EyeWorld Asia-Pacific December 2012 Issue

40 December 2012 EWAP CORNEA combination of an optical beam path that is suspended on a fulcrum with force-feedback servo control of the height of the patient bed and headrest in order to maintain a consistent contact glass force onto the cornea (again minimizing tissue distortion), (5) a high numerical aperture of the beam designed to deliver a tight concentration of femtosecond energy with low per-pulse energy load, and (6) a high pulse repetition rate of 500 kHz in order to minimize treatment time. Advantages of ReLEx SMILE over LASIK These new femtosecond intrastromal lenticule procedures offer a number of potential advantages. 1. More accurate and repeatable tissue removal: Intrastromal lenticule procedures may have advantages over LASIK as all of the potential errors associated with excimer laser ablation are avoided, such as stromal hydration, laser fluence projection and reflection losses, and other environmental factors. In ReLEx, the tissue removal is defined only by the accuracy of the optomechanics of the femtosecond laser, none of which is affected by environmental conditions. Therefore, it is likely that there will be less need for personalized nomograms to be used for different machines, locations, or surgeons. In addition, the accuracy remains similar for low and high corrections as the only variable is the distance between the upper and lower lenticular cuts. 2. Increased biomechanical stability and reduced biomechanical variability: Potential benefits of SMILE are the relatively higher biomechanical strength of the remaining cornea as well as a reduction in the variability of the biomechanical effects produced by refractive tissue removal. Randleman et al. 3 published a study in 2008 in which they measured the tensile strength of strips of stromal lamellae cut from different depths within the cornea. They found a strong negative correlation between stromal depth and tensile strength. The anterior 40% of the central corneal stroma was found to be the strongest region of the cornea, whereas the posterior 60% of the stroma was at least 50% weaker. We are accustomed to calculating the residual stromal thickness in LASIK as the amount of stromal tissue left under the flap, so the first instinct is to apply this rule to SMILE. However, because there is no flap created in SMILE, the anterior stromal lamellae remain intact everywhere except for the small 2-3-mm incision. Therefore, the actual residual stromal thickness in SMILE is calculated as the stromal thickness below the posterior lenticule interface plus the stromal component of the overlying cap— between the anterior lenticule interface and the epithelium. Moreover, because anterior stroma is 50% stronger than posterior stroma, a further 50% of the untouched anterior stromal thickness can be added to get a residual stromal thickness value that can be compared to a LASIK residual stromal thickness. If this concept is applied to an example case (Figure 1), the significant improvement in biomechanics can be appreciated. Consider an eye with a spherical equivalent refraction of –8.00 D and a central corneal pachymetry of 500 microns. We will assume that the epithelial thickness is 50 microns, that the LASIK excimer laser ablation depth and SMILE lenticule thickness are 100 microns, and that both procedures were performed under a 120-micron LASIK flap and with a 120-micron SMILE cap (depth of the anterior lenticule interface). The LASIK RST would be 280 microns, but the SMILE RST would be 385 microns after adding 1.5 times the 70 microns of intact anterior stroma. Recently, Knox Cartwright et al. 4 performed a study on human cadaver 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 significantly greater increase when the depth was increased from 90 to 160 microns. 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 delamination 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 central cornea. The nerves then penetrate 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 superficial stromal nerve bundles in the flap interface are cut by the microkeratome 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 symptoms and decreased corneal sensitivity while the nerves continued on page 49 Scientific - from page 39