Association of Iris Structural Measurements with Corneal Biomechanics in Myopic Eyes

Purpose. To evaluate the relationship between iris sectional parameters on swept-source optical coherence tomography (SS-OCT) with corneal biomechanics measured by Corneal Visualization Scheimpflug Technology (Corvis ST) in young adults with myopia. Methods. 117 patients with myopia aged ≥18 years were recruited from the Eye Hospital of Wenzhou Medical University, who had complete SS-OCT and Corvis ST data. Only the left eye of each participant was selected for analysis. Iris sectional parameters included iris thickness at 750 μm from the scleral spur (IT750), iris sectional area (I-area), and iris curvature (I-curv) measured from four quadrants. Associations between the iris parameters and corneal biomechanics were analyzed using linear regression models. Results. The mean age of the included young adults was 26.26 ± 6.62 years old with 44 males and 73 females. The iris parameters were different among the four quadrants. The nasal, temporal, and inferior quadrants of IT750, together with nasal and temporal quadrants of I-area, were correlated with corneal biomechanical parameters after being adjusted for age, gender, pupil diameter, and axial length. Thicker IT750 and larger I-area were related to a softer cornea. However, no association was found between I-curv and corneal biomechanics. Conclusions. Iris sectional parameters measured from SS-OCT images were associated with corneal biomechanical properties in myopic eyes. Thicker IT750 and larger I-area indicate a softer cornea. IT750 and I-area may provide useful information on corneal biomechanical properties in myopic eyes.

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.

Probing biomechanical properties of the cornea with air-puff-based techniques – an overview

The cornea is a part of the anterior segment of the eye that plays an essential optical role in refracting the light rays on the retina. Cornea also preserves the shape of an eyeball and constitutes a mechanical barrier, protecting the eye against the factors of the external environment. The structure of the cornea influences its biomechanical properties and ensures appropriate mechanical load transfer (that depends on the external environment and the intraocular pressure) while maintaining its shape (to a certain extent) and its transparency. The assessment of the corneal biomechanics is important in clinical ophthalmology, e.g. in the diagnosis of ectatic corneal diseases, for precise planning of the refractive surgery, and in accurate determination of the intraocular pressure. A standard technique to determine corneal biomechanics requires the application of well-defined mechanical stimulus (e.g. air puff) and performing simultaneous imaging of the response of the tissue to the stimulus. A number of methods to assess the biomechanical properties of the cornea have been developed, including ultrasound, magnetic resonance imaging, and optical methods as visualization modalities. Commercially available methods include the ocular response analyzer (ORA) and corneal visualization scheimpflug technology (Corvis ST). Currently advanced research is conducted using optical coherence tomography (OCT). The extension of OCT called optical coherence elastography (OCE) possesses high clinical potential due to the imaging speed, noncontact character, and high resolution of images.

Association between Iris Biological Features and Corneal Biomechanics in Myopic Eyes

Purpose. Iris biological features such as surface features and profile characteristics reflected the development of iris stroma and microvessels. Iris vessels and microcirculation are still lack of effective detection methods, and we can directly observe only the iris surface biological characteristics. This cross-sectional study evaluated the association between iris surface biological features and corneal biomechanics in young adults with myopia. Methods. We recruited 152 patients with myopia aged ≥18 years, from the Eye Hospital of Wenzhou Medical University, who had complete Corneal Visualization Scheimpflug Technology (Corvis ST) data and graded iris surface features. Iris surface features included crypts, furrows, and color measured from digital slit lamp images. The biomechanical properties of the cornea were assessed using Corvis ST. Only 1 eye of each participant was randomly selected for analysis. Associations between the iris surface features and corneal biomechanics were analyzed using linear regression models. The grade of iris crypts, furrows, and color and corneal biomechanical parameters measured with Corvis ST was the main outcome measures. Results. The iris crypts were significantly associated with deflection amplitude at the first applanation (A1 DLA, β = 0.001 , P = 0.013 ), A1 delta arc length (A1 dArcL) ( β = − 0.001 , P = 0.01 ), maximum delta arc length (dArcLM) ( β = − 0.004 , P = 0.03 ), and stiffness at the first applanation (SP-A1) ( β = − 2.092 , P = 0.016 ). The iris furrows were only associated with integrated radius ( β = − 0.212 , P = 0.025 ). Iris color was found not related with corneal biomechanical parameters measured via Corvis ST. Conclusions. Iris surface features were associated with corneal biomechanical properties in myopic eyes; more iris crypts were associated with lower corneal stiffness while more extensive furrows were related with higher corneal stiffness. Iris crypts and furrows may provide useful information on corneal biomechanical properties in myopic eyes.

