Depth-dependent cohesive tensile strength in human donor corneas: implications for refractive surgery

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.

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.

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.

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.