refractive surgery is one of the most commonly performed elective procedures and will likely maintain its popularity as ablation techniques become more refined and understanding of corneal wound healing improves . two of the most common methods of refractive surgery are photorefractive keratectomy ( prk ) and laser in situ keratomileusis ( lasik ) . the rapid improvement in vision and lack of postoperative pain associated with lasik has made this the preferred option with patients compared with prk , which has greater postoperative discomfort and prolonged recovery of visual acuity.1 recently , there has been renewed interest in prk because of increasing concerns of complications associated with lasik flap creation , including dry eye , corneal ectasia , and flap tears.25 thin - flap lasik attempts to gain benefits of both techniques by creating a flap of between 80 and 90 m.68 use of a thinner flap results in a more biomechanically stable cornea and decreases incidence of ectasia given the thicker residual stroma.3,9 cutting a thinner lasik flap is less invasive to the nerves within the corneal stroma , decreasing the severity and duration of dry eye , possibly by preserving corneal sensation and blinking rate.1014 flap creation avoids corneal epithelium removal , allowing reduced healing time and less haze and scarring.15 the present contralateral study compares the outcomes of eyes that have undergone prk or thin - flap lasik using the visx star s4 excimer laser ( visx incorporated , santa clara , ca ) , with flaps created with intended thicknesses of 90 m using the intralase fs60 femtosecond laser ( abbott medical optics [ amo ] , santa ana , ca ) . data from myopic eyes were analyzed , with or without astigmatism , in which the dominant eye was randomized ( research randomizer software urbaniak , www.randomizer.org ) to prk or thin - flap lasik ( 90 m flap ) and the nondominant eye underwent the alternative treatment . all prk and thin - flap lasik treatments were performed using the visx star s4 customvue laser at the john a. moran eye center , salt lake city , utah , between february 2008 and july 2009 . all patients included in this study met the us food and drug administration guidelines for visx customvue lasik . mean age of patient , 13 men and 13 women , was 30.8 years ( range : 2346 ) . twenty - six patients ( 52 eyes ) with stable myopia ( 1.58.5 diopters [ d ] ) and astigmatism ( 0.2423.11 d ) were enrolled in the study . eleven patients excluded from this study had clinically significant lens opacities , previous corneal or intraocular surgery , keratoconus , unstable refraction , autoimmune disease , immunosuppressive therapy , or were pregnant or breastfeeding . contact lenses were discontinued 2 weeks prior to screening for soft contact lens wearers and 6 weeks prior to screening for rigid gas permeable lens wearers . all patients had a preoperative examination including assessment of uncorrected distance visual acuity ( udva ) , corrected distance visual acuity ( cdva ) , tonometry , slitlamp examination of the anterior segment , and dilated fundus examination . manifest and cycloplegic refractions were repeated on 2 separate visits to ensure reliability and stability . corneal topography and thickness were measured using the orbscan ii v.3.0 ( bausch and lomb , rochester , ny ) . all eyes received 5 preoperative wavefront analyses with the visx customvue wavescan aberrometer v.3.62 ( fourier ) ( amo ) , without pharmacologic intervention , under mesopic conditions , with a minimum pupil diameter of 6.0 mm . the contralateral study design was made so that each eye could act as a control for the fellow eye in each patient , allowing for study groups to be well matched . there were no violations in the randomization ; all patients were analyzed as originally assigned . the randomization protocol was generated before the trial and known only to the study coordinator . in all patients , the emmetropic correction target was based on manifest refraction and wavefront analysis . all flaps were created with the intralase fs60 femtosecond laser at 60 khz in a raster pattern with bed energy of 1.15 j , side - cut energy of 2.00 j , and pocket enabled . the flaps were created with an intended thickness of 90 m , diameter of 8.4 to 9.0 mm , superior hinge angle of 55 , and a side - cut angle of 70. intraoperative pachymetry or optical coherence tomography were not performed to evaluate