laser - assisted in situ keratomileusis ( lasik ) is a widely used technique in refractive corneal surgery . many complications can occur during the preparation of the corneal flap , resulting in costly retreatments . a meta - analysis of 30 trials reported an incidence of 31% for intraoperative complications , of which 19% were microkeratome associated . these were cutting failures such as buttonholes , free caps , or irregular or incomplete cuts . the complications required retreatments , comprising the majority of postoperative costs.1,2 application of femtosecond lasers ( fs - lasers ) is safe and predictable . nevertheless , specific complications , such as transient light sensitivity syndrome , rainbow glare , and complications concerning gas bubbles , may arise . the laser devices have been found to be as good as or better than mechanical microkeratomes , but long - term results are still needed.3 therefore , refinement of cutting quality with regard to morphology of cutting surfaces and edges is ongoing . a smooth cutting surface is considered to be one of the main quality markers in lasik . surface irregularities can cause haze and result in lower visual outcome ; furthermore , predictability is reduced because the later excimer laser ablation profile is altered.4,5 sharp cut edges are relevant for the reduction of postoperative epithelial ingrowth , less flap dislocations , and less postoperative inflammation.69 today , different instruments can be used for corneal preparation such as occurs in lasik . one recent option is use of an fs - laser system,10 which is , however , a very expensive surgical tool for lamellar keratotomies . despite the increasing use of fs - lasers , microkeratomes are still common for cutting flaps.3 as long as fs - lasers do not offer a main advantage in terms of visual outcome or complications , microkeratomes can continue to be regarded as a cost - saving alternative . hence it is also necessary to refine the technical parameters of mechanical microkeratomes . in this study , we examined the influence of different oscillation rates , head advances , and cutting depths on the quality of corneal flap cuts . in this experimental study , freshly isolated porcine eyes ( n=32 ) obtained from a local slaughterhouse were used . prior to surgery , eyes were held at room temperature for 2 hours to allow deswelling of the corneal stroma . only macroscopically perfect eyes not showing any epithelial defects were used . after removing adnexa ( lids , extraocular muscles , and conjunctiva ) , the eyes were placed in a suction device in which the intraocular pressure was set to 20 mmhg by vacuum . flaps were created using the carriazo - pendular ( cp ) microkeratome ( schwind eye - tech - solutions gmbh & co , kg , kleinostheim , germany ) . this instrument relies on a pendulum - like cutting action being controlled by a console giving acoustic and visual signs . blade speed can be set within a range of 2 to 5 mm / second , and oscillation frequencies can be adjusted from 9,000 to 15,000 rpm . cutting heads of different sizes ( 90 m , 110 m , 130 m , 150 m , and 170 m ) are also available.11 four flap - cut series including eight cuts each were performed . for each cut , a new blade was used . microkeratome settings were changed so that four different setups ( cp 14 ) could be tested ( table 1 ) . to compare the different head advances , a quotient was created containing the oscillation frequency and head - advance - speed . this quotient was named the oscillation / advance ratio ( oar ) in cuts / mm . with an oscillation frequency of 15,000 rpm , 30,000 cuts per minute or 500 cuts per second were performed , equating to 100 cuts per mm when a head advance of 5 mm per second is applied . after cutting , flaps were lifted using a spatula ( g-16193 ; geuder ag , heidelberg , germany ) , cut at the hinge , and discarded . the interface was washed with 5 ml bss - solution ( alcon pharma gmbh , freiburg i m breisgau , germany ) . with an 11 mm franceschetti trephinator , the corneal beds , including the cutting edges , were removed from the rest of each globe and prepared for scanning electron microscopy ( sem ) . the corneas were immediately immersed in 3% glutaraldehyde , 100 mm hydroxyethyl piperazineethanesulfonic acid ( hepes ) ( ph 7.4 ) , 1 mm cacl2 , 1 mm mgcl2 , and 25 mm nan3 and left at room temperature for 2 hours , and then at 4c overnight . the tissues were treated with 2% tannic acid for 1 hour , 2% osmium tetroxide for 2 hours , and 2% uranyl acetate for 1 hour , with washing steps in between . afterwards , the samples were dehydrated in a graded series of aqueous ethanol solutions ( 20%100% ) . finally , samples were mounted on aluminum stubs , sputtered with gold , and examined by sem in a dsm 940 a ( carl zeiss ag , oberkochen , germany ) . the description of the morphology of the corneal stromal bed as well as the evaluation of their surface quality was carried out on the basis of the electron micrographs . for evaluation , a semiquantitative score system was applied to the results observed in the sem micrographs.12 different criteria were evaluated , with points given corresponding to the cutting quality . therefore , cutting edges ( table 2 ) and cutting surfaces ( table 3 ) were evaluated separately . for evaluating the cutting edges , three different graduations were available ( criteria i  iii ) . a clearly visible cutting edge with no irregularities and a sharp epithelial a visible