laser - assisted in situ keratomileusis ( lasik ) is the most common surgical technique for correcting refractive defects and although its efficacy and safety have already been proven , it is not free of potential complications .
one of the most feared is postoperative ectasia which induces astigmatism and myopia and reduces the quality of vision due to the induced high - order aberrations and in severe cases it reduces visual acuity . in order to detect ectatic change early ,
many authors have studied posterior corneal surface stability after laser refractive surgery [ 1 , 2 ] .
although postoperative ectasia was described in several papers [ 1 , 3 ] , its cause remains unclear .
onset seems to be favored by conditions such as a low residual corneal thickness , an abnormal preoperative corneal topography , a high refractive defect , or the patient 's young age [ 35 ] . today
femtosecond laser ( fsl ) is widely used in lasik surgery to create the corneal flap .
compared with the microkeratome ( mk ) , it has increased the safety of the operation and improved the predictability and homogeneity of the corneal flap thickness .
the latter has been shown to limit loss of corneal tensile strength especially for thin - flap lasik ( 90120  m ) . to our knowledge
, no studies have as yet focused on posterior corneal surface changes after femtosecond laser - assisted keratomileusis ( fsl - lasik ) using a corneal tomography unit .
the purpose of the present study is to evaluate the posterior corneal surface in eyes which underwent fsl - lasik for myopia and myopic astigmatism over a twelve - month follow - up period in order to highlight any eventual postoperative forward shift .
after the local ethics committee had approved the study protocol , patients were enrolled in a prospective noncomparative case series between january 2012 and june 2012 .
inclusion criteria were age of 21 years and older with stable myopia and a myopic astigmatism for at least two years .
exclusion criteria were systemic or ocular pathologies interfering with the healing process of the cornea , keratoconus , corneal dystrophy , glaucoma , previous eye trauma , or surgery .
the study was conducted on patients undergoing fsl - lasik at the department of surgery and biomedical science of university of perugia .
all patients underwent a complete preoperative assessment , including corneal tomography using a scheimpflug camera ( sirius , c.s.o .
four tomographic images were obtained for each eye , and the best image was chosen .
anterior corneal surface data are derived from the integration of both types of images , while data on the posterior corneal surface and the lens are derived only from scheimpflug images .
a corneal flap of 9.0  mm in diameter was made at a depth of 110 microns with the femtosecond laser ( visumax , carl zeiss meditec ag , jena , germany ) .
once the corneal flap was lifted , photoablative excimer laser treatment was applied ( mel 80 , carl zeiss meditec ag , jena , germany ) .
check - ups were scheduled for 1 , 6 , and 12 months after surgery . at each check - up changes in the posterior corneal curvature ( pcc ) , posterior corneal elevation ( pce ) , with respect to the best - fit sphere , and the anterior chamber depth as evaluated from the endothelium ( acd ) were studied by comparing the results of the preoperative evaluation with postoperative data .
the postoperative forward shift was expressed as the difference between preoperative and postoperative pcc values in the central 4  mm areas of the elevation maps , assigning a positive value to anterior shift .
differences in posterior corneal curvature , ablation , residual thickness , and the ablation percentage of the total preoperative corneal thickness were analyzed by spearman 's coefficient of rank correlation (  ) .
the analysis of variance ( anova ) for repeated measures and student 's t - test for paired data with the bonferroni correction were used for data analysis . statistical significance was set at p  0.05 .
forty - two eyes of 42 patients underwent bilateral and simultaneous fsl - lasik surgery , and one eye of each subject was randomly chosen ( coin toss ) and included in this study .
the mean age of patients was 36.05  8.3 years ( range : 2453 years ) .
the mean preoperative spherical equivalent was 5.05  2.07  d ( range : 1.25  d/7.50  d ) and the programmed ablation depth was 117.2  32.2  m ( 36160 ) .
table 1 reports mean values of the posterior corneal curvature , posterior corneal elevation , and anterior chamber depth .
data analysis detected no significant changes in the posterior corneal curvature at 1 , 6 , and 12 months after surgery compared with preoperative values ( f = 1.47 ; p = 0.226 ) .
there was no significant change in posterior corneal elevation during the 12-month follow - up period ( f = 1.89 ; p = 0.135 ) .
anterior chamber depth was stable 1 month and 6 months after fsl - lasik surgery but a significant reduction was observed 12 months after surgery ( 0.03  0.07  mm ) ( f = 4.25 ; p = 0.007 ) ( bonferroni test : t = 2.94 ; p = 0.005 ) .
our data did not show any linear correlation between corneal ablation depth and posterior corneal curvature throughout the follow - up period ( figures 1(a ) , 1(b ) , and 1(c ) ) .
no relationships emerged between posterior corneal curvature , residual corneal thickness ( figures 2(a ) , 2(b ) , and 2(c ) ) , and the percentage of ablated corneal tissue ( figures 3(a ) , 3(b ) , and 3(c ) ) .
our study on patients undergoing fsl - lasik for myopia and myopic astigmatism shows the posterior corneal surface was stable over time .
since refractive surgery reduces corneal thickness , weakening of this layer might possibly have a modification of the posterior corneal surface .
in fact , one potential complication of this type of surgery is posterior corneal ectasia which ranges in frequency from 0.04 to 0.9% depending on several factors such as the accuracy of the preoperative evaluation , the surgical technique , and the surgeon 's skill [ 3 , 5 , 8 , 9 ] . using orbscan ( orbscan , bausch & lomb , tampa , fl , usa ) a forward shift of the posterior corneal surface was observed after photokeratectomy ( prk ) and lasik [ 1 , 4 , 10 ] .