actual flap thicknesses . if the 8.0 mm maximum intended ablation diameter exceeded the flap diameter , the hinge and flap were shielded during ablation . postoperatively , each eye undergoing thin - flap lasik received 1 drop of gatifloxacin 0.3% ( zymar ; allergan inc , irvine , ca ) , prednisolone acetate 1% ( pred forte , allergan inc ) , ketorolac tromethamine 0.4% ( acular ls , allergan inc . ) , and a bandage soft contact lens ( softlens plano t , bausch and lomb , rochester , ny ) . the prednisolone acetate was continued hourly during the first preoperative day and 4 times daily for an additional 6 days . the gatifloxacin was continued 4 times daily for 1 week . in eyes undergoing prk all eyes had their corneas cooled with 15 ml of bss ( 2.83.9c ) immediately following ablation . this was followed by 1 drop of a gatifloxacin 0.3% ( zymar ) , prednisolone acetate 1% ( pred forte ) , ketorolac tromethamine 0.4% ( acular ls ) and a bandage soft contact lens ( softlens plano t ) . gatifloxacin and prednisolone acetate were continued 4 times a day for 1 week with a subsequent steroid taper over 2 to 3 months per surgeon preference . both bandage soft contact lenses were removed simultaneously once re - epithelialization was complete , typically on postoperative days 3 to 5 . patients were seen 1 day , 1 week , 1 month  10 days , 3 months 14 days , and 6 months 14 days . at all follow - up examinations , udva and cdva were tested using a standard snellen eye chart . visual acuity was recorded in both snellen notation and logarithm of the minimum angle of resolution ( logmar ) format . contrast sensitivity was measured in controlled mesopic conditions at 3 , 6 , 12 , and 18 cycles per degree ( cpd ) using the vectorvision csv-1000e chart ( vectorvision , greenville , oh ) . higher - order aberrations ( hoas ) , including coma z(3,1 ) , trefoil z(3,3 ) , and spherical aberration z(4,0 ) , were measured using the customvue wavescan at a mean diameter of 6 mm . undilated scans of both eyes were taken preoperatively and 1 , 3 , and 6 months postoperatively . after the study was completed , the results were compiled and the data unmasked for statistical analysis . refractive error , visual acuity , and hoas were treated as continuous variables and analyzed for significance by independent t - tests . in all tests , p values data analysis was done using microsoft excel ( microsoft corp , redmond , wa ) . mean preoperative measurements of udva , cdva , sphere , and cylinder are shown in table 1 . 25 of 26 patients ( 50 eyes ) completed the study at postoperative 6 months . one eye in the thin - flap lasik group required prk retreatment following a flap tear and both eyes from this patient were therefore removed from analysis of visual acuity , contrast sensitivity , and hoas as the retreatment prevented the ability to distinguish results between the 2 surgical methods . table 2 shows visual acuity outcomes at 1 , 3 , and 6 months postoperatively . statistically significant differences were found between prk and thin - flap lasik in udva at 1 month postoperatively , with thin - flap lasik eyes showing more improvement in udva . table 3 shows stability , efficacy , and predictability outcomes postoperatively at 1 , 3 , and 6 months . cdva was statistically different between groups at 1 month , with 24% of the prk group losing a line or more from preoperative values , while 9% of eyes in the thin - flap lasik group lost only 1 line at 1 month . no eyes in the thin - flap lasik group lost more than 1 line . also , 39% of eyes in the thin - flap group gained a line by 1 month compared with only 12% of eyes in the prk group . at 6 months 64% and 56% of eyes had gained a line or more of cdva in the prk and thin - flap lasik groups , respectively ( p = 0.462 ) . contrast sensitivity measurements at 3 , 6 , 12 , and 18 cycles per degree ( cpd ) in each group are shown in figure 1 . there were no differences between groups at any cpd at any time in the study ( p  0.156 ) . the thin - flap lasik group showed no change in contrast sensitivity postoperatively ( p > 0.131 ) , while patients in the prk group had a slight decrease in contrast sensitivity at 1 month seen at 3 and 12 cpd ( p = 0.004 ) and ( p = 0.025 ) , respectively . at 6 months contrast sensitivity in the prk group was still significantly decreased from baseline at 3 cpd ( p = 0.013 ) , although it did not reach a statistically significant