***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 present the results after simultaneous photorefractive keratectomy ( PRK ) followed by corneal collagen cross-linking ( CXL ) for progressive keratoconus . METHODS Twelve patients ( 14 eyes ) with progressive keratoconus were prospect ively treated with customized topography-guided PRK with the Pulzar Z1 ( wavelength 213 nm , CustomVis ) immediately followed by corneal collagen CXL with the use of riboflavin and ultraviolet A irradiation . RESULTS Mean follow-up was 10.69+/-5.95 months ( range : 3 to 16 months ) . Mean preoperative spherical equivalent refraction ( SE ) was -3.03+/-3.23 diopters ( D ) and defocus was 4.67+/-3.29 D ; at last follow-up SE and defocus were statistically significantly reduced to -1.29+/-2.05 D and 3.04+/-2.53 D , respectively ( P<.01 ) . Preoperative mean ( logMAR ) uncorrected visual acuity was 0.99+/-0.81 and best spectacle-corrected visual acuity was 0.21+/-0.19 , which improved postoperatively to 0.16+/-0.15 and 0.11+/-0.15 , respectively . The mean steepest keratometry was reduced from 48.20+/-3.40 D preoperatively to 45.13+/-1.80 D at last follow-up . CONCLUSIONS Simultaneous PRK followed by CXL seems to be a promising treatment capable of offering functional vision in patients with keratoconus Purpose To determine the variability in Scheimpflug image – derived posterior elevation measurements in eyes with keratoconus or corneal collagen crosslinking ( CXL ) . Setting Eye Institute of Thrace , Alex and roupolis , Greece . Design Prospect i ve observational study . Methods Two operators in 2 sessions obtained posterior elevation measurements using the Pentacam Scheimpflug system . Variability in measurements was assessed using Bl and ‐Altman analysis and intraclass correlation coefficients ( ICCs ) . Measurements were associated with a series of Scheimpflug‐derived keratoconus indices using the Pearson correlation and multivariate linear‐regression analysis . Results Mean interobserver differences for the central elevation point ( e1 ) and the highest elevation point ( HEP ) in the keratoconus group were −0.9 μm ( 95 % limits of agreement [ LoA ] , 5.7 to −7.5 μm ) and 1.7 μm ( 95 % LoA , 26 to −22.6 μm ) , respectively . In the CXL group , the mean differences were 0.2 μm ( 95 % LoA , 6.0 to −5.5 μm ) and −6.1 μm ( 95 % LoA , 16.8 to −29 μm ) , respectively . The ICC values in the keratoconus group were satisfactory for most elevation points ; the ICC values were unsatisfactory for a series of points in the lower corneal hemisphere . The ICC values for points in both hemispheres were unsatisfactory in the CXL group ; HEP had adequate reproducibility . The e1 and HEP parameters correlated significantly with most Scheimpflug‐derived indices ( P<.05 ) in the keratoconus group but not in the CXL group . Regression analysis indicated that the central keratoconus index was the primary determinant of the variability in keratoconus eyes . Conclusions Scheimpflug‐derived posterior elevation measurements had acceptable reproducibility in keratoconus eyes and post‐CXL eyes . Less consistent measurements were obtained in the lower hemisphere . Financial Disclosure No author has a financial or proprietary interest in any material or method mentioned Purpose : To evaluate the clinical and topographic effects after accelerated corneal collagen cross-linking ( CXL ) in keratoconus . Methods : This was a prospect i ve interventional study in 25 eyes of 24 Asian patients with keratoconus , where 10 eyes in group 1 had mild to moderate keratoconus with the steepest keratometry reading ( Kmax ) < 58 diopters ( D ) , and 15 eyes in group 2 had advanced keratoconus with Kmax ≥ 58.0 D. The patients underwent accelerated CXL with 18 W/cm2 UV-A light for 5 minutes  . Clinical and topographic parameters were obtained before and 1 year after treatment . Results : Overall , spectacle-corrected distance visual acuity , refraction , average keratometry readings , Kmax , anterior elevation at the apex , and corneal