Microkeratomes

performed on the remaining stromal bed (8). With these advances and the increasing popularity of the excimer laser, the microkeratome still had its place, but it was no longer used for reshaping. Instead, its primary purpose was the creation of the corneal flap.

The microkeratomes of today have become safer and have been proven successful in performing corneal lamellar cuts; however, complications with their use continue to occur. These complications have been decreasing with surgical experience, advancements, and design elements incorporated into newer microkeratomes. The current microkeratomes are by no means close to being free of complications, nor do they approximately an ideal microkeratome, which would optimize the following parameters: safety, ease of use and sterilization, reliability, moderate increase in IOP and minimal structural deformation during surgery, smooth beds, regular edges, reproducible thickness, sufficient hinge width, suction ring stability, and reasonable cost of hardware and disposables. Although currently used microkeratomes incorporate several features of the ideal microkeratome, they nevertheless sacrifice some obviously useful features in order to achieve others.

Typical components of a modern microkeratome system include the following (9):

Motor Microkeratome head

Applanator lenses to measure the diameter of the exposed cornea Vacuum fixation ring used to secure the eye

Flap stop ring, which limits the travel of the microkeratome head through the fixation ring

Foot switch

B. MICROKERATOME TYPES

Although microkeratomes share several similarities, they can be divided into five basic categories:

1.Nondisposable horizontal

A.Gear driven

B.Automated sliding

C.Manual

2.Nondisposable vertical

A.Automated

B.Manual

3.Disposable

A.Cable driven

B.Turbine driven

C.Gear driven

4.Water jet

5.Laser microkeratomes (picosecond)

C. NONDISPOSABLE HORIZONTAL MICROKERATOMES

Automated horizontal microkeratomes include the Bausch and Lomb Surgical/Chiron Automated Corneal Shaper; Summit Krumeich-Barraquer (SKBM); Herbert Schwind; B.B.- I-T-I, Allergan, and Nidek (similar to Summit), Med-Logics, LaserSight Technologies Ultrashaper, and Innovative Optics Innovatome. Manual microkeratomes in this category

2. Design

Automated horizontal microkeratomes may advance through the use of rotating gears between the microkeratome head and the suction ring (Fig. 4.2A) or through gearless sliding of the microkeratome head on the suction ring. Manual microkeratomes primarily consist of gearless sliding of the microkeratome head on the suction ring (Fig. 4.2B).

Gear-driven microkeratomes may have an increased likelihood of jamming due to operative debris and may also be more difficult to clean between procedures. Sliding designs may decrease these complications. The instances of microkeratome head slippage from the suction ring tracks in the manual microkeratomes have been practically eliminated in the newer automated sliding horizontal designs.

The ACS is a gear-driven nondisposable horizontal microkeratome, while the other instruments in this category utilize the sliding mechanism (Figs. 4.2B and 4.3). The Moria LSK-1 and the SCMD, both manual microkeratomes, run on a gas-turbine mechanism, while the others use an electric motor. Multiple flap sizes and depths are available. Technical features of individual nondisposable horizontal microkeratomes may be seen in Table 1.

3. Safety Issues

The original SCS microkeratome uses multiple components to ensure appropriate depths and flap size. Although this may allow flexibility, the incomplete, inappropriate tightening of this instrument may result in free caps (if stop is not screwed properly) or corneal perforation (if the depth plate is not tightened or mistakenly not inserted). Care must be taken

Table 1 Technical Features of Certain Nondisposable Horizontal Microkeratome

in the use of all microkeratomes to ensure that a variable depth plate is tightened to the proper level or that fixed depth plates are always inserted prior to keratomileusis.

4. Oscillation and Advancement Rate

Automated microkeratomes allow advancement at a constant, appropriately slow speed throughout the keratomileusis procedure. Slow advance rates allow the blades to perform their shearing action with decreased chance of scraping, plowing, or producing areas of the flap that are thinner than others. When using manual microkeratomes, the surgeon must maintain a slow, steady advance rate throughout the cut to ensure consistent flap thickness.

