Corneal Biomechanical Changes Research Articles

Changes in corneal biomechanics in patients with glaucoma: a systematic review and meta-analysis.

Corneal biomechanics has been implicated in a variety of ocular diseases. The purpose of this study was to evaluate the relationship between the glaucoma and corneal biomechanical properties, and exploring the value of corneal biomechanics in the diagnosis and follow-up of glaucoma diseases. We searched studies in PubMed, EMBASE, Web of Science and clinicaltrials.gov., as of October 8, 2022. Only English studies were included, without publication time limit. We also searched the reference lists of published reviews. This meta-analysis was conducted with random-effects models, we used mean difference(MD) to evaluate the outcome, and the heterogeneity was assessed with the I2 statistic. Subgroup analyses were performed under the appearance of high heterogeneity. We used 11 items to describe the characteristics of included studies, publication bias was performed through the Egger's test. The quality assessment were evaluated by Newcastle-Ottawa Scale(NOS) items. A total of 27 eligible studies were identified for data synthesis and assessment. The result of meta-analysis showed that in the comparison of included indicators, the corneal biomechanics values of glaucoma patients were statistically lower than those of normal subjects in a similar age range. The covered indicators included central corneal thickness(CCT) (MD = -8.34, 95% CI: [-11.74, -4.94]; P < 0.001), corneal hysteresis(CH)(MD = -1.54, 95% CI: [-1.88, -1.20]; P < 0.001), corneal resistance factor(CRF)( MD = -0.82, 95% CI: [-1.21, -0.44]; P < 0.001), and intraocular pressure(IOP)( corneal-compensated intraocular pressure (IOPcc): MD = 2.45, 95% CI: [1.51, 3.38]; P < 0.001); Goldmann-correlated intraocular pressure (IOPg): MD = 1.30, 95% CI: [0.41, 2.20]; P = 0.004), they all showed statistical difference. While the value of axial length(AL) did not show statistically different(MD = 0.13, 95% CI: [-0.24, 0.50]; P = 0.48). Corneal biomechanics are associated with glaucoma. The findings can be useful for the design of glaucoma screening, treatment and prognosis.

Quantitative assessment of corneal elasticity distribution after FS-LASIK using optical coherence elastography.

Quantifying corneal elasticity after femtosecond laser-assisted in situ keratomileusis (FS-LASIK) procedure plays an important role in improving surgical safety and quality, since some latent complications may occur ascribing to changes in postoperative corneal biomechanics. Nevertheless, it is suggested that current research has been severely constrained due to the lack of an accurate quantification method to obtain postoperative corneal elasticity distribution. In this paper, an acoustic radiation force optical coherence elastography system combined with the improved phase velocity algorithm was utilized to realize elasticity distribution images of the in vivo rabbit cornea after FS-LASIK under various intraocular pressure levels. As a result, elasticity variations within and between the regions of interest could be identified precisely. This is the first time that elasticity imaging of in vivo cornea after FS-LASIK surgery was demonstrated, and the results suggested that this technology may hold promise in further exploring corneal biomechanical properties after refractive surgery.

Corneal biomechanical changes following small incision lenticule extraction (SMILE) with variable cap thickness in management of mild to moderate myopia

Corneal biomechanical changes following small incision lenticule extraction (SMILE) with variable cap thickness in management of mild to moderate myopia

Changes in corneal biomechanics and posterior corneal surface elevation after FS-LASIK.

