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Sebastien Besner, Giuliano Scarcelli, roberto pineda, Seok H Yun, Patricia Kalout; 3D corneal elasticity mapping in normal and keratoconus patients. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1138.
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© ARVO (1962-2015); The Authors (2016-present)
The balance between the cornea mechanical strength and the intraocular pressure is critical to maintain normal shape. When disrupted, the local stress can lead to a progressive compensatory thinning and bulging of the cornea or keratectasia. Although marked clinical advances have been made in cornea structural imaging, still no method is available to map elasticity in vivo. In this study, we present a non-contact method for in vivo mechanical mapping based on Brillouin scattering and evaluate its diagnostic potential for keratoconus.
A 780-nm confocal Brillouin microscope (P=1.5 mW, rlateral=5 μm, raxial=35 μm) combined with real time eye registration system was used to acquire 6-mm central cornea elasticity maps in 11 normal and 5 advanced keratoconus patients. Axial scans were taken at various lateral positions with a depth scanning interval of 30 μm. The Brillouin anterior modulus was computed by averaging the Brillouin shift value over the anterior portion of the stroma and used to construct lateral elasticity maps. Correlations between pachymetry/curvature (Pentacam, Oculus) and Brillouin map were established.
In normal patients, the cornea mechanical strength decreased with depth and increased toward its periphery. In average, the central Brillouin shift at 80% depth was ~80 MHz lower in comparison to the anterior region. Laterally, the Brillouin modulus increased by 30 MHz. It also increased with age in the range of 20-60 years old (0.7±0.2 MHz/year, p=0.005). When converted into shear modulus, those relative changes corresponded to a 2.4x decrease in depth, a 1.5x increase radially, and a 0.8%/year increase with age. In keratoconus patients, the Brillouin modulus in the cone region was about 100 MHz lower (2.9x drop in shear modulus) and the lateral gradient was 3.8x steeper than the normal control. Away from the cone, the Brillouin modulus was comparable to the normal ones. In all patients, Brillouin modulus was positively correlated with cornea thickness and negatively correlated with front sagittal curvature.
Striking differences could be observed between the normal and keratoconic cornea biomechanics. While normal corneas presented a gradual increase of the elastic modulus from its center toward the periphery, strong mechanical focal weakening was observed in keratoconus cases. Cornea stiffness increased with age, and may explain the high prevalence of keratoconus in teenage years.
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