Fast oscillation speeds allow for a cut that has reduced ridges or “chatter” in the stromal bed, while slow oscillation speeds have increased ridges. Gas turbines have allowed microkeratomes to oscillate at faster rates; however, they require large gas canisters to remain in the operating room.

Postoperative irregular astigmatism may be caused by variation in thickness throughout a flap or increased ridges created in the corneal stroma. Therefore, to minimize induced astigmatism, the surgeon should aim for a consistent flap thickness and minimal ridges or “chatter” especially when using a manual unit (10,11).

The manual horizontal microkeratomes that we have studied, Moria and SCMD, attempt to decrease the chance of induced astigmatism with a faster oscillation speed, while the automated microkeratomes decrease the complication by a programmed advance rate. It may be appropriate to hypothesize that the chance of induced astigmatism can be minimized by combining a fast oscillation with a consistent advance rate, which is the guiding principle of most of the newer automated sliding units.

D. NONDISPOSABLE VERTICAL MICROKERATOMES

These microkeratomes are available in automated and manual forms. The automated microkeratomes include the Carriazo-Barraquer and M-2 made by Moria and the Bausch and Lomb Surgical/Chiron Hansatome. Moria/Schwind also make manual versions of the Car- riazo-Barraquer microkeratome.

1. Basics

In nondisposable vertical microkeratomes, the motor lies directly vertical or above the eye, rather than to the side of the eye. This instrument may be used to create a superior hinge (Fig. 1B). Vertical microkeratomes are equipped with a rotating head that travels to create the surgical arc (flap).

2. Designs

The primary difference between the two popular vertical microkeratomes is the presence of a geared track on the Hansatome compared to the smooth track of the Carriazo-Barra- quer (Fig. 4.4). As with the ACS, the geared track may cause this instrument to be more susceptible to jamming from debris; however, it provides the microkeratome with more torque (torque force* distance). Greater torque allows the microkeratome to perform its keratomileusis with minimal assistance from the surgeon. The Carrazo-Barraquer’s lack

Figure 4.4 Examples of nondisposable vertical microkeratomes. Note that the microkeratome motor is directly above the operating surface. (A) The geared outer track of the Hansatome suction ring. (B) The smooth outer track of the Carriazo-Barraquer suction ring. (C) Fully assembled Hansatome with suction ring, plate, and head. (D) Fully assembled Carriazo-Barraquer with suction ring and head. Note the microkeratome head in B is smaller than A. The M-2 Moria Microkeratome has virtually replaced the Carriazo-Barraquer microkeratome because it avoids the problem of flap thickness variability.

Table 2 Technical Features of Nondisposable Vertical Microkeratomes

of an outside gear, and hence its inferior torque, may require the surgeon to push the microkeratome head gently to initiate keratomileusis. This causes the automatic version of the Carriazo-Barraquer microkeratome to have a manual component.

Both the Hansatome and the Carriazo-Barraquer operate at high oscillation speeds, close to 15,000 rpm. The Hansatome has a faster advancement rate, while the Carriazo-Bar- raquer has a greater selection of flap diameters. Technical features of nondisposable vertical microkeratomes appear in Table 2.

3. Safety Issues

The Hansatome has made a few advancements over the ACS. First of all, it will not operate if the plate is not inserted. Second, the number of gears is reduced and they are better covered from debris. The Carriazo-Barraquer does not require a plate to be inserted and can fit in narrower spaces. In addition, it may produce a hinge in any peripheral corneal location.

E. DISPOSABLE MICROKERATOMES

These microkeratomes include the Refractive Technologies Flapmaker, the LaserSight Technologies Unishaper, and the Moria LSK-1 Disposable.

1. Basics

These are horizontal microkeratomes that have the advantage of requiring no assembly, disassembly, or cleaning. Each microkeratome head, blade, and suction ring combination is sterile, used once, and then discarded.

2. Designs

In disposables, the blade is preassembled in the head, and the head is engaged in the track. The head and suction ring are made of clear plastic, and the instruments operate by utilizing a sliding (cable driven or turbine driven) or a gear driven mechanism (Fig. 4.5).

