EyeWorld is the official news magazine of the American Society of Cataract & Refractive Surgery.
Issue link: https://digital.eyeworld.org/i/111385
46 EW REFRACTIVE SURGERY February 2013 Biomechanics of ReLEx SMILE: A keyhole intrastromal form of keratomileusis by Dan Z. Reinstein, M.D. Keyhole intrastromal keratomileusis S ixty years after the colossal pioneering work by Jose Ignacio Barraquer Moner in developing keratomileusis, Carl Zeiss Meditec (Jena, Germany) introduced the VisuMax femtosecond laser with the 3D intrastromal cutting accuracy that has enabled a keyhole intrastromal form of keratomileusis to be performed. In small incision lenticule extraction (ReLEx SMILE), the femtosecond laser creates two interfaces that define a refractive lenticule of stromal tissue. A dissector is passed through a small 2-3 mm incision to separate the lenticular interfaces and allow the lenticule to be manually removed in one piece (Figure 1), thus eliminating the need to create a flap.1,2 The absence of a flap and the fact that the stromal tissue is removed from within the stroma means that the anterior-most stromal lamellae remain intact after the procedure (except for the region of the small incision). This is in contrast to both LASIK, where the anterior stromal lamellae are severed by the creation of the flap and also by the excimer laser ablation, and PRK, where the anterior stromal lamellae are severed by the excimer laser ablation. Therefore, ReLEx SMILE must leave the cornea with greater biomechanical strength than both LASIK and PRK as the anterior stroma is known to be the strongest part of the stroma, which has been elegantly demonstrated by Randleman et al.3 In their 2008 study, they measured the tensile strength of strips of stromal lamellae cut from different depths within the cornea Figure 1. Incision geometry of the ReLEx SMILE procedure. The lenticule cut is performed (the underside of the lenticule), followed by the lenticule sidecuts. Next, the cap interface is created (the upper side of the lenticule), and finally a 2- to 3-mm small incision is created supertemporally. The lenticule interfaces are separated using a flap separator, and the lenticule is extracted manually, all via the small incision. and 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. Paradigm shift 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, the actual residual stromal thickness in SMILE should be 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) since the anterior stromal lamellae have not been cut. But given the decreasing strength of stroma with depth, we also need to start thinking more in terms of tensile strength rather than simply in terms of residual stromal thickness. For example, a rough adjustment would be to say that anterior stroma is approximately 50% stronger than posterior stroma, so a further 50% of the untouched anterior stromal thickness in ReLEx SMILE can be added to get a residual stromal thickness value that can be compared to a LASIK residual stromal thickness. In reality, we can go further than this by using the actual tensile strength data from the Randleman paper to calculate a post-op total tensile strength for the different procedures. And it is this total tensile strength that should replace residual stromal thickness as the limiting factor for corneal refractive surgery. continued on page 48 Figure 2. Demonstrates the difference in strength of the remaining stroma after PRK, LASIK, and ReLEx SMILE. These diagrams show that the strength of the stroma remaining after ReLEx SMILE is greater than the equivalent treatment as both PRK and LASIK. The fact that Bowman���s layer remains intact will also provide added strength. Finally, the two diagrams for ReLEx SMILE show the even greater strength if the lenticule is removed from deeper within the stroma. Source (all): Dan Z. Reinstein, M.D.