difference at 3 months ( p = 0.101 ) . types of complications differed between the 2 groups . in the prk group , 2 cases of epithelial defects occurred by 1 week , but had completely resolved by 6 months . three eyes in the prk group had mild haze appearing as early as 1 week postoperatively . haze remained in only 1 eye at 6 months , but was classified as minimal and had no effect on udva or cdva . nine eyes ( 35% ) in the thin - flap lasik group experienced complications . in the thin - flap lasik group , flap debris ( 1 eye ) , diffuse lamellar keratitis ( dlk , 1 eye ) , and an epithelial cyst at the edge of 1 flap were observed , with no loss of udva or cdva , and all resolved by 6 months . microstriae were observed in 6 eyes , one of which was the eye described above with flap debris and the other was the eye with dlk , with no associated loss of udva or cdva , with epithelial proliferation noted as filling the microstria and making them less apparent . two eyes in the thin - flap lasik group experienced flap tears intraoperatively  one resulting in mild flap edge scarring by 6 months that had no significant effect on visual function , and the other case affecting vision at 1 month postoperatively which was retreated with prk at 3 months . as a result of the retreatment with the counter surgical technique , the ability to accurately compare visual acuity , contrast sensitivity , and hoas between the 2 surgical methods was limited and both eyes from this patient were removed from analysis of these measures , but were still included in the analysis of complications . at postoperative 1 , 3 , and 6 months , 24 ( 96% ) , 25 ( 100% ) , and 24 ( 96% ) eyes , respectively , in each group completed customvue wavescan analysis . total root - mean square ( rms ) hoas , coma , trefoil , and spherical aberrations are compared in figure 2 . there were no significant differences between groups in any hoas throughout the study ( p  0.101 ) , with all p values at 6 months 0.63 . < 0.002 ) , and coma ( p = 0.008 at 3 months ; p = 0.024 at 6 months ) aberrations were significantly increased compared with preoperative conditions . trefoil showed no significant change throughout the study in either group ( p = 0.298 ) . table 2 shows visual acuity outcomes at 1 , 3 , and 6 months postoperatively . statistically significant differences were found between prk and thin - flap lasik in udva at 1 month postoperatively , with thin - flap lasik eyes showing more improvement in udva . table 3 shows stability , efficacy , and predictability outcomes postoperatively at 1 , 3 , and 6 months . cdva was statistically different between groups at 1 month , with 24% of the prk group losing a line or more from preoperative values , while 9% of eyes in the thin - flap lasik group lost only 1 line at 1 month . no eyes in the thin - flap lasik group lost more than 1 line . also , 39% of eyes in the thin - flap group gained a line by 1 month compared with only 12% of eyes in the prk group . at 6 months 64% and 56% of eyes had gained a line or more of cdva in the prk and thin - flap lasik groups , respectively ( p = 0.462 ) . contrast sensitivity measurements at 3 , 6 , 12 , and 18 cycles per degree ( cpd ) in each group are shown in figure 1 . there were no differences between groups at any cpd at any time in the study ( p  0.156 ) . the thin - flap lasik group showed no change in contrast sensitivity postoperatively ( p > 0.131 ) , while patients in the prk group had a slight decrease in contrast sensitivity at 1 month seen at 3 and 12 cpd ( p = 0.004 ) and ( p = 0.025 ) , respectively . at 6 months contrast sensitivity in the prk group was still significantly decreased from baseline at 3 cpd ( p = 0.013 ) , although it did not reach a statistically significant difference at 3 months ( p = 0.101 ) . types of complications differed between the 2 groups . in the prk group , 2 cases of epithelial defects occurred by 1 week , but had completely resolved by 6 months . three eyes in the prk group had mild haze appearing as early as 1 week postoperatively . haze remained in only 1 eye at 6 months , but was classified as minimal and had no effect on udva or cdva . nine eyes ( 35% ) in the thin - flap lasik group experienced complications . in the thin - flap lasik group , flap debris ( 1 eye ) , diffuse lamellar keratitis ( dlk , 1 eye ) , and an epithelial cyst at the edge of 1 flap were observed , with no loss of udva or cdva , and all resolved by 6 months . microstriae were observed in 6 eyes , one of which was the eye described above with flap debris and the other was the eye with dlk , with no associated loss of udva or cdva , with epithelial proliferation noted as filling the microstria and making them less apparent . two eyes in the thin - flap lasik group experienced flap tears intraoperatively  one resulting in mild flap edge scarring by 6 months that had no significant effect on visual function , and the other case affecting vision at 1 month postoperatively which was retreated with prk at 3 months . as a result of the retreatment with the counter surgical technique , the ability to accurately compare visual acuity , contrast sensitivity , and hoas between the 2 surgical methods was limited and both eyes from this patient were removed from analysis of these measures , but were still included in the analysis of complications . at postoperative 1 , 3 , and 6 months , 24 ( 96% ) , 25 ( 100% ) , and 24 ( 96% ) eyes , respectively , in each group completed customvue wavescan analysis . total root - mean square ( rms ) hoas , coma , trefoil , and spherical aberrations are compared in figure 2 . there were no significant differences between groups in any hoas throughout the study ( p  0.101 ) , with all p values at 6 months 0.63 . in both groups , total hoas ( p < 0.008 ) , spherical ( p < 0.002 ) , and coma ( p = 0.008 at 3 months ; p = 0.024 at 6 months ) aberrations were significantly increased compared with preoperative conditions . trefoil showed no significant change throughout the study in either group ( p = 0.298 ) . the present study confirms that prk and thin - flap lasik are effective surgeries for the correction of low to moderate myopia . although thin - flap lasik showed superior visual results in the early postoperative period there was no statistically significant difference in outcomes of udva , cdva , contrast sensitivity , or total rms hoas between prk and thin - flap lasik by 6 months . in a similar study comparing prk and thin - flap lasik , slade et al also found that udva results were better in the thin - flap group early on and equalized by 6 months.16 our study showed a similar trend , with no significant differences in any of the primary outcomes at 6 months , and with no difference in udva at 3 months . visual regression in our study was similar to outcomes in slade s study in which 42% of the prk group lost a line or more of cdva and 22% of the thin - flap lasik group lost 1 line at 1 month postoperatively . despite the use of custom ablation , postoperative increases in total hoas , sphere , and coma were noted in our study , as also seen by slade et al , although they noted that the increase in sphere and coma aberrations was significantly higher in the prk group at 1 and 3 months postoperatively . as found in previous studies , there was no significant change found in trefoil at any time postoperatively.17,18 our study showed no difference in induction of hoas between groups at any time . although increases in hoas after refractive surgery have been correlated with decreases in contrast sensitivity in other studies , we demonstrate that increases in total rms , sphere , and coma were seen postoperatively in both groups without a reliable decrease in contrast sensitivity.19,20 slade s group found that contrast sensitivity was better in the thin - flap group at all postoperative points in the study , which may have been related to their finding of lower induction of sphere and coma aberrations in the thin - flap group compared with the prk group . the authors recognize that the slade study had a larger population size ( n = 50 per group ) and would have increased power to detect significant differences . our study would have had increased power of analysis with a similar study group size , but results from analysis of hoas would not likely change as p values for all hoas at 6 months were 0.63 . it would be difficult to make any such correlation between contrast sensitivity and hoas from the results of this study . a loss of cdva has been associated with the development of corneal haze in other studies , but as mentioned above none of the patients with visual regression developed haze.2123 findings in other studies showing that the biomechanics of eyes that have received thin - flap lasik treatment are indistinguishable from those of prk have led to suggestions that thin - flap lasik