Dokument

Summary:
Introduction Corneal refractive surgery reshapes the cornea in order to change its refractive power and correct refractive errors such as myopia, hyperopia and astigmatism. An important factor associated with the postoperative quality of vision is the centration of the treatment zone. A treatment zone that is decentered in relation to the visual axis may affect the functional corneal morphology and therefore the quality of the visual outcome after the treatment. Functional deficits, such as reduced corrected distance and near visual acuity, irregular astigmatism, halos, glare, reduced contrast sensitivity and monocular diplopia are associated with decentered treatment zones, even in cases of subclinical decentration (<1.0 mm). Purpose An argument often expressed by refractive surgeons is that in small incision lenticule extraction (SMILE) a precise centration cannot be guaranteed due to the subjective intraoperative alignment and lack of eye tracking. Therefore, the purpose of this study is to investigate and compare the centration of the treatment zone between eyes treated with SMILE and active eye-tracker assisted femtosecond laser-assisted LASIK (FS-LASIK), and evaluate the pattern of the achieved centration. Methods In the present retrospective study, 69 myopic eyes of 36 patients who underwent SMILE were compared to 69 myopic eyes of 36 patients treated with FS-LASIK. All procedures were performed at the Department of Ophthalmology, Philipps University of Marburg, by a single surgeon using the VisuMax® femtosecond-laser and MEL-80® excimer-laser (Carl Zeiss Meditec AG, Jena, Germany). Pentacam (Oculus Optikgeräte GmbH, Wetzlar, Germany) was used for preoperative and 3-month postoperative topography and pachymetry. The centration of the treatment zone was estimated by the distance of the point of the maximum pachymetric difference (PMPD) on the corneal thickness differential map from the coaxially sighted corneal light reflex (CSCLR; reference point of centration of the treatment zone in SMILE) and the topographic centre of the entrance pupil (EPC; reference point of centration of the treatment zone in FS-LASIK). The distribution of angle K (angular distance between visual and pupillary axis) was assessed preoperatively in both groups by depicting the exact location of the CSCLR in relation to the EPC. The pattern of the achieved centration was evaluated by depicting on a Cartesian plane the location of the PMPD in relation to the EPC. The pattern of the achieved centration was compared to the preoperative pattern of angle K (pattern of the preoperative CSCLR in relation to the EPC). Results In SMILE group, the mean decentration of the treatment zone from the EPC was 0.326 ± 0.196 mm, ranging from 0.014 to 1.062 mm, whereas the centration of the treatment zone demonstrated a nasalization pattern. In FS-LASIK group, the mean decentration of the treatment zone from the EPC was 0.452 ± 0.224 mm, ranging from 0.02 to 1.040 mm, whereas the centration of the treatment zone demonstrated a random pattern. In relation to the CSCLR, the decentration in SMILE was 0.315 ± 0.211 mm, ranging from 0.0 to 1.131 mm, whereas FS-LASIK eyes demonstrated a mean decentration of 0.516 ± 0.254 mm, ranging from 0.103 to 1.265 mm. The decentration from the reference point of its technique (CSCLR in SMILE; EPC in FS-LASIK) was significantly more extended in FS-LASIK group (P < 0.001). The evaluation of angle K based on the location of the preoperative CSCLR in relation to the EPC showed in SMILE group 32 right eyes and 24 left eyes with positive angle K, 2 right eyes and 11 left eyes with negative angle K, and no eyes with 0° angle K. In FS-LASIK group, there were 29 right eyes and 22 left eyes with positive angle K, 3 right eyes and 14 left eyes with negative angle K, and 1 right eye with 0° angle K. In both groups, the location of the preoperative CSCLR demonstrated a nasalization pattern. In SMILE group, the mean distance of the point corresponding to the preoperative CSCLR from the EPC was 0.227 ± 0.121 mm, ranging from 0.014 to 0.602 mm, and in FS-LASIK group was 0.206 ± 0.097 mm, ranging from 0.045 to 0.457 mm. Statistical analysis showed no significant difference between the two groups (P = 0.201). After SMILE, the achieved centration followed the preoperative pattern of angle K in 52 out of 69 eyes (75.36%), whereas only 32 of 69 eyes (46.37%) followed that pattern after FS-LASIK (P < 0.001). Conclusions The centration of the treatment zone as evaluated on corneal thickness differential maps was better for patient-controlled fixation during SMILE compared to active eye tracker-assisted FS-LASIK. Moreover, the results suggest that centring the refractive procedure on the CSCLR (as in SMILE), results in a more natural outcome, which follows the preoperative pattern of angle K as opposed to centring the refractive procedure on the EPC (as in FS-LASIK).

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