Flap Technology Review—The Case for Femtosecond Laser Flaps in Laser In Situ Keratomileusis

It has been more than a decade since the femtosecond laser entered the ophthalmic market as an alternative to the mechanical microkeratome for the creation of flaps in laser in situ keratomileusis (LASIK).¹ The first device approved in the US was the IntraLase^® Femtosecond Laser (Abbott Medical Optics) in 2001. The early successes of this device generated considerable interest in the technology, such that there are now five femtosecond laser systems available to surgeons, each with slightly different characteristics. Table 1 summarizes the key features of each system.² The first widely available IntraLase system operated at 15 kHz, a significantly lower pulse rate (with significantly higher pulse energy) than used in more recently available systems. The fundamental theory behind the use of the femtosecond laser for flap creation is that the laser can perform cutting procedures, much like a blade. A focused pulse of the laser causes photodisruption of the target tissue, effectively creating a microseparation that is then followed by an expanding cavitation bubble, which then collapses to a smaller size. When these pulses are contiguously arranged along a common focal plane, they produce a dissection plane. Larger pulses create larger expanding bubbles, which reduce the number of pulses required to cleave the tissue. However, collateral damage from the shock wave in the tissue is higher with these larger, higher energy pulses, and there is a correspondingly higher inflammatory response. ³ A greater number of smaller pulses can achieve the same result. The advantage of a smaller spot size is that the energy per pulse can be significantly lower. To maintain a constant ablation time, the frequency of the laser system must rise as the spot size decreases. The cleavage that occurs with the femtosecond laser is functionally equivalent to that which can be achieved with the cutting of a blade. However, there are histologic differences in the two procedures in both the inflammatory⁴ and healing⁵ responses.
By the end of 2008, it was estimated that 35–40 % of LASIK surgeries in the US were being performed using a femtosecond laser for flap creation.⁶ It is estimated that this number has since increased to up to 70 %. There are now more than 50 publications in the peer-reviewed literature comparing the use of mechanical microkeratomes and femtosecond lasers for LASIK flap creation. The evidence in the literature indicates that there is a good case for adopting femtosecond laser technology for all LASIK flaps. Some of the key arguments are outlined here.

Flap Geometry and Orientation

The general principle of mechanical microkeratome flap creation is as follows. The first step is application of a suction ring to fix the eye into a firm position. Next, an oscillating blade set at an acute angle within a microkeratome head piece is advanced across the cornea to cut the flap. This second step involves a localized flattening (applanation) of the cornea over the area of contact as it advances, but this is not the same as the global corneal applanation of femtosecond lasers. The localized applanation of the cornea over the advancing blade can locally imbricate or compress the tissue to lead to variability of flap thickness.⁷ The blade stops before the final diameter is reached to provide a flap hinge opposite to the point of entry of the blade. The surgeon finally lifts the cut flap with a spatula to expose the stroma for the excimer laser ablation.
The placement of the microkeratome head defines the position of the flap, and the flap diameter. Flap thickness (in the applanated state) is determined by the chosen microkeratome head. Mechanical microkeratomes tend to produce a meniscus (tapering) flap (see Figure 1),⁸ and of course there is no opportunity to create a side wall, since one cannot make an angle at any point in the cut.
The femtosecond laser systems generally involve the same first step of a suction ring to fix the eye in a firm position. The laser is next coupled with the eye in a uniform manner by docking with either flat applanation or a fixed, curved surface of greater radius of curvature than the cornea (see Table 1). The laser is then used to create a specific flap geometry on the eye. The position of the flap can generally be adjusted within the region of docking contact to refine the flap placement despite the initial suction ring position. Flap diameter, thickness, and hinge geometry are programmed and the laser performs the necessary cuts. Vertical side walls are typical, and several systems include a stromal ‘pocket’ unrelated to the flap to allow for the incarceration of the gases generated during the photodisruptive process. Alternatively, an external canal can be designed to expel the gases, as seen with the Wavelight FS200^® system (Alcon Laboratories, Inc). The final step of lifting the flap to expose the stroma for excimer laser ablation involves a fine dissection of the precut flap using a blunted spatula.
Overall, the variability of central flap thickness is greater with mechanical microkeratomes in comparison to femtosecond laser systems. The standard deviation of central flap thickness measured with ultrasound subtraction pachymetry is in the order of 20 μm with mechanical systems and typically around 10 μm with femtosecond laser systems.⁹ Equally important is the flap geometry and its individual predictability. The variability in flap geometry has been measured with anterior segment optical coherence tomography (AS-OCT) post-operatively. Flaps made with femtosecond lasers appear closer to the planned geometry and significantly more planar than flaps made with mechanical microkeratomes.^10–12 This is also consistent with reports using confocal microscopy to evaluate flap dimensions post-operatively.¹³

