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Quan V Hoang, Daniel Rohrbach, Sally A McFadden, Ronald H Silverman, Jonathan Mamou; Inter-Layer Differences in Elastic Properties of Guinea Pig Sclera at the Micrometer Scale with Acoustic Microscopy. Invest. Ophthalmol. Vis. Sci. 2017;58(8):5639.
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© ARVO (1962-2015); The Authors (2016-present)
The anatomical change underlying excessive eye elongation in myopia likely occurs in the collagen-rich sclera. We employed scanning acoustic microscopy (SAM) to study the relationship between the microscopic structural properties of scleral collagen and its macroscopic mechanical outcome in myopic guinea pig (GP) eyes.
Sclera from 6 eyes from three 14-day-old guinea pigs were scanned with SAM. Right eyes had undergone 6-8 days of form deprivation. Flash-frozen eyes were cryosectioned 12-µm thick and mounted on the SAM, which was equipped with an F-1.16, 250-MHz transducer and 7-µm lateral beam width. Maps of the speed of sound (c), acoustic impedance (Z), attenuation (a), bulk modulus (K), and mass density (ρ) were derived from the frequency domain representation of each recorded signal using a model-based approach. Additionally, two GP eyes were processed for light and transmission electron microscopy to assess collagen fibril morphology.
Average material properties among control left eyes were found to be c=1657±85 m/s, Z=1.63±0.04 MRayl, K=2.71±0.18 GPa, and ρ=1.00±0.03 g/cm3. Among right eyes (which were -3.21, -5.54 and -9.29 D relatively myopic), average material properties were c=1638±65, Z=1.64±0.04, K=2.70±0.14 and ρ=1.01±0.02. Maps of K in both control and myopic sclera showed a distinctive 4-layered structure of roughly equal thickness (25.0±5.4% each). When comparing the two central scleral layers to the anterior-most and posterior-most scleral layers in control eyes, central layers had significantly greater K (by +0.28, p=0.0008), lower ρ (-0.04, p=0.004) and greater c (+137, p=0.0005). Among myopic eyes, central scleral layers showed similar trends with greater K (by +0.22, p=0.002), lower ρ (-0.04, p=0.005) and greater c (+106, p=0.0005). Consistent with previous reports, (Cui et al. 2011), microscopy demonstrated variation in collagen fibril diameter and orientation across scleral layers.
This study reports a layered structure of the GP sclera with varying properties in ρ and K. These layers may correspond to noted differences in collagen fibril diameter and orientation between scleral layers. SAM analysis provides fine-resolution biomechanical properties of soft tissues and may be invaluable in the investigation of ocular tissues in diseases where tissue elasticity is believed to play an important role such as in high myopia.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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