May 03, 2016
Save

# Order of Merit: Conquering the post-refractive surgery IOL outcome

### Determining the correct IOL power calculation in post-refractive surgery patients is essential for today's premium surgeon.

The Order of Merit as we know it today is a special honor awarded to individuals for great achievement in the fields of arts, learning, literature and science. Established in 1902 by King Edward VII, admission into this elite order is restricted to a maximum of 24 living recipients from the Commonwealth realms plus a limited number of honorary members. There have been no honorary members since the death of Nelson Mandela in 2013.

For the premium refractive cataract surgeon today, the challenge to earn this similar Order of Merit occurs every time a post-refractive cataract surgery patient is encountered. Tackling the correct IOL power in this subset of patients is becoming increasingly greater as the numbers of baby boomers who have had prior refractive surgery are now filling up our cataract surgery days.

Conventional keratometry has never yielded a good visual outcome in post-refractive surgery patients due to an inaccurate central corneal power because prior myopic laser vision correction gives an IOL power underestimation or unwanted hyperopic surprises. This occurs because the anterior curvature in post-myopic laser vision correction is flatter than the posterior curvature.

#### Calculation methods

The Masket regression method adjusts for IOL power based on a simple formula: LSE × (–0.326) + 0.101, where LSE = excimer laser spherical equivalent treatment. This formula results in an approximate 1 D adjustment for every 3 D of correction. For example, a 6 D myope pre-LASIK would convert by adding 2 D of power to the IOL, so a +18 D IOL would now be a +20 D IOL. This is a quick and fairly accurate way to avoid an undesirable hyperopic surprise.

In the clinical history method (CHM), the change in manifest refraction spherical equivalent due to keratorefractive surgery is subtracted from the preoperative mean keratometry value and is represented as the following formula calculation: K = Pre-RS K + (Pre-RS SEQ – Post-RS SEQ), where RS = refractive surgery, K = keratometry value and SEQ = spheroequivalent. The preference is to use the most recent manifest refraction after the refractive surgery prior to the development of the cataract, as the latter can induce a myopic shift, bringing error to this method.

In the contact lens method (CLM), a gas permeable contact lens (CL) over-refraction is required and is represented as the following formula calculation: K = BC + D + (ORcl – SEQnocl), where BC = base curve of CL used, D = diopter power of CL used, OR = over-refraction with CL, and SEQ = manifest spheroequivalent without CL. The contact lens method and clinical history method are available on the ASCRS website.

#### Current-generation formulae, intraoperative aberrometry

More current-generation formulae are also available on the ASCRS IOL calculator website and include double-K Holladay, Shammas-PL and Haigis L. The ASCRS calculator will give weighted averages of the results of these later-generation formulae.

My personal preference is use of the Haigis L formula because it is conveniently already part of the IOLMaster (Carl Zeiss Meditec) software used in all my cataract patients to determine axial length. The advantage of Haigis L and other later-generation formulae is the lack of the need for clinical history or contact lens over-refractions. Haigis L is based only on whether a patient had prior myopic or hyperopic laser vision correction. Most patients by history or corneal topography imaging will yield whether the patient had prior myopic or hyperopic laser vision correction. Haigis L is not indicated in post-RK patients. Haigis L avoids using corneal power readings to predict postoperative effective lens position and will give predictable outcomes even in the absence of intraoperative aberrometry.

One final technique I use in addition to Haigis L is the use of intraoperative aberrometry (ORA, Alcon). The algorithms established with this technology, especially in post-PRK or post-LASIK patients, have been very accurate; also, post-RK (four incision and eight incision) algorithms were recently added to the ORA armamentarium. Intraoperative aberrometry accounts for the posterior corneal astigmatism effects published by Doug Koch as well in cases in which astigmatism management is critical.

In summary, as the premium refractive cataract surgeon can imagine, conquering the visual outcome and IOL power calculation in this ever-increasing post-refractive cataract surgery population can be daunting. When the target is hit accurately, resulting in a happy patient and surgeon, the Order of Merit may just be granted to us as a living honorary member.

Disclosure: Jackson reports he is a consultant for Carl Zeiss Meditec.