Flap Complications

The types of complications encountered with mechanical microkeratomes versus femtosecond laser technology are significantly different. While overall complication rates appear similar, it seems, at first glance, that intra-operative complications are more frequent with mechanical microkeratomes, while post-operative complications seem to favor the femtosecond laser systems. Specific complication rates vary widely because the criteria for identifying what constitutes a complication are not standardized. The majority of complications resolve without any impact on best corrected vision.¹⁴
One complication encountered with mechanical microkeratomes is epithelial sloughing, thought to be related to the shearing forces of the microkeratome blade transmitted to the basement membrane, disrupting its attachment to the epithelium. Other complications are typically related to the interface between the eye and the microkeratome docking head, and the keratome’s effect on cut geometry. These intra-operative complication rates are generally low, but include button-hole flaps (where the central cornea is not removed with the rest of the flap), partial flaps, and free flaps.¹⁴ Diffuse lamellar keratitis is also a reported complication post-surgery, but the incidence appears much lower than with the early femtosecond laser systems.¹⁵ Epithelial ingrowth is also a complication primarily favoring mechanical microkeratomes, presumably because the steeper side walls of a femtosecond laser flap fit more tightly together and reduce the potential space for ingrowth.¹⁶
Intra-operative complications with femtosecond laser systems have been rare. However, mechanical complications have been observed, specifically the possibility of a bubble breakthrough in the area of a focal scar, where the stromal strength is insufficient to resist the passage of expanding bubbles anteriorly beneath the applanation plate, and flap tears, where resistance in separation of the flap during dissection can lead to unwanted externalization of the spatula through the flap.
The most common early finding was the presence of an opaque bubble layer (OBL) that obscured the features of the anterior segment and rendered iris tracking or pupil tracking unreliable. This OBL resolved over the course of minutes to hours and without any apparent long-term effects.¹⁷ Modern-day femtosecond laser systems, with smaller spot sizes and lower energies, appear to have largely eliminated this problem. The higher incidence of diffuse lamellar keratitis noted with early femtosecond laser systems appeared related to the inflammatory response of photodisruption.¹⁵ As with the incidence of OBL, lower energy, smaller spot systems appear to have the potential to reduce the inflammatory response, as seen in studies comparing progressive upgrades of the IntraLase laser system.^5,18,19
Transient light-sensitivity syndrome is characterized by photophobia and mild pain that can appear days or weeks after surgery and then persist for multiple weeks20 Rainbow glare is an optical side effect due to light scattering from the perfect array of laser spots remaining on the back surface of the flap. It can create a spectral pattern whose visual impact is clinically inconsequential in the majority of patients. Both of these situations are predominately related to earlier, femtosecond laser devices using higher raster energies and lower numerical aperture optics.^21,22

Surgical Considerations

There are several important differences in the procedural steps used in making a flap with a mechanical microkeratome versus the femtosecond laser. The essential step in using a femtosecond laser system is to effectively dock the interface that connects the cornea to the laser. There is no blade to handle, and no mechanical parts to align, such as are present when a mechanical microkeratome is used. Studies suggest that 25–100 eyes are required to become comfortably familiar with the operation of a new microkeratome.^9,23 Flap creation with the mechanical microkeratome can be performed on the same patient bed as is used for the subsequent excimer laser refractive shaping. A separate station, or a translating bed, is required when using a femtosecond laser system. As noted earlier, there are more limitations restricting the use of a mechanical microkeratome versus a femtosecond laser system. Extremely flat or steep eyes are unsuitable for mechanical flap creation because of the potential for buttonhole, free cap or other flap complications.