is the best approach to lasik.16 although the present study did not find any statistically significant differences between thin - flap lasik and prk in terms of visual quality at 6 months , complications dealing with flap integrity in the thin - flap lasik group were present which are not complications found in prk . although prk remains a viable option for those unable to undergo lasik , the use of thinner flaps may eliminate some of the complications seen with traditional lasik . larger studies are needed to better compare the complication rates of both methods and to determine how effective thin - flap lasik will be in achieving the benefits of prk and lasik while avoiding the risks associated with each method . while thinner lasik flaps attempt to preserve the biomechanical stability of the corneal stroma , at the same time , the flap itself becomes less stable , as was noted with the 2 flap tears and other complications occurring in the thin - flap lasik group in this study . a study by espandar and meyer24 showed that most complications in flaps created by intralase femtosecond laser occurred at the hinge , which is where the 2 flap tears that occurred in this study . a thinner flap hinge would be biomechanically less stable and would increase the likelihood of intraoperative flap tear occurrence as well . six of the 9 eyes with complications in the thin - flap lasik group had microstriae , which are caused by the flattening of a weak corneal flap unable to maintain its curvature over the small area of stroma removed during ablation . the biomechanics of the flap and hinge , however , can not be evaluated by the design of this study as analysis was done based on intended flap thickness , which has been shown to vary with the intralase fs60 femtosecond laser.25 the study could have been strengthened had intraoperative pachymetry or oct been performed . creating a flap with increased integrity would help prevent microstriae from forming and would also provide for a stronger hinge that would be less susceptible to flap tear . possible ways to optimize flap integrity include modification of hinge and side - cut angle creation , as well as improved planarity and microdisruption of flap edges . this will allow improved adhesion of the flap to the underlying stroma . continued improvements in laser technology may allow for safer creation of thinner flaps , helping to provide evidence for superior outcomes in thin - flap lasik , permitting the biomechanical stability of prk with the visual recovery of lasik . custom flap formation that minimizes weak areas susceptible to tearing will be helpful in achieving this difficult balance between corneal and flap integrity .
purpose : to compare differences in visual acuity , contrast sensitivity , complications , and higher - order ocular aberrations ( hoas ) in eyes with stable myopia undergoing either photo - refractive keratectomy ( prk ) or thin - flap laser in situ keratomileusis ( lasik ) ( intended flap thickness of 90 m ) using the visx star s4 customvue excimer laser and the intralase fs60 femtosecond laser at 1 , 3 , and 6 months postoperatively.methods:in this prospective , masked , and randomized pilot study , refractive surgery was performed contralaterally on 52 eyes : 26 with prk and 26 with thin - flap lasik . 
 primary outcome measures were uncorrected distance visual acuity ( udva ) , corrected distance visual acuity ( cdva ) , contrast sensitivity , and complications.results:at 6 months , mean values for udva ( logmar ) were 0.043  0.668 and 0.061  0.099 in the prk and thin - flap lasik groups , respectively ( n = 25 , p = 0.466 ) . 
 udva of 20/20 or better was achieved in 96% of eyes undergoing prk and 92% of eyes undergoing thin - flap lasik , whereas 20/15 vision or better was achieved in 73% of eyes undergoing prk and 72% of eyes undergoing thin - flap lasik ( p > 0.600 ) . 
 significant differences were not found between treatment groups in contrast sensitivity ( p  0.156 ) or cdva ( p = 0.800 ) at postoperative 6 months . 
 types of complications differed between groups , notably 35% of eyes in the thin - flap lasik group experiencing complications , including microstriae and 2 flap tears.conclusion:under well - controlled surgical conditions , prk and thin - flap lasik refractive surgeries achieve similar results in visual acuity , contrast sensitivity , and induction of hoas , with differences in experienced complications .