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Judith Birkenfeld, James A Germann, Alberto De Castro, Andres De la Hoz, Andrea Curatolo, Susana Marcos; Assessment of asymmetries in biomechanical properties from corneal deformation imaging. Invest. Ophthalmol. Vis. Sci. 2019;60(9):6809.
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© ARVO (1962-2015); The Authors (2016-present)
Non-invasively capturing the presence of corneal weakness is important in detecting keratoconus. Air-puff corneal deformation imaging (APCDI) may complement topography in biomechanical diagnostics of corneas, yet to date is unable to detect asymmetries in biomechanical properties. We have developed a porcine model for corneal biomechanical asymmetries and metrics to assess those from APCDI.
Freshly enucleated (<48h) porcine eye globes were treated, following standard cross-linking (CXL) protocols with Riboflavin-UVA (UVX) and Rose Bengal-Green light (RGX), but shielding 50% (N=4) or 70% (N=4) of the upper cornea. Eyes were mounted on a rotation base and measured in a Scheimpflug APCDI system (Corvis ST, Oculus) under a constant IOP of 13 mmHg. Measurements were performed after application of photosensitizer (=baseline, BSL) and after partial CXL. Cross-sectional images were obtained for vertical meridians and analyzed at highest concavity with the following metrics: deformation amplitude (DA), peripheral deformation amplitude (PDAx) for x=±0.5, ±1.0, ±1.5 mm, and horizontal distance between apex and peak deformation (PD). Asymmetry ratios (AR) were calculated for PDA and PD for both BSL and CXL conditions in the vertical meridian. To confirm regional variations of corneal elasticity, uniaxial extensiometry of corneal strips was performed.
Uniaxial strip extensiometry measured a 1.95-fold stiffness increase in CXL areas (2.4 & 1.5-fold for UVX and RGX, respectively). For 50% CXL corneas, the mean PDA+x/PDA-x AR in the vertical meridian were 1.0±0.03, 0.98±0.09, 0.99±0.13 (BSL), and 0.95±0.04, 0.88±0.07, 0.77±0.09 (CXL) for x=0.5, 1.0, 1.5 mm, respectively. AR for PD were 1.02±0.06 and 0.95±0.08 for BSL and CXL conditions, respectively. For 30% CXL corneas, the mean PDA+x/PDA-x AR in the vertical meridian were 1.03±0.02, 1.03±0.05, 1.07±0.03 (BSL), and 0.97±0.01, 0.94±0.04, 0.88±0.06 (CXL) for x=0.5, 1.0, 1.5 mm, respectively. AR for PD were 1.08±0.04 and 0.96±0.05 for BSL and CXL respectively.
PDA ratios obtained from Scheimpflug APCDI revealed asymmetries produced by local variations in corneal stiffness (modeled here by local CXL). While in-vivo this instrument is limited to one meridian, other technologies, such as optical coherence tomography (OCT), will allow air puff imaging at multiple meridians, and therefore improve detection of corneal abnormalities.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.
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