Lamellar Keratoplasty Techniques Research Articles

Historical Review and Update of Surgical Treatment for Corneal Endothelial Diseases.

The cornea remains in a state of deturgescence, maintained by endothelial cell Na+/K+ ATPase and by tight junctions between endothelial cells that limit entrance of fluid into the stroma. Fuchs’ endothelial corneal dystrophy (FECD) was initially described by Fuchs in 1910 as a combination of epithelial and stromal edema in older patients. It manifests as bilateral, albeit asymmetric, central corneal guttae, corneal edema, and reduced vision. When edema is severe, the corneal epithelium can detach from its basement membrane, creating painful bullae on the anterior surface of the cornea. The course of this dystrophy can be further accelerated after intraocular surgery, specifically cataract extraction. Pseudophakic bullous keratopathy (PBK) is endothelial cell loss caused by surgery in the anterior chamber. If the corneal endothelium is damaged during surgery, the same spectrum of symptoms as found in FECD can develop. In the nineteenth century, penetrating keratoplasty was the only surgical procedure available for isolated endothelial disease. In the 1960s, Dr. José Barraquer described a method of endothelial keratoplasty using an anterior approach via laser-assisted in situ keratomileusis (LASIK) flap. In 1999, Melles and colleague described their technique of posterior lamellar keratoplasty. Later, Melles et al. started to change host dissection using simple “descemetorhexis” in a procedure known as Descemet’s stripping endothelial keratoplasty. Following the widespread adoption of Descemet’s stripping automated endothelial keratoplasty, the Melles group revisited selective Descemet’s membrane transplantation and reported the results of a new procedure, Descemet’s membrane endothelial keratoplasty (DMEK). Recently, some eye banks have experimented with the preparation of DMEK/Descemet’s membrane automated endothelial keratoplasty donor tissue that may help the surgeon avoid the risk of tissue loss during the stromal separation step. Recently, the authors described a new bimanual technique for insertion and positioning of endothelium–Descemet membrane grafts in DMEK.Electronic supplementary materialThe online version of this article (doi:10.1007/s40123-014-0022-y) contains supplementary material, which is available to authorized users.

Surgical strategies to improve visual outcomes in corneal transplantation

The recent years have brought about a sea change in the field of corneal transplantation with penetrating keratoplasty being phased to newer lamellar keratoplasty techniques for a variety of corneal pathology. Improved and innovative surgical techniques have allowed selective replacement of diseased host corneal layers with pre-prepared healthy donor corneal lamellae for anterior corneal disorders such as keratoconus and posterior corneal disorders such as Fuch's corneal endothelial dystrophy. The results of lamellar techniques are encouraging, with rapid visual rehabilitation and vastly reduced risk of immune-mediated transplant rejection. The techniques of deep anterior lamellar keratoplasty and Descemet's stripping endothelial keratoplasty (DSAEK) continue to evolve with advent of femtosecond lasers and newer concepts such as pre-conditioned donor corneas for Microthin DSAEK and Descemet's membrane keratoplasty. This review describes the current developments in lamellar keratoplasty, including the futuristic approach using cell therapy to restore vision in corneal blindness.

Novel spatula and dissector for safer deep anterior lamellar keratoplasty

Objective: We describe a novel spatula and dissector to facilitate the big-bubble technique in deep anterior lamellar keratoplasty (DALK). Methods: A 29-year-old man who was diagnosed with bilateral keratoconus underwent deep anterior lamellar keratoplasty (DALK). After 350μm partial thickness incision of the recipient cornea, the Bonfadini dissector was inserted at the deepest point in the peripheral incision and could be advanced to the center of the cornea safely because of its semi-sharp tip. After achieving the big-bubble (BB) separation of Descemet membrane (DM) from the overlying stroma, the anterior stromal disc was removed. Viscoelastic material was placed on the stromal bed to prevent uncontrolled collapse and perforation of DM during the paracentesis blade incision into the BB. We could detect the safe opening of the BB using the Bonfadini dissector by the leakage of air bubbles into the viscoelastic material. After injecting viscoelastic material into the BB space, we inserted the Bonfadini spatula into the bigbubble safely because of its curved profile and blunt edges. The groove along the length of the Bonfadini spatula enables safe and efficient incision or the residual stromal tissue using the pointed end of a sharp blade while protecting the underlying DM. After removal of posterior stroma, the donor button was sutured with 16 interrupted 10-0 nylon sutures. Results: This technique and the use of the Bonfadini spatula and dissector facilitate exposure of Descemet membrane. Conclusion: The smooth Bonfadini DALK spatula and dissector facilitate safe and efficient completion of DALK surgery.