endothelial cell density remained stable ( P < 0.102 ) . Posterior elevation at the apex increased at 1 year ( P = 0.002 ) . The thinnest pachymetry readings also showed significant reduction ( P < 0.001 ) . Kmax in group 1 increased by 1.00 ± 1.63 D ( range , −2.2 to 5.4 D ; P = 0.012 ) , whereas Kmax in group 2 reduced by 1.69 ± 2.61 D ( range , −8.4 to 3.2 D ; P = 0.013 ) at 1 year after accelerated CXL . The change in Kmax between the 2 groups was significantly different ( P < 0.001 ) . There was no intergroup difference for the change in other parameters . A significant correlation was found between the preoperative Kmax and postoperative change in Kmax at 1 year ( r = −0.648 ; P < 0.001 ) . Conclusions : Accelerated CXL seems to be effective in preventing progression and causing topographic flattening in advanced cases of keratoconus but not as effective in the less progressed counterparts PURPOSE To compare the long-term outcomes of accelerated and st and ard corneal cross-linking protocol s in the treatment of progressive keratoconus . DESIGN Prospect i ve r and omized clinical trial . METHODS Thirty-one eyes with keratoconus were treated with an accelerated protocol ( 18 mW/cm(2 ) , 5 min ) and all contralateral eyes were treated with the st and ard method ( 3 mW/cm(2 ) , 30 min ) using the same overall fluence of 5.4 J/cm(2 ) . RESULTS At 18 months after the procedure , the st and ard group showed significant improvement in spherical equivalent ( P < .05 ) , K-readings ( P < .05 ) , Q value ( P < .05 ) , index of surface variance ( P < .05 ) , and keratoconus index ( P = .008 ) and decline in central corneal thickness ( P < .05 ) , but no significant change in visual acuity , corneal hysteresis , corneal resistance factor , P2 area , or endothelial cell density . In the accelerated group , central corneal thickness was the only parameter with statistically significant change . However , neither of these parameters showed significant differences between the st and ard and the 18 mW/cm(2 ) accelerated protocol , except K-reading ( P = .059 ) and index surface variance ( P = .034 ) . CONCLUSION An accelerated cross-linking protocol , using 18 mW/cm(2 ) for 5 minutes , shows a comparable outcome and safety profile when compared to the st and ard protocol , but better corneal flattening is achieved with the st and ard method than the accelerated method . Overall , both methods stop the disease progression similarly . This study will continue to examine more long-term results PURPOSE In animal eyes , a significant increase in corneal biomechanical stiffness has been found after collagen crosslinking by combined riboflavin/ultraviolet-A ( UVA ) treatment . The aim of the present study was to evaluate the clinical usefulness of riboflavin/UVA-induced collagen crosslinking for bringing the progression of keratoconus to a halt . DESIGN Prospect i ve , nonr and omized clinical pilot study . METHODS Twenty-three eyes of 22 patients with moderate or advanced progressive keratoconus ( maximum K value , 48 - 72 diopters ) were included . After central corneal abrasion , photosensitizing riboflavin drops were applied and the eyes exposed to UVA ( 370 nm , 3 mW/cm(2 ) ) in a 1-cm distance for 30 minutes . Postoperative examinations were performed in 6-month intervals , including visual acuity testing , corneal topography , slit-lamp examination , measurement of endothelial cell density , and photographic documentation . The follow-up time was between 3 months and 4 years . RESULTS In all treated eyes , the progression of keratoconus was at least stopped . In 16 eyes ( 70 % ) regression with a reduction of the maximal keratometry readings by 2.01 diopters and of the refractive error by 1.14 diopters was found . Corneal and lens transparency , endothelial cell density , and intraocular pressure remained unchanged . Visual acuity improved slightly in 15 eyes ( 65 % ) . CONCLUSIONS Collagen crosslinking may be a new way for stopping the progression of keratectasia in patients with keratoconus . The need for penetrating keratoplasty might then be significantly reduced in keratoconus . Given the simplicity and minimal costs of the treatment , it might also be well-suited for developing countries . Long-term results are necessary to evaluate the duration of the stiffening effect and to exclude long term side-effects PURPOSE To assess the repeatability and agreement of three rotating Scheimpflug cameras , Pentacam , Galilei , and Sirius , in measuring the mean keratometry ( Km ) , thinnest corneal thickness ( TCT ) , anterior chamber depth ( ACD ) , and mean posterior keratometry ( pKm ) in keratoconus patients in a prospect i ve study . METHODS Fifty-five eyes of 55 patients with keratoconus underwent three consecutive scans on each machine , performed by a single operator . Within-subject st and ard deviation ( Sw ) , test-retest repeatability ( TRT ) , and coefficient of variation ( COV ) for assessing repeatability and Bl and -Altman plots for the agreement between the mean measurements of each machine were examined . RESULTS The Sw of Km and pKm measurements with Pentacam ( 0.23 and 0.10 diopters [ D ] , respectively ) were significantly lower ( better ) than those of Galilei ( 0.60 and 0.17 ) and Sirius ( 0.23 and 0.36 ) . The Sw of TCT measurements with Sirius ( 8.88 μm ) was significantly lower than that of Galilei ( 11.64 μm ) . The COV ranged between 0.5 for the Km measurements of Pentacam and 2.8 for the TCT measurements of Galilei . Significant proportional bias in agreement was detected for the pKm measurements with all the three device pairs and for the ACD measurements between Pentacam and Galilei and between Galilei and Sirius . CONCLUSIONS Though Pentacam , Galilei , and Sirius showed repeatable measurements for Km , TCT , ACD , and pKm , repeatabilities with Pentacam and Sirius were better than those with Galilei . There were significant differences in the measurements between the three devices ; hence they can not be used interchangeably for anterior segment measurements in keratoconus patients PURPOSE To evaluate the long-term results of corneal collagen cross-linking ( CXL ) in patients with progressive keratoconus . DESIGN Prospect i ve case series . PARTICIPANTS This study was conducted on 40 eyes of 32 patients with progressive keratoconus between 2006 and 2012 . METHODS Patients underwent CXL no later than 1 month after baseline examinations . For CXL , ultraviolet irradiation was applied for 30 minutes , during which riboflavin instillation was repeated every 3 minutes . MAIN OUTCOME MEASURES Patients were tested for best-corrected visual acuity ( BCVA ) , uncorrected visual acuity ( UCVA ) , manifest refraction spherical equivalent ( MRSE ) , and Scheimpflug imaging from which we extracted maximum keratometry reading ( max-K ) , average of minimum and maximum keratometry readings ( mean-K ) , central corneal thickness ( CCT ) , and anterior and posterior elevation at the apex at baseline , at 1 , 3 , 6 months after CXL , and 1 , 2 , 4 , and 5 years later . We studied results at 5 years after CXL as well as the trend of changes over the 5-year period . RESULTS Mean UCVA was 0.67 ± 0.52 logarithm of the minimum angle of resolution ( logMAR ) at baseline and 0.65 ± 0.51 logMAR at 5 years after the procedure . For mean BCVA , these values were 0.31 ± 0.28 and 0.19 ± 0.20 logMAR , respectively ( P = 0.016 ) . The mean MRSE changed from -3.18±2.23 diopters ( D ) to -2.77 ± 2.18 D , and mean refractive cylinder error changed from -3.14 ± 2.22 to -2.49 ± 1.71 D ( P = 0.089 ) . Mean max-K and mean-K decreased by 0.16 ± 2.20 and 0.10 ± 1.69 D , respectively . The CCT increased from 483.87 ± 29.07 to 485.95 ± 28.43 μm . Mean anterior elevation at the 
    Output:
    
Pentacam repeatability of different indices is not affected by CXL .
However , the change of indices showed high variance , which should be taken into consideration , especially in systematic review s because inter- study differences can be due to low repeatability of the measures of change