Corneal stress-strain index in myopic Indian population.

AIM: The purpose is to study the corneal stress-strain index (SSI) in myopic refractive error among Indian subjects. METHODS: A retrospective study where young myopic subjects aged between 11 and 35 years who had undergone corneal biomechanics assessment using Corvis ST between January 2017 and December 2021 were enrolled. Subjects with central corneal thickness (CCT) <500 µ, intraocular pressure (IOP) >21 mmHg, history of any systemic and ocular disease or any previous ocular surgery, high astigmatism, corneal disease such as keratoconus were excluded. Subjects with missing data or having poor quality scan were excluded. Corneal biomechanical properties and corneal SSI were assessed using Corvis ST. For statistical purposes, eyes were divided into four different groups and were analyzed using one-way ANOVA. RESULTS: Nine hundred and sixty-six myopic eyes with mean ± standard deviation age, IOP, and CCT of 26.89 ± 4.92 years, 16.94 ± 2.00 mmHg, and 540.18 ± 25.23 microns, respectively, were included. There were 311, 388, 172, and 95 eyes that were low, moderate, severe, and extreme myopic. Deformation amplitude ratio at 1 mm and 2 mm were similar across different myopic groups. A significant increase in max inverse radius, ambrosia relational thickness, biomechanically corrected IOP, integrated radius was noted with an increase in myopic refractive error. Corvis biomechanical index, corneal SSI was found to be decreased significantly with an increase in myopic refractive error. We noted a significant positive association between myopic refractive error and SSI (P < 0.001). CONCLUSION: Corneal SSI was found to be reduced in extreme myopic eyes.

Visual outcomes and higher-order aberrations in eyes implanted with hybrid extended depth of focus intraocular lens.

PURPOSE: To study the visual outcomes and higher order aberrations in eyes implanted with Hybrid EDOF IOL, particularly in Indian eyes. MATERIALS AND METHODS: This is retrospective case series where subjects aged between 40-65 years and were implanted with LUCIDIS IOL by single surgeon were included. Subjects with Pre surgery corneal astigmatism > 1.50 D, corneal guttae, IOP >22mmHg, any ocular Co morbidities, Intra or post operative complication were excluded. At 1 month follow up, uncorrected visual acuities at distance (UCDVA), intermediate (UCIVA) and near (UCNVA), refraction were recorded. Internal Higher order aberrations and strehl ratio for a fixed pupil of 4mm and 6mm were calculated using NIDEK OPD Scan. Monocular defocus curve was obtained at 4 meter logMAR chart. RESULTS: Total of 55 eyes of 35 patients with mean±SD age of 58.50±7.49 years were evaluated. 89.09% of the eyes achieved visual acuity of 6/6. 49.09% could read N10 font at intermediate distance without any correction and 85.45% of the eyes had uncorrected near visual acuity of N6. Around 67% of the eyes did not require any refractive correction whereas around 26% of the eyes required correction upto ±0.25D. The defocus curve showed that visual acuity also ranges from 0.05 logMAR to 0.2 logMAR for Plano to -3.00D respectively. Mean±SD Strehl ratio at 4 and 6 mm pupil size was 0.06±0.04 and 0.02±0.02 respectively. CONCLUSION: Hybrid EDOF IOL such as LUCIDIS provides excellent vision at all distances. This could be attributed to IOL design which is spherical aberration neutral lens.

Role of corneal epithelial mapping, Corvis biomechanical index, and artificial intelligence-based tomographic biomechanical index in diagnosing spectrum of keratoconus.

AIM: The aim of the study was to evaluate the utility of epithelial mapping, Corvis biomechanical index (CBI), and tomographic biomechanical index (TBI) in diagnosing the spectrum of keratoconus (KC). METHODS: This was a retrospective study where KC subjects with an age between 11 and 50 years were enrolled. Subjects with ocular diseases, history of previous corneal surgery, corneal scars or hydrops, ocular trauma, ocular surface disorder, systemic disease, and poor-quality scans were excluded. KC was classified using Belin ABCD classification system. Epithelial thickness, corneal tomography, and corneal biomechanical measurements were recorded using Fourier-domain optical coherence tomography Avanti with corneal adaptor module, Pentacam HR, and Corvis® ST, respectively. To understand the utility of various corneal parameters in diagnosing spectrum of keratoconus, cutoff values for epithelial thickness at the thinnest location and its standard deviation (SD) were considered 45 and 3 microns, respectively, CBI of 0.5 and TBI of 0.29 was considered. RESULTS: Sixty-five eyes (45 - KC, 10 - subclinical KC (SBKC), and 10 - forme fruste [FF]) of 34 patients with a mean ± SD age of 30.73 ± 5.71 were included. In our keratoconic sample, epithelial mapping alone helped diagnose the 77.77% of cases, however, when combined with CBI, it helped diagnose 95.5% cases and when combined with TBI, it was useful in diagnosing 100% of cases. In SBKC group, 40% of cases were detected by epithelial mapping alone, and when combined with CBI, it helped diagnose 70% of cases and TBI further helped diagnose 90% of cases. We did not see any role of epithelial mapping in detecting FFKC cases whereas CBI and TBI helped diagnose 60% and 90% of cases, respectively. CONCLUSION: The utility of epithelial mapping as a solitary tool is limited in detecting the spectrum of KC, especially SB and FFKC. However, combining it with corneal biomechanical parameters could help improve the efficacy of diagnosis of KC.

