Lamellar changes in the keratoconic cornea

The purpose of this study was to identify ultrastructural changes associated with ectasia and to determine the association between lamellar count and corneal thinning.

Five surgically removed keratoconic corneal buttons and four, non-keratoconic, normal eye bank control corneas were processed for transmission electron microscopy using an established protocol, ensuring minimal tissue distortion. A sequence of overlapping digital images, spanning the full apical cone corneal thickness, was assembled. A seamless digital montage was printed at 5000× magnification. Lamellae were counted in the anterior-posterior orientation, along a linear line, using established criteria for identification of individual lamellae.

The stromal thickness estimated as a 95% confidence interval for the mean, CI (0.95), in the keratoconic corneas was 372 ± 62 μm, while in the normal cornea, it was 446 ± 89 μm. All keratoconic corneas showed ultrastructural evidence of lamellar splitting and a loss of interweaving anterior lamellae. In the keratoconic corneas, the median total linear stromal lamellar absolute count tangential to the corneal surface was 362, (25th percentile; 75th percentile) = (355; 365) lamellae and in the normal cornea, 246, (25th percentile; 75th percentile) = (239; 251). The linear lamellar density in the keratoconic corneas was estimated as CI (0.95) 117 ± 22 and 86 ±19 lamellae per 100 μm in the anterior and posterior portion of the stroma, respectively. In normal cornea, the linear lamellar density was estimated as CI (0.95) 51 ± 8 and 80 ± 20 lamellae per 100 μm. The mean difference of linear lamellar count between the anterior and the posterior portion of the cornea was estimated as CI (0.95) 31 ± 23 for keratoconic corneas and -29 ± 28 for the normal corneas.

The current morphometric analysis of ultrastructural changes suggests that ectasia and thinning in keratoconus is associated with lamellar splitting into multiple bundles of collagen fibrils and loss of anterior lamellae. These structural changes, possibly in addition to lateral shifting of lamellae due to the pressure gradient over the cornea, are a potential explanation to the central loss of mass.