In this prospective study, 35 patients who had bilateral LASEK were followed for 8 years to assess refractive stability and changes in ocular biometric parameters in long-term follow-up in patients with low to moderate myopia.
Previous studies showed variable results regarding the efficacies of LASIK and LASEK for the correction of myopia.1–3
In a twelve-year follow up study in LASIK for moderate to high myopia, Ikeda et al. found a refractive regression of 0.74 ± 0.99D from 3 months to 12 years postoperatively. 53% and 75% of the eyes were within 0.5 and 1.0 diopter respectively and they found a significant correlation of refractive regression with the changes in keratometric readings, but not with the changes in CCT and amount of regression from 3 months to 12 years. The safety index was 1.09 ± 0.21,12 years after the surgery.4
In a long term observational case series conducted by O'Brart et al. to evaluate the 20 year efficacy and safety of PRK, all eyes underwent − 3.00 or -6.00 diopter corrections. The efficacy index at 20 years was 0.49, and the safety index was 0.97.5
Due to the promising reliability of the optical biometer we tested its accuracy in detecting biometric changes after LASEK. The advantage of optical biometers over ultrasound is being more precise non-contact measurements. Lenstar LS900 is the first optical biometer, which uses the precision of optical low-coherence reflectometry for all its measurements, including ACD and LT.10
Reports in literature regarding the change in biometric parameters of the eye are diverse and in most of them reassessment have been performed in short-term after refractive ablation.
In present study mean postoperative AL at 8 years was 24.9 ± 0.92 mm in comparison with AL of 24.92 ± 0.96 mm preoperatively, showing no significant change during follow up period(p = .664).
In 22 eyes of 11 patients CCT was measured at 6 months post LASEK; they had a mean of 467.05 ± 32.247 µm, which was significantly less than the mean value at 8 years (476.94 ± 29.94 µm)(P˂.001). Although, we had no epithelial map for these patients but it could be contributed to epithelial remodeling hyperplasia during follow up period.
In their retrospective study, Fu D et al. evaluated the refractive regression and changes in CCT following LASEK for high myopia in 76 eyes with thin corneas. At 3 years after the procedure, CCT was significantly greater than the measurements at 3 months post LASEK (with mean change of 40.46 ± 14.02 µm ).Moreover, there was no significant change in mean AL compared with preoperative values.6
Von Mohrenfels et al. conducted the first study on measuring the AL before and 1 month after LASEK. In their study they showed a statistically significant difference in AL 1 month after refractive surgery for myopia.7
In their publication Rosa et al. revealed that IOLMaster does not correlate well with theoretical ablation depth after myopic PRK .8
Another study to evaluate the relationship between programmed ablation depth and AL change after LASIK with IOLMaster (Carl Zeiss Meditec, Dublin, CA) demonstrated that 1 µm increase in ablation depth resulted in 0.00118 ± 0.00005 mm decrease in AL. At one month follow up ,mean AL was less than preoperative measurement (25.11 ± 0.14 mm vs. 25.20 ± 0.14 mm respectively ,P˂ .001).9
In present study at 8 years the mean values of flat ,steep and mean keratometries had a significant increase from postoperative values at 6 month (P˂.001). Nevertheless, we must point that; these changes in refractive parameters were not significant in terms of manifest refraction from 6 months to 8 years (Table 2).
In assessment of refractive stability and safety after PRK for low to moderate myopia by O'Connor et al., 67.0% of eyes achieved a UCDVA of ≥ 20/20 and the percentage of eyes within 0.5D was 62.1%(36 of 58 eyes) at 12 years follow up .11
O'Brart et al. measured preoperative and twenty years postoperative biometric data using ultrasound and and partial coherence interferometric( IOL Master) respectively. At 20 years, they showed a statistically significant increase in AL of 0.84 ± 0.43 mm (P˂.0001). AL increased by a mean of 0.84 mm. Furthermore, there was a statistically significant decrease in ACD of -0.42 ± 0.68 mm (P˂.02). They explained the myopic shift as a consequence of growth in eye's AL, rather than to regression of correction at surface of the cornea.5
Nevertheless, it is known that AL measurements by partial coherence interferometric biometry are mostly more precise than those measured by ultrasound.12
In another study in 10 eyes underwent LASIK to treat myopic refractive errors ranging from − 2.50 to -8.00 diopters of SE (mean:-5.23 ± 1.30), preoperative and postoperative AL measurements using IOL Master showed a change in AL from mean 25.80 ± 1.01 mm to 25.68 ± 0.93 mm ,one month after surgery ,showing a decrease in AL that did not have a good correlation with theoretical ablation depth.13
In Rajan et al. Study to evaluate long term refractive stability for myopic PRK, they reported stability of refraction at 12 years ,with no significant change in mean SE between 1, 6, and 12 years and no late regression in long term.14
Other studies of myopic refractive surgeries have also shown mild regression in long term.15,16
In another report for PRK to treat low to moderate myopia by Guerin et al., they showed a slight regression over 16 years. At 2 years, the mean SE was − 0.25 D and at 16 years, − 0.58 D.17
Ivarsen et al., in a randomized study of PRK and LASIK in high myopia, reported stabilization of corneal power from 1–7 years after PRK but not after LASIK. No significant changes were observed from 1 to 7 years after surgery, and no significant correlation was found between changes in corneal power and spherical equivalent refraction (Pearson's r = 0.00; P = .99) or CCT (Pearsons's r = − 0.31; P = .41).18
A limitation of current study is the small number of patients. Although, initially 240 eyes of 120 patients were included in this cohort, however ,35 patients who were underwent bilateral LASEK attended follow up visits in 8 years. Second limitation is that we did not include patients with high degrees of myopia to compare possible effects of higher ablation depth on myopic regression and associated keratometric and refractive changes. Third, there is a problem that resides in the anterior chamber depth measurements. Lenstar measures ACD from the epithelium to the anterior lens surface. Having done excimer ablation would lead to false impression of drop of ACD; although, aqueous depth measurements similarly revealed a significant decrease in comparison to pre-operative values. The extended follow up (8 years) in this study, enabled analysis of biometric changes including keratometry, pachymetry, ACD, aqueous depth, LT and AL in long term to provide sufficient data regarding refractive stability.