Comparison of clinical results between trans-PRK and femtosecond LASIK for correction of high myopia

The efficacy and safety of corneal refractive surgery has improved since excimer laser photorefractive keratectomy was introduced to treat refractive error in the human eye. New surgical techniques such as corneal lamellar surgery, femtosecond assisted laser in situ keratomileusis (FS-LASIK) and small incision lenticule extraction (SMILE) have emerged as the preferred procedures because of rapid visual recovery, less pain, and less corneal haze [1]. However, LASIK had some adverse outcomes, including intraoperative and late flap-related complications, corneal biomechanical instability, and iatrogenic keratectasia that were more likely to occur in high myopia [2‐4]. Corneal surface laser ablation had advantages of not needing to make a flap and greater stability of postoperative corneal biomechanics than LASIK [5]. Compared with LASIK for correcting high myopia, surface ablation retained more corneal stromal tissue, thus avoiding the potential risk of keratectasia [6].

In recent years, surface ablation techniques have improved. A new procedure, transepithelial photorefractive keratectomy (TPRK), was introduced as an alternative to conventional PRK. This avoided the need for alcohol epithelial debridement or mechanical removal of the epithelium during PRK [7, 8]. TPRK requires only one-step removal of the epithelium and stroma, has no instrument contact with the cornea, takes less surgical time, less postoperative pain, faster wound healing and faster visual recovery than conventional PRK [9].

Studies have shown that TPRK, PRK or LASEK are efficient and safe methods to correct low and moderate myopia [10, 11]. However, for high myopia, the stability and predictability of correction may be reduced. Larger stromal ablation is required causing more extensive wound healing [12, 13]. Correction of high myopia remains a difficult challenge for both corneal lamellar and surface ablation, Furthermore, whether lamellar surgery or surface ablation provides the better outcome remains inconclusive.

The aim of this prospective, non-randomised, cohort study is to compare 12-months clinical outcomes of predictability, safety, efficacy and corneal high order aberration, using TPRK with FS-LASIK for high myopia.

We commenced with 48 patients with high myopia in the TPRK group and 45 patients in FS-LASIK group. Two patients in the TPRK group and three patients in the FS-LASIK group were excluded after loss to the follow-up at 12 months. Finally, 85 eyes in 46 patients receiving TPRK and 80 eyes in 42 patients receiving FS-LASIK were included in the analysis (Table 1). There were no significant differences between the two groups in terms of preoperative variables, including age, gender percentage, UDVA, CDVA, sphere, spherical equivalent refraction, IOP, CCT, flat meridian curvature, steep meridian curvature, optical zone (P > 0.05, except for CCT P = 0.05).

At postoperative 1 month, UDVA in TPRK group was equal to that in the FS-LASIK group. At final follow-up, UDVA was better in the TPRK group than the FS-LASIK group, whereas there was no difference in CDVA between groups. In our study, the efficacy and safety of both procedures was comparable, and no eyes showed 2 or more lines worse CDVA postoperatively, compared to preoperative CDVA. The postoperative UDVA and CDVA in both groups were acceptable, but when compared with other studies, percentage of UDVA achieving 20/40 or better was 95.4 to 100% in TPRK group [14‐16] and the percentage was 88.2 to 99% in other FS-LASIK groups [17‐20]. Comparisons of visual outcomes between TPRK and FS-LASIK group in our studies were similar to two others that described treatment for high myopia. Ghadhfan [16] found that in eyes with high myopia, transepithelial PRK provided better visual outcomes than LASIK, LASEK, or mechanical epithelial removal PRK. LASIK was associated with most major postoperative complications. Aslanides [15] found that TPRK for correction of high myopia demonstrated comparable refractive outcomes to LASIK and PRK, with relatively favorable visual acuity outcomes. Contrary to those studies and ours, Gershoni [20] reported that clinical outcomes of FS-LASIK were slightly better than those of TPRK. Another study compared the results of FS-LASIK and PRK for the correction of high myopia and found that FS-LASIK improved UDVA better than PRK [21].

