Axial mechanical and structural characterization of keratoconus corneas - PubMed (original) (raw)
Axial mechanical and structural characterization of keratoconus corneas
Eric Mikula et al. Exp Eye Res. 2018 Oct.
Abstract
Purpose: Previous studies indicate that there is an axial gradient of collagen lamellar branching and anastomosing leading to regional differences in corneal tissue stiffness that may control corneal shape. To further test this hypothesis we have measured the axial material stiffness and quantified the collagen lamellar complexity in ectatic and mechanically weakened keratoconus corneas (KC).
Methods: Acoustic radiation force elastic microscopy (ARFEM) was used to probe the axial mechanical properties of the cone region of three donor KC buttons. 3 Dimensional second harmonic generation microscopy (3D-SHG) was used to qualitatively evaluate lamellar organization in 3 kC buttons and quantitatively measure lamellar branching point density (BPD) in a separate KC button that had been treated with epikeratophakia (Epi-KP).
Results: The mean elastic modulus for the KC corneas was 1.67 ± 0.44 kPa anteriorly and 0.970 ± 0.30 kPa posteriorly, substantially below that previously measured for normal human cornea. 3D-SHG of KC buttons showed a simplified collagen lamellar structure lacking noticeable angled lamellae in the region of the cone. BPD in the anterior, posterior, central and paracentral regions of the KC cornea were significantly lower than in the overlying Epi-KP lenticule. Additionally, BPD in the cone region was significantly lower than the adjacent paracentral region in the KC button.
Conclusions: The KC cornea exhibits an axial gradient of mechanical stiffness and a BPD that appears substantially lower in the cone region compared to normal cornea. The findings reinforce the hypothesis that collagen architecture may control corneal mechanical stiffness and hence corneal shape.
Copyright © 2018 Elsevier Ltd. All rights reserved.
Figures
Fig. 1.
Elastic moduli in the anterior and posterior portions in keratoconic samples as measured by ARFEM. The anterior region is significantly stiffer than the posterior in all three samples. Elasticity data from a 2016 study in the healthy cornea has been included for comparison (Mikula et al., 2016). Asterisk (*) indicates significance (p < 0.05).
Fig. 2.
SHG backscatter image in a keratoconus cornea. Panel B is zoomed in on a region within the cone revealing a thin cornea with noticeably absent interweaving. Panel C is zoomed in on a region outside of the cone; anterior interweaving in this region is apparent as shown by lamellae forming acute angles to the corneal surface. White arrows indicate areas of branching.
Fig. 3.
SHG backscatter image in a keratoconus cornea. Panel B is zoomed in on the thinnest region within the cone revealing a dearth of interweaving and breaks in Bowman’s layer. Panel C is zoomed in on a region outside of the cone revealing normal looking structure and anterior interweaving as shown by lamellae forming acute angles to the corneal surface. White arrows indicate areas of branching.
Fig. 4.
A. NLO HR-MAC image of a corneal cross section on a single optical plane out of 51 planes. Imposed on top of the single plane is a 3-D reconstruction of bow spring fibers (teal) inserting into Bowman’s layer of the Epi-KP lenticule (green). Bow spring fibers (red) are shown inserting into Bowman’s layer of the KC cornea (yellow) along with a single fiber (magenta) near the cone region. Fibers were segmented manually and rendered. Note: Image B,C,D are from different slices to show image at optimal resolution. B. Zoomed in 3-D reconstruction of a detailed fiber meshwork (teal) from the Epi-KP lenticule showing multiple branching points. C. Zoomed in 3-D reconstruction of a single fiber (magenta) running along the KC cornea near the cone region showing minimal branching compared to the Epi-KP lenticule in (B). Highlighted portion shows a break in Bowman’s layer (yellow) and a lack of bow spring insertion. D. Zoomed in 3-D reconstruction of bow spring fibers (red) inserting into the KC Bowman’s layer (yellow) along with a region of interweaving due to the significant distance from the cone (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 5.
Branching point density (BPD) in the KC button and Epi-KP lenticule as a function of depth. The anterior and posterior are defined as being within 100 μm of the anterior and posterior surfaces of the region.
Fig. 6.
Branching point density (BPD) in the KC button and Epi-KP lenticule as a function of radial position (central vs. para-central).
References
- Andreassen TT, Hjorth Simonsen A, Oxlund H, 1980. Biomechanical properties of keratoconus and normal corneas. Exp. Eye Res 31 (4), 435–441. -PubMed
- Edmund C, 1988. Corneal elasticity and ocular rigidity in normal and keratoconic eyes. Acta Ophthalmol. 66 (2), 134–140. -PubMed
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