Comparative Analysis of Stress and Deformation between One-Fenced and Three-Fenced Dental Implants Using Finite Element Analysis.

Finite element analysis (FEA) has always been an important tool in studying the influences of stress and deformation due to various loads on implants to the surrounding jaws. This study assessed the influence of two different types of dental implant model on stress dissipation in adjoining jaws and on the implant itself by utilizing FEA. This analysis aimed to examine the effects of increasing the number of fences along the implant and to compare the resulting stress distribution and deformation with surrounding bones. When a vertical force of 100 N was applied, the largest displacements found in the three-fenced and single-fenced models were 1.7469 and 2.5267, respectively, showing a drop of 30.8623%. The maximum stress found in the three-fenced and one-fenced models was 13.518 and 22.365 MPa, respectively, showing a drop of 39.557%. Moreover, when an oblique force at 35° was applied, a significant increase in deformation and stress was observed. However, the three-fenced model still had less stress and deformation compared with the single-fenced model. The FEA results suggested that as the number of fences increases, the stress dissipation increases, whereas deformation decreases considerably.

Characterization of Corneal Biomechanical Properties and Determination of Natural Intraocular Pressure Using CID-GAT.

PURPOSE: The intraocular pressure (IOP) measured using Goldmann Applanation Tonometry (GAT) is confounded by individual corneal properties. We investigated a modified method that removes the confoundment by incorporating corneal properties into the Imbert-Fick's law is investigated. METHOD: Porcine eyes were pressurized between 10 and 40 mm Hg using a manometer. The eyes were indented using a flat cylindrical indenter. A modified corneal indentation device (CID) procedure was used to obtain the corneal moduli Eqs . The calculated IOPNC from the Imbert-Fick's Law using the corneal moduli Eqs was compared to the natural IOPN, measured using pressure sensor inserted into the eye. RESULTS: Test results showed that IOP-dependent corneal modulus Eqs is a primary confounding factor in IOP calculation. The average elastic modulus Eqs is 0.173 ± 0.018 MPa at 20 mm Hg, and increases with IOP at a linear rate of 0.0066 MPa per mm Hg (r = 0.997, P < 0.001). Incorporation of individual Eqs into IOPNC calculation showed that IOPNC are in good agreement with reference IOPN (slope = 0.999, r = 0.939, P < 0.001). CONCLUSIONS: The IOP-dependent corneal modulus Eqs is a primary confounding factor in IOP calculation. A modified CID-GAT procedure to obtain natural cornea-independent IOPNC is developed and verified in this study. The CID-GAT IOP modification may be used in place of conventional GAT when the confounding effects in eyes with atypical cornea (e.g., laser-assisted in situ keratomileusis [LASIK] thinned) are significant. TRANSLATIONAL RELEVANCE: Confoundment from corneal properties results in IOP measurement errors. The study showed that the CID-GAT method can significantly reduce the confounding corneal errors.

In vivo measurement of regional corneal tangent modulus.

Currently available clinical devices are unable to measure corneal biomechanics other than at the central region. Corneal stiffness (S), thickness, and radius of curvature was measured at the central cornea (primary fixation) and 3 mm from the temporal limbus (primary and nasal fixations). The corneal tangent modulus (E) of 25 healthy subjects was calculated from these data. After confirming normality, repeated measures analysis of variance (RMANOVA) revealed significant difference in S (F(2, 48) = 21.36, p < 0.001) at different corneal regions and direction of fixations. E also varied significantly at different corneal regions and direction of fixations (RMANOVA: F(2, 48) = 23.06, p < 0.001). A higher S and a lower E were observed at the temporal region compared with the corneal centre. Nasal fixation further increased S and E values compared with primary fixation. Due to the specific arrangement of corneal collagen fibrils, heterogeneity of corneal biomechanical properties is expected. In future clinical practice, localized corneal biomechanical alternation and measurement might assist corneal disease detection and post-surgery management. In addition, practitioners should be aware of the fixation effect on corneal biomechanical measurement.

High myopes have lower normalised corneal tangent moduli (less 'stiff' corneas) than low myopes.

PURPOSE: To compare corneal tangent moduli between low and high myopes. METHODS: Corneal hysteresis (CH) and corneal resistance factor (CRF) of 32 low and 32 high myopes were obtained using an Ocular Response Analyzer, followed by a corneal indentation device that measured corneal stiffness. Corneal topography, pachymetry, Goldmann applanation tonometry intraocular pressure (GAT-IOP), and corneal compensated intraocular pressure (IOPcc) were also obtained. Corneal tangent modulus was calculated on the basis of corneal stiffness, central corneal thickness and corneal radius. Comparisons between groups and associations between corneal biomechanical and ocular parameters were performed. RESULTS: Corneal tangent moduli were positively correlated with GAT-IOP (R2 = 0.078, p = 0.025), and IOPcc (R2 = 0.12, p = 0.006). Despite similarity in corneal thickness and radius, high myopes exhibited a significantly higher IOPcc (16.4 ± 2.51 mmHg) than low myopes (13.1 ± 1.96 mmHg; t(62) = -5.57, p < 0.0001). Both groups had similar corneal stiffness (0.063 ± 0.0085 and 0.063 ± 0.0079 N mm-1 for low and high myopes, respectively) and CRF (9.6 ± 1.58 and 9.5 ± 1.90 mmHg for low and high myopes, respectively). Moreover, high myopes exhibited a significantly lower CH (9.5 ± 1.51 mmHg) than low myopes (10.6 ± 1.38 mmHg; t(62) = 2.92, p = 0.005). After normalising corneal tangent moduli to the mean intraocular pressure in normal eyes (15.5 mmHg) using IOPcc, high myopes showed a significantly lower corneal tangent moduli (0.47 ± 0.087 MPa) than low myopes (0.57 ± 0.099 MPa; t(62) = 4.17, p < 0.0001). CONCLUSIONS: High myopes had lower normalised corneal tangent moduli than low myopes, which indicated that their corneas were less stiff. This is the first in vivo study comparing elastic moduli of the cornea in different refractive groups. Further studies are warranted to understand whether a less stiff cornea is a cause for or an outcome from myopia development.

Diurnal Variation of Corneal Tangent Modulus in Normal Chinese.

PURPOSE: The aim of this study was to investigate the diurnal variation of corneal tangent modulus, measured using a novel corneal indentation device, in healthy Chinese subjects. METHODS: The central corneal thickness (CCT), mean central corneal radius (meanK), intraocular pressure (IOP), and corneal stiffness of 25 young adults aged 21 to 25 years (23.0 ± 1.0 yrs) were measured at 3-hour intervals from 09:00 to 21:00 in the course of 1 day. Corneal tangent modulus was calculated on the basis of corneal stiffness, CCT, and meanK. Repeated-measures analyses of variance were performed to compare the diurnal changes in ocular parameters over time. RESULTS: Significant diurnal variations were observed in CCT and IOP (P < 0.001 and P = 0.025, respectively). Both parameters showed a decreasing trend throughout the day. MeanK and corneal stiffness did not show any significant diurnal changes (P = 0.251 and P = 0.516, respectively). Mean corneal tangent modulus across all measurements was 0.047 ± 0.085 MPa, and its diurnal rhythm ranged from 0.469 to 0.485 MPa. The variation was nonsignificant (P = 0.526). CONCLUSIONS: The elastic properties of the cornea in healthy Chinese subjects were stable during wake time. The present study shows that the corneal indentation device obtains stable corneal biomechanics similar to other clinical devices. Future studies investigating the differences in corneal biomechanics among patients with various ocular conditions are warranted.