Short and long term corneal biomechanical analysis after overnight orthokeratology.

This study aimed to investigate intraocular pressure (IOP) measurements taken using the dynamic contour tonometer (DCT) and the non-contact tonometer (NCT) in subjects with keratoconus. Intraocular pressure was measured in 20 keratoconus subjects and 20 age-matched control subjects using the DCT and NCT instruments. Central and off-centre measures were taken with the DCT in order to highlight any systematic errors associated with corneal biomechanical factors. Measures of anterior and posterior corneal topography and thickness were also taken in each subject. No significant difference was found between the central and off-centre DCT IOP readings for the keratoconus subjects and age-matched controls (p > 0.05). The average DCT IOP was 14.2 ± 1.4 mmHg in the keratoconus subjects and 14.2 ± 1.6 mmHg in the controls. However, the average NCT readings differed significantly (p < 0.001) between the keratoconus subjects (9.2 ± 1.5 mmHg) and the controls (12.9 ± 2.4 mmHg). Intraocular pressure measurements taken with the DCT showed no significant (p > 0.05) correlation with the severity of keratoconus as determined through measures of corneal topography and thickness. Equivalent IOP measurements taken with the NCT correlated significantly with certain measures of corneal curvature and thickness in the keratoconus population. The difference between DCT and NCT IOP was strongly correlated with measures of corneal topography and thickness, with differences increasing in more advanced keratoconus. The measurements obtained with the DCT do not appear to be dependent upon corneal factors, unlike those taken with the NCT. The presence or severity of keratoconus was not correlated with DCT IOP values.

To evaluate the impact of cap thickness for small incision lenticule extraction (SMILE) on changes in corneal curvature and biomechanics. One hundred eyes (50 patients) were enrolled in this prospective contralateral eye study. The difference in manifest refraction spherical equivalent (MRSE) in the same patient was less than 0.50 diopters. SMILE was performed with a randomized cap thickness of 110 µm in one eye and 140 µm in the other eye. MRSE, uncorrected distance visual acuity (UDVA), and corneal curvature and biomechanics were evaluated. The anterior and posterior surfaces of the corneal curvature (mean keratometry [Km] values 2, 4, and 6 mm from the pupil center) were measured by Pentacam HR (Oculus Optikgeräte, Wetzlar, Germany) and changes in corneal biomechanics at 6 months postoperatively by Scheimpflug technology. There was no significant between-group difference in UDVA or MRSE postoperatively. Postoperative changes in Km at the anterior surface (ΔKm-ant) in the 4-mm zone were significantly higher in the 110-µm group than in the 140-µm group at 1 day and 1, 3, and 6 months postoperatively (P = .043, .045, .003, and .049, respectively); at 3 months, the ΔKm-ant in the 6-mm zone was higher in the 110-µm group (P = .035). The changes in second appla-nation time, deformation amplitude, and integrated radius were significantly less in the 110-µm group (P = .031, .049, and < .001, respectively). A thicker corneal cap caused less change in anterior surface curvature after SMILE for moderate or low myopia, with no significant difference in UDVA and MRSE. [J Refract Surg. 2020;36(2):82-88.].

PurposeTo assess the relationship among corneal stiffness, thickness, and biomechanical parameters in keratoconus.SettingThe EENT Hospital of Fudan University, Shanghai, China.DesignComparative study.MethodsIn this cross-sectional prospective study, 75 keratoconic eyes of 44 patients were recruited. Eyes were divided three groups according to the steepest K-readings (Kmax): mild (31 eyes; 42.1–54.5D); moderate (27 eyes, 55.0–61.6D); and severe (17 eyes, 65.2–94.5D). Thirty-one healthy subjects were recruited as the control group. All patients underwent Corvis ST, Pentacam and ORA examinations at the same time. Stiffness parameter A1 (SP-A1) and other dynamic parameters were assessed using the Corvis ST. Kmax and thinnest corneal thickness (TCT) was obtained using the Pentacam. Corneal resistance factor (CRF) and corneal hysteresis (CH) were measured using the ORA. Analysis of correlation was applied to investigate the association between variables.ResultsThere was a decrease in SP-A1 in different stages of keratoconus compared with controls (P ≤ 0.001): with increasing severity, the value of SP-A1 became smaller (P < 0.05). A statistically significant linear relationship was noted between SP-A1 and TCT in each subgroup of keratoconus (P ≤ 0.001). In all three groups, SP-A1 was found to be positively correlated with first applanation time (P < 0.01), while negatively correlated with deformation amplitude (P < 0.05). Analysis of SP-A1 with regard to CRF and CH indicated statistically positive correlation in keratoconus (P < 0.05).ConclusionSignificant decreases in corneal stiffness were noted in kerotoconic eyes compared with normal eyes. The stiffness parameter could be a valuable clinical tool enables biomechanically track progression with keratoconus.SynopsisOur study found that corneal thinning and biomechanical decreasing synchronize with one another throughout the progression of the keratoconus, and SP-A1 could be a potential biomarker evaluating disease progression.

