Abstract
Purpose: :
To compare the ratio of keratometric change (ΔK) to refractive change (ΔSE) induced by refractive laser ablation and to propose an equation to predict the postoperative K prior to surgery.
Methods: :
The charts of 2738 eyes were reviewed of patients that underwent laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) between March 2007 and March 2010. One-thousand and twenty eyes with ΔSE of 0 to 2.99 D (Low ΔSE) were compared with 1253 eyes with ΔSE of 3.00 to 5.99 D (Moderate ΔSE) and with 465 eyes with ΔSE greater than 6.00 diopters (High ΔSE). Statistical analysis was performed using an independent student t-test of equal variance and a best-fit equation was produced from all ΔK/ΔSE data.
Results: :
Significant differences were found in the ratio ΔK/ΔSE between eyes with Low ΔSE vs. Moderate ΔSE (0.98 ± 0.60 vs. 0.80 ± 0.17, p=1.02 x 10-25), Moderate ΔSE vs. High ΔSE (0.80 ± 0.17 vs. 0.77 ± 0.11, p=1.01 x 10-14), and Low ΔSE vs. High ΔSE groups respectively (0.98 ± 0.60 vs. 0.77 ± 0.11, p=4.55 x10-4). The equation that best predicts the postoperative K prior to surgery was found to be K=x-(y*z), where x=preoperative K, y = -0.264*ln(attempted ΔSE) + 1.1788, and z=attempted ΔSE.
Conclusions: :
The change in simulated K required to achieve a diopter of myopic refractive correction decreases as the amount of refractive change increases, is more variable with lower amounts of correction, more constant with higher amounts of correction, and follows a non-linear relationship.
Keywords: refractive surgery • refractive surgery: LASIK • refractive surgery: PRK