New formula allows, simplifies pseudophakic IOL power calculation

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/ (Figure 1).

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."