COMPARISON OF IOL POWER CALCULATION METHODS AND INTRAOPERATIVE WAVEFRONT ABERROMETER IN EYES AFTER REFRACTIVE SURGERY

Canto AP, Chhadva P, Cabot F, et al¹

ABSTRACT SUMMARY

Canto et al compared the predictive accuracy of the ORange intraoperative wavefront aberrometer (now ORA System; WaveTec Vision) with the SRK-T formula with IOLMaster keratometry (Carl Zeiss Meditec), SRK-T formula with corneal topography central keratometry, and the American Society of Cataract and Refractive Surgery (ASCRS) online calculator for eyes that previously underwent refractive surgery. This was a retrospective study of 46 eyes that had undergone myopic PRK, myopic or hyperopic LASIK, or radial keratotomy. For each method, the investigators evaluated the difference between predicted and actual lens power for emmetropia.

The researchers found the ORange to be within 0.50 and 1.00 D of emmetropia more frequently than the other methods. The device predicted IOL power to within 0.50 D for 37% of cases, the IOLMaster keratometry 30%, topographic keratometry 26%, and the ASCRS calculator 17%. When subdivided for previous myopic treatment, prediction within 0.50 D was seen in 39%, 27%, 24%, and 18%, respectively. For eyes that had undergone radial keratotomy, the ORange was less accurate than IOLMaster keratometry (14% vs 43% within 0.50 D).

DISCUSSION

Cataract surgery is increasingly becoming a refractive procedure, and patient expectations are already high. Patients with a history of refractive surgery typically come to the office with even higher expectations, and they assume that cataract surgery offers the same precision and results as their laser refractive surgery did. Whereas LASIK has a prediction accuracy of 91% within 0.50 D and 100% within 1.00 D,² cataract surgery in eyes with a history of myopic refractive surgery offers a level of precision of up to 58% within 0.50 D and 90% within 1.00 D.³ These are extremely low numbers and far from ideal. Intraoperative wavefront aberrometry can increase predictive accuracy by capturing and measuring the entire optical system. It accounts for anterior and posterior corneal curvatures, axial length, and the effect of cataract incision. Intraoperative wavefront aberrometry is performed while the eye is aphakic, without any potential artifact from the cataract.

Canto et al are the first group to publish results specifically on the ORange with postrefractive surgery patients. The device improved postoperative outcomes by making the IOL power calculation more accurate. Although the investigators noted the superior accuracy of the ORange, no single method achieved a prediction within 0.50 D of emmetropia more than 50% of the time.

When the ORange prediction was not at emmetropia, it tended to leave the patient with more postoperative hyperopia, whereas the ASCRS calculator tended to leave the patient with more postoperative myopia. By having these two calculations available simultaneously when selecting an IOL, surgeons can elect to take a middleground approach for an overall balance toward emmetropia.

This study is limited due to its retrospective design and the small number of eyes in each group. It is also unclear if there was a single surgeon or multiple surgeons; surgical technique can influence the effective lens position, and the technique of performing the ORange measurement can also be affected by wound hydration and placement of the speculum.⁴ The ORange has since been upgraded to the ORA with VerifEye, which includes a sharper light source, aspheric optics, and current algorithms that may further improve predictive accuracy for eyes that have undergone refractive surgery.

INTRAOPERATIVE REFRACTIVE BIOMETRY FOR PREDICTING INTRAOCULAR LENS POWER CALCULATION AFTER PRIOR MYOPIC REFRACTIVE SURGERY

Ianchulev T, Hoffer KJ, Yoo SH, et al⁵

ABSTRACT SUMMARY

Ianchulev et al performed a retrospective study of 215 consecutive patients at the time of cataract surgery who had previously undergone myopic laser refractive surgery. The study compared the ORA intraoperative aberrometer with the surgeon’s best preoperative choice (determined by the surgeon with all available data), Haigis L method, and Shammas method. The median absolute error of prediction within 0.50 and 1.00 D was analyzed for each method.

