Cutting-edge LASIK technologies in quest for perfect vision

Take-home message: The various excimer ablation technologies have their pros and cons. Wavefront-guided ablations using aberrometry can provide the vast majority of patients with uncorrected vision of 20/20 with fewer postoperative visual symptoms. 

By Lynda Charters; Reviewed by Steven C. Schallhorn, MD

San Diego - Over the years, LASIK has moved forward by leaps and bounds–with each new added technology further maximizing vision via innovative means. The vast majority of patients can now achieve uncorrected visual acuities of 20/20 or better and, compared with the early days, there is now far better quality of vision.

But surgeons did not reach this level of excellence overnight. Some of the important steps between the introduction of LASIK performed using a conventional procedure and wavefront-guided ablations were the improvements in the laser algorithms, development of sophisticated eye trackers and cyclotorsional alignment, and use of the femtosecond laser to create flaps.

However, the greatest improvements in the quest for perfect vision have been refinements in the ablation profile and the data used to create it, according to Steven Schallhorn, MD.

Conventional ablations

The first foray into refractive correction was the conventional or standard ablation, which was based on the spectacle prescription and utilized the Munnerlyn formula for the ablation profile.

The Munnerlyn formula correctly and precisely calculates the treatment profile to treat the refractive error, but it does not account for the healing response and the variability in the amount of tissue removed by each laser pulse. The conventional correction is a simple and straightforward procedure that removes a minimal amount of tissue. The surgery effectively reduces patients’ need for spectacle or contact lens correction.

However, spherical aberration and other higher-order aberrations (HOAs) can be induced which are associated with night vision difficulties. In addition, complex treatments cannot be performed.

Optimized LASIK treatment

This surgery addressed the induction of spherical aberration with conventional procedures. Dr. Schallhorn, in private practice in San Diego, explained that in an optimized treatment, “extra laser pulses are added in the periphery of a myopic treatment to account for average induction of spherical aberration.”

Like the conventional procedure, this also is simple and straightforward, but it significantly decreases the induction of spherical aberration. It also is similar to the conventional procedure in that the treatment is based on the spectacle refraction.

The disadvantages of the optimized approach, according to Dr. Schallhorn, are that the spherical aberration adjustments area population-based pattern, it cannot address other optical aberrations which may be present and that complex treatments cannot be performed.

Topography-guided ablations

In topography-guided ablations, the patient’s entire corneal shape is taken into account when deriving the ablation profile. Topography information is transferred to the excimer laser to maximize the treatment.

The advantages are that the “cosine effect” on each laser pulse can be precisely determined, resulting in an improved ablation algorithm, and that substantial cornea induced optical aberrations, specifically from previous surgery or trauma, can be addressed.

Disadvantages include the added surgical time and expense of interpreting and transferring topography information to the excimer laser, the correction is based on the manifest refraction and refined nomograms are needed to properly correct the refractive error. Another downside in the United States is that the procedure is approved to treat only low-to-moderate myopia with astigmatism.

Wavefront-guided treatment

This approach differs markedly from conventional and optimized procedures in that it uses an aberrometer to customize the treatment by taking into consideration all optical aberrations present in the eye, including the refraction. This results in improved contrast sensitivity, decreased glare, and better night vision compared to a conventional treatment.

The patient’s eye is imaged with the aberrometer, such as the iDesign (Abbott Medical Optics), the treatment is calculated, the plan is transferred to the excimer laser, and the treatment is aligned with the wavefront capture.

The advantages are that whole-eye aberrations are measured and taken into account in the ablation profile–even those in highly aberrated eyes; the alignment is sophisticated; and adjustment can be made for the angle kappa. The procedure has a broad range of FDA approvals, and the outcomes are well established in the literature.

Dr. Schallhorn describes this surgery as being very accurate with better results than conventional procedures, such as fewer HOAs and improved contrast sensitivity, which translates into better quality of vision.

On the downside, Dr. Schallhorn points out that additional time and expenses are required, such as:

• Purchasing the aberrometer device;

• High-quality wavefront measurement and interpretation is necessary;

• Accurate wavefront capture is not always possible;

• Ablation profile removes more tissue than a conventional procedure;

• Ablation profile can still can induce HOAs.

“It is clear that each ablation profile has its advantages and disadvantages,” Dr. Schallhorn summarized. “Spherical aberration is the most significant HOA induced by laser vision correction and this is being addressed with modern ablation profiles. Enhanced customization to the individual patient has improved the treatments we can now perform.”

Steven C. Schallhorn, MD

e: scschallhorn@yahoo.com

This article was adapted from Dr. Schallhorn’s presentation during the 2015 meeting of the American Society of Cataract and Refractive Surgery. Dr. Schallhorn is a consultant to Abbott Medical Optics (AMO) and Carl Zeiss Meditec.