but partly inhomogeneous rim was evaluated as 2 points ( criterion ii ) . if the edge was barely locatable , it was evaluated as 1 point ( criterion iii ) . an electron micrograph with ten - fold magnification was used to evaluate the cutting surface ( criterion a ) . regularity of surface structure ( criterion b ) , portion of surface irregular ( criterion c ) , and position of the irregular area ( criterion d ) were evaluated on the basis of 50-fold magnification of the sample . each of the eight specimens per series was scored up to a maximum of 3 points for the evaluation of the cut edges ( possible maximum of 24 points ) and 11 points for the evaluation of the cut surfaces ( possible maximum of 88 points ) . as we examined eight cuts per setting , a maximum of 112 points could be achieved per series . statistical analysis was performed using spss software ( v 16 for windows ; ibm corporation , armonk , ny , usa ) . a p - value less than 0.05 was considered to be statistically significant . for each microkeratome setting , the results were averaged and compared using excel software ( microsoft corporation , redmond , wa , usa ) . as new blades were used for each keratotomy and a complex preparation for sem was needed , sample size was limited . regarding cutting edges , the carriazo - pendular microkeratome creates a serrated rim , showing the typical shape ( figure 1a - a ) . another criterion was the visibility of the crossing from the epithelium to the stroma of the cornea ( figure 1a - b ) . in this area , irregularities such as tissue ruptures appeared ( figure 1a - c ) . according to the sharpness of the cutting edges , different scores were available ( criteria i  iii ) , as shown in table 2 . the cutting surface was observed with regard to any surface roughness on the basis of ten - fold magnification electron micrograph ( table 3 , criterion a ) . in case of a homogenous surface , the interface could show waves ( figure 1b - a ) , tissue roughness ( figure 1b - b ) , or tissue ruptures ( figure 1b - c ) . depending on the location of these irregular morphologies , centrally or peripherally , scores from 13 points regarding cutting edges , the carriazo - pendular microkeratome creates a serrated rim , showing the typical shape ( figure 1a - a ) . another criterion was the visibility of the crossing from the epithelium to the stroma of the cornea ( figure 1a - b ) . in this area , irregularities such as tissue ruptures appeared ( figure 1a - c ) . according to the sharpness of the cutting edges , different scores were available ( criteria i  iii ) , as shown in table 2 . the cutting surface was observed with regard to any surface roughness on the basis of ten - fold magnification electron micrograph ( table 3 , criterion a ) . in case of a homogenous surface , the interface could show waves ( figure 1b - a ) , tissue roughness ( figure 1b - b ) , or tissue ruptures ( figure 1b - c ) . depending on the location of these irregular morphologies , centrally or peripherally , scores from 13 points no cutting complications , such as free flaps , buttonholes , or incomplete cuts , occurred . figure 2 shows representative samples of cutting edges and stromal surfaces for each of the series . the amount of the inhomogeneous areas led to different evaluations , as shown in table 4 . evaluating criterion a , which characterized the overall appearance of the interface , there was a tendency toward more irregularities in series cp 1 and 2 , with the 110 m and the 130 m cutting head , respectively . the other cutting surface criteria ( b  d ) were taken together as the regularity index . more specifically , it was the percentage amount of irregular areas ( criterion c ) and the integrity of the surface regularity ( criterion b ) that exhibited the main differences . comparing the surface scores of the different series series cp 4 achieved a total of 77 points , equating to 88% of the theoretically possible maximum points . in comparison , cp 3 scored 72 points ( p=0.407 ) and series cp 1 and 2 scored 65 points each ( p=0.055 and p=0.108 , respectively ) . the sharpness of the transition between epithelium and stroma , in particular , was inspected . comparing the four series with their different cutting heads , deeper keratotomies led to a sharper cutting edge . the 150 m cutting head ( series cp 3 and 4 ) scored significantly higher than the 110 m cutting head used in series cp 1 ( p<0.05 ) . the group with a higher blade cutting rate ( cp 4 ) also scored slightly higher . our investigations revealed that different head - advance speeds and different cutting heads did not influence the surface of the keratotomies significantly , although there was a trend toward better surface quality using cutting heads performing a deeper keratotomy ( figure 4b ) . further , higher head - advance speeds ( lower oar ) seemed to be advantageous for a smoother interface . as such , we observed a higher surface regularity in the series with 5 mm / second advance speed ( figure 4a ) . this trial was scored 77 points , whereas the same settings , except with a 3 mm / second head advance , scored 72 points . overall , the scores of the different series ranged from 65 to 77 points ( theoretical maximum 88 points ) . on the basis of scanning electron micrographs , certain surface morphologies , such as tissue ruptures or parallel waves , could be visualized . changing the cutting heads from 110 m over 130 m to 150 m , the standard deviation decreased ( 