Rate of Cornea Endothelial Cell Loss and Biomechanical Properties in Fuchs' Endothelial Corneal Dystrophy

Background: Our study aimed to determine the correlation between the clinical staging of Fuchs' endothelial corneal dystrophy (FECD), rate of endothelial cell loss, and corneal biomechanical properties.Methods: This study combined a longitudinal retrospective/prospective analysis of corneal endothelial cell loss and a prospective cross-sectional analysis of corneal biomechanics of Fuchs' endothelial dystrophy. The trial was registered at the Thai Clinical Trials Registry as TCTR 20160927004. FECD was diagnosed by the presence of corneal guttata detected by slit lamp microscopy; the disease severity was classified into four stages using the modified Stocker's classification. In vivo confocal microscopy, Scheimpflug imaging, and Corneal Visualization Scheimpflug Technology were performed to evaluate endothelial cell count, central corneal thickness, and corneal biomechanical properties. Linear mixed modeling analyses were used to estimate the endothelial cell densities in a 4-year period. The corneal biomechanics were compared among the stages using Corvis ST parameters.Results: Eighty eyes from eighty subjects were enrolled (42, 26, 12, and none in stages 1, 2, 3, and 4, respectively). The mean endothelial cell density was 1228.35 cells/mm2. The year-by-year reduction rate was 94.3 cells/mm2 (μEMM = −94.3, 95% CI: −115.4 to −73.2, p &lt; 0.001). Corneal endothelial cell losses in Fuchs' endothelial dystrophy were estimated to be 7.7, 7.8, and 8.4% per year for stages 1, 2, and 3, respectively. The mean corneal thicknesses of stages 1, 2, and 3 were 556 ± 32, 623 ± 33, and 648 ± 50 mm, respectively. For the corneal biomechanical parameters, the A1-length and A1-time were significantly different between stages 1 and 3 (A1-length: mean diffstage1vs.3 = 0.10, 95% CI: &lt; 0.001–0.15, p &lt; 0.001, A1-time: mean diffstage1vs.3 = −0.24, 95% CI: −0.41 to −0.07, respectively).Conclusions: In the advanced stage, corneas significantly changed their biomechanical viscoelastic behavior by decreasing resistance, as measured by a longer A1-length and shorter A1-time.

Influence of tomographic and biomechanical corneal indexes on myopic refractive surgery indications: A multicenter study

Purpose To evaluate the influence of corneal tomographic and biomechanical indexes on the refractive technique indication. Methods A total of 251 eyes from 251 patients interested in refractive surgery were enrolled in this cross-sectional and multicenter study. Previous to the surgeon decision, a preoperative protocol was performed by refractive optometrists, containing four sections: refraction, biometry, corneal tomography and biomechanics. The refractive surgeons made a first decision based only on refraction, biometric and tomographic information. Biomechanical indexes were revealed, and refractive surgeons made a second indication. Additionally, for Laser-Assisted in-situ Keratomileusis cases, the percent tissue altered were calculated. Possible indications were no refractive surgery, photorefractive keratectomy, Laser-Assisted in-situ Keratomileusis or intraocular Collamer lens. Results After the first surgery indication, the distribution was photorefractive keratectomy (47.4%), Laser-Assisted in-situ Keratomileusis (48.2%) while intraocular Collamer lens achieved 2.8%. This proportion changed significantly after the second indication regarding corneal biomechanics and photorefractive keratectomy and Laser-Assisted in-situ Keratomileusis decreased by 24% while intraocular Collamer lens increased 19%. A total of 69 eyes changed the indication (27.5%) and 182 eyes (72.5%) remained unchanged. All indications changes were from photorefractive keratectomy or Laser-Assisted in-situ Keratomileusis to intraocular Collamer lens or no surgery. Indication changes to intraocular Collamer lens were observed in 49 eyes (71%). Tomographic, biomechanical indexes, ablation depth and percent tissue altered achieved statistically significant differences between eyes without and with indication changes (all, P < .01). Conclusion New corneal biomechanical indexes could change the indication decision regarding biometric and tomographic data alone. Intraocular Collamer len was the preferred indication for eyes at risk of ectasia or with subclinical keratoconus due to corneal biomechanical parameters.

Evaluation of changes in corneal biomechanics in eyes with keratectasia

Purpose. To make a comparative analysis of topographic, tomographic and biomechanical values, measured by Pentacam HR and Corvis ST, in healthy multirefractive cohort versus patients with keratectasia of various severity. Material and methods. The prospective study comprised 237 patients aged from 18 to 47 years, subdivided into three groups. The first group included 174 healthy persons with various refraction (174 eyes). The second and the third group embraced 63 patients with keratoconus: 36 patients (36 eyes) with keratoconus of the I degree and 27 eyes with keratoconus of the II and III degrees. Besides the conventional comprehensive ocular examination the following studies were performed in all the patients: evaluation of topographic, tomographic and biomechanical corneal properties with application of Pentacam HR and Corvis ST (OCULUS Optikgerate GmbH; Wetzlar, Germany). Results. It was revealed that the following biomechanical values, assessed by means of Pentacam HR and Corvis ST, exhibited statistically significant changes (p<0,001) in all degrees of ketatoconus: a relative corneal thickness by Ambrosio, inverted radius of applanation curvature (IntRadius), DA Ratio Coefficient, values of corneal rigidity – SP-A 1 and SSI. Conclusion. The detected parameters may be applied as indicators, allowing to evaluate corneal biomechanics, to define early features of ectatic process, to assess effectiveness of the performed corneal collagen crosslinking and intralamellar keratoplasty with implantation of corneal segments. Key words: keratoconus, corneal biomechanics, pachymetry, index of pachymetric progression, corneal viscoelasticity.