The Alma Method to Calculate Intraocular Lens Power After Corneal Refractive Surgery

THE ALMA METHOD TO CALCULATE INTRAOCULAR LENS POWER AFTER CORNEAL REFRACTIVE SURGERY Nicola Rosa MD e mail: [email protected]

To calculate the power of the intraocular lens to be implanted in eyes that underwent corneal refractive surgery, namely RK, PRK and LASIK is a great challenge 1.

In 2002 we published a theoretical study2 where we described a corneal radius correcting factor, axial length (AL) related, to be used in these cases. Later on3 we named it R factor, to point out that it has to be multiplied by the corneal radius and not by the K readings.

This method required neither the knowledge of the preoperative parameters or the amount of treatment and this is a very important issue, as regrettably, these parameters are often unavailable. The power of the IOL to be implanted was calculated modifying the corneal radius according to the following theoretical regression formula: R = 0.0276 *AL + 0.3635 Where R is the corneal radius correcting factor and AL is the axial length. To convert the k readings in corneal radius we utilize the formula: r = n-1/k Where r is the corneal radius, n= 1.3375, and k is the mean kearometric reading We multiplied this value by the correcting factor, and the obtained value was converted again in diopters and inserted in the SRK T formula to calculate the IOL power to be implanted. For example, one of our patient presented with an AL of 28.06 mm and a mean K reading of 35.75 D (a corneal radius of 9.44 mm), inserting the AL value in the formula, we calculated the corneal radius correcting factor which allow us to obtain the new corneal radius of 10.74 mm (mean K = 31.42D) which was then inserted in the SRKT formula to calculate the IOL power to be implanted. Utilizing the correcting factor a + 21.5 D IOL was implanted with a resulting final refraction of +0.5. Without the correcting factor the power of the IOL to be implanted would have been +17 D leading to a final refraction of +3.65.

Utilizing this formula, we found that most of the patients came out myopic and in particular those with long AL the refractive outcome was too myopic. The reason for this is that in this method the R factor is related to the AL as, it is assumed that the correction is related to the AL and that the longer the AL, the greater the amount of treatment required. This makes sense because it has been shown that in a normal population, there is a highly significant negative correlation between refraction and AL.4

However, this might not always be true because in some patients, a partial treatment could be planned (eg, to correct anisometropia) or undercorrection or overcorrection occurs. For this reason we found a regression formula to calculate the amount of refractive error obtained after corneal refractive surgery, independent of cataract onset 5: Refractive error = -0.0157 x (AL x K) +16.437.

This new method, that we called ALMA, requires the use of the R factor together with the SRKT formula to calculate the IOL power, but if we plan to get emmetropia, we should not point to emmetropia, but to the opposite of the refractive error we calculated with this new formula (e.g. if the calculated refractive error is -5, we should point to a refractive error of +5 to get emmetropia). This new method improves significantly the results, in fact if we compare our results with those that, according to the most recent literature 6 , should be obtained in eyes that did not undergo refractive surgery, we can see that the results are quite similar. Moreover this method can be used not only in patients that underwent PRK, LASIK, LASEK, but also after radial keratotomy. In the light of the results presented in this paper, our study suggests that this factor could be reliably used , even if further study on larger series of patients, comparing the different methods are needed to show which of these will be more valuable.

ACKNOWLEDGEMENT

The following surgeons who used our method and gave us stabilized refractive results, so making this study possible need to be thanked: Avitabile T. MD; Avvisati M. MD, Bifani M. MD; Bongiorno V. MD, Buratto L. MD; Capasso L. MD, Cerbella R. MD; Ciampa N. MD, Coda T. MD, Costantino A, MD, Cotticelli L. MD, Crisci T. MD; Cusati G. MD; Della Corte M. MD; Di Nardo C. MD; Falco L. MD; Giambrone S. MD, Giugno S. MD, Hughes M. MD; Iuliano G. MD; Kołodziejczyk W. MD. Mancino S. MD; Mastursi B. MD; Mazzilli E. MD, Montesarchio A. MD; Orfeo V. MD; Paolercio F. MD; Pascotto F. MD; Pellegrino F. MD; Piciocchi S. MD; Polisena P. MD Sabetti L. MD; Sammartino A. MD; Savastano S. MD; Sbordone M. MD, Scorcia V. MD, Stanislawska A. MD; Stella A. MD; Teramo P. MD, Thielsen T. MD; Toni F. MD; Tortori A. MD; Trotta E. MD; Vardaro A. MD,Vastarella P. MD, Zeppa L. MD..

REFERENCES 1)Langenbucher A, Haigis W, Seitz B: Difficult lens power calculation. Curr Opin Ophthalmol 2004; 15, 1-9 2)Rosa N, Capasso L, Romano A. A new method of calculating intraocular lens power after PRK. J Refract Surg 2002;18:720-724. 3) Rosa N, Capasso L, Lanza M, Iaccarino G, Romano A. Reliability of a new correcting factor in calculating intraocular lens power after refractive corneal surgery. J Cataract Refract Surg 2005;31:1020-1024 4) Olsen T, Arnarsson A, Sasaki H, Sasaki K, Jonasson F. On the ocular refractive components: the Reykjavik Eye Study. Acta Ophthalmol Scand 2007; 85:361–366 5) Rosa N, MD, De Bernardo, M, Borrelli M , Lanza M, : New factor to improve reliability of the clinical history method for intraocular lens power calculation after refractive surgery J Cataract Refract Surg 2010; 36:2123–2128. 6) Olsen T : Improved accuracy of intraocular lens power calculation with Zeiss IOL Master. Acta Ophthalmol Scand 2007; 85:84-87.