Deep anterior lamellar keratoplasty with a manual spatula: anatomical and functional results.

Objective:Evaluate the anatomical, refractive, and functional results of an innovative technique of deep anterior lamellar keratoplasty with a manual spatula.Materials and Methods:We evaluated the results and examinations of 16 eyes from 14 patients who underwent deep anterior lamellar keratoplasty using the technique described by Ferrara. Residual bed thickness after keratoplasty was evaluated postoperatively using Visante. The measurement was performed using a technique similar to that used to measure flap thickness after laser in situ keratomileusis refractive surgery. The measurements were performed at the center of the cornea in an area comprising the central 3-mm in the 45° and 135° meridians.Results:Best-corrected visual acuity was 0.34 ± 0.18 LogMar (0.09 to 0.60 LogMar), the spherical equivalent was -4.31 ± 3.38 D (+0.25 to -9.50 diopters), and keratometry was 45.75 ± 2.77 D (41.11 to 52.48 diopters) postoperatively. Corneal astigmatism was 3.19 ± 2.78 D (0.18 to 11.81 diopters). Residual stromal bed thickness measured by optical coherence tomography showed values of 67.1 ± 24.3 μm (30 to 109 μm). The statistical correlation by Spearman's test between the best-corrected visual acuity and the residual stromal bed thickness was 0.11 (P = 0.67).Conclusion:Deep anterior lamellar keratoplasty, in which manual dissection was performed using an instrument similar to that used to implant corneal rings, provided good visual and anatomical results.

Modified Femtosecond Laser–Assisted Sutureless Anterior Lamellar Keratoplasty

A variation of the femtosecond laser-assisted sutureless anterior lamellar keratoplasty technique using a femtosecond laser incision for surgical management of anterior corneal disease is described. Six patients with corneal scars were treated with the laser to create a horizontal lamellar bed interface in the cornea of the donor and recipient eyes, with a manual partial-thickness vertical trephination to complete the excisions. This technique allows matching of donor and host tissue dimensions with precise tissue apposition and greater surface area for healing. No intraoperative adverse events were observed. One patient developed epithelial ingrowth, requiring a successful surgical intervention.

Microbubble Incision as a New Rescue Technique for Big-Bubble Deep Anterior Lamellar Keratoplasty With Failed Bubble Formation

To describe a new surgical technique allowing dissection down to Descemet membrane in big-bubble deep anterior lamellar keratoplasty (DALK) with failed big-bubble formation (the "microbubble incision technique"). This is an interventional case series of 10 consecutive patients with keratoconus undergoing intended big-bubble DALK with failure to establish a normal big bubble. In all patients, repeated air injections into the stroma were performed, leaving a whitish colored stroma. Lamellar dissection as far down as possible was then performed within this white tissue. As soon as the anterior chamber was visible, a large remaining intrastromal air bubble was incised with a sharp 15-degree knife introduced perpendicular to the tissue to open up this predescemetic bubble. If deeper air bubbles were still visible, this approach was repeated. Using a blunt spatula, this new layer was then prepared and viscodissection performed. Using this novel approach, in 9 of the 10 patients, it was possible to dissect down to Descemet membrane. Macroperforation made conversion to penetrating keratoplasty necessary in 1 patient. Microperforations not necessitating conversion occurred in 2 patients. All 9 patients with "rescued" DALK had an uneventful postoperative course and had a mean visual acuity of 20/63 ± 20/125 (range, 20/500-20/50) and a mean endothelial cell count of 1672 ± 163 cells per square millimeter (range, 1493-1867 cells/mm) at 3 months. Microbubble incision is a new rescue technique for big-bubble DALK patients without bubble formation allowing for a safer dissection down to Descemet membrane.

