Keratomileusis is a lamellar refractive keratoplasty technique whose object is the correction of spherical refractive errors by modifying the radius of curvature of the anterior surface of the cornea through the modification of the corneal thickness.1 With the original technique of José Barraquer, after a piano lamellar keratectomy, the resected disc was frozen and the stromal tissue was removed from its posterior surface by the cryolathe. After being thawed and replaced back on its bed, the anterior corneal surface assumed a flatter or steeper curvature. The difficulty of the technique, the prolonged training period, the expense of the equipment, and the lack of consistent or satisfactory results have prevented most anterior segment surgeons from using this technique.
Another technique is the nonfreeze planar procedure of Barraquer- Krumeieh-Swinger,2·3 which avoids the cryolathing trauma and produces the refractive cut in parallel planes on the posterior surface of the excised corneal lenticule. In 1986, Ruiz (personal communication, American Academy of Ophthalmology, November 1986, Dallas, Tex) reported a new technique of keratomileusis called myopic keratomileusis-in-situ. Hofmann (personal communication, Ricardo Guimaraes, MD) independently described a sutureless form of KMIS involving an excimer laser photoablation second pass in July 1987 at the International Symposium on the Surgical Correction of High Myopia (Rio De Janeiro). In this technique, the microkeratome removes a primary piano corneal disc 7.25 mm in diameter and 150 µ?? thick and a secondary central piano lenticule from the remaining corneal bed, so that corneal flattening is induced after the primary disc is replaced and sutured on the host.
Most techniques of lamellar refractive corneal surgery are both dependent on and limited by the necessity for a microkeratome section across the central cornea. Technical improvements of the original work of Barraquer1 have eliminated the need to freeze or lathe the corneal cap. Current techniques of keratomileusis, including the excimer laser4 and mechanical'0 variations, and intracomeal lenses6·7 (collagen, hydrogel, or fenestrated high index plastic) cannot proceed to perfection without predictable, safe, consistent, and minimally traumatic microkeratome sections. The microkeratome looms paradoxically as both a pathway and a barrier to progress in lamellar refractive corneal surgery.
The present study evaluates and compares in human cadaver eyes three microkeratome systems (Automatic Corneal Shaper [Steinway Instrument Company, Ine, San Diego, Califl, Draeger Lamellar Keratome [Storz Instrument GmbH, Heidelberg, Germanyl, and Microprecision test model [Microprecision Instrument Company, Ine, Phoenix, Ariz]), comparing the accuracy of the resection diameter and thickness, the ultrastructure of the resected stromal surfaces, and the quality of the blades.
Comparison of Currently Available Microkeratome Systems
MATERIALS AND METHODS
Human cadaver eyes without previous ocular surgery were injected with and soaked in 70% Dextran in saline solution mixed in equal amounts with glycerin (U.S.R) for 24 hours to deturgesce the corneal stroma. Each eye was inflated with air through a 30-gauge needle tract into the optic nerve to a pressure of 20 to 24 mm Hg. The epithelium was removed and the globe was mounted into a Visitec/Zirm (Visitée, Ine, Sarasota, Fla) Visualeyes surgical practice system with the correct orientation relative to muscle insertions. Pachometry was accomplished with a solid head ultrasonic probe at the center and four mid peripheral points and, with the epithelium removed, all the corneal thicknesses were in the 440 µ?? to 500 µ?? range.
Two currently marketed suction microkeratomes, the Automatic Corneal Shaper (Steinway Instrument Company, Ine, San Diego, Calif) and the Draeger Lamellar Keratome (Storz Instrument GmbH, Heidelberg, Germany), and one test model microkeratome (Microprecision Instrument Company, Phoenix, Ariz) were prepared by corporate representatives after sufficient practice by an experienced refractive surgeon (R.F.H.). The Table shows the main characteristics of the four currently available systems for keratomileusis-in-situ. In November, 1990, four series of lamellar keratectomies were done, simulating contemporary surgical conditions: 1) In the first series, each machine performed five keratomileusisin-situ primary keratectomies of variable thicknesses and five secondary 100-micrometer thick keratectomies. Because the Draeger microkeratome is not currently designed for keratomileusis-in-situ (its offset mechanism is adjusted by precision spacer shims of preset dimension, every 50 µm from 100 to 400 µm, rather than by an adjustment screw), it was set to match a primary resection of 150 µm; 2) A second series of four eyes per system had single pass identical setting keratectomies to 150 µm depth with a fresh blade each time; 3) A third series of one eye per system had one deep (400 µm) lamellar keratectomy to simulate hyperopic keratomileusis; 4) The fourth Series, performed only with the Microprecision and Steinway instruments, was approximately 80 µ?? deep to simulate a thin resection.
