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This mathematical model predicts that the postoperative TTS is considerably higher after SMILE than both PRK and LASIK, as expected given that the strongest anterior lamellae remain intact. Consequently, SMILE should be able to correct higher levels of myopia.

The Ectasia Risk Score System is a valid and effective method for detecting eyes at risk for ectasia after LASIK and represents a significant improvement over previously utilized screening strategies.

<h4>PURPOSE</h4><p>To determine the cohesive tensile strength throughout the stroma of normal human donor corneas and evaluate the relevance of these findings within the context of current excimer laser surgical techniques.</p> <h4>METHODS</h4><p>Twenty normal corneoscleral buttons from 11 donors were obtained from the Georgia Eye Bank. The corneas were cut into 3-mm strips, dissected at varying stromal depths, mechanically separated through the dissection plane using a motorized extensometer, and measured for cohesive tensile strength. Central corneal thickness and dissection depth were measured by routine light microscopy and correlated with cohesive tensile strength measurements. </p> <h4>RESULTS</h4><p>A strong negative correlation was noted between stromal depth and cohesive tensile strength (r=–0.93). The anterior corneal stroma directly adjacent to Bowman’s layer followed by the underlying anterior 40% of the corneal stroma had the highest cohesive tensile strength. Cohesive tensile strength plateaued from 40% to 90% corneal stromal depth and then declined rapidly from the posterior 10% of the stroma to Descemet’s membrane. The anterior 40% of the corneal stroma had significantly higher cohesive tensile strength than the posterior 60% (33.3 g/mm vs 19.6 g/mm, <i>P</i><.00001). Within the central 40% to 60% depth, a positive correlation was found between increased age and increased tensile strength (r=0.67), with corneal tensile strength increasing 38% from ages 20 to 78 years</p>. <h4>CONCLUSIONS</h4><p>The anterior 40% of the central corneal stroma is the strongest region of the cornea, whereas the posterior 60% of the stroma is at least 50% weaker. The risk for ectasia may therefore be greater with ablations into the posterior stroma. Increasing age is associated with increased corneal cohesive tensile strength. [<cite>J Refract Surg</cite>. 2008;24:S85-S89.]</p> <h4>ABOUT THE AUTHORS</h4> <p>From Emory University, Department of Ophthalmology (Randleman, Dawson, Grossniklaus, McCarey, Edelhauser) and Emory Vision (Randleman), Atlanta, Ga.</p> <p>Supported in part by Research to Prevent Blindness Inc, New York, NY; NEI grants EY-00933, P30-EY06360, T32-EY07092, Bethesda, Md; and the ASCRS Foundation’s Research Grant Program.</p> <p>The authors have no financial interest in the materials presented herein. </p> <p>Presented at the Sixth International Congress on Advanced Surface Ablation & SBK; May 5, 2007; Ft Lauderdale, Fla.</p> <p>Correspondence: J. Bradley Randleman, MD, 1365 B Clifton Rd NE, Ste 4500, Atlanta, GA 30322. E-mail: <a href="mailto:Jrandle@emory.edu">Jrandle@emory.edu</a></p>

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Since its inception in the late 1990s, corneal cross-linking has grown from an interesting concept to a primary treatment for corneal ectatic disease worldwide. Using a combination of ultraviolet-A light and a chromophore (vitamin B2, riboflavin), the cornea can be stiffened, usually with a single application, and progressive thinning diseases such as keratoconus arrested. Despite being in clinical use for many years, some of the underlying processes, such as the role of oxygen and the optimal treatment times, are still being worked out. More than a treatment technique, corneal cross-links represent a physiological principle of connective tissue, which may explain the enormous versatility of the method. We highlight the history of corneal cross-linking, the scientific underpinnings of current techniques, evolving clinical treatment parameters, and the use of cross-linking in combination with refractive surgery and for the treatment of infectious keratitis.

No author has a financial or proprietary interest in any material or method mentioned.

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