March 2016

 

REFRACTIVE SURGERY

 

Presentation spotlight

Tools for post-refractive surgery eyes


by Louise Gagnon EyeWorld Contributing Writer

 
   
Post-refractive IOL calculator

The ASCRS post-refractive IOL calculator Source: ASCRS

Dr. Holland discusses tools that can help surgeons plan for procedures in eyes that have previously undergone refractive surgery

Ophthalmologists can use tools such as the ASCRS post-refractive IOL calculator as well as intraoperative aberrometry to reduce refractive errors in patients undergoing cataract surgery who have previously had refractive surgery, according to Edward Holland, MD, director of the cornea service, Cincinnati Eye Institute, and professor of ophthalmology, University of Cincinnati.

Its important to manage the expectations of these patients, Dr. Holland said, speaking at the 55th annual Walter Wright Symposium in Toronto. The results are less predictable, but we will do everything we can to get them as close as we can (to an optimal outcome). You can discuss with patients that there may be more variations in their refractive outcomes. They are at a much higher risk of being unhappy than a non-refractive patient would be with a multifocal lens. Discussing the challenges of cataract surgery planning in the population of post-refractive surgery eyes, Dr. Holland explained how the eye is forever changed after refractive surgery, a fact that is critical in planning for cataract surgery. There is a disconnect between the anterior corneal curvature and posterior corneal curvature (after refractive surgery), he said.

The shortcoming with keratometry is that it measures the central anterior corneal curvature and assumes that corneal curvature is uniform and that the anterior and posterior surfaces remain untouched, Dr. Holland explained. In patients who have undergone vision correction with a laser, the procedure modifies the anterior corneal curvature but does not modify the posterior curvature. Moreover, the degree of laser ablation will dictate the index of refraction. Subsequent to myopic ablative procedures, keratometry will appear erroneously high, leading to an underestimation of the IOL power that will result in a difference of between 14 and 25%, Dr. Holland said. Conversely, subsequent to hyperopic ablative surgeries, keratometry will appear erroneously low, leading to an overestimation of the IOL power. There are numerous alternatives to keratometry to derive accurate IOL power for patients who have undergone previous refractive surgery, including the complete reliance on historical data, which offers solutions such as the Feiz-Mannis formula or Walter formula. Another approach is to use historical data as well as current corneal measurements, which presents solutions such as modified computerized videokeratography, the Latkany formula, the Masket formula, and the modified Masket formula. Still another strategy is to use no prior data and only use current measurements, found in solutions such as the Koch-Wang-Maloney method, the Haigis-L formula, intraoperative refraction, and the Gaussian lens formula.

Another approach is a post-refractive IOL calculator, available through the website of the ASCRS, and designed specifically for eyes that have undergone prior myopic LASIK or photorefractive keratectomy (PRK) or prior hyperopic LASIK or PRK or eyes that have undergone prior RK.

The easiest way to calculate the appropriate IOL power for patients with previous refractive surgery is to use the ASCRS calculator, Dr. Holland said. We have found the calculator to be quite accurate by itself. It is a free service and as good as anything else out there. Dr. Holland pointed to a study that assessed several methods of determining IOL power in post-LASIK or post-PRK patients. The investigators found the use of the ASCRS calculator produced better results than methods using pre-LASIK/PRK keratometry values and change in refraction that was surgically induced.1 In addition, Dr. Holland suggested that intraoperative aberrometry be employed in conjunction with the IOL calculator. Confirmation with intraoperative aberrometry has been a very nice adjunct to get you even closer to the desired refractive outcome, Dr. Holland said.

Intraoperative wavefront aberrometry is convenient because it attaches to most surgical microscopes. As a whole, intraoperative wavefront aberrometry measures the true refractive power of the cornea including the posterior surface, incorporates surgically induced astigmatism, compensates for ocular cyclotorsion, measures the visual axis of the cornea, and enhances refractive results in post refractive eyes. Intraoperative aberrometry provides us with real-time information, Dr. Holland said.

Dr. Holland also noted some emerging formulas, such as the Barrett True-K formula, in calculating appropriate IOL power for eyes with previous refractive surgery. Another emerging technology to evaluate accurate IOL power in post-LASIK patients is custom ray tracing including a modified equivalent refractive index.

Reference

1. Wang L, et al. Evaluation of intraocular lens power prediction methods using the American Society of Cataract and Refractive Surgery Post-Keratorefractive Intraocular Lens Power Calculator. J Cataract Refract Surg. 2010;36:14661473.

Editors note: Dr. Holland has financial interests with Alcon (Fort Worth, Texas).

Contact information

Holland
: eholland@holprovision.com

       
Tools for post-refractive surgery eyes Tools for post-refractive surgery eyes
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