Evaluation of Corneal Biomechanics: Stress-Strain Index (SSI) Mapping Using Corvis ST in Normal, Diabetic, and Keratoconic Patients

The Corneal Visualization Scheimpflug Technology by Oculus is a non-contact device that assesses corneal biomechanics by using a high-speed Scheimpflug camera to capture the cornea’s response to an air puff. The corneal biomechanical properties play a crucial role in maintaining ocular shape and visual function. Corneal stiffness, which reflects the resistance of the cornea to deformation, is influenced by factors such as intraocular pressure, collagen structure, and extracellular matrix composition. Understanding the biomechanical behavior of the cornea is essential for accurate diagnosis, monitoring, and management of various corneal disorders.

 The Stress-Strain Indexis a biomechanical parameter that quantifies corneal stiffness by measuring the relationship between stress and strain provided by the Corvis ST to estimate the overall stress–strain behavior of corneal tissue. With this information, the tangent modulus, a measure of the tissue’s material stiffness, could be estimated at any load or stress. This point is of particular importance since biological tissues have nonlinear stress–strain behavior, which means that Et does not have a unique value but increases with applied load and the corresponding deformation.

 SSI mapping, performed using the Corvis ST, provides a non-invasive method to evaluate corneal biomechanics by analyzing the cornea’s dynamic response to an air puff. This work uses Corvis ST to map the SSI in a cohort of healthy people in order to assess age-related changes in corneal biomechanics. The selection criteria for refractive and corneal procedures will be improved, diagnostic accuracy for corneal disorders will be increased, and important insights into corneal health will be gained from an understanding of how corneal stiffness varies with age. The results of this study will help create age-specific normative data for SSI, which may be used as a clinical reference for upcoming treatments and diagnostic procedures.

 Diabetes is a systemic metabolic disease associated with high morbidity and mortality that can affect almost all tissues of the human body, including the most superficial and transparent ocular tissue: the cornea. Diabetes can have a variety of effects on the cornea, frequently without any outward symptoms in the early stages. Because of the accumulation of dangerous chemicals known as advanced glycation end-products, persistently elevated blood sugar levels can alter the cornea’s rigidity and structure. Before any obvious damage happens, SSI mapping assists diabetic patients in detecting early alterations in the cornea’s biomechanics. It also helps assess corneal stiffness between diabetic and non-diabetic persons, and between patients with good and poor blood sugar control. SSI mapping can help us better understand how diabetes impacts the cornea and promote more individualized, high-quality eye treatment. By assessing SSI in relation to HbA1c levels or disease duration, clinicians may gain insight into how metabolic control influence corneal biomechanics. This makes SSI mapping a potentially valuable tool for monitoring the ocular effects of diabetes and tailoring individualized eye care strategies.

 Keratoconus is a progressive corneal disease that usually affects the central and paracentral cornea and is characterized by localized thinning and biomechanical weakening. A single worldwide indicator of corneal stiffness is provided by the stress-strain index, which is derived from the Corvis ST. However, a spatial SSI map that shows changes in stiffness throughout the corneal surface can be created by combining this with geometric data and the known distribution of collagen fibrils. This makes it possible to see the biomechanical variations between the cone and the surrounding tissue, providing a better understanding of how disease behave. Thus, SSI mapping is essential for comprehending the evolution of keratoconus, improving early detection and staging, and detecting individualized therapy techniques like cross-linking. A single SSI value obtained from the Corvis ST for an eye, either healthy or KC, could be translated into a map by adding geometric information and using knowledge about collagen fibril density to show the variation of SSI across corneal surface. This map could enable visualization of the effect of the disease on the affected area, and help improve fundamental understanding of the mechanics of keratoconus progression in individual patients.