The Case for Femtosecond Laser Flaps in Laser In Situ Keratomileusis 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.
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
US OPHTHALMIC REVIEW
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
Table 1: Technical Features of Femtosecond Laser Devices for Flap Creation
Feature
Abbott Carl Zeiss IntraLase VisuMax iFS
400 1
Technolas Alcon Ziemer FS200 LDV
Pulse rate (kHz) 150 kHz 500 kHz 80 kHz 200 kHz >1 mHz Pulse duration (fs) >500 Spot size (µm) 1–5
>500 >1
Pulse energy (nJ) 500–1,300 <300 >500
350 5
300–1500 <100
Interface suction Manual Computer Manual Manual Computer controlled controlled low pressure
Laser-cornea Flat coupling
Customizable features
High Curved Curved Flat Very high High Source: Reggiani-Mello and Krueger, 2011.2
Figure 1: Meniscus versus Planar Flaps A
Planar flap High
within hand piece Flat
Very limited
200–300 <2
Microkeratome flaps are thicker in the periphery and thinner in the center B Meniscus flap
Sub-Bowman’s keratomileusis (A) creates a uniform flap profile, whereas microkeratome-made flaps (B) are thickner in the periphery and thinner in the center.2
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,
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