The resected discs and the corneal bases were fixated immediately after the resections in 3% glutaraldehyde and prepared for scanning electron microscopy by sectioning the discs into quadrants, followed by critical point drying.8 The corneal stromal beds were examined at frontal view to measure the diameters of resection areas. The four mounted quadrant sections of each primary and secondary disc were oriented with the cut edge directly facing the detector to measure the attained thickness. A calibrated measuring reticle was utilized to determine edge thickness in the dehydrated state; this was multiplied by the in vivo human corneal hydration factor of 1.7681 to determine the net resection of tissue.9
Each manufacturer's blades were evaluated with the scanning electron microscope after a single resection, looking for manufacturing defects as well as surgically-induced changes. The Barraquer style systems made by Microprecision and Steinway used disposable steel blades. Only one Draeger, round, nondisposable stainless steel blade was available, and it was examined after all the resections.
The most striking finding on scanning electron microscopy of all the microkeratomes was vibrationinduced chatter, which created a periodic nonuniform thickness (Fig 1), present diffusely in all specimens. The chatter lines were arranged parallel to the edge of the blade; they had a regular pattern with gear or screw driven translational systems and an irregular pattern with hand advanced assemblies (Figs 2-3). The surfaces appeared smoother after sections made by the Draeger system (Fig 4).
Figure 1: Scanning electron micrograph of the periphery of a corneal bed after a primary keratectomy with the Microprecision test model. Chatter lines secondary to a periodic nonuniform thickness are present (x 600).
Figure 2: Scanning electron micrograph of a corneal bed after primary and secondary keratectomies with the Steinway system. Chatter lines (arrow) are straight and parallel, with regular periodicity (x 80).
Figure 3: Scanning electron micrograph of a corneal bed after a primary keratectomy with the Microprecision model. Chatter lines (arrow) are parallel and present a variable pattern ( x 80).
Figure 4: Scanning electron micrograph of a corneal bed after a primary keratectomy with the Draeger rotary system. The surface is granular and relatively smooth, with some peripheral chatter (arrow) (x 80).
Accuracy of Cut
All resected beds appeared round, with variation of ±0.10 mm along different diameters. Primary resections were easy to center and the secondary sections were more difficult. AU of the instruments created mean primary section diameters 10% to 12% smaller than the stated size of 7.20 mm regardless of depth. The average secondary resection diameters were about 10% smaller than attempted. However, the variation among systems for the 4.50-millimeter secondary cut diameter was remarkable: the Steinway and Draeger instruments had an error range of 6% and 8.9%, respectively, while the hand-pushed Microprecision system had an error range of 25%.
The accuracy of the thickness of the primary resected disc did not seem to be affected by the attempted depth of resection. Shallow cuts (less than 100 µm) showed increased chatter and surface irregularities. The Microprecision turbine manual system tended to cut thinner on primary resections and had a wide deviation from the attempted thickness (Fig 5). The most accurate instrument in terms of mean primary section thickness was the Barraquer Steinway (Fig 6). However, averages may be misleading, because all three machines demonstrated ranges of deviation from the attempted thickness of resection over 20 µm. The Draeger system cut an average 12.2 µm thinner than desired on primary resections and had a range of deviation similar to the Steinway system (Fig 7). It did not have a sufficient number of adjustment shims to attempt more precisely graded resection depths.
Figure 8 shows the secondary section thickness accuracy of the three systems : on average, Steinway tended to cut thicker (mean + 4.1 µm) and Microprecisión (mean -7.4 µm) and Draeger (mean -7.5 µm) tended to cut thinner than attempted. The range of deviation from the attempted secondary section thickness was wider with Steinway (22.0 µm) and Microprecision (24.1 µm) than with the Draeger system (7.8 µm).