0.77 over 0.73 to 0.66 ) . the surface scored a mean of 8.125 points for the 110 m and 130 m cutting head . the 150 m head scored 9 points ( theoretical maximum of 11 points ) . after reducing the oar , the amount of the inhomogeneous areas led to different evaluations , as shown in table 4 . evaluating criterion a , which characterized the overall appearance of the interface , there was a tendency toward more irregularities in series cp 1 and 2 , with the 110 m and the 130 m cutting head , respectively . the other cutting surface criteria ( b  d ) were taken together as the regularity index . more specifically , it was the percentage amount of irregular areas ( criterion c ) and the integrity of the surface regularity ( criterion b ) that exhibited the main differences . comparing the surface scores of the different series series cp 4 achieved a total of 77 points , equating to 88% of the theoretically possible maximum points . in comparison , cp 3 scored 72 points ( p=0.407 ) and series cp 1 and 2 scored 65 points each ( p=0.055 and p=0.108 , respectively ) . the sharpness of the transition between epithelium and stroma , in particular , was inspected . comparing the four series with their different cutting heads , deeper keratotomies led to a sharper cutting edge . the 150 m cutting head ( series cp 3 and 4 ) scored significantly higher than the 110 m cutting head used in series cp 1 ( p<0.05 ) . the group with a higher blade cutting rate ( cp 4 ) also scored slightly higher . our investigations revealed that different head - advance speeds and different cutting heads did not influence the surface of the keratotomies significantly , although there was a trend toward better surface quality using cutting heads performing a deeper keratotomy ( figure 4b ) . further , higher head - advance speeds ( lower oar ) seemed to be advantageous for a smoother interface . as such , we observed a higher surface regularity in the series with 5 mm / second advance speed ( figure 4a ) . this trial was scored 77 points , whereas the same settings , except with a 3 mm / second head advance , scored 72 points . overall , the scores of the different series ranged from 65 to 77 points ( theoretical maximum 88 points ) . on the basis of scanning electron micrographs , certain surface morphologies , such as tissue ruptures or parallel waves , the reproducibility increased with the depths of keratotomy . changing the cutting heads from 110 m over 130 m to 150 m , the standard deviation decreased ( 0.77 over 0.73 to 0.66 ) . the surface scored a mean of 8.125 points for the 110 m and 130 m cutting head . the 150 m head scored 9 points ( theoretical maximum of 11 points ) . after reducing the oar , involved instruments have to provide a maximum of safety , accuracy , and simplicity with regards to handling and performance . this study showed morphologic characteristics of keratotomies performed by the carriazo - pendular , a modern mechanical microkeratome . this study assessed the influence of variable depths of keratotomies and different head - advance speeds while using a constant oscillation frequency . the carriazo - pendular microkeratome offers a unique and modern design for preparation of lamellar keratotomies such as lasik or descemet s stripping automated endothelial keratoplasty ( dsaek ) . it is one of the most used devices for mechanical flap preparation in lasik procedure in germany.13 an advantage is its special type of cutting ( molding technique ) . realized by the cutting head design fulfilling a convex applanation of the cornea , the created flap shows a planar configuration.11,14 this offers advantages for the later ablation by the excimer laser . because of the constant pressure on the cornea , fewer buttonholes appear.15 a homogenous stromal surface after flap preparation leads to better visual outcome , and high oscillation frequencies combined with a fast blade rate lead to higher reproducibility,4,5 while an elevation of head advance using the same oscillation frequencies created worse surface qualities . using lower oars , better cutting results were observed.16 higher oscillation frequencies combined with lower advance speed possibly increases the horizontal tissue dislocation because of more blade movement without real cutting the collagen fibers . on the other hand , if the head advancement per oscillation is lower , the tissue is cut more cleanly and with less dragging and chattering.17 contrary to our results , no influence of different head advance speeds on the interface smoothness has been previously revealed in porcine eyes.18 characteristic serrations have been observed on the cutting edges , as described in earlier studies.12,19 reducing the cutting depth , the edge configuration becomes increasingly blurred . the trial series with the 110 m head ( 3 mm / second head advance ) was scored 10 out of the theoretical maximum of 24 points ( 42% ) . in comparison to the 150 m head ( 3 mm / second head advance ) , a cutting head of 130 m scored 12 points , which is 50% of the theoretical maximum . the head advance did not affect the cutting edge quality significantly , but there was a trend toward better results in the trial using 5 mm / second head advance ; here , 19 of 24 points were achieved ( 79% ) . the series with 3 mm / second scored 17 of 24 points ( 71% ) . to our knowledge , clinical studies proving the impact of keratotomy depth on postoperative complications such as flap dislocation are not available to date . nevertheless , it is essential to leave a residual stromal thickness of at least 250 m after cutting and laser ablation in order to avoid iatrogenous corneal ectasia . it would be desirable to investigate the lowest cutting depth with which cutting edges present the best morphology.20 sarayba et al investigated nine human corneas using sem micrographs and showed a smoother stromal bed after application of a 30 khz fs - laser when flaps with 110 m thickness were created.21 this may have been due to the fact that they used human corneas and different parameters of photodisruptive preparation technique . it seems very probable that fs - lasers can generate smoother cutting surfaces , although thinner flaps are created . the small sample size in sarayba et al s study reduces its validity.21 in this context , fs - lasers might be advantageous over microkeratomes as they can produce very thin flaps while the cutting edges remain extremely sharp.22 porcine corneas are established models simulating human tissue;12,19,22,23 however , it would have been desirable to investigate human eyes because there are ultrastructural differences that limit the comparability of both tissues . the human cornea , having a strictly organized bowman s layer , is more rigid . as the porcine corneal tissue is smoother it might visualize cutting characteristics better.24 considering this aspect , porcine eyes could be regarded as a sensitive model showing the characteristics of lasik instruments on surface quality of the cuts . in contrast , there are investigations demonstrating more surface irregularities in human corneas than in porcine corneas.18 the animal model can serve its purpose for investigation of different technical parameters , especially since large numbers of samples are available . the next step could be in situ investigations , such as in the study of ramrez et al , who studied reflective particles in the interface after either bilateral lasik by fs - laser or microkeratome on one eye using confocal microscopy , found that there was no statistically significant difference in the numbers of particles between each eye.25 the scoring system used in this study provided a very detailed description of cutting quality , but depended on the subjective evalu ation of the surgeons . to avoid this bias iatrogenic keratectasia is an ongoing problem , even if the residual stroma thickness is more than 300 m.26,27 the preparation of ultrathin flaps may lead to problems like microstriae , tissue particle dislocation into the interface , or flap dislocation.11,26 a sharp cutting edge is regarded as advantageous , avoiding corneal erosion , epithelial ingrowth into the interface , and scarring , as well as flap dislocation.19 serrations of the cutting edge could further support the flap adaptation . the steeper the cut , the less epithelial cells are altered , which could decrease epithelial ingrowth and unreasonably wound healing reaction . stronger tissue alteration caused by blurred cutting can cause more inflammation leading to stronger scaring as well as myopic regression.7,9 additionally , the cutting head design has an important effect concerning the development of epithelial defects.8 another possible application for microkeratomes is the preparation of corneal grafts for dsaek , in which it is desirable to create a thin and homogenous lenticule for transplantation . the carriazo - pendular microkeratome device in combination with an artificial anterior chamber is available for this type of surgery . in comparison to fs - laser - assisted preparation , the microkeratome seemed to be superior in regards to postoperative visual outcome.28,29 further developments in dsaek are made in creating an ultrathin flap by a microkeratome . after a first cut of 300 m , a second cut formed a 100 m lenticule , which is used for dsaek . with this technique , comparable visual outcomes to descemet s membrane endothelial keratoplasty could be reached , requiring less time and with fewer difficulties with donor tissue preparation.3032 good cutting results while preparing deeper keratotomies , in particular , could be advantageous . a reason for the better outcome after micro - keratome preparation in comparison to fs - laser preparation might be less tissue bridges.23,33 material and equipment costs play an important role in daily medical routine . microkeratomes such as the carriazo - pendular device could be a cost - saving alternative to the very expensive and vulnerable fs - laser devices . in a prospective randomized clinical study of visual outcome , higher - order aberrations and flap thicknesses of fellow eyes treated with fs - laser- or mechanical keratome - created flaps during laser in situ keratomileusis in myopes was investigated.34 besides more predictable flap thicknesses in the fs - laser group , there were no significant differences concerning visual outcome , whereas higher - order aberrations were lower in the micro - keratome group . in a meta - analysis , there were more epithelial defects after microkeratome use , but more cases of deep lamellar keratitis after fs - laser application.10 a deeper keratotomy enables a safer sliding of the blade in the corneal stroma . microkeratomes may offer a cost - saving alternative to fs - lasers . it still remains a wide field for indications for micro - keratome use such as lasik or dsaek . further studies should compare both techniques to provide more information about their advantages and disadvantages .