To investigate the changes in corneal biomechanics and posterior corneal surface elevation after femtosecond laser-assisted in situ keratomileusis (FS-LASIK). Totally 197 eyes of 100 patients who underwent the FS-LASIK from April 2022 to November 2022 were included. They were divided into three groups according to the ratio of residual corneal stroma thickness/corneal thickness (RCST/CT): Group I (50%≤RCST/CT<55%, 63 eyes of 32 patients), Group II (55%≤RCST/CT<60%, 67 eyes of 34 patients), and Group III (RCST/CT≥60%, 67 eyes of 34 patients). The intraocular pressure (IOP), corneal compensated IOP (IOPcc), corneal hysteresis (CH) and corneal resistance factor (CRF) were measured immediately, 1, and 3mo postoperatively by ocular response analyzer (ORA) and the posterior elevation difference (PED) was measured by Pentacam. After operation, IOP, CH, CRF, and PED were statistically different among the three groups (F=12.99, 31.148, 23.998, all P<0.0001). There was no statistically significant difference in IOPcc among the three groups (F=0.603, P>0.05). The IOP, IOPcc, CH, and CRF were statistical changed after surgery (F=699.635, 104.125, 308.474, 640.145, all P<0.0001). The PED of Group I was significantly higher than that of Group II (P<0.05), and Group II was significantly higher than that of Group III (P<0.05). The PED value of 3mo after surgery decreased in each group compared with 1mo after surgery, but there was no statistical difference (Group I: t=0.82, P=0.41; Group II: t=0.17, P=0.87; Group III: t=1.35, P=0.18). The correlation analysis of corneal biomechanical parameter changes with PED at 1mo and 3mo after surgery showed that ΔIOP, ΔIOPcc, ΔCH, and ΔCRF were not correlated with PED value in three groups (P>0.05). The smaller the RCST/CT, the greater effect on corneal biomechanics and posterior surface elevation. There is no correlation between changes in corneal biomechanics and posterior corneal surface elevation in the range of RCST/CT≥50%.

The corneal biomechanical differences after wearing orthokeratology lenses and multifocal soft lenses in children: A self-control study

BackgroundTo compare the changes in corneal biomechanics after orthokeratology (OK) lens and Defocus Incorporated Soft Contact (DISC) lens treatment. MethodsOf 28 myopic children were recruited, with one eye wearing OK lens and the other eye wearing DISC lens for one year, and the data after discontinued for 4 weeks were also collected. Major outcomes were corneal biomechanics and axial length (AL) elongation. ResultsThroughout the follow-up period, the DISC group had longer the first applanation (A1) time, larger A1 deformation amplitude, A1 deflection length (A1 DLL), and A1 deflection amplitude than the OK group. AL elongation was less in the OK group at each visit (all P < 0.05) but faster in the OK group than in the DISC group after discontinuation (P = 0.006). Moreover, AL elongation was related to baseline A1 time, A1 velocity and whole eye movement max in the DISC group, and in the OK group, was related to the baseline the second applanation (A2) DLL, A2 delta arc length and stiffness parameter A1 (all P < 0.05). ConclusionsThe cornea was more deformable after wearing DISC lens than OK lens, and corneal biomechanical parameters were associated with AL elongation. Eyes showed less AL elongation during OK lens treatment while faster AL elongation after discontinuation than DISC lens. The baseline corneal biomechanics may help to predict AL elongation in myopic control strategies.

Comparing corneal biomechanic changes among solo cataract surgery, microhook ab interno trabeculotomy and iStent implantation

Minimally invasive glaucoma surgery has expanded the surgical treatment options in glaucoma, particularly when combined with cataract surgery. It is clinically relevant to understand the associated postoperative changes in biomechanical properties because they are influential on the measurement of intraocular pressure (IOP) and play an important role in the pathogenesis of open-angle glaucoma (OAG). This retrospective case–control study included OAG patients who underwent cataract surgery combined with microhook ab interno trabeculotomy (µLOT group: 53 eyes of 36 patients) or iStent implantation (iStent group: 59 eyes of 37 patients) and 62 eyes of 42 solo cataract patients without glaucoma as a control group. Changes in ten biomechanical parameters measured with the Ocular Response Analyzer and Corneal Visualization Scheimpflug Technology (Corvis ST) at 3 and 6 months postoperatively relative to baseline were compared among the 3 groups. In all the groups, IOP significantly decreased postoperatively. In the µLOT and control groups, significant changes in Corvis ST-related parameters, including stiffness parameter A1 and stress‒strain index, indicated that the cornea became softer postoperatively. In contrast, these parameters were unchanged in the iStent group. Apart from IOP reduction, the results show variations in corneal biomechanical changes from minimally invasive glaucoma surgery combined with cataract surgery.

Localized Corneal Biomechanical Alteration Detected In Early Keratoconus Based on Corneal Deformation Using Artificial Intelligence.

