In their study of the accuracy of intraocular lens (IOL) power calculations in eyes having phacotrabeculectomy for macular hole, Patel et al.1 failed to address certain issues before drawing conclusions. The accuracy of formulas in predicting IOL power depends on the estimated lens position, also known as estimated postoperative anterior chamber depth (ACD).2 The SRK/T formula3 used by the authors is based on the thin lens optics and relies on a single A-constant to determine ACD. The actual ACD measurement is not taken into account. Formulas such as Haigis4 and Holladay 2 (Holladay LASIK Institute, Houston, Texas, USA) incorporate ACD measurements in the calculations. As a result, with Haigis and Holladay 2 formulas, a difference in measurement of the optical ACD would give a different IOL power, even when axial length and keratometry measurements remained the same. Since Patel et al. hypothesized that myopic overcorrection after phacovitrectomy might be a result of the gas bubble causing forward displacement of the capsular bag, the authors should reconsider the use of the SRK/T formula in these patients.
Even for the SRK/T formula, surgeons should optimize the A-constant in patients having phacovitrectomy rather than aim for residual hyperopia to counteract the overcorrection. Also, if surgeons perform biometry in these patients preoperatively and postoperatively, it will assist in determining whether the myopic shift is caused by forward displacement of the capsular bag by the gas bubble or by inaccuracy in the axial length measurement caused by fixation problems because of the macular hole.
Finally, although the authors claim that the achieved refractions in their patients are comparable to those after phacoemulsification alone, the available data suggest that much better accuracy can be obtained with optimization. Using optimized constants, Eleftheriadis5 found that with the Holladay 2 formula and the SRK/T formula, 96% of patients and 95% of patients, respectively, achieved a refraction within ±1.0 diopter of the targeted refraction. Depending on which IOL is used, with the optimized Haigis formula, 93% of the patients could achieve a refraction within ±1.0 diopter.2
1. Patel D, Rahman R, Kumarasamy M. Accuracy of intraocular lens power estimation in eyes having phacovitrectomy for macular holes. J Cataract Refract Surg. 2007;33:1760-1762.
2. Shammas HJ, editor. Intraocular Lens Power Calculations. Thorofare, NJ: Slack; 2003.
3. Retzlaff J, Sanders DR, Kraff MC. Development of the SRK/T intraocular lens implant power calculation formula. J Cataract Refract Surg. 1990;16:333-340. correction, 528.
4. Haigis W. IOL calculation according to Haigis. Available at: http://www.augenklinik.uni-wuerzburg.de/uslab/ioltxt/haie.htm
. Accessed January 11, 2008
5. Eleftheriadis H. IOLMaster biometry: refractive results of 100 consecutive cases. Br J Ophthalmol. 2003;87:960-963.