[Orbscan II-assisted intraocular lens power calculation after LASIK].

OBJECTIVE: To evaluate the accuracy of keratometric value derived from one zone of Orbscan II mean power map after Laser in situ keratomileusis (LASIK) in combination with Holladay IOL Consultant software to calculate the intraocular lens (IOL) power. METHODS: A two-part study was conducted at a referral practice. Part 1 was a prospective study of 118 eyes undergoing LASIK. The changes in Orbscan II mean power maps at four central zones (1.5, 2.0, 2.5 and 3.0 mm) were compared with the cornea power calculated from pre-LASIK data to determine the optimum Orbscan II correlation zone. In Part 2, the power of optimum measured by Orbscan II after LASIK was applied to IOL calculations for 62 eyes undergoing LASIK. And the results were compared with the IOL power calculated by the pre-LASIK data. RESULTS: (1) An analysis at the Orbscan II 1.5 mm measurement zone demonstrated an underestimation of net cornea power after LASIK while the 3.0 mm zones demonstrated an overestimation. The 2.0 mm and 2.5 mm zones best approximated the net cornea power calculated from pre-LASIK data; (2) The cornea power at 2.5 mm from Orbscan II was selected for IOL calculations in combination with Holladay IOL Consultant software Holladay II and HofferQ formula. The refractive error calculated by Holladay II and HofferQ formula were (0.47 ± 0.75) D and (0.52 ± 0.83) D versus the IOL power calculated by clinical history method. The refractive errors of two formula within ± 0.50 D were 48.4% and 43.5% and within ± 1.0 D 80.6% and 74.2%. CONCLUSIONS: The cornea power from 2.5 mm Orbscan II zone after LASIK in combination with Holladay II or HofferQ formula can accurately predict the IOL power for cataract surgery.

[Estimation of preoperative corneal power in patients with myopic LASIK based on postoperative corneal data measured from Orbscan II].

OBJECTIVE: Compare the change of pre- and post-operative posterior corneal curvature measurements in peripheral fitting zones using the Orbscan II topographer in patients undergoing myopic laser in situ keratomileusis (LASIK). To establish a new ratio of the anterior and posterior corneal curvature for the calculation of preoperative total corneal power using postoperative corneal data alone in patients after LASIK. METHODS: Retrospectively analyzed the changes of posterior corneal curvature in peripheral 7 to 10 mm fitting zones in 151 right eyes (151 individuals) 3 month after LASIK as measured by Orbscan II. The new ratio between the anterior and posterior corneal curvature was measured. The new ratio was used to estimate pre-operative total corneal refractive power in 30 eyes. The estimated total corneal refractive power was compared with the value measured from Orbscan II preoperatively. The accuracy of intraocular lens power estimated in 2 cases underwent cataract surgery after corneal refractive surgery was studied. RESULTS: The mean difference in estimated pre- and post-operative power of the posterior cornea was (-0.005 ± 0.154) mm (t = 0.417, P = 0.677). The ratio between the preoperative anterior central (0 - 7 mm) corneal curvature and the posterior peripheral (7 to 10 mm) corneal curvature was 1.167 ± 0.030. Estimated mean pre-operative total corneal power was (43.49 ± 1.79) D, whereas the value measuring from Orbscan II preoperatively was (43.77 ± 1.53) D. The mean difference between these two measurements was (-0.28 ± 1.00) D (t = -1.523, P = 0.139). The spherical equivalent refraction after cataract extraction of the two cases undertaken cataract surgery after corneal refractive surgery was 0.54 D and 0.69 D, respectively. CONCLUSIONS: The difference in posterior corneal curvature measurement following myopic LASIK using the peripheral fitting zone with the Orbscan II, as compared to the preoperative values, is clinically insignificant. The pre-LASIK corneal power can be estimated using post-LASIK data together with the new calculated ratio. This will be useful in post-LASIK patients requiring cataract surgery but without the availability of pre-LASIK corneal data for the estimation of the intraocular lens power.