Astigmatism measurement based on total corneal power

Astigmatism measurement based on total corneal power
Cheryl Guttman Krader
Cheryl Guttman Krader
Published: Tuesday, November 3, 2015

Corneal topographic astigmatism based on total corneal power (CorT total) provides an accurate and consistent measure of astigmatism, said Noel Alpins MD at the XXXIII Congress of the ESCRS in Barcelona, Spain.

CorT total is derived using individual data for anterior and posterior corneal power measurements. In 2012, Dr Alpins and colleagues reported that CorT anterior, a corneal power measure based on anterior corneal power measurements only, corresponded better with manifest refractive cylinder in both magnitude and orientation than simulated keratometry, corneal wavefront, manual keratometry, and automated keratometry (Alpins N et al. J Cataract Refract Surg. 2012;38(11):1978-88).

Now, a study evaluating the performance of CorT total showed that it corresponded better with manifest refractive cylinder, both in variability and closeness, than CorT anterior and consequently the other measures also.

“Accuracy for the total astigmatic power of the cornea and its meridian is of utmost importance when correcting astigmatism, and CorT gives more accurate information than other parameters because it is derived from all measured data rather than from only a limited area of the cornea,” said Dr Alpins, Medical Director, NewVision Clinics, Melbourne, Australia. (see Figure 1)

“Therefore, when planning toric intraocular lens selection and orientation, or the length and orientation of limbal relaxing incisions, we believe cataract surgeons should use the CorT parameter, total or anterior depending on whether their tomography/topography system measures the posterior and anterior cornea or just the anterior surface,” he added.

The study comparing the performance of CorT total and CorT anterior has also been published (Alpins N et al. J Refract Surg. 2015;31(3):182-6). It included data from 526 surgically-naïve eyes of patients presenting for laser vision correction.

Corneal power measurements obtained with the Sirius tomographer (Costruzione Strumenti Oftalmici) were used to determine total and CorT anterior values for each eye. Correspondence of the CorT values with refractive cylinder was analysed based on calculations for ocular residual astigmatism (ORA) mean magnitude and standard deviation. ORA is the vectorial difference between the corneal astigmatism and the refractive cylinder at the corneal plane. (see Figure 2)

Compared with CorT anterior, CorT total had significantly lower values for both mean ORA magnitude (0.30 vs 0.32D) and ORA variability (SD 0.53 vs 0.64D), that is the CorT total was closer to the manifest refractive cylinder than CorT anterior. (see Figure 3)

“A lower mean ORA magnitude indicates closer correlation between refractive cylinder and corneal astigmatism, and a lower standard deviation indicates lower variability,” Dr Alpins explained.

CorT total is now available on these tomography platforms that measure total corneal power – CSO Sirius, Pentacam (Oculus), and Galilei (Ziemer). Those three systems, as well as the Atlas (Carl Zeiss Meditec) and OPD-Scan III (Nidek), have the capability of providing CorT anterior. The iAssort software is required for calculation of the CorT parameter (www.assort.com).

 

Noel Alpins: alpins@newvisionclinics.com.au

Tags: cornea
Latest Articles
ESCRS Today 2025: Happy Anniversaries!

ESCRS celebrates milestones with pioneers in IOLs, LASIK, femtosecond lasers, and corneal transplantation.

Read more...

ESCRS Today 2025: A Congress for Everyone

From YOs to families, the ESCRS Annual Meeting embraces full participation through inclusivity.

Read more...

ESCRS Today 2025: All Eyes on Innovation

Watching out for obstacles and opportunities

Read more...

Beyond the Numbers

Empowering patient participation fosters continuous innovation in cataract surgery.

Read more...

Thinking Beyond the Surgery Room

Practice management workshop focuses on financial operations and AI business applications.

Read more...

Aid Cuts Threaten Global Eye Care Progress

USAID closure leads retreat in development assistance.

Read more...

Supplement: ESCRS Clinical Trends Series: Presbyopia

Read more...

Debate: FS-LASIK or KLEx for Hyperopia?

FS-LASIK has more of a track record, but KLEx offers advantages.

Read more...

Four AI Applications Ready for Practice

Commercial offerings may save time, improve practice and research.

Read more...

Perioperative Medication Regimens for Cataract Surgery

Randomised controlled clinical trial results provide evidence-based guidance.

Read more...