To evaluate the corneal biomechanical changes due to small incision lenticule extraction (SMILE) measured by Scheimpflug-based dynamic tonometry and to assess the impact of the corneal thickness. Sixty-eight patients measured with the Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) preoperatively and 1 month after SMILE were included in this retrospective observational study. Patients were divided into three groups depending on the preoperative spherical equivalent: low from -1.00 to -3.00 diopters (D), medium from -3.25 to -5.00 D, and high from -5.25 to -7.25 D. Changes in Corvis ST parameters due to the surgery were analyzed and new indexes for correcting the impact of corneal thickness were proposed. First and second applanation times changed after SMILE (P < .0001) but no differences were found in the comparison between these relative changes (P = .31). First applanation time was correlated with central corneal thickness (r = 0.368, P = .002) but not second applanation time (r = -0.149, P = .23). The change in first applanation time due to SMILE was different among myopic groups (P = .007) but equal when a new index that considers the removed central corneal thickness was used for comparison (P = .31). Deformation amplitude was also increased after SMILE (P < .0001), but after subtracting the removed corneal thickness from the postoperative deformation amplitude the result was equal to the preoperative deformation amplitude (P = .26). SMILE produces significant changes in the Corvis ST parameters of time and deformation amplitude, but these changes are mainly explained by the confounding variable of corneal thickness. [J Refract Surg. 2016;32(12):821-828.].

To compare the long-term changes in corneal biomechanics, topography, and tomography before and 4 years after corneal cross-linking (CXL) with the Dresden protocol and correlate these changes with visual acuity. In this longitudinal study, 18 eyes of 18 patients with progressive keratoconus who were treated with CXL were included. All patients received a standard ophthalmological examination and were examined by Placido disc-based topography, Scheimpflug tomography, and biomechanical assessments with the Corvis ST (OCULUS Optikgeräte GmbH, Wetzlar, Germany) and Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments, Buffalo, NY) before and 4 years after CXL. The main outcome measures were dynamic corneal response (DCR) parameters obtained from the Corvis ST, corneal hysteresis (CH), corneal resistance factor (CRF), visual acuity, refraction, corneal curvature, and corneal thickness. There were no significant differences in mean visual acuity, refraction, intraocular pressure, corneal topography, corneal astigmatism in both corneal surfaces, maximum keratometry, corneal thickness at apical and thinnest points, thickness profile indices, corneal volume, and specular microscopy before and 4 years after CXL (P > .05). Significant changes were observed in many DCR parameters, including radius at highest concavity and integrated inverse radius, both of which were consistent with stiffening. The CH and CRF values after CXL were not statistically significant. The new parameters using the Corvis ST include integrated inverse concave radius, which showed a significant decrease 1.07 ± 0.93 mm-1, consistent with stiffening. The corneal stiffness parameter at the first applanation, Ambrósio's Relational Thickness to the horizontal profile, deformation amplitude ratio, and Corvis Biomechanical Index as a combined biomechanical screening parameter did not show significant changes. CXL is a minimally invasive treatment option to prevent keratoconus progression over 4 years. Pressure-derived biomechanical parameters obtained from the ORA did not show any change following CXL at 4 years of follow-up, whereas the Corvis ST DCR parameters detected changes in corneal biomechanical properties. [J Refract Surg. 2018;34(12):849-856.].

