Intrastromal AK outcomes with laser-based procedure favorable, improving

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Using the most recent nomogram for intrastromal astigmatic keratotomy using a proprietary femtosecond laser platform, the mean vector magnitude achieved was 87%.

By Cheryl Guttman Krader; Reviewed by Nicola Lau, MD, and Julian Stevens, MD

London-Intrastromal astigmatic keratotomy (AK) performed with a proprietary femtosecond laser platform (Catalys Precision Laser System, Abbott Medical Optics) and a dedicated nomogram is an effective method for correcting up to 2.5 D of astigmatism during cataract surgery, according to the experience of surgeons from Moorfields Eye Hospital, London.

Results from follow-up to 6 months indicate that the effect of the intrastromal AKs is fairly stable, and with refinements to the nomogram over time, the accuracy and predictability of the treatment is continuing to improve.

“The ability to use the femtosecond laser for intrastromal astigmatic keratotomy is an attractive option because compared to manual AK, the technique offers potential safety and comfort benefits along with the opportunity for more reproducible and accurate results,” said Nicola Lau, MD, specialist registrar, Moorfields Eye Hospital.

“Our nomogram generates the intended vector magnitude and was specifically created for the Catalys, using the manufacturer’s standard laser spot and energy settings. It allows the intended vector magnitude to be refined and is aiming for 75% vector magnitude as a safety measure, taking into account the potential for long-term progression of astigmatic effect.”

All of the procedures were performed by Julian Stevens, MD, consultant ophthalmologist, Moorfields Eye Hospital, and involved the creation of paired symmetrical instrastromal incisions using an 8.0 mm optical zone with limbal centration. The laser was also used to perform capsulotomy and lens conditioning.

Initially, the paired instrastromal cyclinders were created at between 30% to 80% depth from the anterior corneal surface. Subsequently, the depth was modified to be between 20% and 80%, and the nomogram was also refined based on multiple regression analysis of topography and refraction outcomes data.

Dr. Stevens and Dr. Lau reported that for a series of 61 eyes operated with nomogram version 2, mean cylinder was 1.24 D preoperatively and 0.80 D at 1 month with a standard deviation of 0.5 D. Vector analysis showed the mean vector magnitude achieved was 59% of intended with a range between 12% and 164%. Among 28 eyes followed to 6 months, the mean vector magnitude achieved was 56% (range, 3% to 150%).

“Based on the 1- and 6-month data, we are concluding that these intrastromal AKs are stable,” Dr. Lau said.

With implementation of the next version of the nomogram, analyses of data from 80 eyes followed to 1 month showed the mean vector magnitude achieved improved to 87%.

“The slight undercorrection of vector cylinder is probably desirable because overcorrection would flip the axis,” said Dr. Lau.

Limitations of the technique relate to the standard deviation or scatter in the magnitude of effect outcome along with angle alignment error. The causes for outcome scatter are multifactorial and include:

• Separation of the intrastromal cylinders by the laser, within the corneal stroma

• Accuracy of the depth of the instrastromal cylinders

• Variation of the corneal biomechanics with thickness, geometry, curvature, elastic modulus and centration of cylinders

• Patient movement during femosecond application with incomplete cylinder edge separation

“Angle error remains a major source of undercorrection as does standard deviation and variation between corneas in terms of laser delivery, biomechanics, and healing,” Dr. Lau said.

Dr. Stevens noted a forthcoming version of software for the Catalys laser (version 4) that will feature automatic corneal power meridian alignment is being looked to for its potential to provide better results.

“This new feature is expected to help solve the problem with angle error. Then, vector magnitude and standard deviation will be critical in determining the predictability of the nomogram,” Dr. Stevens said.

Available for adoption

The nomogram is available as a web-based program at www.femtoemulsification.com. Surgeons need to input patient age, cylinder magnitude and meridian, intended cylinder, and the surgeon’s own surgically induced refractive change (SIRC) from the primary incision and paracenteses. The nomogram generates the final arc length angle and the meridian in which the cylinder is to be placed. Those values are programmed into the Catalys laser.

Dr. Stevens said that a future version of the nomogram will allow surgeons to input their own outcomes data to personalize the nomogram, and they will also be able to compare their outcomes to peer group data.

Nicola Lau, MB BS BSc(Hons) AICSM

E: Nicola.Lau@moorfields.nhs.uk

This article was adapted from Dr. Lau's presentation at the 2014 meeting of the American Society of Cataract and Refractive Surgery. Dr. Lau has no financial interest to disclose.

Julian Stevens, MD

Dr. Stevens was a previous consultant to OptiMedica.