# Laser Vision Correction for Regular Myopia and Supracor Presbyopia: A Comparison Study
## 4 Discussion  
In this study, femto-LASIK refractive surgical treatments were performed in two groups of study populations. While one of the groups received a standard myopia treatment, the second group was treated with a specific multifocal presbyLASIK algorithm to treat presbyopia. Study data, such as uncorrected visual acuity (UCVA), were carefully analyzed and presented as study results. Figure 3: Comparing planned to achieved shape change of anterior corneal surface. Presbyopia treatments (**b**) showed a better match (R\({}^{2}\) = 0.93 and slope 0.67) than (**a**) myopia (R\({}^{2}\) = 0.45 and slope 0.47). In almost all the cases (>93%), UCVA was improved after the surgery, with respect to the baseline assessment before the treatment. Thereby, the standard femto-LASIK myopia treatment showed slightly better results than the presbyLASIK treatment, where outcome UCVA was lower. This most likely arises from the fact that the subjective tolerance to eye surgical treatments including multifocal optics is greater than in monofocal treatments. Even a small deviation of 0.5D from the target value can be disturbing for the patient. Accordingly, there was a small but significant trend in recent results, where higher correction of presbyLASIK lead to lower postoperative UCVA. No similar trend was observed in the standard myopia treatment cases, where even high corrections of up to \(-\)10.5D had perfect vision with UCVA result of decimal 1.0. The completely positive UCVA outcome was achieved, although the results suggest that astigmatism was drastically undertreated. Even though the laser was programmed with astigmatism correcting ablation patterns for all cases with treatable preoperative astigmatism, significant postoperative corneal astigmatism was observed in the topography measurements of both study groups. This observation is not supported by estimates of apparent refraction, where the cylinder of astigmatism decreased from \(-\)0.80D \(\pm\) 0.79 (SD) to \(-\)0.18 D \(\pm\) 0.33 (SD) (\(p\) < 0.00001). Thus, it could be assumed that the topography measurements were not accurate enough, but we did not have additional data to examine this in more details. After surgical treatment, the mechanical strength of the cornea depends only on the residual stroma. It is well known that when the residual stroma is too thin to handle IOP, both its anterior and posterior surface will bulge forward as in ectasia. Nowadays, residual stroma thickness (RST) is accepted to be at least 250 \(\upmu\)m. Conventionally, thickness less than 480 is considered abnormal, while some surgeons consider preoperative corneal thickness less than 500 \(\upmu\)m unsuitable for LASIK. While planning the treatments, 250 \(\upmu\)m of the RST was suggested as it accounts for 50% of 500 \(\upmu\)m, which is the minimum corneal thickness recommended for LASIK. Also, high percentage of tissue altered (PTA) is the most common cause of ectasia after LASIK in patients with normal preoperative corneal topography. According to [PERSON] et al. [13], high values of PTA, especially higher than 40% is important factor in the development post-Lasik ectasia with normal preoperative corneal topography. For that reasonPTA should be taken as one of the screening parameter for refractive surgery candidates. In addition to this criterion, we have aimed at a postoperative thickness of 300 \(\upmu\)m residual cornea stroma or more for all cases to allow re-treatment if necessary. None of the eyes in this study developed postoperative ectasia. The observation, that postoperative central corneal curvature was steeper than planned comes from the fact that the algorithm used for the multifocality add-on causes a central steepening within the 3.0 mm zone. This is because the central zone is intended to be used for near vision. The 3.0 to 6.0 mm zone is adjusted to correct intermediate and distance vision. Both eyes thereby are treated the same way, except that the non-dominant eye is targeted at \(-\)0.5D from plano.