Lamellar Keratoplasty: A Literature Review

The concept of lamellar keratoplasty (LK) is not new. However, newer forms of lamellar keratoplasty techniques have emerged in the last decade or so revolving around the concept of targeted replacement of diseased corneal layers. These include anterior lamellar keratoplasty (ALK) techniques that aim to selectively replace diseased corneal stroma and endothelial keratoplasty techniques aiming to replaced damaged endothelium in endothelial disorders. Recent improvements in surgical instruments and introduction of new techniques as well as inherent advantages such as preservation of globe integrity and decreased graft rejection have resulted in the reintroduction of LK as an acceptable alternative to conventional PK. In this review, indications, benefits, limitations, and outcomes of various anterior and posterior lamellar keratoplasty techniques are discussed.

Ferrara Technique of Deep Anterior Lamellar Keratoplasty for Keratoconus Treatment

Abstract Purpose To report the first clinical results of application Ferrara's technique of deep anterior lamellar keratoplasty (FDALK) in patients with keratoconus. Materials and methods Thirty-four eyes of 34 patients with advanced keratoconus were included in the study. All patients were operated by the FDALK technique. Preoperative and postoperative uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), corneal topography and corneal astigmatism were evaluated. Intra- and postoperative complications are described. Results Uncorrected visual acuity improved in from 20/400 to 20/125; corrected visual acuity improved from 20/300 to 20/48. The mean follow-up was 19.3 months (sd: 7.1). Corneal topography (Pentacam®) showed significant corneal flattening in all cases. Microperforations of the Descemet's membrane, requiring conversion to penetrating keratoplasty, occurred in two cases. Conclusion The FDALK technique is a safe and effective technique to be applied in patients with keratoconus. Visual outcome is comparable to PK, with advantage of preserving recipient endothelium, which lessens the risk of endothelium rejection. How to cite this article Ferrara P, Ferrara G, Torquetti L. Ferrara Technique of Deep Anterior Lamellar Keratoplasty for Keratoconus Treatment. Int J Kerat Ect Cor Dis 2013;2(2):51-55.

Hybrid Technique of Lamellar Keratoplasty (DMEK-S)

Purpose: To evaluate the outcomes of the hybrid technique of posterior lamellar keratoplasty (DMEK-S). Materials and Methods: 71 eyes of 55 patients enrolled in a single-center study underwent posterior lamellar keratoplasty with a hybrid lamella DMEK-S implanted using a solution implantation technique, owing to endothelial dysfunction. The outcome measures studied were visual acuity and endothelial cell density. Results: The rate of endothelial cell loss caused by surgery was 43.8%. During followups, we observed the stabilization of postoperative findings, or at minimum a very low rate of corneal endothelial cell loss. The UCDVA and BCDVA dramatically improved postoperatively. The rebubbling rate in our group of patients was 61.9%. We replaced the lamella due to its failure or malfunction in 17 patients (23.9%). Conclusion: In summary, DMEK-S combines the advantages of DSEK/DSAEK and DMEK. The central zone of bare Descemet's membrane and endothelium allows for very good visual outcomes, and the peripheral rim allows for better manipulation of the lamella during implantation. It is an effective method of treating the endothelial dysfunction of various etiologies, but the high complication rate needs to be addressed before widespread implementation of the technique in the future.

Rupture Pressure of the Posterior Lamella Obtained During Big-Bubble Deep Anterior Lamellar Keratoplasty

To investigate the pressure required to rupture the posterior lamella obtained during the presumed Descemet membrane-baring big-bubble technique of deep anterior lamellar keratoplasty (DALK). DALK using the big-bubble technique was carried out on donor corneoscleral discs mounted on an artificial anterior chamber. Once the anterior lamella was removed, the chamber was connected to a mercury manometer. The pressure inside the chamber was increased until rupture occurred or the manometer reached its measurement limit. The deep lamella ruptured at 252 mm Hg in one cornea, at 270 mm Hg in another, and had not ruptured at 300 mm Hg (upper limit of the manometer) in the remaining 18 donor corneas. The posterior lamella of big-bubble DALK probably confers significant structural integrity on the globe.