The ORA provided the highest predicted accuracy of the compared methods after myopic refractive surgery. Of the 246 eyes included in the study, 67% of the device’s predicted errors were within 0.50 D, and 94% were within 1.00 D. The median error was 0.35 D for the ORA, which was significantly lower than that of the other methods (P<.0001): 0.60 D for the surgeon’s best choice, 0.53 D for the Haigis L method, and 0.51 D for the Shammas method. The ORA calculation was chosen or influenced the lens power selection in 68% of cases. The lens power originally selected agreed with the ORA’s selection in only 13%.

DISCUSSION

Ianchulev et al published the largest series assessing the predictive accuracy of the ORA intraoperative aberrometer and one of the largest studies examining power calculations after prior myopic refractive surgery. The results provide additional evidence of the benefit of and improvement in accuracy provided by this device in eyes that have undergone myopic refractive surgery. The ORA was more accurate than the surgeon’s best choice (this method was not well described or characterized) and the ASCRS calculator for the Haigis L and Shammas methods.

The study yielded a higher level of accuracy than the previously discussed study. These results are even comparable to those of cataract surgery on eyes that have not undergone refractive surgery.⁶ However, one limitation of the study is its large number of investigators (more than 60) without a unified protocol, as some physicians used the IOLMaster, whereas others used the Lenstar (Haag-Streit) for preoperative measurements. These loose criteria, however, also produced results that are more applicable to average ophthalmologic practices that may be considering using an ORA.

The unit significantly improved outcomes but still is not a perfect tool; the algorithms are constantly being refined as more data are obtained. The better outcomes likely result from the device’s ability to capture a perfect aphakic refraction. The limitation lies in the ORA’s ability to predict the effective lens position based upon its measurements and the preoperative measurements provided. One could expect the device’s results to continue to improve over time.

Jeffrey Liu, MD, is a fellow in cornea, external disease, and refractive surgery at the Gavin Herbert Eye Institute at the University of California, Irvine. He states that he has no financial interest in the products or companies mentioned. Dr. Liu may be reached at tel: +1 949 824 2020; e-mail: jeffrey.liu@uci.edu.

Marjan Farid, MD, is the Director of Cornea, Refractive, and Cataract Surgery; Vice-Chair of Ophthalmic Faculty; Codirector of the Cornea Fellowship Program; and an Associate Professor of Ophthalmology at the Gavin Herbert Eye Institute at the University of California, Irvine. She states that she has no financial interest in the products or companies mentioned. Dr. Farid may be reached at tel: +1 949 824 2020; e-mail: mfarid@ uci.edu.

1. Canto AP, Chhadva P, Cabot F, et al. Comparison of IOL power calculation methods and intraoperative wavefront aberrometer in eyes after refractive surgery. J Refract Surg. 2013;29(7):484-489.
2. He L, Liu A, Manche EE. Wavefront-guided versus wavefront-optimized laser in situ keratomileusis for patients with myopia: a prospective randomized contralateral eye study. Am J Ophthalmol. 2014;157(6):1170-1178.e1.
3. Yang R, Yeh A, George MR, et al. Comparison of intraocular lens power calculation methods after myopic laser refractive surgery without previous refractive surgery data. J Cataract Refract Surg. 2013;39(9):1327-1335.
4. Stringham J, Pettey J, Olson RJ. Evaluation of variables affecting intraoperative aberrometry. J Cataract Refract Surg. 2012;38(3):470-474.
5. Ianchulev T, Hoffer KJ, Yoo SH, et al. Intraoperative refractive biometry for predicting intraocular lens power calculation after prior myopic refractive surgery. Ophthalmology. 2014;121(1):56-60.
6. Aristodemou P, Knox Cartwright NE, Sparrow JM, Johnston RL. Formula choice: Hoffer Q, Holladay 1, or SRK/T and refractive outcomes in 8108 eyes after cataract surgery with biometry by partial coherence interferometry. J Cataract Refract Surg. 2011;37(1):63-71.