The Moria LSK-1 Disposable is a manual microkeratome and uses the same control unit as the other Moria microkeratomes. The Flapmaker and Unishaper are automated and use electric motors. Technical features of disposable microkeratomes appear in Table 3.

3. Safety Issues

Disposable systems have the advantage of requiring fewer components to be set up by the surgeon; they also have improved sterility, thus decreasing the chance of operational error or surgical infection. In addition, the clear plastic of the disposable systems ensures good visibility for the keratomileusis procedure.

F. WATER JET MICROKERATOMES

These instruments include the Medjet Hydrokeratome, the Medjet Hydrorefractive Keratectomy hydrokeratome, and the Visijet hydrokeratome.

Figure 4.5 Examples of disposable microkeratomes. Note the pre-engaged plastic suction ring and head. See the smooth tracks of A and B and the geared track in C. (A) Refractive Technologies Flapmaker. (B) Moria LSK-1 disposable. (C) LaserSight Unishaper.

Table 3 Technical Features of Disposable Microkeratomes

1. Basics

The theory behind the water systems is that blades cause collateral tissue damage, while a beam of water can precisely cut between layers without leaving debris and provoking minimal wound response. In addition, the hydrokeratome can perform the cut without requiring an increase in intraocular pressure.

2. Designs

The Medjet Hydroblade Keratome uses a 30 m beam, and advances at 6.7 mm/s. It has a fixed depth plate and can produce superior, temporal, or nasal hinges. The Medjet Hydrorefractive Keratectomy (HRK) System is a water system that makes a bi-hinged flap with a C-shaped needle. The system attempts to ablate without the need of a laser. Instead, the shape and pressure of the template can be changed, along with the scan rate, in order to remove different amounts of tissue. The Visijet Hydrokeratome is also a water jet system that uses a 36 m beam and advances at 6.7 mm/s. It has a variable depth plate of 120–200m and a clear applanation lens for good visualization.

G. LASER MICROKERATOMES

These instruments include the Novatek Laser Microkeratome and the IntraLase Microkeratome. The Novatek microkeratome has not been used clinically, whereas the IntraLase Microkeratome (picosecond MK) has been in clinical use since early 2000 with outstanding results.

1. Basics

Laser systems create a precise flap with a laser or perform an intrastromal cavitation by carving a corneal lenticule with a laser beam. The vapor and fluid residues from intrastromal ablations dissipate a few hours after surgery.

2. Designs

The IntraLase Laser Microkeratome depends upon femtosecond pulses, so less energy is needed to photodisrupt corneal tissue, thus allowing more localized effects. It also allows a wide variety of corneal flap shapes. It also depends on an applanation plate to ensure consistent flap thickness. The Novatec Laser Systems Laser Microkeratome is expected to have eye tracking systems to monitor eye movements, giving a three-dimensional map for the laser’s location within the cornea. The IntraLase Microkeratome has been continously upgraded, so as to allow complete versatility and reproducibility, and may become the most commonly used system for corneal flaps in the future.

H. OTHER MICROKERATOMES

The Ophthalmic Technologies Inc./Loktal Medical Clear Corneal Molder allows for correction without photoablation. It can perform convex or concave cuts, thus correcting for

myopia and hyperopia. It has minimal vibration, allowing use of steel or diamond blades. It also has gearless tracks, is a manual microkeratome, and has separate motors for oscillation and advancement. It is adjustable, 1–500 m in depth, and the standard is fixed at 180m. Its blade speed is also adjustable to 24,000 rpm. It has a viewing window and automatically stops if suction is lost. In addition, it only cuts in forward motion.

Mastel Precision Surgical Instruments’ Buzard Barraqueratome has a diamond blade and needs no assembly. It provides good visualization of the flap and is compact, so it can fit small eyes. It has a two-chambered vacuum, which acts on the cornea and sclera to allow for a lower IOP. The depth plate is fixed at 160 m and the instrument makes nasally oriented flaps.

