BESST II IOL POWER CALCULATOR                                    

 
 
How accurate was BESSt 1.0 Formula compared to other techniques?
In our original published case series involving eyes undergoing phakoemulsification following laser refractive surgery, the target refractions calculated with BESSt Formula were statistically significantly closer to the post-operative manifest refractions than those calculated with other methods.

Mean deviatios between Target and Achieved refractions after phako: 
BESSt Formula (0.08 D±0.62 SD)
Clinical History Method(1) (-0.07 D ±1.92 SD; p=0.05)
Clinical History Method(1) with Double-K adjustment1 (0.13 D ±2.39 SD; p=0.05)
Holladay 2 Formula(2) using K-values from the Hard Contact-Lens over-refraction Method(3) (-0.76 D ±1.36 SD; p=0.03)
Holladay 2 Formula(2) using K-values from the Atlas™ Topographer, Carl Zeiss Meditec Inc™ (-0.55 D ±0.61 SD; p<0.01).

Notes: (1) The Clinical History Method and the Double-K adjustment both require pre-keratorefractive surgery information (i.e. refraction and K values) in order to perform the calculations; (2) The Holladay 2 Formula (Holladay IOL Consultant program, Holladay Consulting Inc, Bell Air, TX) seems to provide unprecedented level of accuracy in post-kerato refractive surgery eyes. However, it requires the knowledge of pre-operative Refraction, horizontal White-To-White distance (WTW), phakic ACD and phakic Lens Thickness in order to perform the calculations; (3) The Hard Contact lens over-refraction method is time consuming and often inaccurate.

  
Figure 1. Mean Deviation between Target Refractions calculated with various techniques and the Actual Postoperative refractions after phako+IOL in eyes which previously underwent keratorefractive surgery for myopia or hyperopia. The zero line represents a perfect match between the Target and the Achieved post-phako refraction. The numbers represent the mean deviations.

Legend: *= indicates a statistically significant difference (p<0.05) between the current technique and the BESSt© Formula; Conf. Interv. (C.I.) =confidence interval; Abs. Mean=absolute mean; d-K SRKT= History technique with double-K SRK/T adjustment; History= History technique; Holl.2 CL =contact lens over-refraction technique using Holladay 2 formula; GOF=Gaussian optics formula; Holl.2 Topo = Holladay 2 formula using central K values provided by Atlas™ Corneal Topography (Carl Zeiss Meditec, Inc.).

BESSt Formula provided the greatest accuracy compared to the other formulae analysed. Using the Holladay 2 Formula with K-values taken from the Atlas Topographer, the mean postoperative refraction was -0.76 D more myopic than the target refraction, compared to an error of just 0.08 D using BESSt Formula.




Figure 2. Mean Absolute Deviation between Target Refractions calculated with various techniques and the Actual Postoperative refractions after phako+IOL in eyes which previously underwent keratorefractive surgery for myopia or hyperopia. Standard Deviations (SD) are reported in brackets.

BESSt Formula provided the greatest accuracy with an Absolute SD nearly as half as the one of the Holladay 2 Formula with K-vlaues from the Atlas topographer (±0.26 versus ±0.49 D SD, respectively).

Proportion of eyes <±0.50 D of the intended post-operative refraction:
BESSt Formula (46%)
Clinical History Method (23%)
Clinical History Method with Double-K adjustment (31%)
Holladay 2 Formula using K-values from the Hard Contact-Lens over-refraction Method (39%)
Holladay 2 Formula using K-values from the Atlas™ Topographer, Carl Zeiss Meditec Inc™ (54%)
Proportion of eyes <±0.75 D of the intended post-operative refraction:
BESSt Formula (77%)
Clinical History Method (23%)
Clinical History Method with Double-K adjustment (38%)
Holladay 2 Formula using K-values from the Hard Contact-Lens over-refraction Method (39%)
Holladay 2 Formula using K-values from the Atlas™ Topographer, Carl Zeiss Meditec Inc™ (61%) 

Proportion of eyes <±1.00 D of the intended post-operative refraction:
BESSt Formula (100%)
Clinical History Method (31%)
Clinical History Method with Double-K adjustment (38%)
Holladay 2 Formula using K-values from the Hard Contact-Lens over-refraction Method (61%)
Holladay 2 Formula using K-values from the Atlas™ Topographer, Carl Zeiss Meditec Inc™ (69%)

Proportion of eyes >±1.50 D of the intended post-operative refraction:
BESSt Formula (0%)
Holladay 2 Formula using K-values from the Atlas™ Topographer, Carl Zeiss Meditec Inc™ (8%)

In summary:
BESSt 1.0 Formula was statistically significantly more accurate than the above described techniques without requiring any pre-refractive surgery information. In our practice it has significantly improved IOL power calculation accuracy and reduced the SDs in eyes after laser keratorefractive surgery and is an extremely valuable tool especially in those cases when pre-refractive surgery data is unavailable. 

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