J Refract Surg. 2008 Jan;24(1):S85-9.
Depth-dependent cohesive tensile strength in human donor corneas: implications for refractive surgery.
Randleman JB, Dawson DG, Grossniklaus HE, McCarey BE, Edelhauser HF.
Emory University, Department of Ophthalmology, Atlanta, GA, USA.
Jrandle@emory.edu
PURPOSE: 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.
METHODS: 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. RESULTS: 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, P < .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.
CONCLUSIONS:
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.
From the full text:
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Excimer laser corneal refractive surgery inevitably reduces keratocyte density and corneal tensile strength. Although both surface ablation techniques and LASIK reduce overall tensile strength through tissue removal, LASIK further weakens the cornea through lamellar flap creation because the anterior lamellar flap does not contribute significantly to postoperative corneal tensile strength.
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As these results demonstrate, excimer laser ablation reduces not only the quantity of the remaining load-bearing corneal tissue, but also the quality of the residual load-bearing corneal tissue (measured as reduced cohesive tensile strength), especially when laser ablation extends deeper than the anterior 40% of the corneal stroma. Ablation beyond this anterior 40% rarely occurs with surface ablation procedures; however, standard LASIK more frequently extends into the posterior 60% of the stroma through a combination of laser ablation and flap creation with average thicknesses of approximately 150 μm that can be highly variable. These findings agree with reported ectasia studies, where more than 90% of reported corneal ectasia cases have occurred after standard LASIK.
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Cohesive tensile strength studies in human donor corneas demonstrate that the anterior 40% of the central corneal stroma has the highest cohesive tensile strength, whereas the tensile strength in the posterior 60% of the cornea is approximately 50% less. Cohesive corneal tensile strength increases with age. Based on these results, the maximal amount of anterior stroma should be left intact to maximize postoperative corneal integrity.
