Biomechanics are important to the functioning of the eye. For example, the biomechanical properties of the cornea are responsible for its functionality and play an important role in the refractive properties of the eye. The shape of the cornea relies on the equilibrium between its stiffness and the internal and external forces acting on it. The mechanical stiffness of the cornea depends on its geometry (thickness and shape or topography) and material properties.
Biomechanical imaging with BOSS will address a large range of ophthalmic applications. In addition to imaging the cornea and lens, Confocal Brillouin Spectroscopy will allow eye care professionals to image and examine the sclera, vitreous, and even the retina. Individually examined locations can then be joined to build spatially resolved profiles or map images of ocular stiffness in vivo.
As with most soft tissues in the eye, collagen is often considered the primary biomechanical element of the cornea, as it provides tensile strength through its long, dense fibrous bundle organization. The ground substance matrix, wherein collagen fibers are embedded, contains the elastin fibers, proteoglycans, fibroblasts, tissue fluid and all other tissue constituents except for collagen.
The stroma comprises over 200 lamellae, which form a proteoglycan-rich matrix containing tightly packed and ordered collagen fibrils. The arrangement and density of collagen fibrils in the stroma are the primary contributors to the biomechanical stiffness of corneal tissue.
It is well-documented in the literature that corneal tissue does not have a constant modulus of elasticity. Its modulus is non-linear and depends on the degree of deformation at a specific location. This means that the more the cornea is deformed, the stiffer it becomes.
Responsible for this behavior is the organization (distribution and orientation) of collagen fibrils within the cellular matrix. The distribution of collagen fibrils in the cornea is inhomogeneous, as it varies between the center of the cornea and the periphery. Furthermore, the fiber structure of the anterior section of the cornea differs from that of the deeper layers on the posterior cornea. Currently, there are no established non-invasive methods for identifying the elastic properties of the cornea in vivo. BOSS is here to change that.
Biomechanics and Morphology. (a) Normal subject (b) Mild Keratoconus (c) Advanced Keratoconus.
All: Top images are pachymetry (thickness) and topography (elevation and steepness) images recorded with the Oculus Pentacam®. Below are readings of stiffness from BOSS with morphologically matched regions of interest.
Increasing stiffness of the crystalline lens with age has long been recognized as one of the major factors causing presbyopia – a failure of the eye to accommodate and focus on nearby objects. More-recent studies indicate that lens stiffening alone can be responsible for the inability of the lens to change its refractive power at an older age. BOSS can provide critical information regarding the progression of presbyopia and potential surgical or therapeutic interventions.
In experiments, stiffness of the lens center (nucleus) was found to depend on the type of cataract (clouding) and the age of the patient. Lenses with nuclear cataracts (clouding in the nucleus) were generally stiffer than lenses extracted from patients with predominantly cortical cataracts (clouding in the cortex), with some in the latter group appearing not to differ significantly from age‐matched normals. At age 40–50, the nuclear region of lenses with an advanced nuclear cataract were found to be approximately 46 times harder than that of normal lenses in individuals of the same age. By age 70–80, the stiffness of advanced nuclear cataract lenses had doubled. However, by this age, normal lenses had also increased significantly in stiffness, such that the difference between normal lenses and those with a cataract were much less pronounced, with a factor of approximately 2.5.
BOSS can provide new possibilities for identifying the lens-clouding process in cataracts in different parts of the lens, and will potentially be useful in the guidance of lens-extraction surgery and therapeutic lens-softening procedures.
Age-related crystalline lens thickening with (a) Model of thickening (b) Crystalline lens stiffness profiles measured with BOSS in 19-, 31-, 43- and 61-year-old humans showing the aqueous humor (A.H.), the vitreous humor (V.H.), the lens cortex, and the lens nucleus (central) plateau.