Consideration of the posterior corneal curvature for assessment of corneal power after myopic LASIK

Purpose: To evaluate the effect of a separate measurement of the anterior and posterior corneal surface to calculate the total refractive power of the cornea after myopic laser in situ keratomileusis (LASIK). Methods: A total of 39 eyes of 21 patients (aged 33 ± 9 years) were included in this prospective, non-randomized, comparative study. These involved 19 myopic corrections (-3.5 ± 1.6 dioptres) and 23 refractive corrections of myopic astigmatism (sphere: -3.7 ± 1.6 D, cylinder: -1.2 ± 0.4 D). All procedures were accomplished with the Keratom II®. Coherent-Schwind excimer laser and the Moria Model One® microkeratome (150 μm head) at the Medical Education Centre, La Trinidad, Caracas, Venezuela. Subjective refractometry, Bausch & Lomb® keratometry and Orbscan® slit-scanning corneal topography analysis were performed before and 3 months after LASIK. Corneal power was assessed directly using keratometry (K1) and Orbscan videokeratography (T1). Corneal power was calculated using the preoperative keratometric (K2, 'gold standard', clinical history method) or topographic power (T2, clinical history method) and spherical equivalent change. A composite value was derived from the Orbscan anterior and posterior surface power and central pachymetry (T3). Results: Three months postoperatively, corneal power ranged in a descending order from T1 (42.33 ± 1.78 D), K1 (40.82 ± 2.20 D), K2 (40.42 ± 2.36 D), T2 (40.03 ± 2.51 D) to T3 (38.78 ± 2.23 D). On average, T1 exceeded the gold standard by 1.9 D and the gold standard exceeded T3 by 1.6 D. K2, T1, T2 and T3 correlated significantly with K1 (r = 0.975, p < 0.001; r = 0.909, p < 0.001; r = 0.963, p < 0.001; r = 0.853, p < 0.001, respectively). The differences T1-K2 (r = -0.699, p < 0.001) and T3-K2 (r = -0.499, p = 0.001) correlated highly inversely and K1-K2 correlated borderline inversely (r = -0.325, p = 0.043) with the intended refractive correction. Conclusion: After myopic LASIK, refractive corneal power is overestimated by direct keratometric and especially videokeratoscopic measurements. The higher the intended refractive correction, the greater is this error. A separate measurement of both refractive surfaces of the cornea tends to underestimate but may enhance accuracy of the total refractive corneal power if the history of the patient is unknown.