Age-related variations in corneal stress-strain index in the Indian population.

Purpose: To report age-related variations in corneal stress-strain index (SSI) in healthy Indians. Methods: It was a retrospective study where healthy Indian individuals aged between 11 and 70 years who had undergone corneal biomechanics assessment using Corvis ST between January 2017 and December 2021 were enrolled. Composite corneal biomechanical parameters and corneal SSI were abstracted from Corvis ST and compared across different age groups using one-way analysis of variance (ANOVA). Also, Pearson's correlation was used to evaluate the association between age and SSI. Results: Nine hundred and thirty-six eyes of 936 patients with ages between 11 and 77 years with mean ± SD intraocular pressure (IOP) and pachymetry of 16.52 ± 2.10 mmHg and 541.13 ± 26.39 µs, respectively. Composite corneal biomechanical parameters such as deformation amplitude ratio max at 1 mm (P < 0.001) and 2 mm (P < 0.001), biomechanically corrected IOP (P = 0.004), stiffness parameter at A1 (P < 0.001, Corvis biomechanical index (P < 0.018), and SSI (P < 0.001) were found to be significantly different as a function of age group. We noted a statistically significant positive association of SSI with age (P < 0.001), spherical equivalent refractive error (P < 0.001), and IOP (P < 0.001) and a significant negative association with anterior corneal astigmatism (P < 0.001) and Anterior chamber depth (ACD) (P < 0.001). Also, SSI was positively associated with SPA1 and bIOP, whereas negatively associated with integrated radius, max inverse radius, and Max Deformation amplitude (DA) ratio at 1 mm and 2 mm. Conclusion: We noted a positive association of corneal SSI with age in normal healthy Indian eyes. This information could be helpful for future corneal biomechanical research.

Short term changes in corneal stress-strain index and other corneal biomechanical parameters post-laser in situ keratomileusis.

PURPOSE: To report the short-term changes in a corneal stress-strain index (SSI) and other corneal biomechanical parameters post-laser in situ keratomileusis (LASIK) surgery. METHODS: A retrospective study was conducted at a tertiary eye care center wherein patients who had undergone LASIK (microkeratome blade and femtosecond bladeless LASIK) between July and December 2019 were enrolled. Patients of age group 20-40 years, best-corrected visual acuity of 20/20, intraocular pressure (IOP) <22 mmHg, pre-LASIK pachymetry >500 microns, and corneal astigmatism ≤3.00 D were included. Subjects with a prior history of refractive surgery, any other ocular or systemic disease, poor-quality scans, intraoperative complications, and missing data were excluded. Corneal biomechanical properties including SSI were analyzed using Corvis ST and compared using the Paired T-test for each group separately at pre-LASIK, and 1-month post-operatively. RESULTS: Overall, 202 eyes were reviewed, and 79 eyes fulfilled the inclusion criteria. Forty-three and 36 eyes had undergone Microkeratome Blade LASIK (Group I) and Femto LASIK (Group II), respectively. Overall, 29 and 26 corneal biomechanical parameters out of 33 changed significantly post-Microkeratome Blade LASIK and Femto LASIK, respectively. Statistically significant changes were noted in all the parameters at A1, maximum and Vinciguerra screening parameters (P < 0.001), however, no changes were noted in SSI in both the groups when compared with the pre-surgery data. CONCLUSION: Though the reduction in SSI was not statistically significant, other biomechanical parameters showed significant biomechanical changes pre- and post-LASIK surgeries in both the groups. However, a long-term study with a larger sample size would be required to understand the changes and stability in SSI post-refractive surgery.

Age-related variation in corneal biomechanical parameters in healthy Indians.