This study aimed to develop a novel method to diagnose early keratoconus by detecting localized corneal biomechanical changes based on dynamic deformation videos using machine learning. Diagnostic research study. We included 917 corneal videos from the Tianjin Eye Hospital (Tianjin, China) and Shanxi Eye Hospital (Xi'an, China) from February 6, 2015, to August 25, 2022. Scheimpflug technology was used to obtain dynamic deformation videos under forced puffs of air. Fourteen new pixel-level biomechanical parameters were calculated based on a spline curve equation fitting by 115,200-pixel points from the corneal contour extracted from videos to characterize localized biomechanics. An ensemble learning model was developed, external validation was performed, and the diagnostic performance was compared with that of existing clinical diagnostic indices. The performance of the developed machine learning model was evaluated using precision, recall, F1 score, and the area under the receiver operating characteristic curve. The ensemble learning model successfully diagnosed early keratoconus (area under the curve = 0.9997) with 95.73% precision, 95.61% recall, and 95.50% F1 score in the sample set (n=802). External validation on an independent dataset (n=115) achieved 91.38% precision, 92.11% recall, and 91.18% F1 score. Diagnostic accuracy was significantly better than that of existing clinical diagnostic indices (from 86.28% to 93.36%, all P <0.01). Localized corneal biomechanical changes detected using dynamic deformation videos combined with machine learning algorithms were useful for diagnosing early keratoconus. Focusing on localized biomechanical changes may guide ophthalmologists, aiding the timely diagnosis of early keratoconus and benefiting the patient's vision.

Keratometric Study of Cornea in Allergic Conjunctivitis Patients

Background: To explore corneal biomechanical changes, identify related factors and determine early indicators of keratoconus (KC) development risk in allergic conjunctivitis (AC) patients. Methods: A total of 50 patients, including 20 eyes without AC and 30 eyes with AC were enrolled in this study. All patients underwent a complete ocular examination, including evaluations of clinical manifestations of AC, corneal tomography and densitometry by Pentacam, corneal biomechanics by Corvis ST, and corneal and epithelial thickness mapping by RTvue optical coherence tomography (OCT). Results: The index of surface variance (ISV), index of vertical asymmetry (IVA), keratoconus index (KI), index of height decentration (IHD) and Belin/Ambrosio enhanced ectasia total deviation index (BAD-D) were significantly higher in the AC group than in the non-allergic conjunctivitis (NAC) group (P &lt; 0.05). The tomography and biomechanical index (TBI) was also significantly higher in the AC group (P =?0.04). The average epithelial thickness in the 2–7?mm annulus was significantly thinner in the AC group than in the NAC group (P &lt; 0.05). The average densitometry of the total cornea and the anterior layer were higher in the AC group than in the NAC group (P &lt; 0.001). The ISV, IVA, KI, IHD and BAD-D were significantly correlated with the TBI and changes in corneal epithelial thickness in AC patients (P &lt; 0.05). The changes in epithelial thickness were closely related to the eye rubbing frequency and allergic sign scores (P &lt; 0.05). CONCLUSIONS: AC patients should be advised to routinely undergo corneal tomographic and biochemical measurements and the TBI could be used an indicator of KC devolopment risk in AC patients.

Corneal Morphological and Biomechanical Changes in Thyroid-Associated Ophthalmopathy.

This study aimed to evaluate corneal morphological and biomechanical changes in patients with thyroid-associated ophthalmopathy (TAO) and their correlations with activity and severity. Patients diagnosed with TAO were recruited and divided into groups by activity and severity. All subjects underwent a complete ophthalmic examination, including magnetic resonance imaging. Corneal topography was measured using a Pentacam device, and biomechanical parameters were obtained using a CorVis ST tonometer. Correlations among the corneal parameters, clinical activity score, and NOSPECS score were analyzed. Areas under the receiver operating characteristic curves were calculated to evaluate the diagnostic accuracy of corneal changes for active and severe TAO. Fifty-three eyes with TAO and 16 healthy eyes were enrolled in our study. The back elevation, CorVis biomechanical index, tomographic and biomechanical index, stiffness parameter at the first applanation, deviation from normality in back elevation, relational thickness, and overall deviation from normality were significantly increased in patients with TAO (all P <0.05), whereas the smallest corneal thickness, maximum Ambrósio relational thickness, and deformation amplitude (DA) ratio were significantly decreased (all P <0.05). The clinical activity score was strongly positively correlated with back elevation (γ = 0.515, P <0.001). The NOSPECS score was strongly positively correlated with relational thickness and tomographic and biomechanical index (γ = 0.429 and 0.515, P <0.001) and negatively correlated with maximum Ambrósio relational thickness (γ = -0.53, P <0.001). Moreover, maximum Ambrósio relational thickness and the Ambrósio relational thickness through the horizontal meridian showed desirable diagnostic capacity in distinguishing mild TAO from moderate-severe TAO (areas under the receiver operating characteristic curve, 0.799 and 0.769). Corneal morphological and biomechanical changes were found in patients with TAO, which might be related to the presence of inflammation. Measurements of corneal morphological and biomechanical parameters could serve as references in evaluating TAO.

