New measurement method quantifies corneal astigmatism
A study compared the parameter of CorT against SimK, manual keratometry, corneal wavefront and paraxial curvature matching.
A new measure of corneal astigmatism known as CorT more accurately assesses corneal astigmatism when compared to the manifest refractive cylinder than other commonly used measurement methods, according to its developer.
After becoming frustrated with the variability of simulated keratometry (SimK), Noel A. Alpins, FRACO, FRCOphth, FACS, OSN Refractive Surgery Board Member, developed the CorT parameter.
“This is the most effective way of assessing the corneal astigmatism. Why wouldn’t you want to use all the numbers that you gain from the cornea instead of just one [Placido] ring as is done with SimK calculation?” Alpins asked in an interview with Ocular Surgery News at the American Society of Cataract and Refractive Surgery meeting.
A common difficulty with the SimK value is that the measurement is taken from a small 3-mm region of the cornea, consequently not creating the most accurate representation of the existing corneal astigmatism.
“The CorT value is intended to provide a consistent measure of corneal astigmatism across regular and irregular corneas, which can then be implemented in corneal incisional and refractive laser surgery to better correct astigmatism,” Alpins and colleagues said in a study.
To derive the CorT measurement, Alpins and colleagues compared all adjacent sets of Placido rings to find the set of rings with the lowest standard deviation of the ocular residual astigmatism (ORA). The ORA is defined as the vectorial difference between the refractive cylinder (corneal plane) and the corneal astigmatism, and it is expressed in diopters. The CorT value was then derived by combining the astigmatism value for each Placido ring with a vector summation.
“The CorT value is neither as jumpy nor as variable as SimK values,” Alpins said.
When compared with four parameters — corneal wavefront, manual keratometry, SimK and paraxial curvature matching — CorT not only had the least variability but also was the statistically closest to the manifest refractive cylinder at the corneal plane in all cases, Alpins said, as it is the standard for total astigmatism for treatments and several refractive device assessments such as ocular wavefront.
This closer correlation to the manifest refractive cylinder is more representative of the whole cornea than parameters derived from other means, the authors said.
The retrospective review examined 971 eyes of 498 patients aged 19 to 64 years who had later undergone refractive surgery. CorT was compared with manual keratometry, SimK, corneal wavefront and paraxial curvature matching.
The standard deviation for CorT was significantly less than for the other measures of astigmatism (P < .001).
Using the ORA, CorT had a better correlation with manifest refractive cylinder (ORA 0.62 ± 0.33 D) than manual keratometry (ORA 0.68 ± 0.38 D), SimK (ORA 0.70 ± 0.35 D), corneal wavefront (ORA 0.74 ± 0.36 D) and paraxial curvature matching (ORA 0.85 ± 0.48 D).
According to the study, a benefit of CorT is that the resulting ORA magnitude is lower than that produced by other astigmatism measures.
“This may indicate that estimates of ORA are commonly larger than should normally prevail because these three other measures of corneal astigmatism do not consistently represent the corneal astigmatism that is actually perceived across wider regions of the cornea,” Alpins and colleagues said.
CorT may also provide additional safety and accuracy in determining patient suitability for corneal astigmatic surgery because the method is based on multiple points of the entire cornea, thus lessening the impact of a single outlier, according to the study.
In the future, CorT may have the potential to improve outcomes of astigmatism treatments, Alpins said.
“Patients with high ORA amounts can be counseled before surgery that expectations for a complete correction of their existing cylindrical refractive error with refractive surgery may have to be lowered to realistic levels,” the authors said.
“When the ORA may be significant, the treatment paradigm can be safely adjusted to combine the corneal astigmatism into the refractive treatment plan when performing excimer laser surgery with the technique of vector planning,” Alpins and colleagues said. – by Ashley Biro