Our efficacy and safety indices in both groups (all more than 1.00) were superior to those of previous studies that corrected myopia. Gershoni [20] reported that efficacy index values were 0.92 in their TPRK group and 0.95 in their FS-LASIK group. Corresponding safety index values were 0.95 and 0.97. Hashemi [22] found the efficacy indices of 0.99 and 0.93 in FS-LASIK and PRK group, respectively, and safety indices of 1.01 and 1.01, respectively. The difference in outcomes may be due to the smart pulse technology used by Amaris 750S excimer laser, which improved residual bed smoothness and reduced irregularity. Vinciguerra [23] found that excimer laser coupled with smart pulse technology led to improvement of 6-month uncorrected visual acuity, compared with use of conventional techniques. The smart pulse technology software features a particular characterization of the ablative spot geometry. This avoids the thermal load and ablation effect of pulse energy not effectively applied in the ablation process [24].

For surface ablation, the use of mitomycin could surely reduce the possibility of haze formation, especially when correcting high myopia. However, in our study we did not use mitomycin in TPRK group. There was no haze above 0.5 level in the TPRK group at the last follow-up. We assumed that it was due to improved surface smoothness and less ineffective energy invested in the ablation process induced by the smart pulse technology. After laser ablation, using cold balanced salt solution is also beneficial, because it could lessen the thermal effect of laser ablation, which in turn decreases formation of corneal haze. Furthermore, after surgery all patients were treated with 0.1% fluoromethane drops for 4 months and strictly followed up every month. If necessary, short-term 1% prednisone acetate drops were used to inhibit haze formation. All patients were also required to wear sunglasses to prevent ultraviolet rays which could increase the incidence of haze.

At postoperative 12 months, we found refractive predictability to be higher in the TPRK group than the FS-LASIK group. Postoperative refraction was never more than ±1.00 D in the TPRK group. Six eyes were outside ±1.00 D in the FS-LASIK group, where UDVA was decreased. The percentage of eyes within ±0.50D was higher than that in Gershoni’s study [20], but lower than that in Aslanides’ study [15]. Refractive stability was also better in the TPRK group. During postoperative follow-up, the spherical equivalent refraction changed from + 0.11 D to − 0.05 D in the TPRK group and + 0.08 D to − 0.26 D in the FS-LASIK group. At 12 months, refractive status was more minus in the FS-LASIK group than the TPRK group, with some undercorrection observed in the FS-LASIK group. In Luger’s study [25], small overcorrection was also observed in the eyes that had TPRK, and some undercorrection was observed in the eyes that had FS-LASIK. Better UDVA in the TPRK group might have occurred because of slight overcorrection. Unlike spherical refraction, changes in refractive astigmatism were similar in both groups. The large reduction in astigmatism at postoperative 1 month was similar in both groups and remained unchanged until postoperative 12 months. TPRK and FS-LASIK were both efficient and relatively safe procedures for the correction of astigmatism.

In this study, the corneal RMS HOA increased after surgery in both groups, the amount of change was larger in the FS-LASIK group at 12 months postoperative. Another difference was vertical coma values, which were higher in the FS-LASIK group, compared with the TPRK group. Changes of spherical aberration and horizontal coma were similar in both groups. There was a corneal flap created in the FS-LASIK group, but there was no incision in the TPRK group, so corneal integrity was maintained. The flap made with the hinge at the superior location in this study may explain why the vertical coma and corneal HOA were higher in the FS-LASIK group. Similar findings [22, 26‐28] reported that horizontal coma was induced when the flap was made on the nasal side and vertical coma was induced with the flap at the superior location.

The main limitations of our study were a relatively short-term follow-up of 12 months and some omission of visual quality data (such as contrast sensitivity). A longer follow-up period is necessary for comprehensive evaluation of visual acuity. Although we found that HOA changed, especially vertical coma and spherical aberration, it remained unclear how HOA increase influenced quality of vision. HOA was related to perception of shadows, halos and night vision glare, and a reduction in contrast sensitivity. Visual quality might better reflect the clinical outcomes compared with visual acuity, but a reliable metric for quality must be established. We believed that due to no haze formation and smaller coma, the contrast sensitivity of TPRK group may be better than that of FS-LASIK group. This needs further research to confirm.

The current study found that TPRK and FS-LASIK is safe, efficacious, and predictable for correction of high myopia. TPRK are slightly better than FS-LASIK.