SEP 2012

EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.

Issue link: https://digital.eyeworld.org/i/82503

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laser intrastromal keratomileusis September 2012 EWINTERNATIONAL 81 120 µm LASIK flap and with a 120 µm SMILE cap (depth of the anterior lenticule interface). The LASIK RST would be 280 µm, but the SMILE RST would be 385 µm after adding 1.5 times the 70 µm of intact ante- rior stroma. Recently, Knox Cartwright et al4 performed a study on human ca- daver eyes and found that creating a sidecut only resulted in a similar increase in strain to that found after creating a whole flap, with a signifi- cantly greater increase when the depth was increased from 90 to 160 µm. On the other hand, the increase in strain was the same at both depths when a delamination cut only was performed. Applying this finding to SMILE, since no anterior corneal sidecut is created, there will be slightly less increase in corneal strain in SMILE compared to thin flap LASIK and a significant difference in corneal strain compared to LASIK with a thicker flap. Also, given the finding that the increase in corneal strain with a de- lamination cut only is independent of depth, this means that the SMILE lenticule can be created at any depth within the stroma. Therefore, putting this finding together in context of the varying tensile strength of stroma at different depths as described above, the effective post-op corneal biomechanical strength will increase as the lenticule is moved deeper. Therefore, it is possible that SMILE might be used to extend the range of myopia that can be corrected by corneal excimer laser surgery. 3. Reduction in post-op dry eye: The other major potential advantage of the flapless ReLEx SMILE procedure is the reduction in post-op dry eye compared with that observed after PRK and LASIK. The cornea is one of the most densely innervated peripheral tissues in humans. Nerve bundles within the anterior stroma grow radially in from the periphery toward the cen- tral cornea. The nerves then pene- trate Bowman's layer and create a network of nerve fibers, known as the sub-basal nerve plexus, by branching both vertically and horizontally between Bowman's layer and basal epithelial cells. In LASIK (as shown in Figure 2), sub-basal nerve bundles and superfi- cial stromal nerve bundles in the flap interface are cut by the micro- keratome or femtosecond laser, with only nerves entering the flap through the hinge region being spared. Subsequent excimer laser ablation severs stromal nerve fiber bundles. Post-op, this means that the patient may have dry eye symp- toms and decreased corneal sensitiv- ity while the nerves regenerate. A number of studies have reported the recovery of corneal sensation after continued on page 82

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