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  • Creating LASIK Flaps: Femtosecond Laser vs. Mechanical Microkeratome

    Refractive Mgmt/Intervention
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    The emergence of femtosecond laser technology has revolutionized lamellar flap creation. The refractive surgery community continues to debate whether to use either a blade or laser to create corneal flaps. This article discusses the clinical advantages and disadvantages of the femtosecond laser and mechanical microkeratome for LASIK flap creation.

    Mechanical Microkeratomes

    Mechanical microkeratome devices (Figure 1) use high-precision oscillating-blade systems that dock to a suction ring to create a lamellar corneal flap while the cornea is held under high pressure. These devices have a proven history of excellent use and safety for LASIK flap creation. Translational microkeratomes create nasal hinges whereas pivoting systems create superior hinges. Disposable systems have single-use microkeratome heads housing the blade, and in some cases, single-use suction rings.

    Image courtesy Bausch & Lomb.
    Figure 1. Zyoptix XP microkeratome.
     

    Femtosecond Laser

    This system uses a YAG laser operating in the infrared wavelength to produce ultrashort pulses of energy to create adjacent areas of microcavitation (separation of tissue at the molecular level) at a specified depth in the cornea (Figures 2 and 3). The bubble created leaves a cavitation volume of 2 to 3 cubic µm. Thousands of these tiny bubbles, created in a raster pattern across the cornea, define the interface plane between the flap and the stromal bed. Bubbles are then stacked, starting around the edge of the interface, proceeding up through the epithelium to the corneal surface, creating the side cut and completing the flap creation.

    Image courtesy IntraLase.
    Figure 2. IntraLase FS 30.

    Image courtesy IntraLase.
    Figure 3. Photodisruption with a femtosecond laser.

    Advantages of Mechanical Microkeratomes


    1. Proven history

    Microkeratome technology is more than 20 years old (beginning with the ALK procedure) with many surgeons having performed thousands of procedures with very low complication rates.1,2 Femotsecond laser technology is a more recent development with clinical use approaching 9 years currently.

    2. Lower cost

    Mechanical microkeratome systems are relatively inexpensive compared with a femtosecond laser system.

    3. More efficient surgical flow

    Creation of a microkeratome flap typically takes less than 30 seconds from the time of suction-on to suction-off and is done just prior to the excimer ablation without moving the patient. The fourth-generation 60kHz IntraLase system typically requires about 30 to 45 seconds of suction-on time to create the flap (the laser portion of the flap creation takes only 15 to 20 seconds). Some excimer system designs, however, do not allow the same surgical bed to be used for the femtosecond laser flap creation and the excimer ablation. This necessitates moving the patient, sometimes between rooms, after the flap creation, before the excimer ablation, when the femtosecond laser is used.

    4. Ability to create flap in eye with anterior stromal opacity/scar

    A mechanical system can cut through opacities in many cases without difficulty or complication. Femtosecond laser flap creation is susceptible to vertical gas breakthrough in corneas with anterior stromal opacities or previous incisional surgery. This occurs because the gas that develops from cavitation following each laser pulse builds pressure and must escape from the cornea through the path of least resistance. A previous incision or scar anterior to the interface may represent the path of least resistance, thus leading to gas escaping to the surface of the flap in an undesirable location. Should this occur towards the center of the optical zone, consideration should be given to not lifting the flap. Once the cornea has healed, the surgeon can determine whether a mechanical microkeratome or a surface ablation is the most appropriate subsequent procedure. Use caution in setting a flap thickness less than 100 µm using the IntraLase as there is a higher chance of vertical gas breakthrough, even in the absence of scars/opacities evident at the slit lamp.

    5. Potential for epikeratome epithelial flap creation

    Several mechanical microkeratome systems have auxiliary equipment that allows for automated epithelial flap creation or removal (eg, epi-LASIK). This type of epithelial flap creation is not possible currently with the femtosecond laser.

    6. Less inflammation induced

    Diffuse lamellar keratitis (DLK), sterile inflammation confined to the lamellar interface following LASIK, is quite rare with today’s sterilization protocols in LASIK using a microkeratome. Early generation femtosecond lasers required significantly higher energy settings to create the flap in a clinically acceptable time due to the lower repetition rate. This led to opaque bubble layer (OBL) formation and increased incidence of DLK following surgery, requiring higher dosing of more potent corticosteroid drops than what is typically used for LASIK using a microkeratome. In addition, occasionally patients would note transient light-sensitivity (TLS) around 2 to 6 weeks following surgery. The latest generation IntraLase system operates at 60 kHz. This increased speed allows for lower energy settings with smaller spot sizes and less induced inflammation, essentially resolving the DLK, OBL, and TLS issues. OBL that occurs with the 60 kHz system is of a lower magnitude, resolves more quickly, and does not appear to be associated with increased incidence of DLK.

