An unfortunate irony of corneal refractive surgery is the difficulty it causes in the accurate calculation of intraocular lens (IOL) power. To date, most work has dealt with improving methods to accurately measure or calculate corneal refractive power.
In this issue, however, Aramberri (pages 2063–2068) introduces an issue that has been relatively neglected. He points out that corneal power values (which we will refer to as “corneal power”) are used in 2 ways in IOL calculation formulas: (1) in a vergence formula that calculates the refractive power of the eye and (2) as one of the values used for calculation of the effective lens position (ELP). In eyes that have had refractive surgery for the correction of myopia, use of the corneal power to calculate the ELP results in underestimation of the anterior chamber depth and selection of an IOL of insufficient power. Aramberri proposes the “double-K method,” in which the corneal power before refractive surgery is used to calculate the ELP, whereas corneal power after refractive surgery is used in the vergence formula. Applying this method to the SRK/T formula, he reports greatly improved accuracy and, in particular, documents the importance of the double-K method in reducing the likelihood of a hyperopic refractive surprise.
Aramberri's insight was recognized by Holladay in the development of the Holladay 2 formula. In this formula, one can enter the corneal power before refractive surgery for calculation of the ELP. If this value is not known, one can check the “previous RK” box, which will instruct the formula to use a corneal power of 44 diopters (D) to calculate the ELP.
For surgeons who are accustomed to using third-generation theoretical formulas, the “single-K method” (ie, using post refractive surgery corneal power in both portions of the formulas) will introduce the errors that Aramberri describes. Therefore, the double-K method is indicated when using these formulas. Third-generation and fourth-generation formulas do not use corneal power in a uniform fashion as part of the calculation of ELP. In the SRK/T formula, corneal power is used in 2 ways to calculate ELP: (1) to compute corneal width and (2) together with corneal width to estimate corneal height. In the Hoffer Q formula, corneal power is used in the ELP formula in the form of (tan K).21 In the Holladay 1 formula, the power of the cornea is used to calculate corneal height, which is used to estimate ELP. In the fourth-generation Holladay 2 formula, 7 factors including corneal power are taken into account in calculating ELP.
Surgeons with a working knowledge of these formulas can insert the appropriate values into the formulas to apply the double-K method, as Aramberri describes. However, as a short cut for surgeons who are less familiar with the formulas, we have prepared Tables 1 and Table 2 to assist them in modifying IOL power according to the double-K method. These tables assume a pre-refractive-surgery K-reading of 43 D and implantation of IOL model SA60AT (Alcon).
The effect of incorrect calculation of ELP on IOL power, which we call the ELP-related IOL prediction error, can be determined by subtracting the IOL power calculated using the standard formula from the power calculated using the double-K formula. In the tables, we investigated the ELP-related prediction error as a function of the formula used, axial length, and change in spherical equivalent induced by the refractive surgery.
Comparing the SRK/T, Hoffer Q, and Holladay 1 and Holladay 2 formulas, the single-K SRK/T formula produced the largest ELP-related prediction error, while the single-K Hoffer Q formula yielded the smallest. With all 4 formulas, the ELP-related prediction error increased with increasing amounts of refractive correction; standard formulas underestimated IOL power in eyes after myopic surgery and overestimated IOL power in eyes after hyperopic surgery. The ELP-related prediction error also changed with the axial length. The ELP-related prediction errors using the Hoffer Q formula are shown in Figure 1 as an example.
How should surgeons incorporate the double-K method into their practices?
- If at all possible, obtain the values for corneal power before refractive surgery.
- Insert these values into the Holladay 2 formula or, if knowledgeable about third-generation formulas, insert them directly into these formulas.
- Calculate corneal refractive power as described using the clinical history method or the modified topography approach (in which the effective refractive power [EffRP] from the EyeSys Corneal Analysis System is reduced by [0.15 × Δ spherical equivalent]; for example, if EffRP = 38 and Δ MR = 6, subtract [6 × 0.15] or 0.9 D from 38.0 D to get 37.1 D).1 Recently, we have had good results using an approach modified by Robert Maloney (personal communication, October 2002) in which the power in the center of the axial map of the Humphrey Atlas is inserted into the following formula: Corneal power = [(central topographic power) × 1.1] − 6.0]. Alternatively, one can use the method described by Feiz et al.2
- If the values for corneal power before refractive surgery are not known, use the Holladay 2 formula, checking the “previous RK” box, or for third-generation formulas, adjust the IOL power using Table 1 or Table 2, assuming that some information about the patient's refractive error before refractive surgery can be obtained.
- Warn patients of the relatively poor accuracy of IOL calculations in this setting and advise them that additional surgery may be required, including LASIK enhancement, IOL exchange, or piggyback IOLs.
1. Hamed AM, Wang L, Misra M, Koch DD. A comparative analysis of five methods of determining corneal refractive power in eyes that have undergone myopic laser in situ keratomileusis. Ophthalmology 2002; 109:651-658
2. Feiz V, Mannis MJ, Garcia-Ferrer F, et al. Intraocular lens power calculation after laser in situ keratomileusis for myopia and hyperopia: a standardized approach. Cornea 2001; 20:792-797