BESST II IOL POWER CALCULATOR                                    

 
 
What is BESSt Formula?
BESSt is a biometry formula designed to better estimate true corneal and IOL power following laser keratorefractive surgery for Myopia, Hypermetropia or Astigmatism. It should not be used for non-excimer laser refractive surgery cases (such as Conductive Keratoplasty, Astigmatic Keratotomy, Limbal Relaxing Incisions or other types of refractive surgery).

The formula has been included in a Microsoft Windows™ compatible Software program called BESSt II IOL Power Calculator, which allows more accurate calculation of the IOL power required for cataract (or clear lens) surgery following laser refractive surgery compared to the methods currently available. In the Download Area you can view a Slide Presentation with the results of the formula.

What is the scientific basis behind it?
BESSt Formula was developed on the basis of the outcomes of 170 eyes which underwent laser refractive surgery for Myopia and Hyperopia and is based on an improved version of the Thick Lens Equation (or Gaussian Optics Formula).

The findings have been published on the December issue of the Journal of Cataract and Refractive Surgery (JCRS).










Advantages of BESSt Formula over other techniques:
These are the major advantages of BESSt Formula over other techniques used to estimate corneal power after laser refractive surgery:

More accurate estimation of corneal/IOL power after refractive surgery.
Significantly smaller Standard Deviations with less risk of a refractive  surprises after cataract surgery.
 No need of any pre-refractive surgery information.
Automatic detection of SRK/T formula’s negative square root and cusp phenomena in eyes with steep corneas.
BESSt II: an improved algorithm is finally available!
Since its first release in 2006, we have worked hard trying to improve what was already one of the most accurate formulas available, and we are now very proud to announce that a new and more accurate algorithm is finally available. BESSt 2.0 allows greater accuracy and smaller SDs in both Myopic and Hyperopic laser-treated eyes compared to BESSt 1.0. In post-Myopic treatments, BESSt 2.0 is statistically significantly more accurate than the Haigis-L formula (p: 0.007) and in post-Hypermetropic treatments the new formula is 1.10 D more accurate than BESSt 1.0 (p: 0.002), further reducing the risk of refractive surprises. The new software release includes the new algorithm and an amazing new feature which shows real-time IOL power plots for any desired biometry formula. Learn more about BESSt 2.0.

The results have been presented at several International Meetings:
 ASCRS Annual Meeting, San Francisco, Mar 2006
 ESCRS XXIV Congress, London, Sept 2006
 AAO Refractive Surgery Subspecialty Day, Las Vegas, Nov 2006 
 Moorfields Eye Hospital Alumni Meeting, London, Jan 2007
 ASIA-ARVO, Singapore, Mar 2007
 ASCRS-ASOA, San Diego, Apr 2007
 XLII Congr. Polish Society Ophthalmol, Bydgoszcz, June 2007 
 Moorfields Eye Hospital Dubai Meeting, Dubai, Aug 2007
 Dubai Health Care City Meeting, Dubai, Apr 2008
 WOC Congress, Hong Kong, July 2008
 ESCRS XXVI Meeting, Berlin, Sept 2008
 APAO-AAO, Bali, May 2009
 ASCRS Annual Meeting, Boston, Apr 2010
 Next: ESCRS Congress, Berlin, June 2010 
What do I need to use BESSt 2.0 Formula?
First of all you need to install the latest version of the software on your PC. This is available in the Download Area of the website. Then you’ll need an Oculus Pentacam Scheimpflug camera (either standard or HR model). 

The Oculus Pentacam™ (website)










From the Pentacam scans you will need to retrieve the Anterior and Posterior mean radii of curvature and the Central Corneal Thickness of the laser-treated eye (with the Direct Import version this is done automatically). You’ll find these values in the Pentacam’s Refractive View (fields highlighted in blue in the image below). 

Pentacam’s Refractive View












If you know that the eye has undergone a laser treatment for Myopia or Hyperopia, please choose accordingly from the drop-down menu so that the new BESSt 2.0 algorithm can be used for greater accuracy. In fact BESSt 2.0 is actually TWO separate algorithms, one for Myopia and one for Hypermetropia). If you are unsure about the refractive status of the eye, just choose the appropriate option from the menu so that the old (BESSt 1.0) algorithm will be used. BESSt 2.0 can provide more accurate results when the Spherical Aberration of the eye is known. You can retrieve this information from the Zernike Analysis View of the Pentacam. 

Pentacam’s Zernike Analysis View












The Spherical Aberration value corresponds to the Z (m=0; n=4) value, as highlighted in the image above. If the aberration function is not available on your Pentacam unit, then please enter 0 (zero) in the Spherical Aberration field. WARNING: if you are not sure about this value you must enter zero (0) as a wrong value can greatly affect the BESSt 2 IOL power. We advise never entering values >2.50 or < -2.50. 



The OCULUS Pentacam™ images the anterior segment of the eye by a rotating Scheimpflug camera. Anterior and Posterior radius of curvature and Corneal Thickness are measured from over 25,000 true elevation points acquired from digitally-processed slit-images.

Can I use radii of curvature taken from other devices (e.g. the Orbscan)?
Corneal curvature measurements taken with other instruments are not supported and may lead to serious refractive errors as BESSt formula originates from very fine tuning and regression analysis based on measurements taken only with the Oculus Pentacam.
Axial Length (AL) measurements:
For IOL power calculation, a value for Axial Length must be entered. This should ideally be taken with the IOL Master (Carl Zeiss Meditec™, Germany) for grater accuracy.
Standard or Customized A-Constants?
Finally, the desired post-operative Refraction and the A-Constant of the IOL that you want to implant must be entered. For better accuracy we advise to always use optimised A-Constants values rather than the labelled ones. 

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  1. Borasio E, Stevens J, Smith GT

  2. Estimation of true corneal power after keratorefractive surgery in eyes requiring cataract surgery: BESSt formula.


  3. J Cataract Refract Surgery.

  4. 2006 Dec; 32(12):2004-14. PMID:17137976


  5. BESSt Formula JCRS Paper