Lamellar flap creation can be performed using a mechanical microkeratome or a femtosecond laser. Many surgeons make asymmetric sterile ink marks in the corneal periphery, away from the intended flap hinge, just before placement of the suction ring. These marks can aid in alignment of the flap at the end of surgery and in proper orientation in the rare event of a free cap.
Before each surgery, the microkeratome and vacuum unit are assembled, carefully inspected, and tested to ensure proper functioning. The importance of meticulously maintaining the microkeratome and carefully following the manufacturer’s recommendations cannot be overemphasized.
The basic principles of the microkeratome and the role of the suction ring and cutting head are illustrated in Figure 5-3. The suction ring has 2 functions: (1) to adhere to the globe, providing a stable platform for the microkeratome cutting head, and (2) to raise the IOP to a high level, which stabilizes the cornea. The dimensions of the suction ring determine the diameter of the flap and the size of the stabilizing hinge. The thicker the vertical dimension of the suction ring and the smaller the diameter of the ring opening, the less the cornea will protrude, and hence a smaller-diameter flap will be produced. The suction ring is connected to a vacuum pump, which typically is controlled by an on–off foot pedal.
Figure 5-3 Schematic representation of the principles of a microkeratome. A, The suction ring serves as a platform for the microkeratome head, gripping the conjunctiva and sclera adjacent to the limbus. B, Simplified cross-section schematic of a typical microkeratome head. C, Creation of the flap. When the microkeratome head passes across the cornea, the applanating surface of the head flattens the cornea in advance of the blade.
(Illustration by Jeanne Koelling.)
The microkeratome cutting head has several key components. Its highly sharpened, disposable cutting blade is discarded after each patient, either after treatment of a single eye (if the patient is only having a single eye treated or if the surgeon chooses to discard the blade after the first eye) or after bilateral treatment. It is common practice to use the same blade on the second eye of the same patient.
The applanation head, or plate, serves to flatten the cornea in advance of the cutting blade. The length of the blade that extends beyond the applanation plate and the clearance between the blade and the applanation surface are the principal determinants of flap thickness. The motor, either electrical or gas-driven turbine, oscillates the blade rapidly, typically between 6000 and 15,000 cycles per minute. The same motor or a second motor is used to mechanically advance the cutting head, which is attached to the suction ring, across the cornea, although in some models the surgeon manually controls the advance of the cutting head. Smaller and thinner flap size and longer hinge cord length may be more important than hinge location in sparing the nerves and reducing the incidence and severity of dry eyes. Regardless of hinge type, patients generally recover most corneal sensation to preoperative levels within 6–12 months after surgery.
Once the ring is properly positioned, suction is activated (Fig 5-4). The patient should be notified prior to surgery that when the suction is applied, there may be some discomfort and vision may diminish temporarily. The IOP should be assessed at this point; low IOP can result in a poor-quality, thin, or incomplete flap. It is essential to have both excellent exposure of the eye, allowing free movement of the microkeratome, and proper suction ring fixation. Inadequate suction may result from blockage of the suction ports caused by eyelashes under the suction ring or redundant or scarred conjunctiva. To avoid the possibility of pseudosuction (occlusion of the suction port with conjunctiva but not sclera), the surgeon can confirm the presence of true suction by observing that the eye moves when the suction ring is gently moved, the pupil is mildly dilated, and the patient can no longer see the fixation light. Methods used to assess whether the IOP is adequately elevated include use of a handheld Barraquer plastic applanator or a pneumotonometer and palpation of the eye by the surgeon. Surgeons without extensive experience are advised to use an objective rather than a subjective method.
Before the lamellar cut is made, the surface of the cornea is moistened with proparacaine with glycerin or with nonpreserved artificial tears. Use of balanced salt solution should be avoided at this point because mineral deposits may develop within the microkeratome and interfere with its proper function. The surgeon places the microkeratome on the suction ring (if it is a 2-piece system) and checks that its path is free of obstacles such as the eyelid speculum, drape, or overhanging eyelid. The microkeratome is then activated, passed over the cornea (Fig 5-5) until it is halted by the hinge-creating stopper, and then reversed off the cornea.
Figure 5-4 Placement of a suction ring.
(Courtesy of Roger F. Steinert, MD.)
Figure 5-5 Movement of the microkeratome head across the cornea.
(Courtesy of Roger F. Steinert, MD.)
In addition, the surgeon should be aware that, regardless of the label describing the flap thickness of a specific device, the actual flap thickness varies with the type of microkeratome, patient age, preoperative corneal thickness, preoperative keratometry reading, preoperative astigmatism, corneal diameter, and translation speed of the microkeratome pass. It is important to maintain a steady translation speed to avoid creating irregularities in the stromal bed.
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Hardten DR, Feder RS, Rosenfeld SI. Mechanical microkeratomes. In: Feder RS, ed. The LASIKHandbook: A Case-Based Approach. 2nd ed. Philadelphia: Lippincott Williams & Wilkins; 2013:chap 4.