PURPOSE: To report age-related variations in corneal biomechanical parameters in healthy Indians. METHODS: A retrospective study where healthy Indian individuals aged between 5 and 70 years having undergone corneal biomechanics assessment using Corvis ST between January 2017 and December 2018 and having best corrected visual acuity of 20/20 were enrolled. Subjects with central corneal thickness <500 microns, intra-ocular pressure (IOP) ≥ 22 mmHg, refractive error ≥ 6.00D, history of any systemic and ocular disease, previous ocular surgery, poor scans quality, and subjects with any missing data were also excluded. Corneal biomechanical parameters were noted and compared across different age groups. RESULTS: Total of 3125 eyes had undergone the Corvis ST analysis. After applying exclusion criteria, 718 right eyes of 718 patients were included for the analysis and were further divided into different age groups as per each decade (sample size), such as 5-10 (37), 11-20 (113), 21-30 (396), 31-40 (116), 41-50 (39), 50 and above (17). All the subjects were matched for IOP and central corneal thickness (p > 0.05). A total of 19 out of 26 corneal biomechanical parameters were significantly different across age groups (p < 0.05). Vinciguerra screening parameters, such as deformation amplitude ratio max, biomechanically corrected IOP, and stiffness parameter A1 were significantly different across different age groups (p < 0.05). CONCLUSION: Corneal biomechanical parameters are affected by age as cornea becomes progressively stiffer. The information reported here would serve as a reference for future corneal biomechanical researches and would help in differentiating the abnormal eyes from normal healthy eyes.

Association between corneal biomechanical parameters and myopic refractive errors in young Indian individuals.

PURPOSE: To report corneal biomechanical parameters in young myopic Indian individuals. METHODS: It is a retrospective study where young myopic individuals aged between 19 and 36 years who have undergone corneal biomechanics assessment using Corvis ST between January 2017 and December 2017 were enrolled. Individuals with central corneal thickness (CCT) <500 microns, intraocular pressure (IOP) >21 mmHg, history of any systemic and ocular disease, any previous ocular surgery, high astigmatism, corneal disease such as keratoconus, poor scans quality, and individuals with any missing data were also excluded. Corneal biomechanical parameters were noted in mild to moderate and high myopia. RESULTS: We analyzed the 266 eyes of 266 myopic individuals, of which 167 and 99 eyes had mild to moderate and high myopia, respectively. All the individuals were matched for age, IOP, and CCT (P > 0.05). Twenty-three of 32 parameters were similar in different degrees of myopia whereas 9 parameters were significantly different in high myopes as compared to low to moderate myopes. First applanation (A1) parameters and Vinciguerra screening parameters were similar in both the groups (P > 0.05). Second applanation (A2) parameters were similar in both the groups (P > 0.05) except A2 time, A2 deformation, amplitude (DA) (P < 0.05). Highest concavity (HC) parameters were significantly different in both the groups (P < 0.05) except HCDA, HC deflection length, and HC delta arc length (P > 0.05). CONCLUSIONS: High myopic eyes showed a significantly higher maximum deflection amplitude, lesser A2 time and HC time, less A2DA, smaller HC radius than mild to moderate myopia indicating softer, more deformable corneas. However, better predictor of corneal biomechanics such as Stiffness parameters at A1 (SPA1), DA ratio max, integrated radius, and Corvis Biomechanical Index were similar among both the groups of myopia.

Influence of Target Parameters on Fixation Stability in Normal and Strabismic Monkeys.

PURPOSE: The purpose of this study was to assess the effect of fixation target parameters on fixation instability in strabismic monkeys. METHODS: One normal and three exotropic monkeys were presented with four differently shaped fixation targets, with three diameters, during monocular or binocular viewing. Fixation targets were white on a black background or vice versa. Binocular eye movements were recorded using the magnetic search coil technique and fixation stability quantified by calculating the bivariate contour ellipse area (BCEA). RESULTS: Fixation instability was greater in all the strabismic monkeys compared with the normal monkey. During monocular viewing, strabismic monkeys showed significantly greater instability in the covered eye compared to the fixating eye. Multifactorial ANOVA suggested statistically significant target parameter influences, although effect sizes were small. Thus, a disk-shaped target resulted in greater instability than other target shapes in the viewing eyes of the normal monkey and two of three strabismic monkeys. A similar target-shape effect was also observed in the covered eye. Least instability was elicited with a 0.5° target in the normal monkey and a 1.0° target in the strabismic monkeys, both in the viewing and the covered eye. Target/background polarity effects were idiosyncratic. In strabismic monkeys, stability of the fixating eye during binocular viewing was not different from the stability of the same eye during monocular viewing. CONCLUSIONS: Abnormal drifts and nystagmus contribute to increased fixation instability in strabismic monkeys. Target parameters (shape and size) that influence fixation stability in a normal animal also affected fixation stability in our sample of strabismic monkeys.