Prior to surgery, the ophthalmologist discusses the refractive goals with the patient and determines the desired refractive result. This information with the IOL power calculation formulas can then be used to determine the power of the IOL to be implanted. It is important to make sure that the patient realizes that the formulas are not perfect. Despite extensive research to determine the optimal IOL calculation formulas, large case series show that significant percentages of postoperative results miss the refractive target by up to 0.50 D, while smaller percentages miss by over 1.00 D. The dependability of the calculations varies among subgroups of patients; the greatest challenges occur in postrefractive patients or patients with axial myopia or axial hyperopia. The Hoffer Q formula has been shown to be more reliable for eyes with a short AL, while the SRK/T and Haigis formulas have shown to be more reliable in eyes with a long AL. Fourth-generation formulas (eg, Holladay 2, Haigis, Barrett True-K, and Olsen) refine refractive results by using additional variables such as preoperative refraction and age, preoperative anterior chamber depth, and lens thickness to further improve ELP accuracy. The Hill-RBF (radial base function) method is unique in that its database is continually updated, helping its adaptive learning improve its accuracy. Version 2.0 has a database of over 12,000 eyes and has been optimized for IOLs from +30.00 D to −5.00 D (Fig 7-2).
Acknowledging that no single formula is perfect, many surgeons use multiple IOL calculations to narrow in on the best choice. The American Society of Cataract and Refractive Surgery (ASCRS) hosts a website (http://iolcalc.ascrs.org/) that allows surgeons to run several calculations simultaneously.
IOL calculations following refractive surgery
Calculating the IOL power for eyes that have undergone refractive surgery can present problems for both patients and surgeons. These patients were initially motivated to have refractive surgery because they did not want to be dependent on glasses, and they may have higher expectations regarding postoperative refractive results. It is important to inform patients who have undergone previous corneal refractive surgery about potential problems with IOL selection, as well as the potential for refractive surprise due to overcorrection or undercorrection. Refractive correction with glasses, contact lenses, refractive surgery, or IOL exchange may be required following the surgery. Documenting the discussion with the patient is extremely important.
As previously mentioned, determining the central keratometric power, a key element in lens power calculations, is complicated in eyes that have undergone previous refractive surgery because of the corneal change resulting from the original refractive procedure. In eyes in which radial keratotomy (RK) has been performed, a greater flattening of the posterior cornea relative to the anterior cornea can lead to an overestimation of corneal power, with hyperopic results. After myopic laser vision correction (LVC), conventional keratometry overestimates corneal power, with subsequent hyperopic results if adjustments are not made. The converse is true for patients with prior hyperopic LVC; in addition, the ELP (as calculated in many IOL power calculation formulas) may be incorrectly estimated.
The main challenge in calculating IOL power after refractive surgery is estimating or measuring the true posterior cornea refractive power. The challenge is compounded by the cornea change’s impact on ELP estimations. As previously discussed in the section Corneal Power, traditional calculation methods assume that the relationship between anterior and posterior curvatures is fixed. However, newer corneal topography/tomography systems measure both anterior and posterior corneal curvatures, thereby improving the accuracy of IOL power calculations. Intraoperative aberrometry, which relies not on keratometric power but rather on the total refractive error of the eye, is a good option for determining IOL power in post-LVC eyes.
A variety of methods have also been developed to better estimate central corneal power in eyes that have undergone refractive surgery. Initially, many of these methods used historical data (pre–laser in situ keratomileusis [LASIK]/photorefractive keratectomy [PRK] K-values and the surgically induced change in manifest reaction [ΔMR]). After the discovery that these historical data models were not highly accurate, their use declined; they are no longer routinely used and are no longer incorporated into web-based calculators. Recent studies have demonstrated that OCT-based IOL calculations and the Haigis-L and Barrett True-K No History formulas give the lowest rate of error, and that averaging results can further reduce error. Intraoperative aberrometry has been shown to have similar accuracy. However, the ideal method has not yet been determined. Because each method has advantages and disadvantages, the ophthalmic surgeon may wish to consider using more than 1 method or an average value to calculate corneal power. Selecting the highest IOL power from a tightly clustered group of estimations may help avoid an undercorrection result. The IOL calculator on the ASCRS website allows users to select IOL power for eyes that have previously undergone refractive surgery (http://iolcalc.ascrs.org). The Barrett True-K No History formula is available from the Asia-Pacific Association of Cataract & Refractive Surgeons (www.apacrs.org). See also BCSC Section 13, Refractive Surgery.
Fram NR, Masket S, Wang L. Comparison of intraoperative aberrometry, OCT-based IOL formula, Haigis-L, and Masket formulae for IOL power calculation after laser vision correction. Ophthalmology. 2015;122(6):1096–1101.
Wang L, Booth MA, Koch DD. Comparison of intraocular lens power calculation methods in eyes that have undergone LASIK. Ophthalmology. 2004;111(10):1825–1831.
Wang L, Tang M, Huang D, Weikert MP, Koch DD. Comparison of newer intraocular lens power calculation methods for eyes after corneal refractive surgery. Ophthalmology. 2015;122(12):2443–2449.
Excerpted from BCSC 2020-2021 series: Section 11 - Lens and Cataract. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.