Evaluation Of Corneal Ultrastructure Following Microwave Keratoplasty

Abstract

Purpose: : Microwave keratoplasty is a new non-invasive therapy for the correction of myopia and treatment of corneal ectasia. An annular pulse of microwave energy is delivered to the corneal mid-periphery, which is intended to cause localised shrinkage of anterior stromal lamellae, thereby achieving flattening of the central cornea. However nothing is known of the potential effects of the therapy on stromal collagen at the sub-lamellar level. Here we present the first evaluation of ultrastructural changes in the stromal matrix following experimental microwave keratoplasty.

Methods: : At all times animals were treated in accordance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research. Under anaesthesia, four adult New Zealand White rabbits received 915 MHz microwave treatment unilaterally as a 3.8mm (inner) to 4.3mm (outer) diameter annulus with a Vedera KXS machine (Avedro Inc. USA). This application should treat myopia of approximately -6D. Following treatment eyes received antibiotic eye drops. The untreated contralateral eyes acted as normal controls. After three weeks animals were euthanized by intravascular injection of pentobarbital sodium (100mg/kg) and the corneas harvested and fixed in 4% paraformaldehyde to preserve collagen ultrastructure. Small-angle x-ray scattering was performed using Beamline I22 at the Diamond Light Source, UK. Data analysis provided quantitative profiles of collagen fibril spacing, diameter, axial period and spatial order index at 0.25mm intervals across each specimen from limbus to limbus.

Results: : Collagen fibril spacing, diameter and axial period, as an average through the stromal depth, were unaffected by the therapy. In contrast a highly significant (22%-50%) increase in local fibril disorder was measured in all four treated corneas. This effect was restricted to within the microwave treatment region, with normal collagen architecture retained away from the treated tissue. The treatment area displayed a visible increase in opacity compared to the untreated tissue.

Conclusions: : Microwave keratoplasty causes localised corneal opacity within the treatment area that is likely due to spatial disruption of stromal collagen at the fibrillar level, with implications for peripheral vision. The stromal fibril changes induced by the therapy may also impact on corneal biomechanics.

Keywords: cornea: basic science • cornea: stroma and keratocytes • refractive surgery: other technologies