الفهرس 
Anatomy of the corneaOnly 14 pages are availabe for public view
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Abstract
Excimer laser refractive surgery is the most commonly used operation for correcting myopia. This refractive surgery uses excimer laser to remove corneal tissue and to reshape the cornea, changing its refractive power. These refractive procedures are divided into two main groups: laser assisted in-situ keratomileusis (LASIK) and surface treatments.
Laser in situ keratomileusis (LASIK) has become the refractive procedure of choice for moderate to high myopic corrections because of the fast visual recovery making it more practical , the painless early postoperative period, and the rare occurrence of subepithelial haze.
On the other hand, LASIK exposes the eye to intra- and postoperative flap-related complications including inadequate intraoperative flaps, flap folds, ectasia, that’s why it’s understandable why many refractive surgeons would begin to favor surface ablation to preserve corneal tissue in the hope of reducing the incidence of post-LASIK ectasia. Moreover, eyes with insufficient residual corneal stromal thickness are not candidates for LASIK.
Also it has been well documented that LASIK induces higher order aberrations; whereas lower order refractive errors such as hyperopia, myopia, and astigmatism are corrected successfully with highly predictable results.
Higher order aberrations induced during LASIK can be during any of its steps either flap creation or laser ablation itself, however studies performed proved with no doubt that lots of these aberrations are induced during flap creation, however, in a study that compared flaps created using femtosecond laser and mechanical microkeratome , changes in higher order aberrations after the flap were observed only for the mechanical microkeratome and not for the femtosecond laser . This study therefore imply that aberration induction depends on factors controlling flap creation during flap formation.
Sub-Bowman keratomileusis (SBK) technique on the other hand is a laser in situ keratomileusis (LASIK) procedure in which the flap is thinner ranging between 90 µm and 110 µm in thickness.
A major advantage of creating a thin flap during SBK is leaving sufficient stromal tissue to allow safer excimer laser ablation, especially in patients with moderate or high myopia.
Recent retrospective studies evaluated the effect of thin flaps on the outcomes of LASIK one, three and six months after surgery and proposed that intended thin flaps may be advantageous over thicker flaps for myopic LASIK, studies supporting such data mentioned before as in those conducted by Prandi et al on 2004 and Cobo-Soriano et al on 2005, all in favor of SBK.
These studies paved the way to SBK, which may combine the advantages of LASIK and surface ablation.
The role of photorefractive keratectomy (PRK) has been reappraised because of insufficient preoperative corneal thickness, corneal instability, and the trend towards greater higher order aberrations in LASIK procedures.
In general, PRK with MMC eyes has showed better contrast sensitivity values than LASIK eyes. Many authors reported a decrease in contrast sensitivity and glare after conventional LASIK and PRK and some authors believe that higher order aberrations are responsible for decreased contrast sensitivity postoperatively and here comes a large part of our study importance where it’s proved here that PRK has better contrast sensitivity postoperative than preoperatively.
Studies reported that wavefront-guided surgeries and the modulation of the healing response in PRK with MMC eyes contributed to the better postoperative contrast sensitivity results. Customized ablation results in less increase in higher order aberrations than conventional photoablation.
Tuan on 2006 Studied patient satisfaction with wavefront-guided LASIK and PRK, it was reported that postoperative refractive error, postoperative UCVA, and mesopic contrast sensitivity function were predictors of postoperative satisfaction, PRK with MMC eyes tended to be rated higher than LASIK eyes in terms of visual satisfaction and postoperative visual symptoms.
Our study contributes to this research road as well by conducting this wide based comparison, for as despite all these previous studies, we find only few ones who had comprehensively compared the three groups of the most widely performed laser ablations now.
During our analysis we came to the conclusion that certain points are now well established, no debate needed, LASIK induces higher order aberrations, mainly during flap creation, this is less whenever femtosecond laser is being used for flap formation , any procedure including flap formation whether LASIK or SBK totally changes human aberration profile increasing coma and spherical aberrations in particular.
Surface ablations on the other hand change aberrations in term of increasing spherical aberrations mainly with less influence on coma.
The point to be mentioned here is the importance of contrast sensitivity in assessing postoperative patient’s satisfaction as quality of vision is highly influenced by contrast sensitivity as much as aberration profile.
We found here multiple changes in contrast sensitivity values but mainly is that levels were better for the LASIK group, but this wasn’t taken much into account as LASIK group had better preoperative contrast sensitivity levels but more importantly is the comparison between sensitivity levels of each group pre and postoperatively.
It was found that postoperative contrast sensitivity levels showed no significant changes between pre and post operative levels whereas SBK showed a decline in contrast sensitivity and PRK showed an improvement of such results.
And by this only one fact remains different refractive solutions surpass each other in different aspects and so we can only reach the ultimate option and the perfect refractive procedure by continued research work and endless number of studies pursuing the best choice for our patients, passing by such a stage of decision making, which way to go.