Kumano Y, Matsui H, Zushi I, et al. Recovery of corneal sensation after myopic correction by laser in situ keratomileusis with a nasal or superior hinge. J Cataract Refract Surg. 2003; 29(4):757–761.
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The femtosecond laser can also be used to create a lamellar dissection within the stroma. Each laser pulse creates a discrete area of photodisruption of the collagen. The greater the number of laser spots and the more the spots overlap, the more easily the tissue will separate when lifted. The femtosecond laser allows adjustments for several variables involved in making the flap, including flap thickness, flap diameter, hinge location, hinge angle, bed energy, and spot separation. Although the goal is to try to minimize the total energy used in flap creation, a certain level of power is necessary to ensure complete photodisruption. With the computer programmed for flap diameter, depth, and hinge location and size, thousands of adjacent pulses are scanned across the cornea in a controlled pattern that results in creation of a flap. Some potential advantages of the femtosecond laser include excellent depth control, reduction of complications such as buttonhole perforations, precise control of flap dimensions and location, and the ability to create pockets and channels within the cornea. Utilization of the femtosecond laser allows the geometry of the side cut to be modified in a manner that may reduce the incidence of epithelial ingrowth and flap slippage.
Femtosecond laser complications can occur, however. One study of 208 eyes showed that 1.9% had a loss of suction during femtosecond laser flap creation but that all had successful flap creation after reapplanation of the eye. Occasionally, an opaque bubble layer (OBL) may form from gas expansion into in the stroma adjacent to the flap interface and lead to improper flap creation. To prevent an OBL, most lasers now create a pocket deep within the cornea to disperse the gas away from the flap interface.
Although some variation exists between femtosecond lasers, all systems require centration and vacuum adherence to the patient’s cornea. Complete applanation of the cornea must be achieved, or an incomplete flap or incomplete side cut may result. Figures 5-6, 5-7, and 5-8 illustrate some components of the femtosecond laser. Video 5-2 demonstrates the use of a femtosecond laser for flap creation and the subsequent treatment with the excimer laser.
Figure 5-6 IntraLase femtosecond laser with cone attached.
(Reproduced with permission from Feder RS, Rapuano CJ. The LASIK Handbook: A Case-Based Approach. Philadelphia: Lippincott Williams & Wilkins; 2007:45, fig 2.7. Image courtesy of Robert Feder, MD.)
Figure 5-7 IntraLase suction ring.
(Reproduced with permission from Feder RS, Rapuano CJ. The LASIK Handbook: A Case-Based Approach. Philadelphia: Lippincott Williams & Wilkins; 2007:45, fig 2.8. Image courtesy of Robert Feder, MD.)
Figure 5-8 Docking of IntraLase cone with suction ring positioned on the eye.
(Reproduced with permission from Feder RS, Rapuano CJ. The LASIK Handbook: A Case-Based Approach. Philadelphia: Lippincott Williams & Wilkins; 2007:46, fig 2.9. Image courtesy of Robert Feder, MD.)
Once centration is confirmed on the laser, the surgeon administers the femtosecond laser treatment. The vacuum is then released, the suction ring is removed, and the patient is positioned under the excimer laser. A spatula with a semisharp edge is used to identify and score the flap edge near the hinge (Fig 5-9). The instrument is then passed across the flap along the base of the hinge, and the flap is lifted by sweeping inferiorly and separating the flap interface, dissecting one-third of the flap at a time and thus reducing the risk of tearing.
Several studies have compared the benefits of the mechanical microkeratome with those of femtosecond lasers for creating flaps. Minimal differences between the techniques have been found for most patients (Table 5-2).
Femtosecond laser procedure. Courtesy of George O. Waring IV, MD.
Figure 5-9 Flap lift technique following femtosecond laser application. A, After the flap edge is scored near the hinge on either side (black ovals), a spatula is passed across the flap. B, The interface is separated by starting at the superior hinge and sweeping inferiorly. C, Dissecting one-third of the flap at a time reduces the risk of tearing the hinge.
(Reproduced with permission from Feder RS, Rapuano CJ. The LASIK Handbook: A Case-Based Approach. Philadelphia: Lippincott Williams & Wilkins; 2007:48, fig 2.12. Image courtesy of Robert Feder, MD.)
Bryar PJ, Hardten DR, Vrabec M. Femtosecond laser flap creation. In: Feder RS, ed. The LASIK Handbook: A Case-Based Approach. 2nd ed. Philadelphia: Lippincott Williams & Wilkins; 2013:chap 5.
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Zhang ZH, Jin HY, Suo Y, et al. Femtosecond laser versus mechanical microkeratome laser in situ keratomileusis for myopia: metaanalysis of randomized controlled trials. J CataractRefract Surg. 2011;37(12):2151–2159.
Table 5-2 Advantages and Disadvantages of the Femtosecond Laser
Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.