    Advantages of Femtosecond Laser


    1. Fewer flap-related complications

    Mechanical blade systems are susceptible to malfunction due to loss of power, jamming of mechanical parts, loss of suction during the pass, and operator error in use or assembly (improper assembly is mainly an issue with older mechanical systems). In addition, unusually shaped eyes are at risk for irregular flap creation. With very rare exceptions, these complications are either not possible to obtain or are clinically insignificant when using a femtosecond laser system.

    • Incomplete flap. A partial flap may be created if the mechanical microkeratome movement is compromised during the pass. This can occur if the oscillating blade stops functioning, the translation or pivoting motor malfunctions (eg, an eyelid drape binds in the translating mechanism), or if suction is lost. The presence of an incomplete flap usually contraindicates the excimer laser ablation at the time of surgery due to insufficient stromal bed exposure.3,4

    • Buttonhole flap. A buttonhole is a flap with a full-thickness incision from the interface to the surface located central to the intended flap edge. This can occur in eyes that have a relatively small white-to-white and/or steep central keratometry reading. The presence of a buttonhole flap contraindicates the excimer laser ablation at the time of surgery due to the irregular stromal ablation that will occur in the area of the buttonhole. In addition, the patient would be at increased risk of visually significant epithelial ingrowth in the future.5

    • Free flap. A free flap is a flap without a hinge resulting in a free-floating cap. This can occur in eyes that have large white-to-white and/or flat central keratometry reading. The presence of a free flap usually does not contraindicate the excimer laser ablation at the time of surgery due to adequate stromal exposure for the treatment. However, orienting the free cap can be challenging following ablation, particularly if the cornea was not marked prior to the pass or if the marks have worn off. A rotated free cap can induce irregular astigmatism.6

    • Irregular flap shape/contour. If a mechanical system binds and slows its translation and then releases and speeds up, both the edge of the flap and the thickness of the flap can become irregular. Upon lifting, it may be evident that there are areas of the flap that are much thinner than other areas, possibly with an incursion into Bowman’s layer (eg, a pseudobutton hole). Excimer laser ablation in these situations may result in an irregular ablation leading to induced aberrations and irregular astigmatism.

    2. Hinge position flexibility

    A mechanical microkeratome typically does not allow the surgeon to switch the position of the hinge (eg, nasal, superior, temporal) without switching to a different system. It may be advantageous to customize the hinge position in certain patients (eg, patient with high with-the-rule astigmatism may benefit from superior hinge position due to the deeper ablation required peripherally at 3 and 9 o’clock; dry eye patient may benefit from nasal or temporal hinge position due to decreased severing of afferent nerve trunks;7 hyperopic patient may benefit from temporal hinge position due to nasal angle kappa). The femtosecond laser systems allow flexibility in the hinge location for each patient through the software interface.

    3. Lower incidence of epithelial ingrowth

    Epithelial ingrowth may be seen more commonly in a microkeratome created flap than with a femtosecond laser created flap.8,9 This may occur due to the difference in the flap morphology at the edge. A mechanical microkeratome has a flatter entrance angle and a shallower transition angle between the side cut and the interface. The femtosecond laser allows the surgeon to program precisely the side cut angle, allowing for angles up to 90 degrees. One hypothesis is that the steeper transition angle created by the femtosecond laser system may inhibit the passage of epithelial cells into the interface.

    4. Less variability in flap thickness

    Although this is an extremely controversial topic, the general consensus in the literature is that there is a tighter standard deviation on flap thickness with the femtosecond laser vs. mechanical microkeratome.10-14 Flap thickness is a difficult parameter to measure as there is significant variability in ultrasound measurements based on position of the probe, stromal bed hydration, hydration status of the flap, etc. At our center, regardless of the method of flap creation, we have always measured flap thickness using the same subtraction ultrasound pachymetry technique and found that the measurements are tighter using a femtosecond laser (standard deviation of 11 µm) vs. a mechanical microkeratome (standard deviation of 14µm).

    5. Precise control of flap diameter and hinge width

    Although mechanical microkeratomes offer adjustable parameters to alter flap diameter (eg, suction ring size, suction ring thickness, stop setting, and changeable width head), variability in flap diameter and hinge width is largely out of the surgeon’s control, determined primarily by the corneal curvature and white-to-white measurement. In general, flap diameters and hinge widths are smaller on flat, large corneas and larger on steep, narrow corneas. The femtosecond laser allows the surgeon to precisely control both the flap diameter and the hinge width. This allows for customization of flap sizes for different corneal sizes as well as different refractive errors where ablation patterns may extend further peripherally and nasally (eg, hyperopia) or more at 3 and 9 o’clock (high with-the-rule astigmatism). In addition, recent studies in corneal biomechanics suggest that the anterior, peripheral cornea provides the most biomechanical support (Paper presented at: the XXIV Congress of the ESCRS; September 9, 2006; London, England). Consequently, there may be safety advantages to smaller, more central flaps that can be precisely specified only by the femtosecond laser.

    6. Patient preference

    Subjective questionnaires have shown that patients prefer the laser flap creation to the blade system (Figure 4), removing some of the anxiety associated with the procedure.15

    Figure 4. Patients show continued preference for femtosecond laser flap creation.
     

