New formula allows, simplifies pseudophakic IOL power calculation

Las Vegas-The BESSt formula allows pseudophakic IOL power calculation in postrefractive surgery eyes without any prerefractive surgery data and is significantly more accurate than a variety of other techniques, said Edmondo Borasio, MD, at the Refractive Surgery Subspecialty Day during the American Academy of Ophthalmology annual meeting.

Based on an improved version of the Gaussian Optics Formula (GOF) for Paraxial Imagery, the BESSt formula allows corneal power estimation using only corneal thickness and anterior and posterior corneal radii measurements obtained through corneal scanning with the rotating Scheimpflug camera anterior segment imaging system (Oculus, Pentacam).

Dr. Borasio, refractive surgery fellow, Moorfields Eye Hospital, London, developed the BESSt formula in collaboration with Julian Stevens, MD, and team from a data set of 170 myopic and hyperopic eyes undergoing wavefront-guided LASIK or LASEK for refractive errors between –10.0 and +4.5 D. Using regression analysis, the GOF variables were adjusted for closest fit with K values calculated from the clinical history method to derive the BESSt formula. This formula has been embedded in a software program for IOL power calculations (BESSt Corneal Power Calculator) that can be downloaded for free at http://www. besstformula.com.

“Topography and keratometry can provide inaccurate determinations of true refractive corneal power in eyes postrefractive surgery so that using data from those techniques for pseudophakic IOL power calculation can result in refractive surprises,” said Dr. Borasio. “Although a number of methods have been described to improve estimation of keratometric power in these eyes, we believe the ideal method should be based on direct corneal measurements without any need for preoperative information. The GOF calculates corneal power based on corneal anterior and posterior corneal radii, and information on posterior corneal radii is now available using the rotating Scheimpflug camera imaging system.”

Evaluation of BESSt formula

Performance of the BESSt formula in calculating corneal power in postrefractive surgery eyes was evaluated in a study involving 13 eyes with a prior history of refractive surgery (LASIK, PRK, epi-LASIK, and CK) for myopia (spherical equivalent [SE] –2.89 to –8.62 D) or hyperopia (SE +1.15 to +8.55 D) that subsequently underwent phacoemulsification.

Target refractions calculated using the BESSt formula corneal power were compared with the actual postoperative manifest refraction and the results were compared with those calculated using the clinical history method alone, the clinical history method with double K adjustment, and use of the Holladay 2 formula with K values derived from either hard contact lens overrefraction or corneal topography (Atlas, Carl Zeiss Meditec).

The mean ± SD difference between the target and achieved refraction using the BESSt formula was only 0.08 ± 0.62 D. Using the history technique, the mean difference from the actual postoperative refraction was –0.07 D, but the standard deviation (SD) was 1.92 D. For the other methods, the SDs were significantly greater, ranging between ±1.36 and ±2.39 D.

“Tighter standard deviations translate into a decreased risk of refractive surprises after cataract surgery. The standard deviation of the mean prediction error was similar comparing the BESSt formula with the Holladay 2 formula using the K values from the Atlas topographer, ±0.62 versus ±0.61 D, respectively,” Dr. Borasio said. “However, compared with all other methods, the standard deviation was 2- to 4-fold smaller with the BESSt formula, and it was also half of the standard deviation of the Holladay 2 formula with the Atlas K values in the analysis of mean absolute error, ±0.26 versus ±0.49 D, respectively. The Holladay 2 formula also requires refractive data as well as phakic anterior chamber depth (ACD), lens thickness, and white-to-white (WTW) distance measurements, which are not always available.”

Using the BESSt formula, 46% of eyes were within 0.50 D of intended postop refraction, 77% were within 0.75 D of target, and 100% were ±1.0 D. Using the Holladay 2 with the Atlas K value, 54% of eyes were ±0.50 D of intended refraction, but only 62% were ±0.75 D, and only 69% were ±1.00 D. Only between 31% and 62% of eyes would have been within 1.00 D of target using the corneal power information from the other methods for IOL power calculation.

“The BESSt formula appears to be an advance for significantly improving corneal and IOL power calculation accuracy in eyes after keratorefractive surgery,” Dr. Borasio explained.

The program also has an algorithm to detect automatically when IOL power calculation is inaccurate due to the “zero argument” or “cusp phenomena” that can occur using the SRK/T formula. These phenomena arise, Dr. Borasio explained, when the argument of the square root in the estimated anterior chamber depth (ACD) equation of the SRK/T formula becomes negative due to particular combinations of axial length (AL) and K values (typically in very steep corneas, such as in eyes postpenetrating keratoplasty).“The program will automatically identify those situations and advise to use alternative formulae to minimize further the risk of a refractive surprise,” he said.OT