Shape is the key to aberration-free ablative surgery

September 17, 2004

Contrary to usual thinking, the ablation profile used during refractive surgery is programmed with shape, rather than visual acuity, refractive error, or wavefront error. The latter two are converted to a depth profile that is imposed on the cornea by the laser and the end target is shape.

Paris—Contrary to usual thinking, the ablation profile used during refractive surgery is programmed with shape, rather than visual acuity, refractive error, or wavefront error. The latter two are converted to a depth profile that is imposed on the cornea by the laser and the end target is shape.

"Shape and vision are intimately connected and both must be considered as we move forward in this field," said Cynthia Roberts, PhD, Columbus, OH, at the European Society of Cataract and Refractive Surgeons meeting.

Wavefront-guided ablations, she noted, have fallen short of the goal of achieving perfect vision by measuring and eliminating aberrations after refractive surgery and the expectations for the procedures have decreased. What is missing from customized refractive surgery is the ability to predict individual responses to refractive procedures, rather than using mean population responses as is done now.

In addition, with customized procedures, the appropriate shape must be achieved over a much larger region of the cornea than previously to eliminate aberrations that arise as the result of the enlarging pupil. After refractive surgery, the continuous series of stacked lamella that make up the cornea are severed, which results in an expansion of the peripheral lamellar segments, flattening of the central cornea, and induced spherical aberration.

"Understanding corneal shape changes induced by refractive surgery will help us understand how to reduce them; 100% of induced aberrations come from the cornea," she said.

In a retrospective study of 2,380 patients who underwent LASIK for myopia 6 months previously, the peripheral curvature increase and the amount of spherical aberration were found to be greater as the amount of correction increased. Achieving the desired corneal shape that produces the optimal wavefront will hinge on knowledge of how biomechanics influences the final outcome; wavefront and topography independent of each other are insufficient.

"The biggest challenge is predicting the corneal biomechanical response due to the ablation," she emphasized.