A study of corneal thickness, shape and collagen organisation in keratoconus using videokeratography and X-ray scattering techniques

Abstract

In keratoconus, the cornea becomes progressively ectactic resulting in severe visual impairment. Here, we use a combination of videokeratography and synchrotron X-ray diffraction to investigate the relationship between corneal shape and thickness, and the distribution and predominant orientation of stromal fibrillar collagen in five keratoconus corneas. In all but the least advanced case, the thinning and ectasia measured in vivo using corneal videokeratography was accompanied by corresponding changes in the relative distribution and orientation of stromal collagen in the excised corneal buttons. Although the most severe case of keratoconus possessed the most pronounced stromal collagen alterations, and only a minor disruption to stromal collagen arrangement was seen in the least advanced case, a variability in the extent of stromal collagen alteration was seen between these clinical extremes. The observed abnormalities in collagen distribution and orientation are consistent with a mechanism of keratoconus progression that involves inter-fibrillar or inter-lamellar slippage causing a redistribution of tissue within the cornea.

A structural investigation of corneal graft failure in suspected recurrent keratoconus

“…Despite the considerable variability known to exist between keratoconus corneas in terms of the extent of stromal collagen disruption, 14 X-ray scattering and scanning electron microscopy studies are in agreement that the severity of disruption decreases with distance from the cone apex. 13,14,18,19 In contrast to most keratoconus corneal buttons, the majority of the failed-graft tissue appeared normal in terms of collagen orientation and mass distribution, with regions of abnormality being limited to the inferior aspect of the peripheral cornea and coinciding in each case with the location of the original graft-host interface. The most marked abnormalities were observed in the grossly vascularised infero-nasal quadrant of the peripheral cornea, where the normal tangential alignment of collagen was replaced by radially orientated collagen and the lamellae appeared indistinct and disordered when viewed by electron microscopy.…”

“…These findings are consistent with our previously published data from six keratoconus corneas of varying disease severity, cone shape, and location. 13,14 Collagen distribution…”

“…The findings were compared with our previously published work (using the same experimental protocol as here), which examined six normal corneas 12 and six keratoconus corneas. 13,14 Measurements of fibril diameter in the failed-graft corneal tissue were compared with those obtained from an 8.5-mm button excised from a normal post-mortem cornea stored for 23 days in culture medium and then de-swelled for 2 days in dextran (obtained from the Corneal Transplant Service Eye Bank, Bristol, UK). Our previously published results from five healthy corneas 15 were used to confirm the normal range of fibril diameters in the human cornea.…”

“…The X-ray scatter patterns were normalised against variations in X-ray beam intensity and exposure time and analysed using methods previously described. 14 For each cornea, the analysed data were assembled to show the relative mass distribution of total fibrillar collagen and that of preferentially aligned collagen (defined as additional X-ray scatter from collagen lying in a specific orientation after removal of X-ray scatter from collagen disposed in all directions). To show the predominant orientation of collagen at each sampling position, vector coordinates/ plots were formed in which the distance from the centre of the plot in a given direction represents the relative amount of aligned collagen lying in that direction.…”

Corneal changes after a single session of selective laser trabeculoplasty for open-angle glaucoma

“…It has been demonstrated that corneal edema after phacoemulsification has been associated with a relatively lower IOP by Goldmann applanation tonometry because of the reduction of corneal hysteresis and alterations of the stromal lamellar architecture. [18][19][20] Another possibility is the use of Alphagan P immediately after SLT but as only a single drop was instilled, it is unlikely to cause a significant drop in day 1 IOP. Thus, the IOP at 1 month post SLT may more accurately reflect the true change in IOP as a result of the SLT procedure.…”