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For LASIK planning, study finds manifest refraction more accurate than wavefront refraction
Attempted correction was more accurate with manifest refraction than two types of high-resolution aberrometric refraction.
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Wavefront-guided treatment relies on aberrometric refraction for programming the refractive correction of the eye, but manifest refraction LASIK continues to yield the most accurate refractive results, according to a study.
In the prospective study, LASIK based on manifest refraction was performed on 869 myopic eyes and 413 hyperopic eyes. Two methods of calculating aberrometric refraction with the Wavefront Supported Custom Ablation (WASCA) aberrometer (Carl Zeiss Meditec), known as the COAS in the United States, were obtained preoperatively: one using the Seidel method, including spherical aberration in the sphere calculation, and the other non-Seidel. Manifest refraction was then performed again 3 months after surgery.
The study was published in the Journal of Cataract and Refractive Surgery.
Predicting outcomes
“We calculated a prediction of what the outcome would have been, based on the vector difference between the manifest and aberrometric refractions,” lead author Dan Z. Reinstein, MD, MA(Cantab), FRCSC, DABO, FRCOphth, FEBO, founder and medical director of the London Vision Clinic, said in an interview with Ocular Surgery News.
For myopic eyes, 81% were within ±0.5 D of the attempted correction with manifest refraction, compared with 70% with Seidel and 67% with non-Seidel.
Dan Z. Reinstein
For hyperopia, 71% of eyes were within ±0.5 D of the attempted correction with manifest refraction, in contrast to 61% with Seidel and 67% with non-Seidel.
“There are some laser platforms that force the surgeon to use the aberrometric refraction when performing a wavefront-guided ablation, whereas other laser platforms allow the surgeon to enter the manifest refraction and only take the higher-order aberrations from the aberrometer,” Reinstein said. “The question we wanted to ask was whether the aberrometric refraction could improve the accuracy of the treatment, or whether manifest refraction was still the gold standard.”
Reinstein said that, although a surgeon’s instinct might be to trust the measurement obtained from aberrometry, all instruments have an associated measurement error, as well as a maximum resolution.
The main difference for manifest refraction is that it includes the influence of the brain on filtering the image received by the retina, he said.
“The brain has many complex image processing functions, such as edge detection and blur adaptation, which means that the perceived image is often significantly improved compared with the retinal image. This gives manifest refraction a huge advantage over aberrometric refraction because manifest refraction is based on the patient’s subjective responses and so includes all of the neural image processing,” Reinstein said. “An aberrometric refraction does not, as it is simply an objective measurement of the refraction of the refractive media and length of the eye.”
Reinstein said that refractive outcomes could have been even worse if a lower-resolution aberrometer than the WASCA aberrometer had been used in the study.
Maximizing accuracy
According to Reinstein, the best way to increase the accuracy of refractive outcomes is to continuously audit results and use them to derive a personalized nomogram.
“Ideally, a multivariate regression analysis should be done to get the best possible refinement for your particular laser system, surgical technique and environment,” Reinstein said.
Additionally, he said that it is preferable to use a laser that allows the operator to enter the manifest refraction when planning the treatment, rather than forcing the use of the aberrometric refraction when performing wavefront-guided treatment.
Previous studies have demonstrated that the Seidel method for calculating the aberrometric refraction has a better correlation with manifest refraction.
“It was expected that the accuracy of the treatments would have been higher using the Seidel method compared with the standard aberrometric refraction,” Reinstein said. “We found this to be true for myopic treatments, but we found the accuracy to be slightly worse using the Seidel method for hyperopic treatments.” – by Bob Kronemyer