Specular microscopy can provide a non-invasive morphological analysis of the corneal endothelial cell layer from subjects enrolled in clinical trials. The analysis provides a measure of the endothelial cell physiological reserve from aging, ocular surgical procedures, pharmaceutical exposure, and general health of the corneal endothelium. The purpose of this review is to discuss normal and stressed endothelial cell morphology, the techniques for determining the morphology parameters, and clinical trial applications.

Injection into the suprachoroidal space using a microneedle offers a simple and minimally invasive way to target the delivery of drugs to the choroid and retina.

<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>

This study demonstrated for the first time that coated microneedles can deliver drugs into the eye via intrascleral and intracorneal routes. This minimally invasive approach may avoid the complications associated with intraocular injection and systemic administration.

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NaF reaches the choroid/retina by transscleral diffusion from the periocular depot. The orbital and conjunctival vasculature and lymphatics have a larger role in NaF clearance than does the choroid. NaF diffuses into the vitreous from the choroid and the anterior segment; the periocular depot location determines the predominant diffusion pathway. The duration of high NaF levels in the choroid/retina or the anterior segment determines vitreous NaF levels. PST is the best periocular route for vitreous NaF delivery with minimal systemic levels. Increasing the volume of NaF PST depot enhances transscleral drug delivery.