Microwave treatment of the cornea leads to localised disruption of the extracellular matrix

Abstract

Microwave keratoplasty is a thermo-refractive surgical procedure that can correct myopia (short-sightedness) and pathologic corneal steepening by using microwave energy to cause localised shrinkage around an annulus of the cornea leading to its flattening and vision correction. The effects on the corneal extracellular matrix, however, have not yet been evaluated, thus the current study to assess post-procedure ultrastructural changes in an in-vivo rabbit model. To achieve this a series of small-angle x-ray scattering (SAXS) experiments were carried out across whole transects of treated and untreated rabbit corneas at 0.25 mm intervals, which indicated no significant change in collagen intra-fibrillar parameters (i.e. collagen fibril diameter or axial D-period), whereas inter-fibrillar measures (i.e. fibril spacing and the degree of spatial order) were markedly altered in microwave-treated regions of the cornea. These structural matrix alterations in microwave-treated corneas have predicted implications for corneal biomechanical strength and tissue transparency, and, we contend, potentially render microwave-treated corneas resistant to surgical stabilization using corneal cross-linking procedures currently employed to combat refractive error caused by corneal steepening.

Figure 1

X-ray beam linear scans. Each red dot corresponds to a sampling location on control (a) and microwave treated (b) rabbit corneas. The scans traversed the discernible microwave region (arrow) at two points.

Figure 2

An example parameter profile for collagen fibril disorder. The 0.5 mm wide treatment-affected regions where values were averaged and statistically compared is shaded. Each black dot corresponds to a sampling location on the treated specimen.

Figure 3

Axial corneal topography maps of rabbit corneas. Maps of pre-microwave treatment (0 weeks) and 2 and 5 weeks post-treatment are displayed (bottom two rows). The maps for control corneas at the same time points are also displayed for comparison (top two rows). The colours visually correspond to “flatness” and “steepness,” with hot colours representing steeper regions of the cornea and cool colours representing flatter regions of the cornea. Variation in surface curvature is evident in both control and treated corneas over the 5 week time frame.

Figure 4

Parameter profiles for fibril disorder across individual treated rabbit corneas. Averaged (n = 3) control values are also shown. Note the marked increase in fibril spatial disorder in the microwaved region for all specimens (red arrows).

Figure 5

Parameter profiles for fibril spacing across individual treated rabbit corneas. Averaged (n = 3) control values are also shown. Note the increase in fibril spacing for specimens M1and M2 (red arrows). Changes in fibril spacing in the microwave regions of M3 and M4 are less distinct (green arrows).

Figure 6

Parameter profiles for fibril diameter across individual treated rabbit corneas. Averaged (n = 3) control values are also shown. No obvious change in fibril diameter is evident within the microwaved region of any of the treated corneas (green arrows).