An interferometric ex vivo study of corneal biomechanics under physiologically representative loading, highlighting the role of the limbus in pressure compensation

Abstract

Background: The mechanical properties of the cornea are complex and regionally variable. This paper uses an original method to investigate the biomechanics of the cornea in response to hydrostatic loading over the typical physiological range of intra-ocular pressure (IOP) fluctuations thereby increasing understanding of clinically relevant corneal biomechanical properties and their contributions to the refractive properties of the cornea. Methods: Displacement speckle pattern interferometry (DSPI) was used to measure the total surface displacement of 40 porcine and 6 human corneal-scleral specimens in response to pressure variations up to 1 mmHg from a baseline of 16.5 mmHg. All specimens were mounted in a modified artificial anterior chamber (AAC) and loaded hydrostatically. Areas of high strain in response to loading were identified by comparing the displacements across different regions. Results: The nature of the response of the corneal surface to loading demonstrated high regional topographic variation. Mechanical properties were shown to be asymmetrical, and deformation of the limbal and pre-limbal regions dominated these responses respectively with over 90% (N-T) and 60% (S-I) of the total maximum displacement occurring in these regions indicating high-strain. In contrast, the curvature of the central cornea remained relatively unchanged merely translating in position. Conclusions: The limbal and pre-limbal regions of the cornea appear to be fundamental to the absorption of small pressure fluctuations facilitating the curvature of the central cornea to remain relatively unchanged. The differential mechanical properties of this region could have important implications for the application of corneal surgery and corneal crosslinking, warranting further investigation.

Regional variation of corneal stromal deformation measured by high-frequency ultrasound elastography

“…Previous studies using optical methods also reported a reduced surface tangential strain around the apex during inflation. [12][13][14] Our results expanded upon this observation and confirmed that corneal stroma around the apex indeed stretched minimally under IOP elevation. This may be important for maintaining stable vision during IOP fluctuations.…”

“…Optical imaging was used to quantify corneal surface strains. [12][13][14] Tomographic imaging methods such as optic coherence tomography (OCT) [15][16][17] and high-frequency ultrasound 18,19 have also been applied to characterize corneal deformation under various loads. However, few studies have reported the regional response of the cornea stroma to IOP elevations.…”

The Potential Roles of Genetic Testing and Biomechanical Evaluation in Keratoconus

“…The cornea has a complex and heterogeneous structure resulting in significant spatial and directional variability in mechanical properties. Recent studies [99][100][101] have demonstrated regional variability in mechanical properties of the cornea, especially highlighting differences in the properties of the limbus and peripheral cornea versus the central cornea [101]. These differences are extremely important, and their quantification will be key to the development of customised and targeted treatments for KC.…”

“…Analysis of the distribution of the response is helpful especially when trying to identify localised abnormalities, as the relative deformation in one region can be evaluated relative to the overall deformation of the cornea, removing the reliance on accurate quantification of specific mechanical properties. In fact, it has already been possible to identify similarities in the distribution of the anterior surface displacement of corneas to IOP variations and from this identify corneas that show abnormalities [101,102].…”

“…Data can also be obtained across the entire cornea and surrounding sclera providing a larger field of view and allowing the response at the limbus to be taken into account. Displacement speckle pattern interferometry, as has been used in previous studies [101,102], has a displacement sensitivity of several nanometres and can therefore capture very high resolution displacement data over very small variations in IOP from 0.05 to 1 mmHg [101]. In contrast, OCT and HFU have slightly lower sensitivity requiring pressure variations of between 1 and 3 mmHg.…”