***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: PURPOSE To evaluate the safety and efficacy of laser in situ keratomileusis ( LASIK ) compared to photorefractive keratectomy ( PRK ) for the correction of low or moderate myopia ( -0.50 to -6.00 D ) at 6 months after surgery . METHODS The study population comprised a non-r and omized consecutive series of 622 eyes of 392 patients who were treated with the Nidek EC-5000 excimer laser . LASIK was performed using the ACS Chiron microkeratome on 314 eyes and surface PRK on 308 eyes . All patients were treated using a st and ard protocol , then assessed at 1 , 3 , and 6 months postoperatively . RESULTS Forty-four percent of the LASIK group and 67 % of the PRK group attended their 6-month examination . Eighty percent of patients ( 111 eyes ) after LASIK and 65 % ( 136 eyes ) after PRK had an uncorrected visual acuity of 20/20 or better . Spherical equivalent refraction was within + /-0.50 D of intended refraction in 78 % ( 109 eyes ) for LASIK and 82 % ( 170 eyes ) for PRK . Loss of two more lines of best spectacle-corrected visual acuity at 6 months occurred in 1.4 % ( 2 eyes ) of the LASIK group and 1.0 % ( 2 eyes ) of the PRK group . CONCLUSION At 1 month follow-up , the percentage of eyes that achieved 20/20 uncorrected visual acuity was greater in the LASIK group than in the PRK group . At 6 months , visual and refractive outcomes of LASIK and PRK were similar . Although flap related complications occurred only after LASIK , the overall risk of loss of best spectacle-corrected visual acuity was not significantly greater than for PRK OBJECTIVE This report presents patient-reported optical symptoms after photorefractive keratectomy ( PRK ) and laser in situ keratomileusis ( LASIK ) .  DESIGN Preoperative and postoperative patient surveys in a prospect i ve , multicenter , r and omized clinical trial . PARTICIPANTS Two hundred twenty eyes of 220 patients entered the study ; 105 were r and omized to PRK and 115 were r and omized to LASIK .  INTERVENTION All patients received a one-pass , multizone excimer laser ablation as part of either a PRK or LASIK procedure . Attempted corrections ranged from 6.00 to 15.00 diopters ( D ) . MAIN OUTCOME MEASURES Glare , halo , and monocular diplopia symptoms as reported by patients on question naires before surgery and at the 6-month follow-up . Comparison was made between symptoms when using optical correction before surgery and symptoms without correction after surgery . RESULTS For both the PRK and LASIK groups analyzed individually , the difference in average glare index before surgery and after surgery was not statistically significant ( P = 0.54 for PRK ; P = 0.15 for LASIK ; t test ) . Twenty-four PRK patients ( 41.4 % ) reported worsening of glare symptoms from baseline compared with 11 LASIK patients ( 21.6 % ) ; however , the difference between the two groups was not statistically significant ( P = 0.086 , chi-square test ) . Within the PRK group , the difference in average halo index before and after surgery was statistically significant ( P = 0.0003 , t test ) ; in the LASIK group , it was not statistically significant ( P = 0.1 1 , t test ) . Thirty-four PRK patients ( 58.6 % ) reported worsening of halo symptoms from baseline compared with 26 LASIK patients ( 50.0 % ) ; this difference was not statistically significant ( P = 0.086 , chi-square test ) . For both the PRK and LASIK groups , the difference in average diplopia index before and after surgery was statistically significant ( P < 0.0001 for PRK ; 0.047 for LASIK ; t test ) . Twenty-six PRK patients ( 44.8 % ) reported a worsening of monocular diplopia symptoms from baseline compared with 19 LASIK patients ( 35.8 % ) ; this difference was not statistically significant ( P = 0.39 , chi-square test ) . When changes in glare and halo from before surgery to after surgery were pooled as a glare-halo index , however , the PRK group did show a significantly greater likelihood of demonstrating an increase in symptoms compared with the LASIK group ( P = 0.048 , chi-square test ) . CONCLUSIONS Optical sequelae of glare , halo , and monocular diplopia may occur in some patients after either both PRK or LASIK for moderate to high myopia ; in contradistinction , many other patients ' preoperative symptoms improve after surgery . On average , PRK patients show an increase in halo and diplopia symptoms , but not glare , after surgery , and LASIK patients show an increase in diplopia , but not glare and halo symptoms . There is a suggestion of a somewhat lesser tendency toward postoperative optical symptoms in LASIK compared with PRK treated eyes OBJECTIVE Determine the outcomes of single-zone photorefractive keratectomy ( SZPRK ) , aspherical photorefractive keratectomy ( ASPRK ) , and laser in-situ keratomileusis ( LASIK ) for the correction of myopia between -6 and -12 diopters . DESIGN Two simultaneous prospect i ve , r and omized , multi-center clinical trials . PARTICIPANTS 286 first-treated eyes of 286 patients enrolled in one of two studies . In Study I , 134 eyes were r and omized to SZPRK ( 58 eyes ) or ASPRK ( 76 eyes ) . In Study II , 152 eyes were r and omized to ASPRK ( 76 eyes ) or to LASIK ( 76 eyes ) . INTERVENTION All eyes received spherical one-pass excimer laser ablation as part of PRK or LASIK performed with the Summit Technologies Apex laser under an investigational device exemption , with attempted corrections between -6 and -12 diopters . MAIN OUTCOME MEASURES Data on uncorrected and best spectacle-corrected visual acuity , predictability and stability of refraction , and complications were analyzed . Follow-up was 12 months . RESULTS At 1 month postoperatively , more eyes in the LASIK group achieved 20/20 and 20/25 or better uncorrected visual acuity than PRK-treated eyes ; at the 20/25 or better level , the difference was