Key Points
- Oval flaps with a temporal hinge allow wider access to the stromal bed and reduce the incidence of opaque bubble layers. Based
on positive study results, Dr Kerry Assil believes that, by using a femtosecond laser to perform Sub-Bowman's Keratomileusis
(SBK), he is able to offer patients more pleasing postoperative outcomes in terms of both comfort and safety.
Customizing the flap dimensions appears to have several advantages over traditional LASIK. In our experience, visual recovery
is significantly faster and retinal image quality is higher following this LASIK approach, as compared with PRK. The femtosecond
laser further provides us with control over many variables in flap customization.
Customizing parameters such as thickness, diameter, shape, hinge-width and side-cut architecture of the flap in a LASIK procedure
is a new development that has been proven to maximize residual corneal thickness, which also decreases the risk of post-LASIK
ectasia. We refer to the control of this constellation of variables as Custom-Tailored All Laser LASIK.
Can we mimic the eye's natural anatomy?
Technology is improving, and more patients than ever before are opting for LASIK. Because of this, we believe it is important
to continue exploring how to create LASIK flaps — using the femtosecond laser — that are a better match for the anatomy of
individual eyes. Better matching the flap to the patient improves the exposed stromal bed area and could even diminish the
incidence of opaque bubble layers (OBLs). We have been very excited about femtosecond technology, because these lasers can cut thin flaps precisely, further reducing
the likelihood of ectasia. The technology also offers us the possibility of reducing the incidence of dry eye and decreasing
enhancement rates, and facilitates the performance of future enhancements. The widened hinge capability further reduces the
potential for epithelial ingrowth or formation of striae.
 Figure 1
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Before we attempt to customize the flap, we must first consider anterior segment morphology, taking into account that the
cornea is oval in shape, not circular, and the pupil tends to be superior and nasally displaced. Therefore, housing the hinge
complex in either of these quadrants (nasal or superior) is not very logical, since these are the most cramped quadrants of
the cornea. Furthermore, the limbal region is also the most immunologically active portion of the cornea: placing the flap
margin close to the limbus increases the degree of marginal infiltrates and the likelihood of peripheral Diffuse Lamellar
Keratitis (DLK). Hinge ablation is yet another issue to consider; placing a hinge cover over the stromal surface of a superior
hinge is not an ideal option, as it does not allow us to apply the full (intended) diameter of the laser ablation to the stromal
bed, which is the optimal way to perform the procedure.
Superior vs. temporal
 Figure 2
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As we have explored in the past,1 it is possible to create a round flap with a temporal hinge by performing a 90° rotation of a metal microkeratome. Even
in the presence of a widened hinge, this technique provides the surgeon with larger bed exposure. In our new study, we evaluated
this same process using the IntraLase femtosecond laser (AMO); this laser has the capacity of a second order centration correction,
centring the flap over the entranced pupil. In our original study, we had noted a significant difference between temporal
and superior hinges (Figures 1 & 2) , in favour of temporal; these results were corroborated by our subsequent study. We therefore now recommend creating temporal
rather than superior hinged flaps in most patients.