To evaluate the effects of 0.5% tropicamide on corneal topography and biomechanics in participants with keratoconus (KC), using the Pentacam static Scheimpflug imaging system and Corvis ST dynamic Scheimpflug imaging technology. This study included 83 right eyes from 83 participants with KC. Mydriasis was induced by instilling 0.5% tropicamide eye drops three times at 10-min intervals. Thirty minutes after the final instillation, examinations were repeated using the Pentacam and Corvis ST. Measurement parameters included corneal topographic data (curvature-based parameters, elevation-based data, pachymetric values, integrated keratoconus indices and the Belin/Ambrósio total deviation value [BAD-D]) and biomechanical data (dynamic corneal response parameters, Vinciguerra screening parameters, Corvis biomechanical index and tomographic-biomechanical index). Violin plots were used to visualise the distribution patterns of the measured parameters. Differences between pre- and post-mydriatic measurements were analysed using paired t-tests. Following mydriasis, the Pentacam analysis revealed statistically significant decreases in front mean keratometry, front maximum keratometry, back mean keratometry, index of surface variance, central keratoconus index and BAD-D (p < 0.05). In contrast, central corneal thickness, apex corneal thickness, thinnest corneal thickness and minimum sagittal curvature showed significant increases. Corvis ST biomechanical analysis demonstrated significant reductions in the time from the start to the second applanation, deformation amplitude of the corneal apex at the highest concavity and Corvis biomechanical index (p < 0.05). Mydriasis produced significant alterations in corneal topographic and biomechanical parameters in participants with KC. Although these changes do not affect the diagnosis of the condition, clinicians should be aware of them and adjust their treatment planning accordingly.

To compare early changes in the corneal biomechanical parameters after photorefractive keratectomy (PRK) and small incision lenticule extraction (SMILE) and their correlations with corneal shape parameters. One hundred twenty four eyes received myopic PRK and SMILE for similar amounts of myopia. Corneal tomography with Pentacam HR, biomechanical parameters using Corvis ST, and Ocular Response Analyzer (ORA) were evaluated before and 2weeks after surgery. The change in each parameter was compared between groups, while the difference in central corneal thickness and cornea-compensated intraocular pressure measured before and after surgery were considered as covariates. A significant reduction was seen in the corneal stiffness parameter at first applanation, and an increase in deformation amplitude ratio (DAR), and integrated inverse radius (IIR) in both groups after surgery (p < 0.001) Changes in DAR, and IIR were significantly greater in the SMILE than in the PRK group (p < 0.001) Corneal hysteresis (CH) and corneal resistance factor(CRF) decreased in both SMILE and PRK groups after surgery, (p < 0.001) with no statistically significant difference between groups (p > 0.05) Among new Corvis ST parameters, DAR showed a significant correlation with changes in Ambrosio relational thickness in both groups (p < 0.05). Both techniques caused significant changes in corneal biomechanics in the early postoperative period, with greater elastic changes in the SMILE group compared to the PRK group, likely due to lower tension in the SMILE cap and thinner residual stromal bed in SMILE. There were no differences in viscoelastic changes between them, so the lower CH may reflect the volume of tissue removed.

To evaluate endothelial cell density (ECD) and corneal biomechanical changes following implantation of Veriflex and Implantable Collamer Lens (ICL) phakic intraocular lenses (pIOLs) over a 12-month period. Sixty patients (117 eyes) were included in this prospective comparative study. 58 eyes underwent implantation with the Veriflex lens (Group I), while 59 eyes received the ICL (Group II). Preoperative and postoperative ECD and corneal biomechanical values were assessed using specular microscopy and Corvis St at baseline, 6 months, and 1 year. Endothelial cell loss percentages and corneal biomechanical changes were calculated and compared between the 2 groups. Preoperatively, the mean ECD was 2683.4 ± 340.6 cells/mm2 in Group I and 2732 ± 322.8 cells/mm2 in Group II (P = 0.718). At 1 year, Group I exhibited a significantly greater reduction in ECD compared to Group II (P = 0.021), indicating higher endothelial cell loss with Veriflex. The mean endothelial cell loss at 12 months was 5.4% (145.7 ± 62.8 cells/mm2) in Group I compared to 1.53% (42 ± 17.3 cells/mm2) in Group II (P < 0.001). The Veriflex group showed greater corneal biomechanical changes, with a deformation amplitude (DA) increase to 1.15 ± 0.11 mm and a highest concavity time (HCT) of 17.2 ± 1.1 ms, while the ICL group demonstrated more stable biomechanics, with a DA of 1.09 ± 0.09 mm and an HCT of 16.6 ± 1.0 ms at one year postoperatively. Both Veriflex and ICL pIOLs resulted in a decrease in ECD over one year, with Veriflex showing a significantly higher rate of endothelial cell loss. These findings suggest that ICL may be a safer option for long-term endothelial preservation. Long-term studies are required to assess the continued impact of both lenses on corneal endothelial health.