Clinical Outcomes

There is ample evidence that laser refractive surgery is a safe and effective treatment for the correction of refractive error in the vast majority of appropriately selected patients. There appear to be no major differences in the basic surgical outcomes between mechanical microkeratomes and femtosecond laser systems,^24,25 but minor differences have been reported. One study of hyperopic patients shows significantly better refractive results with flaps created with a femtosecond laser versus a microkeratome.²⁶ It is possible that the generally flatter geometry of the hyperopic eye is a factor. In a study of over 2,000 eyes, Tanna et al. reported similar refractive accuracy between the two flap creation methods. However, they noted a higher percentage of eyes with 20/20 visual acuity when using the femtosecond laser at all time-points (one week to three months post-operatively), and a higher percentage of eyes with 20/16 acuity at three months (see Figure 2).²⁷
More detailed analyses of optical quality have also been performed, suggesting that femtosecond laser flaps improve the overall optical quality. Higher order aberrations have been reported to be significantly lower with femtosecond laser flaps than with mechanical microkeratome flaps.^28,29 Although in one study the results were equivalent,³⁰ there are no studies that suggest better wavefront results with a mechanical microkeratome. When spherical aberration is combined with decentration, the resulting aberration is coma, so the laser’s ability to better center the flap may explain lower levels of observed coma with femtosecond laser LASIK.³¹
Alió and Piñero noted differences in the flap geometry for microkeratomes and femtosecond laser flaps, and reported relatively better contrast sensitivity at higher spatial frequencies with the femtosecond flaps, despite equivalent correction of sphere and cylinder. The improvement was ascribed to differences in the post-operative higher order aberrations (see Figure 3).³²

Upside Potential

While the potential for future improvements in a current technology may not affect the patient undergoing LASIK surgery today, the surgeon’s adoption of any given technology is often made with a longer-term view.
With regard to mechanical microkeratomes, there appears to be little that can be done to address the current limitations they face. Improved blades and designs will not affect the uniformity of the flap created—the flap thickness variability and meniscus shaped profile cannot be overcome. Mechanical microkeratomes are affected by the physiological variability (e.g., extent of compression of the corneal tissue on applanation, which affects achieved flap thickness and diameter) and anatomical variability (e.g., very steep or very flat) of the human cornea. This places a lower limit on the overall variability that can be achieved. Flap complications, while rare, tend to be mechanical in nature (partial flaps, buttonholes, flap tears) and these can have a permanent effect on the optical quality of the eye. These factors are difficult to ‘design out’ of mechanical microkeratomes, suggesting that any technological improvements are unlikely to affect the principal limitations of the procedure.
Further improvement in femtosecond laser technology, on the other hand, would appear less constrained. Continued refinement of laser pulse energy, frequency and spot size has occurred over the past decade and there is no evidence that a limit to this has yet been reached. The electronic systems that measure the eye and control the procedure may benefit from new imaging and biometric technologies that have become available in the past few years (e.g., spectral domain OCT). Because flaps can be programmed in a wide variety of configurations there is the potential to discover an optimal edge profile, or an optimal overall shape. Specific flap configuration based on pre-operative keratometry or refractive error is theoretically possible with a femtosecond laser, but not with a mechanical microkeratome; the clinical advantage of this has not yet been determined. An important step in this ability to customize the flap is connectivity. As an example of preparing for this capability, the Wavelight FS200 system can connect to the Wavelight Allegretto^® excimer laser, allowing the surgeon to view the planned ablation profile on the femtosecond laser screen. Given the above, it may be stated that the potential for technological improvement would appear higher for femtosecond laser systems than for mechanical microkeratomes.

Conclusion

The ability to control flap size, location and overall thickness is significantly higher with femtosecond laser systems. Variability in flap geometry is significantly higher with mechanical microkeratomes. The vertical side walls that most femtosecond laser systems create improves flap adhesion post-operatively and reduces the likelihood of epithelial ingrowth. The ability to precisely program the flap pattern minimizes the likelihood of a buttonhole or free flap. The limitations on how steep or flat an eye may be with a mechanical microkeratome do not apply with the femtosecond laser systems. Measurement of higher-order aberrations and quality of vision (e.g., low contrast acuity) suggest that flap creation with a femtosecond laser improves post-operative optical quality. Early generation laser systems introduced post-operative inflammatory complications in a percentage of patients that have largely been addressed through subsequent system modifications (lower energy, smaller spot sizes). Operative complications are significantly lower than with blades and patient appreciation of bladeless surgery appears high. It can be stated, then, that the body of evidence in the literature now supports femtosecond laser technology as superior to mechanical microkeratomes for the creation of LASIK flaps in refractive surgery. ■