Publication Exclusive: CorT an accurate, consistent measure of corneal astigmatism

The measure of corneal astigmatism magnitude and its meridian has become increasingly important with toric IOLs, femtosecond laser limbal relaxing incisions and high patient expectations to be independent of spectacles. Noel Alpins, my guest columnist in this issue, will explain this in detail.

Amar Agarwal, MS, FRCS, FRCOphth, OSN Complications Consult Editor

The technology available to measure corneal astigmatism is multiple. It includes manual keratometry, automated keratometry (Lenstar from Haag-Streit and IOLMaster from Carl Zeiss Meditec), simulated keratometry (topographers), total corneal astigmatism (tomographers) and corneal wavefront (aberrometry). In addition, there are sophisticated toric IOL calculators available to obtain the most accurate toric IOL power using many parameters such as axial length, anterior chamber depth and personalized IOL constants.

The calculated parameters for corneal astigmatism involve the measurement from a specific but differing area of the cornea and in many cases only the anterior cornea. For simulated keratometry (Sim K), this is around the 3-mm region, for manual keratometry at the 3- to 4-mm zone, for the IOLMaster at six points around the 2.5-mm zone, and for the Lenstar 32 markers on two concentric rings.

Topography systems measure the radius of curvature of most of the entire cornea with the exception of eyelid cover superiorly and tear film inferiorly. Using all this information rather than the limited data measure from one particular zone on the cornea provides much more information as to the overall amount of astigmatism on the cornea and its orientation.

A good benchmark measure of how much astigmatism a particular eye has is the refractive cylinder. The refractive cylinder includes the optical elements of the eye as well as the visual cortex because the patient’s non-optical perception during a subjective refraction is used to tell the practitioner the patient’s preference.

The subjective refraction is based on all the light entering the pupil from both straight ahead and peripherally; therefore, a greater area of the cornea than the pupil size alone under all lighting conditions would be more advantageous than the limited calculations of corneal astigmatism in the central zones commonly used.

Corneal topographic astigmatism (CorT) is calculated using all the valid data that is captured during topography/tomography. This is particularly important in irregular corneas in which the Sim K calculation measured at the 3-mm corneal zone may have been based on an irregular or less than smooth section that is not representative of the whole cornea. Furthermore, the Sim K calculation at the 3-mm zone is not consistent for all corneas because the “3 mm” varies depending on whether a steep or flat cornea is measured; in steep corneas, the Sim K will be calculated using data acquired from a region that is less than 3 mm in diameter, and in a flat cornea, from a region that is more than 3 mm.

Any measure of corneal astigmatism, regardless of how accurate it may be, will still not be equivalent to the refractive cylinder in magnitude and/or orientation in most cases because there is lenticular astigmatism, retinal tilt and the non-optical component of the visual cortex to consider. The vectorial difference between the refractive cylinder (at the corneal plane) and the corneal astigmatism is known as the ocular residual astigmatism (ORA) and has a magnitude and axis expressed in diopters and degrees.

CorT provides a consistent measure of corneal astigmatism for both regular and irregular corneas. This can then be used in the calculation of the most appropriate toric IOL in cataract surgery or the placement of the center of the arc in limbal relaxing incisions.

The CorT value is calculated using a summated vector mean of the astigmatism values determined from all the measured data of corneal curvature/power exported from the topography acquisition.

Click here to read the full publication exclusive, Complications Consult, published in Ocular Surgery News U.S. Edition, October 10, 2015.