Comparison of Mechanical and Laser LASIK Flap Creation

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Flap Anatomy and Implications for Clinical Practice

Planar versus Meniscus

With a microkeratome, the flap diameter is a function of suction ring thickness, corneal diameter and curvature.

An FS laser only requires that the corneal surface be compressed flat against the glass, so it does not matter whether the corneal curvature is steep or flat. Thus, FS lasers can create a planar flap, whereas microkeratomes create a meniscus-shaped flap due to the fact that they cut from the side (Slide 6). Suggestive evidence shows that planar flaps may offer better refractive outcomes, as less induced cylinder and higher-order aberrations seem to occur when compared to meniscus-shaped flaps created by mechanical microkeratomes.^1-3

Hinge Size and Shape

A microkeratome creates a D-shaped stromal bed, leaving less room for the flap diameter. With FS lasers, software settings are used to adjust the hinge size, and the use of a stromal pocket allows the flap to be retracted further, thus making more surface area available for a flap of any given diameter (Slide 7). Because there is no risk of inadvertently amputating a hinge with a microkeratome blade, smaller hinge diameters and well-centered 8.5-mm flaps with an FS laser will suffice for an 8-mm diameter custom myopic treatment where a 9.5-mm or 10-mm flap would be necessary with a microkeratome.⁴

Side Cut Angle

Another major difference between a microkeratome- and an FS laser-created flap anatomy is the angle of side cut. With a microkeratome, the edge configuration is 25º to 35º. It is necessary to enter at that angle with a blade to make a flap without risking deep penetration or anterior perforation. With an FS laser, the side cut angle can be varied from 0º to 90º, and angles ranging from 40º to 70º seem to offer the most advantages. An increased side cut angle can allow the flap to seat more easily and, more importantly, create a powerful barrier to epithelial cell migration.

Flap Thickness

Predictable flap thickness is important in LASIK to avoid complications such as buttonhole defects in the setting of thin flaps or corneal ectasia in the setting of thick flaps. Many variables can affect microkeratome flap thickness. Microkeratomes work with a loosened, oscillating blade, and tissue compressibility varies at high pressures. Corneal curvature and keratome translation speed also can affect flap thickness.

Perhaps because of the fixed reference plane provided by corneal applanation, flap thickness appears to be more predictable with FS lasers. Studies have shown that flap creation with the FS laser offers surgeons more predictable flap thickness and other dimensional parameters compared to current mechanical microkeratomes.^1,4 The narrow range of achieved flap thickness and smaller standard deviation of flap thickness are superior to published results of current microkeratomes.^5-8

Visual Outcomes

As visual results of modern LASIK surgery are excellent, it is difficult to demonstrate clear superiority of one flap creation technique over another. However, two recent peer-reviewed publications (one of which was a prospective, randomized, double-blinded study) suggested that FS laser-assisted LASIK produced both subjective and objective visual outcomes superior to mechanical LASIK in terms of both uncorrected visual acuity and magnitude of induced higher-order aberrations.^1-3

Complications

While many flap related LASIK complications are not unique to the type of keratectomy performed, differences exist between mechanical and FS laser-assisted LASIK. With mechanical LASIK, buttonhole formation and free-cap formation are rare but potentially serious problems that relate directly to the mechanism of flap formation. With FS lasers, it is almost impossible to create a buttonhole, but flap avulsion can occur because residual adhesions must be manually severed before the flap can be lifted.

My experience with all-laser LASIK suggests that epithelial defects are much less common (Slide 8) and that diffuse lamellar keratitis (DLK) is no more commonly observed than after mechanical LASIK (Slide 9). Although this remains an area of controversy, others are validating these findings at national and international meetings with increasing frequency.

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One complication unique to FS laser-assisted LASIK is a phenomenon known as transient light sensitivity syndrome (TLSS). TLSS typically occurs between 3 and 6 weeks after surgery and is characterized by photophobia in the absence of decreased visual acuity, conjunctival injection or intraocular inflammation. Occasionally, fine lamellar fleck-like opacities are seen in the flap interface. These changes likely represent activated keratocytes, and the appearance is different than DLK. As no published data are available, the incidence is unclear, but seems to range from 1% to 10%. Symptoms will resolve spontaneously over 1 to 2 months and are generally highly responsive to a short course of mild topical corticosteroid such as fluorometholone 0.1% four times a day which is tapered over 4 to 6 weeks. The etiology is unknown but is most likely inflammatory and related to infrared laser energy exposure. Some surgeons have linked the incidence of TLSS to energy settings, observing that a reduction of these values decreased the incidence of symptoms.

Maintenance and Cost Considerations

Two areas where microkeratomes have a clear advantage are those of routine maintenance and economics. Modern mechanical microkeratomes have few moving parts, are highly reliable, and require minimal scheduled maintenance. In contrast, FS lasers are highly complex, with numerous systems that must be monitored and calibrated regularly to ensure optimal function. Additionally, FS lasers cost somewhere between $300,000 and $400,000, as contrasted with $40,000-$60,000 for a mechanical microkeratome. Per use procedure fees are also higher: a microkeratome blade costs $60 to $85, while a single use kit for the most commonly used FS laser is about $150.

Conclusion

Femtosecond lasers represent an exciting new technology for the performance of the important first step of LASIK surgery. Clear advantages and disadvantages have emerged after several years of widespread usage, and it is likely that further experience and refinement will make femotsecond lasers the predominant method of LASIK flap creation.

References

1. Kezirian GM, Stonecipher KG. Comparison of the IntraLase femtosecond laser and mechanical keratomes for laser in situ keratomileusis. J Cataract Refract Surg. 2004; 30:804-811.
2. 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; 31:120-126.
3. Tran DB, Sarayba MA, Bor Z, et al. 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; 31:97-105.
4. Binder PS. Flap dimensions created with the IntraLase FS laser. J Cataract Refract Surg. 2004; 30:26-32.
5. Arbelaez MC. Nidek MK 2000 Microkeratome clinical evaluation. J Refract Surg. 2002; 18:S357-S360.
6. Shemesh G, Dotan G, Lipshitz I. Predictability of corneal flap thickness in laser in situ keratomileusis using three different microkeratomes. J Refract Surg. 2002; 18:S347-S351.
7. Yildirim R, Aras C, Ozdamar A, Bahcecioglu H, Ozkan S. Reproducibility of corneal flap thickness in laser in situ keratomileusis using the Hansatome microkeratome. J Cataract Refract Surg. 2000; 26:1729-1732.
8. Talamo JH, Meltzer J, Gardner J. Reproducibility of flap thickness with femotsecond laser and mechanical microkeratomes. J Refract Surg. In press.