EyeWorld Asia-Pacific September 2014 Issue

60 EWAP rEfrActivE September 2014 Views from Asia-Pacific Cordelia CHAN, MD Medical Director and Senior Consultant, International Specialist Eye Centre Visiting Senior Consultant, Singapore National Eye Centre International Specialist Eye Centre Mount Elizabeth Novena Specialist Centre 38 Irrawaddy Road, #08-58 to #08-63 Singapore 329563 Tel. no. +65-96391686 cordelia_chan@snec.com.sg T he ReLEx SMILE concept of “flapless, small incision surgery” affords patients the theoretical advantage of faster surface healing, less dry eye, and increased cornea biomechanical stability. LASIK is still the most popular refractive surgical procedure among surgeons and patients today, as it is technically simple, safe, induces minimal intraoperative and postoperative discomfort, and gives rapid visual recovery. In order for SMILE to cause a shift in refractive surgery preferences and patterns, it has to outdo LASIK in most if not all these aspects. SMILE is technically more demanding than LASIK, with a learning curve required for lenticule dissection and extraction. Studies so far have shown good visual outcomes for SMILE, but more studies are required to determine if its immediate and long-term visual outcomes can surpass those of LASIK. The “flapless” concept may appeal to patients fearful of flaps andflap dislodgement, but with femtosecond-assisted flap creation, flaps are now more stable and adherent, and surgeons no longer consider flap dislodgment to be a major issue in LASIK. Like most LASIK patients, SMILE patients still get dry eye, albeit less than their LASIK counterparts. A number of recently published articles have alluded to the advantage of the increased biomechanical stability of SMILE over LASIK. Wu et al.’s study 1 looking into corneal hysteresis, corneal resistance factor, and other biomechanical waveform parameters with the Ocular Response Analyzer concluded that both SMILE and FemtoLASIK cause biomechanical changes in the cornea, but these changes in the cornea’s viscoelastic properties were less after SMILE. Reinstein et al. 2 predicted a higher postoperative total stromal tensile strength in SMILE compared to LASIK, and Sinha Roy et al. 3 concluded that there was less biomechanical risk to the residual stromal bed in SMILE. So will these findings affect the mindset of refractive surgeons? The incidence of post-LASIK keratectasia is low, and has been reduced further in recent years by a heightened awareness of the problem, better risk assessment systems for early subclinical keratoconus detection and better flap-thickness predictability afforded by femtosecond-laser assisted flap creation. Yet, it is still one of the most feared complications of LASIK, especially since it has been described in cases with normal preoperative topography and minimal risk factors. In my opinion, if the greater biomechanical stability afforded by SMILE, translates into a lower or minimal risk of keratectasia, then this would be the main advantage of SMILE over LASIK. Moreover, with deeper corrections in the stroma and higher myopic treatments postulated to be possible in SMILE 2,3 , this could persuade more surgeons to include it in their armamentarium of refractive surgical procedures and make it available to their patients. References 1. Wu D, Wang Y, Zhang L, Wei S, Tang X. Corneal biomechanical effects: small-incision lenticule extraction versus femtosecond laser-assisted laser in situ keratomileusis. J Cataract Refract Surg. 2014;40(6):954-62. 2. Reinstein DZ, Archer TJ, Randleman JB. Mathematical model to compare the relative tensile strength of the cornea after PRK, LASIK and small incision lenticule extraction. J Refract Surg. 2013;29(7):454-60. 3. Sinha Roy A, Dupps WJ Jr, Roberts CJ. Comparison of biomechanical effects of small-incision lenticule extraction and laser in situ keratomileusis: finite-element analysis. J Cataract Refract Surg. 2014;40(6):971-80. Editors’ note: Dr. Chan is a consultant for Alcon but has no financial interests related to the comments. Biomechanics - from page 59 cohorts. In a prospective study of 80 myopic patients, Wu et al. examined biomechanical corneal properties following ReLEx SMILE and compared these results with femtosecond laser-assisted LASIK (FS-LASIK). The hypothesis of the paper, and presumed benefit of flapless intrastromal surgery, is that SMILE provides superior corneal biomechanics postoperatively. Theoretically, a well-preserved stroma should minimize long-term risk of ectasia. 8 To measure corneal biomechanics, the authors enlisted the Ocular Response Analyzer (ORA, Reichert Technologies, Depew, NY, U.S.), currently the only device available that allows for direct analysis of corneal biomechanics in the clinic setting. The main parameters of the ORA included corneal hysteresis (CH) and corneal resistance factor (CRF). Corneal hysteresis, from a mathematical perspective, reflects the difference in air pressure between force-in (P1) and force- out (P2) applanation but can also be derived from corneal properties including corneal thickness (CCT), rigidity, intraocular pressure (IOP), and hydration. Clinically, abnormally low CH is demonstrated in a variety of pathologic states, most notably in keratoconus. 9 Corneal resistance factor is a derived measurement using P1, P2 and k, a constant developed based on relationships between P1, P2 and CCT. CRF measures corneal resistance theoretically isolated from the effects of IOP. Patients were divided evenly in the two groups such that