To determine 1-year corneal biomechanical changes after accelerated corneal cross-linking in patients with progressive keratoconus and compare them between 5-minute (18 mW/cm2) and 10-minute (9 mW/cm2) protocols. In this non-randomized clinical trial, cases in both groups were examined with the Corneal Visualization Scheimpflug Technology (Corvis ST; Oculus Optikgeräte GmbH, Wetzlar, Germany) at baseline and at 6 and 12 months after treatment. Extracted indices included intraocular pressure (IOP), central corneal thickness (CCT), first and second applanation times, lengths, and velocities (T1, T2, L1, L2, V1, and V2), highest concavity time (HCT), deformation amplitude (DA), peak distance between bending points, and radius of curvature. Mean patient age, baseline maximum keratometry, CCT, and IOP were similar between groups. After adjusting for CCT and baseline values with repeated measures analysis of covariance, at 1 year after the procedure, IOP (13.14 ± 1.41 vs 12.12 ± 1.49 mm Hg, P = .034) and T1 (6.84 ± 0.20 vs 6.67 ± 0.23 ms, P = .036) were higher in the 5-minute group, but T2 (21.31 ± 0.27 vs 21.58 ± 0.28 ms, P = .007), HCT (16.06 ± 0.51 vs 16.31 ± 0.48 ms, P = .017), and DA (1.03 ± 0.09 vs 1.10 ± 0.08 mm, P = .028) were lower. Other inter-group differences were not statistically significant (all P > .050). All 1-year changes were independent of cone position (all P > .050). At 1 year after cross-linking in cases of mild and moderate keratoconus, corneal biomechanics appeared stable or stronger than baseline with both 5- and 10-minute protocols. However, mild cases who had the 5-minute protocol showed better improvement based on Corvis ST indices. [J Refract Surg. 2017;33(8):558-562.].

To compare the changes encountered in corneal biomechanics and aberration profile following accelerated corneal collagen cross-linking (CXL) using hypo-osmolar and iso-osmolar riboflavin in corneal thicknesses of <400 and >400 microns, respectively. This is a prospective, interventional, comparative study involving 100 eyes of 75 patients with progressive keratoconus. Eyes were divided into two groups based on corneal thickness: group 1 included eyes with a corneal thickness of <400 microns who underwent hypo-osmolar CXL, and group 2 included eyes with a corneal thickness of >400 microns who underwent iso-osmolar CXL. Corneal biomechanical and aberration profiles were evaluated and compared between groups. In group 1, all higher-order aberrations (HOA) except secondary astigmatism significantly decreased from baseline; however, in group 2, only coma and trefoil decreased. The corneal resistance factor and corneal hysteresis significantly improved in both groups, which was significantly greater in group 2 than in group 1. The change in inverse radius, deformation amplitude, and tomographic biomechanical index was significantly improved in group 2 as compared to group 1. Improvement in corrected distance visual acuity and decrease in HOA were significantly better in the hypo-osmolar CXL group; however, the improvement in biomechanical strength of the cornea was significantly better in the iso-osmolar group.

