Novel Method of Measuring Corneal Viscoelasticity Using the Corvis ST Tonometer

Background: The process of rapid propagation of the corneal deformation in air puff tonometer depends not only on intraocular pressure, but also on the biomechanical properties of the cornea and anterior eye. One of the biomechanical properties of the cornea is viscoelasticity, which is the most visible in its high-speed deformations. It seems reasonable to link the corneal viscoelasticity parameter to two moments of the highest speed of corneal deformations, when the cornea buckles. The aim of this work is to present a method of determining the time and place of occurrence of corneal buckling, examine spatial and temporal dependencies between two corneal applanations and bucklings in the Corvis ST tonometer, and correlate these dependencies with corneal viscoelastic properties. Methods: Images of the horizontal cross section of the Corvis ST deformed cornea from the air puff tonometer Corvis ST were used. 14 volunteers participated in the study, each of them had one eye measured eight times. Mutual changes in the profile slopes of the deformed corneas were numerically determined. They describe pure corneal deformation, eliminating the influence of rotation, and displacement of the entire eyeball. For each point in the central area of the corneal profile, the maximum velocities of mutual slope changes accompanying the applanations were estimated. The times of their occurrence were adopted as buckling times. Results: The propagation of buckling along the corneal profile is presented, as well as the repeatability and mutual correlations between the buckling parameters and intraocular pressure. Based on the relationship between them, a new parameter describing corneal hysteresis: Corvis Viscoelasticity (CVE) is introduced. It is characterized by high repeatability: ICC = 0.82 (0.69–0.93 CI) and low and insignificant correlation with intraocular pressure: r = 0.25 (p-value = 0.38). Conclusion: The results show for the first time how to measure the corneal buckling and viscoelastic effects with Corvis ST. CVE is a new proposed biomechanical parameter related to the viscoelastic properties of the cornea, which has high repeatability for the examined subject. The distribution of its values is planned to be tested on different groups of patients in order to investigate its clinical applicability.

Association of Corneal Biomechanics Properties with Myopia in a Child and a Parent Cohort: Hong Kong Children Eye Study

Associations between corneal biomechanics, axial elongation and myopia are important but previous results are conflicting. Our population-based study aimed to investigate factors associated with corneal biomechanics, and their relationships with myopia in children and adults. Data from 3643 children and 1994 parents showed that children had smaller deformation amplitudes (DA) than parents (p < 0.001). A larger DA was significantly associated with elongated axial length (AL; children: ß = 0.011; adults: ß = 0.0013), higher corneal curvature (children: ß = 0.0086; adults: ß = 0.0096), older age (children: ß = 0.010; adults: ß = 0.0013), and lower intraocular pressure (IOP; children: ß = −0.029; adults: ß = −0.031) in both cohorts. The coefficient of age for DA in children was larger than in adults (p < 0.001), indicating that the DA change with age in children is faster than in adults. DA was significantly associated with spherical equivalent (p < 0.001) resulting from its correlation with AL and corneal curvature. In conclusion, the cornea is more deformable in adults than in children, whereas corneal deformation amplitude increases faster with age in children than that in adults, along with AL elongation. Longer AL, steeper corneal curvature, older age and smaller IOP correspond to a more deformable cornea. The association between corneal deformation amplitude and refraction was mediated via AL and corneal curvature.

Correction of refractive disorders in the surgical treatment of keratoconus by MyoRing implantation

Keratoconus is a progressive degenerative disease in which, due to the weakening of the biomechanical strength of the cornea, its cone-shaped deformation occurs, leading to the development of induced myopia and astigmatism, as well as to a decrease in corrected visual acuity. Treatment of this disease consists in achieving stabilization of the process by strengthening the structure of the cornea, as well as in correcting refractive disorders caused by corneal deformation. Purpose. The purpose is to evaluate the effectiveness of correction of refractive disorders by implanting a closed MyoRing ring in the corneal stroma of patients with keratoconus. Material and methods. The data of 98 MyoRing implantation operations using CISIS technology using the PoketMaker ultraceratom for stage 3-4 keratoconus were analyzed. The observation period is up to 9 years. The initial myopia averaged -9.23 D ±3.82, astigmatism-6.13 ±2.51. Results. Stabilization of the refractive effect occurred usually within 3-6 months after surgery. On average, the following correction of refractive disorders was achieved: myopia-8.14 D ± 2.67 and astigmatism-4.28 D ± 2.12. Keratometry indicators also in most cases stabilized within 3-6 months after surgery and remained unchanged throughout the entire follow-up period. Conclusions. Implantation of a closed MyoRing ring in keratoconus makes it possible to significantly correct refractive disorders and prevents further progression of the disease. Key words: keratoconus, MyoRing, SISI, ring implantation in keratoconus.

Dynamics Modeling of Corneal Deformation under Air Puff with Linear and Nonlinear Viscoelastic Model

Background: The aim of the study is to model the corneal dynamic deformation under an air puff excitation. The deformation response of the cornea was modeled by using linear and nonlinear viscoelastic models. The corneal deformation responses generated from the linear and nonlinear viscoelastic model were correlated with the clinical results, which were obtained from Corneal Visualization Scheimpflug Tonometer (Corvis ST) to evaluate the comparable biomechanical parameters of the cornea. Methods: A prompt deformation occurs when the external force applied to the cornea. Then a continuous deformation follows. A simple mass, spring and dashpot system were used to model human eyeball. Results: In linear viscoelastic model, the corneal elastic stiffness commanded behavior of the corneal deformation and its maximum, when the viscous component affected for its lateral shifting and marginally alter the magnitude.Whereas, in the nonlinear viscoelastic model, the corneal material nonlinearity commanded the behavior and maximum of the corneal deformation, while the viscous component marginally contributed for its lateral shifting and demonstrated the minimum affect on the magnitude and form. A multi-objective genetic algorithm-based optimization procedure was used to identify the material properties in the nonlinear viscoelastic model for 29 eyes of 20 normal people. Conclusion: The corneal deformation response model with nonlinear viscoelastic model showed to have a better fit with the corneal dynamic deformation behavior under an air pulse excitation. The biomechanical properties of the cornea in vivo can be evaluated by using and analysing dynamic deformation of the cornea under an air puff excitation model.