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: to adhere to the globe, providing a stable platform for the microkeratome cutting head; and 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.
The microkeratome cutting head has several key components. The highly sharpened disposable cutting blade is discarded after each patient, either after a single eye or after bilateral treatment. The applanation head, or plate, flattens 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 corneal sensation to preoperative levels within 6–12 months after surgery.
Once the ring is properly positioned, suction is activated (Fig 5-4). The IOP should be assessed at this point because 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 from eyelashes under the suction ring or from 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 hand-held Barraquer plastic applanator or a pneumotonometer or palpation of the eye by the surgeon. Beginning surgeons are advised to use an objective rather than only a subjective method.
Prior to the lamellar cut, the surface of the cornea is moistened with proparacaine with glycerin or with nonpreserved artificial tears. 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 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 halted by the hinge-creating stopper, and then reversed off the cornea.
Before surgery, the microkeratome and vacuum unit are assembled, carefully inspected, and tested to ensure proper function. The importance of meticulously maintaining the microkeratome and carefully following the manufacturer’s recommendations cannot be overemphasized.
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, 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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