The effect of lateral tarsal strip surgery on corneal biomechanical parameters measured by Corvis ST

To verify corneal biomechanical changes, poststromal augmentation using myopic small-incision lenticule extraction's (SMILEs) lenticules in advanced keratoconus (KCN) through Corvis ST (Oculus, Wetzlar, Germany). A clinical trial enrolled 22 advanced KCN patients. We implanted lenticules exceeding 100 μ according to a nomogram and evaluated biomechanical factors through Corvis ST at 3-, 6-, and 24-month postimplantation. We examined parameters during the first applanation (A1), second applanation (A2), highest concavity (HC)/max concavity events, and Vinciguerra screening parameters, as recently established criteria derived from the ideal blend of diverse biomechanical and ocular factors and formulated through the application of logistic regression. Regression analyses explored relationships with age, mean keratometry value, thickness, sphere, cylinder, and best-corrected visual acuity. Patients were well matched for age, intraocular pressure, and central corneal thickness (CCT). The mean spherical equivalent decreased from -13.48 ± 2.86 Diopters (D) to -8.59 ± 2.17 D (P < 0.007), and mean keratometry decreased from 54.68 ± 2.77 D to 51.95 ± 2.21 D (P < 0.006). Significant increases were observed in HC time (HCT), Radius-central curvature radius at the HC state-, peak distance (PD) during HC state, CCT, first applanation time, and stiffness parameter (A1T and SP-A1), whereas HC deformation amplitude, maximum deformation amplitude ratio at 2 mm, Corvis Biomechanical Index (CBI), integrated radius (IR), second applanation deformation amplitude (A2DA), first applanation velocity and deflection amplitude (A1V and A1DeflA) significantly decreased postlenticule implantation. Multivariable regression revealed age positively correlated with SP-A1 (P = 0.003) and negatively with HC delta Arc length (P = 0.007). Mean K positively correlated with CCT (P = 0.05) and negatively with CBI (P = 0.032). Best-corrected visual acuity positively correlated with HCT (P = 0.044), and the cylinder positively correlated with PD (P = 0.05) and CCT (P = 0.05) whereas negatively with IR (P = 0.025). Stromal augmentation using myopic SMILE lenticules induces significant corneal biomechanical changes in KCN.

Objective: To explore the characteristics and significance of corneal biomechanics in patients with suspicious total deviation value in Belin/Ambrosio Enhanced Ectasia Display of Pentacam corneal topography. Methods: In this cross-sectional study, 206 patients (325 eyes) with myopia or myopic astigmatism were collected from refractive surgery center of Tianjin Eye Hospital. Among them, 110 males(213 eyes) and 96 females (153 eyes) were aged (22.68±5.20) years. 57 patients (71 eyes) with a total deviation value of 1.6-3.0 in the Pentacam corneal topography were included in the suspicious group, and 149 patients (254 eyes) of<1.6 were included in the control group. The values of the first applanation time (A1-Time), first applanation length (A1-Length), first applanation velocity (A1-Velocity), second applanation time (A2-Time), second applanation length (A2-Length), second applanation velocity(A2-Velocity), highest concavity time (HC-time), highest concavity deformation amplitude (DA), highest concavity peak distance (PD), highest concavity radius (HC-Radius, RoC), intraocular pressure were measured using the corneal visualization Scheimpflug technology. The total deviation value in Belin/Ambrosio Enhanced Ectasia Display were measured with a Pentacam. The biomechanical parameters of the suspicious group were compared with the normal group by the two independent samples t test and the identity regression analyses. The data correlation was performed using Pearson linear correlation analysis. Results: Compared with the D normal group, the K1, K2 and Km of D suspicious group were significantly increased (P<0.001). A1-Velocity [(0.152±0.015) m/s] and DA[(1.050±0.090) mm] were significantly increased (t=4.348, 2.708; P<0.001, 0.007), while the CCT, A1-Length, A2-Length and RoC were significantly decreased (P<0.01) in suspicious group. In suspicious group, there was a significant correlation between A1-Time, A1-Length, A2-Length, A2-Velocity, IOP and CCT(r=0.305, 0.324, 0.238, 0.346, 0.316; P<0.05). There was a significant correlation between the anterior surface refractive power K1 and the A1-Length (r=-0.361, P=0.002), and there was a significant correlation between the A1-Length, RoC and anterior surface refractive power K2 (r=-0.369, -0.242; P=0.002, 0.043). There was a significant correlation between anterior surface refractive power Km and A1-Length (r=-0.373, P=0.001). After adjusting the confounding factors between two groups by the identity regression analyses, the A1-Length (P=0.003), A1-Velocity (P<0.001) and DA (P=0.002) were still significant differences. Conclusions: In total deviation value suspicious group, the corneal thickness was thinner, corneal curvature became steeper and the corneal biomechanical properties were weaker. It is important to screen keratoconus before corneal refractive surgery and choose a reasonable surgical procedure to reduce corneal dilatation after refractive surgery. (Chin J Ophthalmol,2019, 55: 442-447).

