Dear Sir,
In this regard, we wish to clarify the following points:
Normality tests were applied using one-sample Kolmogorov-Smirnov test. Data were found to be normally distributed including the sub-group (P < 0.05)
The primary factors in lens calculation accuracy are often thought to be primarily due to biometry issues: If the axial length and keratometry values are accurately measured, then we should be able to calculate the best power intraocular lens (IOL).[1]
Effective lens position: The Hoffer Q, the Holladay 1 and the SRK/T are two-variable theoretical formulae that indirectly determine the effective lens position. If the eye has a longer axial length and a steep K measurement, it is likely to have a deeper anterior chamber, and the IOL is assumed to have a more posterior effective lens position.[1]
To overcome this issue, Jack Holladay, MD, created a new seven-variable formula, the Holladay 2, which takes into account the standard axial length and keratometry measurements, but also incorporates anterior chamber depth, lens thickness, white-to-white size, patient refraction and age.[1]
Lens calculations in extreme eyes require more attention to detail, and newer multivariable formulae such as the Holladay 2 may provide a higher level of accuracy.[1]
But in a study by Ghanem and El-Sayed reported that in patients with high axial myopia, the performance of SRK-T, Hoffer Q, Holladay 2, and Haigis formulas are comparable in low plus powered IOL implantation and Haigis formula was the best formula when minus power IOL is implanted.[2]
In a study by Narváez et al., which compared the accuracy of IOL power calculations using 4 formulas: Hoffer Q, Holladay 1, Holladay 2, and SRK/T found no formula was more accurate than the others as measured by mean absolute error.[3]
In a study by Stopyra, analyzed various formulas in axial length ranged between 24.51 and 26.72 mm found Holladay 1 as the best with (88.5%) achieved full visual acuity (1.0 on Snellen chart) after cataract surgery followed by SRK/T - 63.9%, Hoffer O - 36.1%, Binkhorst - 34.4%, Haigis - 11.5%, SRK II - 8.2%.[4]
In studies by Narváez et al., Ghanem and El-Sayed and Stopyra even the performance of newer generations formula like Holladay 2 and Haigis was not found superior.[2,3,4]
So with these limitations and no conclusive data regarding IOL power calculation in Indian myopic patients we did analyze four formulas and came up with our results of Holladay 1 being the most accurate. We did not include Holladay 2 and Haigis formula in our study.
We statistically analyzed mean error (ME) with all the formulas
With each formula, ME was calculated from the difference between the formula predicted refractive error and actual postoperative refractive error. The smallest ME indicated the formula with the best predictive accuracy. In literature, various other studies have used ME for analyzing formulas regarding IOL power calculation accuracy.[5]
References
- 1.Devgan U. IOL Calculations in Extreme Eyes Require Meticulous Attention. Ocular Surgery News. [Last cited on 2009 Apr 25]. Available from: http://www.healio.com/ophthalmology/cataract.surgery/news/print/ocular-surgery-news .
- 2.Ghanem AA, El-Sayed HM. Accuracy of intraocular lens power calculation in high myopia. Oman J Ophthalmol. 2010;3:126–30. doi: 10.4103/0974-620X.71888. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Narváez J, Zimmerman G, Stulting RD, Chang DH. Accuracy of intraocular lens power prediction using the Hoffer Q, Holladay 1, Holladay 2, and SRK/T formulas. J Cataract Refract Surg. 2006;32:2050–3. doi: 10.1016/j.jcrs.2006.09.009. [DOI] [PubMed] [Google Scholar]
- 4.Stopyra W. The accuracy of IOL power calculation formulas for eyes of axial length exceeding 24.5 mm. Klin Oczna. 2013;115:93–5. [PubMed] [Google Scholar]
- 5.El-Nafees R, Moawad A, Kishk H, Gaafar W. Intra-ocular lens power calculation in patients with high axial myopia before cataract surgery. Saudi J Ophthalmol. 2010;24:77–80. doi: 10.1016/j.sjopt.2010.03.006. [DOI] [PMC free article] [PubMed] [Google Scholar]