Figure 5: Microprecision turbine system - accuracy of the primary keratectomy thickness. The y-axis shows the difference between measured and attempted primary disc thicknesses, and the x-axis shows the attempted primary disc thickness.
Figure 6: Steinway electric system - accuracy of the primary keratectomy thickness. The y-axis shows the difference between measured and attempted primary disc thicknesses, and the x-axis shows the attempted primary disc thickness.
Figure 7: Draeger rotary system - accuracy of the primary keratectomy thickness. The y-axis shows the difference between measured and attempted primary disc thicknesses, and the x-axis shows the attempted primary disc thickness.
Figure 8: Accuracy of the secondary keratectomy thickness in the three systems. The graph shows, for each group, the mean and the range of deviation from the attempted secondary lenticule thickness.
Quality of Blades
The blade of the two Barraquer style systems manifested consistently rough, scored, or fractured cutting edges (Fig 9). The round reusable Draeger circular blade was exquisitely and smoothly honed. No fracture lines, debris, or score lines could be found even after 15 repeated practice and experimental procedures with the same blade (Fig 10).
Lamellar refractive corneal surgery was introduced by José Barraquer1 in 1949 and involves the creation of "thick sections" (80 to 350 µm) of the cornea. While the theoretical basis and mechanical apparatus for the preparation of ultrathin tissue sections (20 µm) for electron microscopy have evolved to a refined level,8 corneal sectioning has made little progress in 40 years, despite the periodic appearance of new instruments.
The suction microkeratome developed by Barraquer is an elegant machine combining a suction ring, dovetail guide, planar compression plate, and motorized oscillating blade. With various combinations of rings and plates, the surgeon can create corneal discs with different diameters and thicknesses. The utilization of the original system is an art subject to a long learning curve punctuated by occasional tragedies, even for an acknowledged master. Suction rings can be difficult to center or reapply, can fail to generate sufficient adherence, and can create different cuts in different eyes. The dovetail slide drive can jam and sometimes create thickness variability due to the surgeon-dependent manual advancement rate of the cutting assembly.
Figure 9: Scanning electron micrograph of a Microprecision blade after a single use. The edge contour is disrupted and the surface is irregular ( x 700).
Figure 10: Scanning electron micrograph of a rotary blade from the Draeger system after 15 passes. The edge is regular and the surface is smooth ( x 3240).
The applanating plate that flattens the dome to create a surface parallel incision is so bulky that it forces a suction break in patients with tight eyelids and deep orbits. The offset plates block the surgeon's view of the blade passage and don't allow for continuously variable depth settings. The electric or gas turbine motors are subject to vibration, occasional loss of torque or failure, and are awkward to hold due to weight imbalance. The oscillating steel blade can be dull, rough, damaged in handling, and prone to warpage or material failure. These and other flaws must be overcome prior to wide dissemination of the lamellar techniques to the general ophthalmologist or to the public.
Microkeratome corneal sectioning occurs when an oscillating or rotating blade edge acts as a wedge.8 The splitting effect of the edge as a pure wedge is unlikely, especially in a fibrous, hydrated, nonembedded, resilient mesh work such as the cornea. Optimally, a section is produced through a continuous displacement (compression) of a surface layer in the direction of advancement of the cutting apparatus. The compression yields a crack due to the shearing forces at the boundary between the stable noncompressed corneal base and the compressed surface. Continued advancement of the sharp wedge causes compression, splitting, and displacement of the surface layer due to plastic and elastic flow.8
Just as with the microscopic sectioning of embedded tissues,8 certain factors are of prime importance to microkeratome sectioning:
1. The tissue should be firm and densely packed. Plastic embedding is impossible, so the easiest way to handle the tissue is to increase intraocular pressure with a suction ring, which is used in all present microkeratome systems.
2. The area of the excised tissue should be as small as possible, preferably less than 2.00 mm diameter, to avoid periodic thickness variations (chatter). Unfortunately, keratomileusis procedures require a large area of section. If the depth of cut is of moderate thickness O200 µ??), as with intracorneal lenses or lamellar keratoplasty, then chatter is less of a problem with large area excision. Thin cut procedures (80 to 150 µ??) such as keratomileusis-in-situ are usually plagued by chatter with any mechanical cutting system over a large area.