    7. Improved visual outcomes

    At our center, we have increased our percentage of eyes seeing 20/20 or better uncorrected following custom LASIK for myopic astigmatism from 80% using the mechanical microkeratome to 93% with the IntraLase. In addition, 2 randomized prospective studies conclude that flaps created with IntraLase resulted in fewer higher-order aberrations when compared with mechanical microkeratomes, suggesting improved quality of vision when the femtosecond laser is used.16,17

    Conclusion

    The latest-generation femtosecond laser system offers significant advantages over a mechanical microkeratome in the vast majority of clinical situations. In addition, a new study from our center suggests that flaps created using the femtosecond laser result in smaller, more consistent, and more predictable changes in corneal biomechanics compared with flaps created using a mechanical microkeratome.18 This potential additional advantage may turn out to be the most important of all. Developing technology that reduces the risk of post-LASIK ectasia is the most important priority in refractive surgery today. A method that creates flaps in the most predictable way with the least effect on corneal biomechanics will undoubtedly be the choice for most LASIK surgeons in the future.

    References

    1. Chan C, Moshegov C. Amadeus microkeratome: experience with the first 2000 cases and lessons learned.Clin Experiment Ophthalmol. 2005;33(4):356-359
    2. Knorz, MC. Flap and interface complications in LASIK. Curr Opin Ophthalmol. 2002;13(4):242-245.
    3. Katsanevaki V, Tsiklis N, Astyrakakis N, Pallikaris I. Intraoperative management of partial flap during LASIK: a small case series report. Ophthalmology. 2005;112(10):1710.e1-1710.e5.
    4. Pallikaris I. Laser in situ keratomileusis intraoperative complications using one type of microkeratome. Ophthalmology. 2002;109(1):57-63.
    5. Vajpayee RB, Gupta V, Sharma N. PRK for epithelial ingrowth in buttonhole after LASIK. Cornea. 2003;22(3):259-261
    6. Hovanesian JA, Maloney RK. Treating astigmatism after a free laser in situ keratomileusis cap by rotating the cap. J Cataract Refract Surg. 2005;31(10):1870-1876.
    7. Donnenfeld E. The effect of hinge position on corneal sensation and dry eye after LASIK. Ophthalmology. 2003;110(5):1023-1029.
    8. Asano-Kato N, Toda I, Hori-Komai Y, Takano Y, Tsubota K. Epithelial ingrowth after laser in situ keratomileusis: clinical features and possible mechanisms. Am J Ophthalmol. 2002;134(6):801-807.
    9. Kezirian GM, Stonecipher,KG. Comparison of the IntraLase femtosecond laser and mechanical keratomes for laser in situ keratomileusis. J Cataract Refract Surg. 2004;30(4):804-811.
    10. Durrie, DS, Kezirian GM. Femtosecond laser versus mechanical keratome flaps in wavefront-guided laser in situ keratomileusis: prospective contralateral eye study. J Cataract Refract Surg. 2005;31(1):120-126.
    11. Kezirian GM, Stonecipher KG. Comparison of the IntraLase femtosecond laser and mechanical keratomes for laser in situ keratomileusis. J Cataract Refract Surg. 2004;30(4):804-811
    12. Kim JY, Kim MJ, Kim T, Choi H, Ho Pak JH, Tchah H. A femtosecond laser creates a stronger flap than a mechanical microkeratome. Invest Ophthalmol Vis Sci. 2006;47(2):599-604.
    13. Krueger RR, Dupps WJ. Clinical utility of very high frequency ultrasound imaging in refractive surgery. Invest Ophthalmol Vis Sci. 2005;46: E-Abstract 2761
    14. Marshall J. Wound healing and biomechanics of corneal flap creation. Paper presented at: the XXIV Congress of the ESCRS; September 9, 2006; London, England.
    15. Mahdavi S. IntraLase: coming of age. How femtosecond technology impacts the business side of refractive surgery. Cataract Refract Surg Today. October 2005; 117-120.
    16. Durrie, DS, Kezirian GM. Femtosecond laser versus mechanical keratome flaps in wavefront-guided laser in situ keratomileusis: prospective contralateral eye study. J Cataract Refract Surg. 2005;31(1):120-126.
    17. Tran DB, Sarayba MA, Bor Z, Garufis C, Duh YJ, Juhasz T, Kurtz RM. Randomized prospective clinical study comparing induced aberrations with IntraLase and Hansatome flap creation in fellow eyes: potential impact on wavefront-guided laser in situ keratomileusis. J Cataract Refract Surg. 2005:31(1);97-105.
    18. Hamilton DR, Lee N, Bourla N. LASIK using a microkeratome versus a femtosecond laser: Determination of differences in corneal biomechanical effects. Paper presented at: ASCRS 2007, San Diego, California (Best Paper of Session Award).

    Author Disclosure

    The author discloses that he has received honoraria for educational lectures from Alcon Laboratories, Allergan Inc., IntraLase (AMO), and Reichert Instruments.