significant for LASIK ( 29/76 eyes , 38 % ) over SZPRK ( 10/58 eyes , 17 % ) ( P = .0064 ) . At all subsequent postoperative intervals , no difference was seen between treatment groups . Similarly , best corrected visual acuities were better for LASIK than all PRK eyes at 1 month postoperatively , and LASIK was better than SZPRK at 3 months follow-up ( e.g. , for 20/20 or better at 1 month , LASIK 50/76 eyes ( 66 % ) versus SZPRK 24/57 eyes ( 42 % ) , P = .0066 ) . PRK eyes had a mean loss of BCVA through 6 months , while LASIK eyes had a slight gain of mean BCVA through month 6 ; at 12 months , both ASPRK groups but not SZPRK continued to have a small mean loss of BCVA ( e.g. , compared to preoperative , mean BCVA at 12 months for SZPRK was + 0.3 , LASIK was + .21 , ASPRK I was -0.11 , and ASPRK II -0.31 ( SZPRK versus ASPRK II , P = .0116 ) . Predictability was better for PRK than LASIK at all follow-up intervals ( e.g. , for manifest refraction spherical equivalent + /- 1.0 diopters at 6 months , ASPRK I 42/62 eyes ( 68 % ) versus LASIK 29/72 eyes ( 40 % ) , P = .0014 % ) . Stability was slightly but insignificantly less in the LASIK eyes compared to PRK eyes . All visual outcome measures were better for eyes with preoperative myopia between -6 and -8.9 D compared with eyes with myopia between -9 and -12 D. No consistent differences in refractive outcomes or postoperative corneal haze were seen between aspherical and single-zone ablations ; haze diminished over 12 months and was judged to be vision-impairing in only one ASPRK eye . Microkeratome and flap complications occurred in 4 eyes , result ing in delay of completion of the procedure in 3 eyes but not causing long-term impairment . CONCLUSIONS Improvement in uncorrected visual acuity and return of best corrected visual acuity was more rapid for LASIK than PRK , but efficacy outcomes in the longer term through 12 months were similar for all treatment groups . LASIK eyes tended toward undercorrection with the nomogram employed in this study compared to PRK , but the scatter was similar , suggesting little difference between these procedures for most patients by 6 months and thereafter . No consistent advantage was demonstrated between aspherical and single-zone ablation patterns . Predictability was much better for all procedures for corrections of -6 to -8.9 D compared with -9 to -12 D. Sporadic loss of best corrected vision in the PRK eyes not found in the LASIK eyes and other measures of visual function require further study PURPOSE To review cases of corneal ectasia after laser in situ keratomileusis ( LASIK )  , identify preoperative risk factors , and evaluate methods and success rates of visual rehabilitation for these cases . DESIGN Retrospective nonr and omized comparative trial . PARTICIPANTS Ten eyes from seven patients identified as developing corneal ectasia after LASIK , 33 previously reported ectasia cases , and two control groups with uneventful LASIK and normal postoperative courses : 100 consecutive cases ( first control group ) , and 100 consecutive cases with high myopia ( > 8 diopters [ D ] ) preoperatively ( second control group ) . METHODS Retrospective review of preoperative and postoperative data for each case compared with that of previously reported cases and cases with uneventful postoperative courses . MAIN OUTCOME MEASURES Preoperative refraction , topographic features , residual stromal bed thickness ( RSB ) , time to the development of ectasia , number of enhancements , final best-corrected visual acuity ( BCVA ) , and method of final correction . RESULTS Length of follow-up averaged 23.4 months ( range , 6 - 48 months ) after LASIK . Mean time to the development of ectasia averaged 16.3 months ( range , 1 - 45 months ) . Preoperative refraction averaged -8.69 D compared with -5.37 D for the first control group ( P = 0.005 ) . Preoperatively , 88 % of ectasia cases met criteria for forme fruste keratoconus , compared with 2 % of the first control group ( P < 0.0000001 ) and 4 % of the second control group ( P = 0.0000001 ) . Seven eyes ( 70 % ) had RSB < 250 microm , as did 16 % of eyes in the first control group and 46 % of the second control group . The mean RSB for ectasia cases ( 222.8 microm ) was significantly less than that for the first control group ( 293.6 micro m , P = 0.0004 ) and the second control group ( 256.5 microm ; P = 0.04 ) . Seven eyes ( 70 % ) had enhancements . Only 10 % of eyes lost more than one line of BCVA , and all patients eventually achieved corrected vision of 20/30 or better . One case required penetrating keratoplasty ( 10 % ) , while all others required rigid gas-permeable contact lenses for correction . CONCLUSIONS Significant risk factors for the development of ectasia after LASIK include high myopia , forme fruste keratoconus , and low RSB . All patients had at least one risk factor other than high myopia , and significant differences remained even when controlling for myopia . Multiple enhancements were common among affected cases , but their causative role remains unknown . We did not identify any patients who developed ectasia  without recognizable preoperative risk factors 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 μm ) using the VISX Star S4 CustomVue excimer laser and the IntraLase FS60 femtosecond laser at 1 , 3 , and 6 months postoperatively . Methods : In this prospect i ve , masked , and r and omized 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 
    Output:
    
There was evidence that LASIK gives a faster visual recovery than PRK and is a less painful technique .
Results at one year after surgery were comparable : most analyses favoured LASIK but they were not statistically significant .
LASIK gives a faster visual recovery and is a less painful technique than PRK .
The two techniques appear to give similar outcomes one year after surgery .