3. The cutting edge of the blade should be smooth, sharp, and free of debris.
4. All movements of the microkeratome should be free from vibration. Sources of vibration include the blade motion, gear meshing, movement of the motor, and the surgeon's hand. The motion of an oscillating blade creates an accelerationdeceleration phenomenon, which sets up a harmonic (sinusoidal high frequency vibration) that propagates at different velocities and magnitude in different media. The machine, blade, and tissue are not synchronous, so chatter can occur with the current designs regardless of the motor speed, mechanism of translation of the assembly, angle of attack, or blade sharpness.
5. The advancing mechanism of the microkeratome should eliminate static friction or jamming. Safety interlock mechanisms for each component and between components do not exist on current models.
The Draeger rotary microkeratome features a smooth translation mechanism and a sharp blade. Its resections are quite atraumatic and consistent, although 10% under target for both thickness and diameter of primary resections. The handpiece is bulky and the applanation plate difficult to see through, especially with the indistinct calibration marks for optic zone size. The resected corneal discs have no protective catchment device and, if the surgeon does not hold it with a notched forceps, the corneal disc can fly off onto the floor or into the translation mechanism. The spacer shims do not presently allow for resections less than 50 µ thick. A micrometer adjustment, instead of shims, would improve its versatility. The Draeger system best fits into the market as a lamellar keratoplasty device for transplants or intracorneal lenses. With improvements in applanation plates, depth adjustment, and disc control, it has potential as a keratomileusis-insitu machine.
The Steinway oscillating microkeratome features some innovations, such as the automated translation that reduces surgeon's manual passage of the instrument, and the adjustable height of the suction rings, which eliminates the need for multiple dovetail rings. Suggested improvements include reduced vibration, sharper blades, and a smoother translational mechanism (screw or hydraulic), moving the bulky electric motor off the eye.
The Microprecision model introduced the gas turbine motor, similar to the SCMD. The other components, except for the depth adjustment, are similar to the classic Barraquer instrument. The manual translation may increase the variability of the resections. The blades were rough and chipped, creating significant trauma. The currently designed machine and its similar competitor with the gas turbine need improvements before they can be recommended for human use. As a result of this study, Microprecision has redesigned its unit significantly. A one-piece low vibration head, sharper blades, a flatter approach angle, and a video nontouch micrometer are being introduced to replace the test model of this study.
According to the scanning electron microscopic examination, the smoothest and most consistent sections occurred with the Draeger system (Fig 4) that incorporated the following features:
1) Machine driven smooth advance of 1 mm/sec.
2) Nearly flat clearance angle (3° to 5°).
3) Unidirectional slicing rotation.
4) Low vibration power source distant from the cutting and fixation assembly.
5) Well-honed round cutting edge blade.
6) Water coolant and lubrication of the resected tissue.
7) Solid nonvibrational blade mount.
Unpredictability of refractive outcome is an unresolved problem in keratomileusis.10,11 Section thicknesses measured in this study showed a significant variation for both primary (Figs 5-7) and secondary (Fig 8) resections. At a depth of 150 µ with a 4.50-millimeter secondary resection zone, based on keratomileusis-in-situ tables for myopia correction, every 10 µ represents approximately 1.00 diopter of myopic correction. Therefore, considering the results from the secondary resections (Fig 8), the Steinway machine would tend to overcorrect by 0.50 D with a variance of ± 1.00 D, the Draeger machine would tend to undercorrect by approximately 0.75 D with a variance of ±0.50 D, and the Microprecision machine would generally undercorrect by approximately 0.75 D with a wider range of ± 1.25 D. A further surgical thickness measurement error of 20 to 30 µ worsens the predictability.
In reported series of patients who underwent keratomileusis for myopia, loss of spectacle corrected visual acuity was a complication of variable frequency.12,13 Chatter lines, such as those observed in this study, could serve as an intrastromal diffractive optical source, sufficient to produce irregular astigmatism and reduce corrected visual acuity, particularly if the cap is not replaced with perfect alignment.
Microkeratome system manufacturers need to develop predictable, nonchatter-producing machines that any skilled eye surgeon can use. With greater attention to detail in parts manufacture, drive system design, blade sharpness, and low vibration power trains, each manufacturer has the potential ability to build a facile, efficient, and consistent microkeratome.
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Comparison of Currently Available Microkeratome Systems