Anwar versus Melles Deep Anterior Lamellar Keratoplasty for Keratoconus: A Prospective Randomized Clinical Trial

To compare the outcomes of 2 techniques (Anwar vs. Melles) of deep anterior lamellar keratoplasty (DALK) in patients with keratoconus. Randomized, double-blind clinical trial. Fifty-seven eyes of 57 patients 20 to 35 years of age were enrolled. Patients with clinical diagnosis of keratoconus who were contact lens intolerant and whose corrected distance visual acuity (CDVA) was less than 20/80 were enrolled. Eligible eyes were allocated randomly into 2 groups: the Anwar technique (23 eyes) or the Melles technique (25 eyes). The primary outcome measure was CDVA. Secondary outcomes were spherical equivalent, contrast sensitivity, corneal aberrations, corneal biomechanical properties, endothelial cell count, and central corneal thickness. All outcomes were compared 15 months after surgery. The CDVA was 0.17 ± 0.09 logarithm of the minimum angle of resolution (logMAR) units and 0.18 ± 0.11 logMAR units in the Anwar and Melles groups, respectively (P = 0.803). Spherical equivalent was -1.82 ± 2.7 diopters (D) and -2.69 ± 3.94 D in the Anwar and Melles groups, respectively (P = 0.155). Overall, the difference in photopic and mesopic contrast sensitivity function between the 2 groups was statistically significant (P<0.05). There was no significant difference between 2 groups in total and higher-order aberrations up to the fifth order (P>0.05 for all parameters). Corneal hysteresis was not significantly different between the 2 groups (9.9 ± 0.8 vs. 9.9 ± 0.6; P = 0.606). The corneal resistance factor was 10.02 ± 0.8 and 10.13 ± 0.76 (P = 0.509). There was no significant difference in percentage of endothelial cell loss between the 2 groups (1 ± 2% vs. 1 ± 3% in the Anwar and Melles groups, respectively; P = 0.869). Mean central corneal thickness was 525.56 ± 47.87 μm versus 504.64 ± 54.20 μm in the Anwar and Melles groups, respectively (P = 0.155). The Anwar and Melles techniques of DALK have comparable visual acuity and refractive outcomes, aberrometric profiles, biomechanical properties, corneal thicknesses, and endothelial cell densities. However, patients who underwent the Anwar technique showed better contrast sensitivity.

Light microscopic evaluation and scanning electron microscopic analysis of horse eyes following deep anterior lamellar keratectomy

OBJECTIVE To describe the technique of deep anterior lamellar keratoplasty (DALK) with Descemet's membrane (DM) exposure in horse eyes. Also, to compare the efficacy and safety of viscodissection and big-bubble techniques for DALK. ANIMALS STUDIED Thirty-four ex vivo horse eyes. PROCEDURE Deep anterior lamellar keratoplasty was performed in 34 ex vivo horse eyes. Two groups (Group V--viscodissection--2% sodium hyaluronate; Group A--air--big-bubble) of 17 eyes were studied. Other than the substance used, the surgical technique was similar for both groups. Nonperforated eyes were submitted for light microscopic histologic evaluation and scanning electron microscopic (SEM) analysis. RESULTS Group V--Perforations occurred in 18% of the eyes during surgery. Light microscopy revealed exposure of DM in 28% of the eyes with mean thickness of the remaining stroma being 70.4 μm. Group A--Perforations occurred in 42% of the eyes. Light microscopy revealed exposure of DM in 60% of the eyes with mean thickness of the remaining stroma being 23.3 μm. No significant differences in safety, efficacy and thickness of the remaining stroma (including all eyes or excluding those with DM exposure) were observed. SEM of the surgical site revealed a more even surface in those eyes with DM exposure compared to eyes with thicker remaining stroma in both groups. CONCLUSIONs We describe two DALK techniques (viscodissection and big-bubble) for use in horses. No significant differences in safety, efficacy and thickness of the remaining stroma were observed. However, a nonsignificant trend toward the big-bubble technique being more efficacious but less safe was observed.

Big-Bubble Deep Anterior Lamellar Keratoplasty Assisted by Femtosecond Laser in Children

