Developing a multiscale in silico cornea for understanding the role of cell mechanics in corneal pathologies

Biomechanics of keratoconus: Two numerical studies

BACKGROUND

The steep cornea in keratoconus can greatly impair eyesight. The etiology of keratoconus remains unclear but early injury that weakens the corneal stromal architecture has been implicated. To explore keratoconus mechanics, we conducted two numerical simulation studies.

METHODS

A finite-element model describing the five corneal layers and the heterogeneous mechanical behaviors of the ground substance and lamellar collagen-fiber architecture in the anterior and posterior stroma was developed using the Holzapfel-Gasser-Ogden constitutive model. The geometry was from a healthy subject. Its stroma was divided into anterior, middle, and posterior layers to assess the effect of changing regional mechanical parameters on corneal displacement and maximum principal stress under intraocular pressure. Specifically, the effect of softening an inferocentral corneal button, the collagen-based tissues throughout the whole cornea, or specific stromal layers in the button was examined. The effect of simply disorganizing the orthogonally-oriented posterior stromal fibers in the button was also assessed. The healthy cornea was also subjected to eye rubbing-like loading to identify the corneal layer(s) that experienced the most tensional stress.

RESULTS

Conical deformation and corneal thinning emerged when the corneal button or the mid-posterior stroma of the button underwent gradual softening or when the collagen fibers in the mid-posterior stroma of the button were dispersed. Softening the anterior layers of the button or the whole cornea did not evoke conical deformation. Button softening greatly increased and disrupted the stress on Bowman's membrane while mid-posterior stromal softening increased stress in the anterior layers. Eye rubbing profoundly stressed the deep posterior stroma while other layers were negligibly affected.

DISCUSSION

These observations suggest that keratoconus could be initiated, at least partly, by mechanical instability/damage in the mid-posterior stroma that then imposes stress on the anterior layers. This may explain why subclinical keratoconus is marked by posterior but not anterior elevation on videokeratoscopy.

Collagen Film with Bionic Layered Structure and High Light Transmittance for Personalized Corneal Repair Fabricated by Controlled Solvent Evaporation Technique

Corneal blindness is a common phenomenon, and corneal transplantation is an effective treatment for corneal defects. However, there is usually a mismatch between the corneal repair material and the degree of the patient’s corneal defect. Therefore, patients with different corneal defects need suitable corneal repair materials with a specific microstructure for personalized treatment. In this research, collagen films with bionic structures were fabricated through ethanol evaporation technique by regulating the volume ratios of collagen solution: ethanol = 10:0(Col)/9:1(CC91)/8:2(CC82)/CC73(CC73). Under various preparation conditions, the obtained collagen films contain layered structures of different density. SEM photos show that the CC73 film with a dense layer arrangement has a microstructure similar to that of the corneal epithelial layer, whereas the Col film has a loose layered structure similar to that of the corneal stroma layer. Four kinds of collagen films showed different optical properties and water absorption ability. A more ordered arrangement of internal layer structure leads to better mechanical properties of the collagen film. In view of this, we think that these collagen films with different microstructures and different interlayer spacing may have huge potential applications for personalized corneal repair.