Left graphic depicts geometry of lenticule in a pure-sphere SMILE procedure for reference. Right image is a photographic image of an extracted lenticule, preserved in formalin, prior to electron microscopy. A curvature is readily observed in the lenticule sample, enabling the identification of anterior and posterior surfaces
Samples were imaged using a FEI/Philips XL-30 Field Emission eSEM/SEM microscope . Images were collected from one side of a sample, with the opposite side sample subsequently imaged after removing and reorienting the sample. Images were acquired at three magnifications: 100×, 250×, and 500×. Exemplary eSEM images of five spherical correction lenticules at 100× magnification are shown in Fig. 16.2. The left column of Fig. 16.2 shows views of the convex surfaces and the middle column shows the concave surfaces.
eSEM images of ten lenticule surfaces of five lenticules extracted from SMILE patients. Each row depicts a separate lenticule. Left column is the convex (top) surface. Middle column is concave (bottom) surface. Spherical correction size for each row is labeled in rightmost column. (a) −9.75 D. (b) −10.0 D. (c) −6.75 D. (d) −7.00 D. (e) −7.25 D. All images are 250× magnification
The surfaces of the lenticules were identified as convex and concave by inspection during mounting. Since no orientation marks were made to identify the surfaces as posterior or anterior, a positive identification of the surface orientation cannot be made. We assume that the corneal lenticules are sufficiently stiff to maintain their natural curvature after removal so that the concave surfaces were likely to correspond to the posterior surfaces. Concave and convex surfaces of the lenticules appear smooth and absent of surface irregularities. Concave and convex surfaces appear equally smooth, with no obvious differences between them. No holes from cavitation bubbles were seen in any of the images. Attachments or tissue bridges appear to be minimal, and the surface morphology appears regular and intact in the highest magnification views.
In SMILE procedures, the quality of the lenticule edge cuts is as important as the general quality of the lamellar-type cut surfaces. Keeping the lenticules intact without tearing or breaks at the edges during lenticule extraction is key to a successful and problem-free procedure. Ideally, the edges of the lenticules would be cut with a clean and well-defined edge. Turning to the images in Fig. 16.3, the lenticule edges are observed at 100× to be well defined and integral for the most part. In Fig. 16.3a, the lenticule sample is folded in such a way that the edge overlays the convex central portion of a lenticule. In contrast, the edge may be jagged in areas where the surgeon has removed or manipulated the lenticule with forceps, as can be seen at one portion of another lenticule edge (Fig. 16.3b). In Fig. 16.3c, the intrinsically good quality of lenticule edge or side cuts can be seen from the concave side. The sample in Fig. 16.3c is mounted on a wire grid, while the other two lenticules in Fig. 16.3 are mounted on flat surfaces.
Lenticule edge quality. (a) Lenticule from the −9.75 D correction previously shown in Fig. 16.2a is folded, with edge overlaying convex surface. (b) Edge of a lenticule from a −7 D correction is seen from concave side, with edges damaged, likely by forceps used to surgically manipulate and extract lenticule. (c) Lenticule from the −7.25 D correction previously shown in Fig. 16.2e has cleanly cut edges as seen from convex surface. Circular periphery seen in (a) and (b) is the microscope field aperture. All images are 100× magnification
A direct study of these surfaces by SEM imaging would not be possible in patients, since the cornea itself would have to be processed and imaged. The strength of the imaging technique we have described here is that it allows for an indirect characterization of both lamellar cut surfaces in the patient’s eye using a single sample. A weakness is that this characterization depends upon the assumption that the cut surfaces remaining in the patient’s cornea are faithfully represented by the quality of the matched lenticule cut surfaces. While a complete validation of this technique would be a comparative study of lenticules from cadaver or animal eyes with the matched corneal beds or corneal cut surfaces, it seems reasonable to infer the cut quality in the cornea by characterizing the cut quality of extracted lenticules.