To report the early results of big-bubble deep anterior lamellar keratoplasty assisted by femtosecond laser in children. Five eyes of 5 pediatric patients were enrolled in the study; 3 had keratoconus and 2 corneal opacities. An IntraLase 60 KHz femtosecond laser (Abbott Medical Optics, Inc) was used to create mushroom incisions on both donor and recipient corneas. All patients were evaluated for best-corrected visual acuity, spherical equivalent, refractive astigmatism, keratometric astigmatism, mean K value, and corneal thinnest point. The big bubble was always achieved, and all eyes were treated successfully without intraoperative complications. The follow-up was of 10 months. At 10 months (at least 3 months after complete suture removal), the mean postoperative best-corrected visual acuity was 20/30 (range, 20/25 to 20/30), mean spherical equivalent was -1.8 ± 1.2 diopters (D) (range, -0.25 to 1.25 D), mean refractive astigmatism was 1.8 ± 1.4 D (range, 0 to 4.0 D), mean keratometric astigmatism was 5.1 ± 2.1 D (range, 3.5 to 8.59 D), mean K value was 46.2 ± 0.8D, and mean corneal thinnest point was 581 ± 46 μm (range, 511-638 μm). Our early findings suggest that the big-bubble technique in deep anterior lamellar keratoplasty assisted by femtosecond laser is safe and effective also in pediatric patients in the attempt to decrease the rejection percentage, improve the refractive outcome, and then provide an antiamblyopic effect.

Dual Laser-Assisted Lamellar Anterior Keratoplasty With Top Hat Graft

To develop a dual laser-assisted lamellar anterior keratoplasty (LALAK) technique, using excimer and femtosecond lasers to perform surgery on eye bank eyes. First, we compared corneal stromal surfaces produced by (1) deep excimer ablation, (2) femtosecond lamellar cuts, and (3) manual dissection and evaluated the effect of excimer laser smoothing with fluid masking on each surface. Masked observers graded scanning electron microscopy images on a 5-point roughness scale. Then, we performed a 6-mm diameter excimer laser phototherapeutic keratectomy ablation to a residual bed thickness of 200 μm, followed by laser smoothing. We used the femtosecond laser to cut donors in a modified top hat design with a thin tapered brim, which fitted into a manually dissected circumferential pocket at the base of the recipient bed. Fourier domain optical coherence tomography was used to measure corneal pachymetry and evaluate graft fit. Deep excimer ablation with smoothing (n = 4) produced a significantly (P < 0.05) smoother surface (grade = 3.5) than deep excimer alone (n = 4, grade = 3.8) or manual dissection with (n = 1, grade = 3.8) and without smoothing (n = 1, grade = 4.8). Deep femtosecond cuts (n = 2) produced macroscopic concentric ridges on the stromal surface. Experimental LALAK was performed on 4 recipients prepared by deep excimer ablation and 4 donors cut with the femtosecond laser. After suturing, good peripheral graft-host match was observed on Fourier domain optical coherence tomography imaging. These preliminary studies show that the LALAK technique permits improved interface smoothness and graft edge matching. Clinical trials are needed to determine whether these improvements can translate to better vision.

Klinische Ergebnisse nach Descemet-Membran-Endothelzelltransplantation

Diseases of the endothelial cell layer represent a common indication for perforating keratoplasty. In recent years posterior lamellar keratoplasty techniques have undergone a revival. The latest and most promising advancement is the isolated transplantation of Descemet's membrane (DM) with the endothelial layer - also known as Descemet's membrane endothelial keratoplasty (DMEK). This study was conducted to evaluate the clinical results of our DMEK patients and to assess the perioperative management. 70 patients (75 eyes) with endothelial cell decompensation (50 eyes with Fuchs endothelial dystrophy and 25 eyes with bullous keratopathy) had undergone DMEK surgery at the Tübingen Eye Clinic. Visual acuity, refractive error, intraocular pressure, slit lamp examination, pachymetry and endothelial cell density were considered and re-examined at intervals of 1, 2 and 4 weeks postoperatively with further 3 monthly follow-ups. The mean age of the 45 female and 25 male patients at time of surgery was 73 years (36 to 91 years). The mean follow-up period was 12.1 months. One patient received an autologous and 4 patients a triple procedure. The mean preoperative LogMAR visual acuity was 0.87 ± 0.41. After 1 week a LogMAR visual acuity of 0.82 ± 0.4 was observed (p = 0.544). At the final examination the LogMAR visual acuity was 0.32 ± 0.35 (p < 0.001, a highly significant result as compared to the preoperative value). The most common and important complication was the dislocation of the transplant which was seen in 23 eyes (31 %). The use of intracameral air pressurisation re-appositioned most transplants. Complications such as highly elevated intraocular pressure, epithelial inclusions or endophthalmitis were not noted in any patient. DMEK surgery lead to a significant visual rehabilitation in a majority of patients in a relatively short postoperative period. It may be considered as a gold standard to treat isolated endothelial diseases as has been implicated by other studies. Thus, the safety and efficiency of this new type of posterior lamellar keratoplasty technique has been confirmed.