orbscan sensitivity in studying the posterior corneal surface after refractive surgery was , however , questioned because a change in optical features does not allow the instrument to provide accurate corneal mapping [ 11 , 12 ] .
orbscan uses mathematical calculations to recreate the posterior corneal surface and this strategy , in patients undergoing refractive surgery , can cause false positive readings of the posterior corneal elevation .
corneal morphology is assessed better by rotating scheimpflug cameras that analyze the posterior corneal elevation without mathematical calculations .
evaluations by pentacam ( oculus gmbh , wetzlar , germany ) , which is the first instrument to be based on this technology , appear more reliable as they seem less influenced by the effects of refractive surgery .
the different sensitivities of the scheimpflug camera and orbscan in studying the corneal morphology were demonstrated in 2007 by nishimura et al .   and later confirmed by others .
studies conducted with pentacam showed that the risk of presenting a forward displacement of the posterior corneal surface after refractive surgery was lower than that which had been suggested using orbscan . in using pentacam to monitor myopic eyes undergoing prk or mk - lasik , ciolino and belin   did not observe any case of forward shift at 2 months after surgery .
this observation was confirmed by nishimura et al .   and by sun et al .  
although reduced , the forward shift complication did not completely disappear . in 2010 , using pentacam , zhang and wang   observed a forward shift of posterior corneal elevation in patients 1 month and 6 months after epi - lasik surgery .
nowadays , cases of iatrogenic corneal ectasia after lasik are still reported even in absence of detectable preoperative risk factors in eyes studied with orbscan or with the scheimpflug camera [ 16 , 17 ] . in our opinion , some limitations emerge from these reports .
most studies on posterior corneal surface stability after lasik have short follow - ups [ 2 , 11 , 12 ] , even though it is well known that a change in the corneal morphology may be observed even a long time after refractive surgery has been performed .
moreover , many studies were conducted on patients who underwent lasik using a microkeratome which does not ensure a high repeatability and regularity of the corneal cut to the detriment of flap thickness homogeneity . on the contrary ,
the femtosecond laser allows the cut to be made at a lower depth , with greater predictability of and homogeneity in thickness .
this probably reduces the loss of corneal tensile strength [ 6 , 10 ] and should minimize the risk of postoperative ectasia . in a retrospective study with a long follow - up period , moshirfar et al
.   used orbscan to study eyes undergoing fsl - lasik and observed 0.05% incidence of iatrogenic ectasia and 0.25% total incidence of postoperative ectasia . in our study
, we used a recently introduced rotating scheimpflug camera that was proven to be accurate and reliable in studies on healthy eyes   and on eyes affected by pathological ectasia   with high repeatability and reproducibility .
moreover , sirius system demonstrated high repeatability and reproducibility for anterior chamber depth measurements [ 22 , 23 ] . in our patients who underwent fsl - lask , we did not observe any change in the posterior corneal surface even one year after surgery .
our findings concur with other authors who reported that a scheimpflug camera after mk - lasik demonstrated posterior corneal surface stability after a long follow - up period [ 2 , 24 ] .
furthermore , in agreement with other authors [ 2 , 11 , 12 ] , the difference in posterior corneal curvature in our study did not correlate with residual corneal thickness , corneal ablation depth , or the percentage of ablated corneal tissue .
lasik surgery induces a change in the corneal morphology with an increase in peripheral corneal curvature and posterior central curvature without a forward shift . observed soon after surgery
, these effects tend to decrease over time but can be confused with a bulge of the posterior corneal surface . in our work
, we did not observe any changes in the posterior corneal surface probably because our postoperative examinations began one month after surgery .
moreover , use of the femtosecond laser to create the corneal flap may have lessened these effects on corneal morphology .
unexpectedly , we observed a reduction in the anterior chamber depth 1 year after surgery , without any clinical consequences , as was reported by nishimura et al .   in 2007 and by sun et al .   in 2009
this paradoxical observation may be one result of the magnification effect that the cornea has on the lens front surface after lasik , or it may be caused by the change in lens morphology due to the increased accommodative effort that patients must make once the myopic refractive error is corrected .
in fact , its early appearance is a consequence of peripheral stromal thickening which recovers the normal morphology within three months , but we can not account for its onset one year after surgery .
using a rotating scheimpflug camera , our study demonstrated posterior corneal surface stability 12 months after surgery in patients who underwent fsl - lasik for myopia and myopic astigmatism .
although these data should be confirmed by further studies , with more patients , our work seems to confirm that the femtosecond laser plays a role in making this surgery safer and more predictable , probably by reducing the onset of forward shift of the posterior corneal surface .
 the purpose of this study was to evaluate posterior corneal surface variation after femtosecond laser - assisted keratomileusis in patients with myopia and myopic astigmatism . 
 patients were evaluated by corneal tomography preoperatively and at 1 , 6 , and 12 months . 
 we analyzed changes in the posterior corneal curvature , posterior corneal elevation , and anterior chamber depth . 
 moreover , we explored correlation between corneal ablation depth , residual corneal thickness , percentage of ablated corneal tissue , and preoperative corneal thickness . during follow - up , the posterior corneal surface did not have a significant forward corneal shift : no significant linear relationships emerged between the anterior displacement of the posterior corneal surface and corneal ablation depth , residual corneal thickness , or percentage of ablated corneal tissue . 