Stability of refractive outcome over the 9-month follow-up period in the initial hyperopic study (Reprinted from Blum et al. )
19.1.2 The Second (Ongoing) Hyperopic FLEx Study
An improved lenticule shape was developed (Fig. 19.2) that had identical optical properties within the optical zone as in the original study, but with a number of differences mainly relating to the creation of a dedicated transition zone. The key features of the larger transition zone were based on the positive long-term experience with hyperopic ablation profiles of the MEL 80 excimer laser [2, 3]. The transition zone was selected for each case individually, according to the corneal curvature, lenticule optical zone diameter, and the dioptric power of treatment (Fig. 19.2).
Diagram of the improved lenticule shape used in the present study. Note a large transition zone and the minimal lenticule thickness at the edge and in the centre
However, the total extent of the optical zone and transition zone is limited by the size of the VisuMax® contact glass being used, as the contact glass is applied to the cornea for suction. Therefore, in order to maximize the total lenticule diameter, we used a medium (M)-size contact glass in all cases rather than a small (S)-size contact glass as in the majority of eyes in the previous study (similarly to myopic treatments). In order to obtain treatment zones as large as possible, the clearance between the edge of the lenticule and the edge of the flap was reduced to as low as 0.5 mm, compared to the 1 mm clearance that was used in the first study. When planning the lenticule and flap dimensions, the corneal diameter was also considered; as the treatments were centred on the corneal vertex, the flap diameter was confirmed to be within the diameter of the cornea in cases with an angle kappa, where the centre of the treatment would not be aligned with the centre of the cornea.
The other change between the two studies was that the pulse frequency of the VisuMax® had been increased from 200 to 500 kHz. This increase in pulse frequency meant that the new lenticule shape (with increased optical zone, transition zone, and flap diameters) could be performed without increasing the total treatment time and therefore did not affect safety in relation to the risk of suction loss.
Because of the above-mentioned complication in the first study, where one eye developed a central buttonhole in the lenticule , we paid a lot of attention to the question of the central minimum lenticule thickness. After extensive ex vivo experiments with pig eyes and human corneas not eligible for corneal transplantation, this value was set at 25 μm in all eyes without any exception, rather than sometimes using 20 μm in the first study.
The Ethics Committees recommended that the study be divided into a pilot study with ten eyes (initial spherical cohort) to be followed by a larger study of 40 eyes (second, spherocylindrical cohort) only after the 9-month data had been reported for the first treatment group. Therefore, only the results of the nine eyes of five patients of the pilot study are available at present (N.B. one patient was emmetropic in one eye).
Patients’ average age at the time of surgery was 55.5 (range: 46–63) years. One patient (two eyes) was male and the other four patients were female. The mean preoperative manifest spherical equivalent refraction (SE) was +1.82 ± 0.56 D (range: +1.25 to +2.75 D) with mean preoperative sphere of +1.89 ± 0.59 D (range: +1.25 to +3.00 D) and mean astigmatism of −0.14 ± 0.18 D (range: 0 to −0.50 D). As all patients were of presbyopic age, an overcorrection was intended in all cases: mean target SE was −0.86 ± 0.41D (range −1.25 to 0.00 D) such that mean attempted SE was +2.69 ± 0.39 D (range: +2.25 to +3.50 D).
The mean flap diameter was 8.46 ± 0.09 mm (range: 8.4–8.6 mm), the optical zone was 5.75 mm for all eyes, and the mean transition zone was 2.02 ± 0.14 mm (range: 1.78–2.29 mm). The femtosecond laser energy was adjusted between 180 and 160 nJ during the study with a fixed laser spot and track distance of 4.5 μm.
At 1 month, 3 months, 6 months, and 9 months, 33 %, 67 %, 22 %, and 22 % of eyes were within 0.50 D of intended correction, and 78 % of eyes were within 1.00 D of target refraction. The analysis of attempted versus achieved refraction over time shows a regression between month 3 and month 9 of approximately 0.50 D in all but two eyes which were overcorrected. There were neither decentrations nor any other adverse effects (Fig. 19.3). Despite a mean refractive undercorrection of approximately 0.17 D, all patients were happy with the achieved results, simply because we purposely aimed at low myopia and, in this way, managed to bring all eyes into a comfortable zone for the presbyopic age (Fig. 19.4). As planned, larger cohorts will be treated in 2015.