PurposeTo assess the diurnal changes in corneal biomechanics in healthy young adults.MethodsAt Wenzhou Medical University Eye Hospital in China, a prospective case series. Each healthy subject had six Corvis ST examinations, from 10:00 on a first day to 6:00 on the following day, at 4-hour intervals. The data collected included the biomechanically corrected IOP (bIOP), the central corneal thickness (CCT) and six dynamic corneal response (DCR) parameters, namely deformation amplitude (DA), the ratio between DA values at the apex and 2 mm from the apex (DAR2), integrated inverse radius (IIR), the stiffness parameter at first applanation (SP-A1), Corvis biomechanical index (CBI) and the updated stress-strain index (SSIv2). A multilevel model was used to assess the changes in DCR parameters of patients over time during the examination period.ResultsSixty-one subjects were included and intra-operator reproducibility was good for most DCR parameters at the 6 time points. DA, DAR2, IIR and CBI showed small fluctuations during the 24-hour cycle with mean values of 0.1, 0.16, 0.34, and 0.04 units, respectively, while SP-A1 showed higher variations averaging 20.57 units and SSIv2 had the least fluctuation within 0.10 units during 24-hour cycle. After correcting for the effects of CCT and bIOP, the diurnal fluctuations in IIR, CBI and SSIv2 were not statistically significant, unlike the fluctuations in DA, DAR2 and SP-A1, which remained significant.ConclusionsCompared with the DA, DAR2, IIR and CBI, SP-A1 was more affected while SSIv2 was the less affected by the diurnal variations and associated corneal edema.

BackgroundCorneal refractive laser surgery is widely used to correct myopia and astigmatism due to its safety and effectiveness. However, postoperative changes in corneal biomechanics, such as corneal ectasia, can occur, necessitating a deeper understanding of these changes. Finite Element Analysis has shown promise in predicting surgical outcomes based on corneal biomechanics. Devices like the Ocular Response Analyser (ORA) and Corvis ST provide noninvasive ways to measure corneal biomechanics, aiding in the assessment of corneal behavior post-surgery. Young's modulus and tangent modulus are crucial parameters for describing corneal elasticity, but there is limited data on the changes in tangent modulus following Femtosecond Laser-Assisted LASIK (FS-LASIK) in humans. This study aimed to investigate the effect of FS-LASIK on the corneal tangent modulus using a novel corneal indentation device (CID). The study sought to explore changes in corneal tangent modulus after FS-LASIK, taking into account central corneal thickness (CCT) and corneal radius, to enhance our understanding of the biomechanical changes induced by this surgical procedure.ResultsSixty-six patients (66 eyes) underwent FS-LASIK, resulting in significant changes in CCT, corneal radius, and Goldmann intraocular pressure (GAT IOP) 6 months post-surgery (△CCT = − 88 ± 31 µm, △corneal radius = 0.81 ± 0.30 mm, △GAT IOP = − 3.2 ± 2.4 mmHg, p < 0.001) 6 months after surgery. However, corneal stiffness did not significantly change (△ = − 0.002 ± 0.011, p < 0.2). The corneal tangent modulus showed a significant increase post-surgery (△ = 0.263 ± 0.146, p < 0.001), exhibiting a negative correlation with CCT (r = − 0.68, P < 0.001) and a positive correlation with corneal radius (r = 0.71, P < 0.001). For each 1 mm increase in corneal radius, there was a 0.23 MPa increase in corneal modulus, and for every 100 µm reduction in corneal thickness, there was a 0.14 MPa increase in corneal modulus.ConclusionsThe corneal tangent modulus, influenced by corneal radius and CCT, increased significantly following FS-LASIK. This study highlights the biomechanical changes induced by FS-LASIK, with implications for understanding corneal behavior post-surgery and its potential impact on patient outcomes.