Split-Cornea-Transplantation - ein neues Konzept gegen Spendermangel

Penetrating keratoplasty is at present the gold standard for corneal transplantation, but tremendous progress has been made in recent years in improving lamellar keratoplasty techniques, such as deep anterior lamellar keratoplasty (DALK) and Descemet membrane endothelial keratoplasty (DMEK). The purpose of this report is to describe split-cornea transplantation by combining DALK and DMEK as a novel concept to reduce donor shortage. The study consists of a PUBMED literature review and our own clinical results. Splitting of a single donor cornea into an anterior part (including epithelium, its basement membrane, Bowman layer, and stroma) for use in a DALK procedure in a patient with anterior-stromal disease (e. g., keratoconus) and into a posterior part (endothelium-Descemet membrane layer) for use in a DMEK procedure in a patient with endothelial disease (e. g., Fuchs endothelial dystrophy) can reduce the need for corneal donor tissue by around 50 %. A short-term 6-months follow-up has revealed good visual and refractive outcomes with low complication rates and acceptable endothelial cell loss. Split-cornea transplantation by combining DALK and DMEK surgeries is a promising concept to reduce donor shortage in corneal transplantation surgery in the future.

Turning the tide of corneal blindness

Corneal diseases represent the second leading cause of blindness in most developing world countries. Worldwide, major investments in public health infrastructure and primary eye care services have built a strong foundation for preventing future corneal blindness. However, there are an estimated 4.9 million bilaterally corneal blind persons worldwide who could potentially have their sight restored through corneal transplantation. Traditionally, barriers to increased corneal transplantation have been daunting, with limited tissue availability and lack of trained corneal surgeons making widespread keratoplasty services cost prohibitive and logistically unfeasible. The ascendancy of cataract surgical rates and more robust eye care infrastructure of several Asian and African countries now provide a solid base from which to dramatically expand corneal transplantation rates. India emerges as a clear global priority as it has the world's largest corneal blind population and strong infrastructural readiness to rapidly scale its keratoplasty numbers. Technological modernization of the eye bank infrastructure must follow suit. Two key factors are the development of professional eye bank managers and the establishment of Hospital Cornea Recovery Programs. Recent adaptation of these modern eye banking models in India have led to corresponding high growth rates in the procurement of transplantable tissues, improved utilization rates, operating efficiency realization, and increased financial sustainability. The widespread adaptation of lamellar keratoplasty techniques also holds promise to improve corneal transplant success rates. The global ophthalmic community is now poised to scale up widespread access to corneal transplantation to meet the needs of the millions who are currently blind.

Outcomes of deep anterior lamellar keratoplasty following successful and failed ‘big bubble’

AimThe most popular technique for deep anterior lamellar keratoplasty (DALK) is the ‘big bubble’ (BB) technique wherein air is injected in the cornea to create a bubble that separates Descemet's...

Modified Deep Anterior Lamellar Keratoplasty in the Management of Small and Large Epithelialized Descemetoceles

Descemetocele is a severe complication of corneal ulceration associated with a high risk of perforation. We describe a modified air-assisted deep anterior lamellar keratoplasty (DALK) technique for the surgical management of small and large epithelialized descemetoceles. Two representative cases are presented, one with a small (2 mm) and one with a large (4 mm) descemetocele. In both cases, the air-assisted lamellar dissection allowed a superficial keratectomy and access to the Descemet membrane. Viscoelastic was then directly injected through the stromal hole to separate the Descemet membrane from the posterior stroma, thereby allowing its complete removal. DALK was successfully performed using this modified air-assisted DALK technique. Visual acuity improved from counting fingers and hand motion to 20/40 and was maintained for at least 1 year after surgery. These cases demonstrate that the modified air-assisted DALK technique may be proposed for the management of small and large descemetoceles, allowing both tectonic and visual rehabilitation.

Hemi-automated lamellar keratoplasty (HALK)

AimTo describe a hybrid technique for anterior lamellar keratoplasty in corneas with topographical irregularities that circumvents the limitations of the microkeratome, namely reproduction of surface irregularities in the lamellar cut...