Hanna also makes a microkeratome. This instrument relies upon rotational motion and is known for the formation of a smooth bed.

I. DISCUSSION

Each of the above-mentioned microkeratomes attempts to decrease complications seen in LASIK surgery. Although no microkeratome is ideal, certain features are desirable in microkeratome systems and physicians should be encouraged to ask manufacturers about them:

Low level of flap complications

Consistent flap thickness

Good visibility

Fits into small eyes

Fixed depth plate

Loss of suction indicators

Table 4 Flap Combinations (Numbers indicate percent complication rates).

First and foremost, a suitable microkeratome should have a low level of flap complications. These complications depend on a multitude of factors, including experience and microkeratome safety features. As refractive surgeons know, there is a steep learning curve for LASIK, and regardless of microkeratome brand complications will decrease with time.

However, complications may be more apt to occur in certain microkeratomes (12–38). Farah’s work quantitates flap complications, and by including data from further studies, one may see the number of flap complications that have occurred with different microkeratome designs (1), specifically the ACS and the Hansatome (Table 4). In these studies, serious flap-related complications occurred in 4% of the ACS cases and 1% of the Hansatome cases. The Hansatome studies contained less severe complications. There were no free caps in the 440 eyes analyzed, and 10 of the 20 complications were a decentered flap. Flap decentration appears to be an inherent possibility in the Hansatome cases. Oftentimes, suction ring application causes movement of the globe. The suction must be stopped and the area recentered. However, in some cases, scleral indentation occurs when the ring fixes on the globe on the first cut attempt and the indentation may prevent adjustment (12). In these cases, a different suction ring or repeated attempts at recentration will solve the problem.

Consistent flap thickness is also important in performing LASIK. Most surgeons aim to leave a minimum of 250 m of residual cornea after ablation to prevent irreversible corneal edema. For patients whose residual corneal thickness is predicted to be close to 250m, according to refractive correction and preoperative pachymetry, a predictable flap thickness is essential. Many microkeratome companies have performed studies to indicate consistent flap thickness. Summit’s two studies showed varied results on flap thickness. A 12 eye study with the Summit Krumeich–Barraquer microkeratome indicated a 190 17 m cut with the 160 m head, while a 24 eye study indicated a 164 13 m cut with the same head. One study involving 93 eyes that underwent LASIK with the Moria LSK-1 microkeratome indicated that the 160 m head averaged 159 28 m (13). Pineda’s work on 25 eyes reveals a 117 17 m cut with the ACS, and his 21 eye study shows a 12729 m cut with the Hansatome; both studies attempted a 160 m cut (14). Pineda’s work and our experience indicate that the ACS and Hansatome provide cuts that are more often thinner than expected.

In addition to consistent flap thickness, design features that offer good visibility are an asset to all surgeons, especially those who are still in the steep portion of the learning curve (15–38). Proper visibility allows the beginner to learn correctly the positions and technical details of microkeratome operation and aids the expert in avoiding debris in the surgical path. Also, designs that are slim have benefits, since they can fit into the widest variety of patients, such as those with narrow eyes or redundant eyelid skin.

Microkeratomes that rely on fixed depth plates, rather than heads that can achieve variable depths, are preferable. First, if a surgeon needs to cut at a different depth, he or she may still do so using a different head. Second, human error may occur when inserting the depth plate into variable depth systems, thus leading to corneal perforation with high intraocular pressure. Such a complication may cause damage to various structures in the eye, including the iris, the ciliary body, and the lens (37).

Finally, safety features such as alarms, automatic shut-sown buttons, or suction displays are very beneficial. If the microkeratome continues forward progress after suction is lost, an irregular cap can occur with a cut through the central cornea. Scarring in this location would be in the visual axis.

As for future methods to perform lamellar cuts, more accurate blades with consistent

depths can be expected, along with laser systems that may cause less corneal disturbance. Time will tell where the future of microkeratomes lies, but one can be assured that microkeratome advances will continue (36–38).

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