterior corneal astigmatism. This is important as less proportional adjustment of keratometry is indicated as anterior astigmatism increases. These findings will help refine decision-making in cataract surgery and may act as a standard against which new technology aimed at assessing the posterior cornea can be compared.

Financial Disclosure:

NONE