Interlamellar cohesive strength in the vertical meridian of human eye bank corneas

Previously, human corneal stromal interlamellar cohesive strength in the horizontal meridian was shown to be twice as strong peripherally as centrally (approximately 2.90 x 10(-1) versus 1.40 x 10(-1) N/mm). In the current study, stromal samples excised from the vertical meridian were studied to determine if meridional differences also exist. Precise knowledge about corneal stromal structure is warranted, because anisotropy can influence postoperative healing strength and corneal shape.

Limbus-to-limbus stromal strips with a constant 2.2-mm width were obtained from the vertical meridians of 52 eyebank corneas (8 single, 22 pairs). Cohesive strength was recorded as samples were split at a constant 1.6 mm/s at 50% stromal depth.

The mean cohesive strength in the inferior periphery was found to be only two thirds the strength observed in the nasal or temporal periphery (1.96 x 10(-1) N/mm versus approximately 2.94 x 10(-1) N/mm @ 5 mm). The mean cohesive strength in the inferior periphery was also significantly less than the strength of the superior periphery (1.85 x 10(-1) N/mm +/- 8.83 x 10(-3) SEM versus 2.34 x 10(-1) +/- 1.37 x 10(-2) @ 4 mm from the central cornea; P = 0.0027). Unlike the samples from the horizontal meridian, which could be described by a profile symmetric about the central cornea, force profiles for the vertical data could be characterized with distinct, classifiable patterns that were generally asymmetric. Fellow corneas from a single donor tended to have strength profiles belonging to the same class (P = 0.035; chi-square), although some paired eyes exhibited profiles from distinctly different classes.

These data strongly support the concept of an anisotropic collagen macrostructure that is more complex than previously believed. This inherent structural anisotropy may become a significant determinant of corneal shape during ectatic disease and some forms of keratorefractive surgery.