Corneal Biomechanical Evaluation After Meniscus-Shaped Stromal Lenticule Addition Keratoplasty (MS-SLAK) for Keratoconus.

To evaluate corneal biomechanical changes after meniscus-shaped stromal lenticule addition keratoplasty (MS-SLAK) performed for the treatment of keratoconus. This interventional study included patients affected by advanced keratoconus (stage III and IV) who underwent examination with a dynamic Scheimpflug analyzer and non-contact tonometer (Corvis ST; Oculus Optikgeräte GmbH) at baseline and 12 months after MS-SLAK. The biomechanical parameters evaluated in this study were integrated inverse radius (1/R), deformation amplitude ratio (DA ratio), stiffness parameter at first applanation (SP-A1), biomechanical intraocular pressure (bIOP), central corneal thickness (CCT), and stress-strain index (SSI). Sixteen patients were enrolled in the study. The analysis was ultimately conducted on 15 patients. Comparative analyses showed an increase in corneal stiffness as demonstrated by a rise in SSI (P < .0001) and SP-A1 (P < .0001) and a decrease in DA ratio (P < .0001) and 1/R (P = .01). A significant increase in CCT was found (P < .0001). No statistically significant modification was found for bIOP (P = .43). The corneal biomechanical analyses evaluated by the Corvis ST showed that MS-SLAK for advanced keratoconus is able to increase corneal overall stiffness. This result is explained by the significant increase in thickness induced by MS-SLAK. [J Refract Surg. 2023;39(7):499-504.].

Influence of a Prostaglandin F2α Analogue on Corneal Hysteresis and Expression of Extracellular Matrix Metalloproteinases 3 and 9.

Low corneal hysteresis (CH) is associated with an increased risk of glaucoma. Prostaglandin analogue (PGA) eye drops may exert their intraocular pressure (IOP)-lowering effect partially by increasing CH. Twelve pairs of organ-cultured human donor corneas were used in an ex vivo model. In each case, one cornea was treated with PGA (Travoprost) for 30 days, whereas the other served as an untreated control. IOP levels were simulated in an artificial anterior chamber model. CH was measured using the Ocular Response Analyzer (ORA). Corneal expression of matrix-metalloproteinases (MMPs) was assessed by immunhistochemistry and real-time polymerase chain reaction (RT-PCR). Increased CH was observed in the PGA-treated corneas. However, at IOP between 10 and 20 mm Hg, CH was increased in PGA-treated corneas (13.12 ± 0.63 mm Hg; control: 12.34 ± 0.49 mm Hg), although not significantly (P = 0.14). CH was significantly increased at higher IOP levels (21-40 mm Hg; PGA-treated: 17.62 ± 0.40 mm Hg; control: 11.60 ± 0.39, P < 0.0001). Treatment with PGA resulted in increased expression of MMP-3 and MMP-9. CH was increased after exposure to PGA. However, this increase was significant only in eyes with higher IOP (>21 mm Hg). A significant increase in MMP-3 and -9 was observed in PGA-treated corneas, indicating structural changes in corneal biomechanics caused by PGA. PGAs alter biomechanical structures by directly upregulating MMP-3 and -9, and the increase in CH is dependent on the level of IOP. Therefore, PGAs may have a greater effect when baseline IOP is higher.

Corneal biomechanical changes after myopic and hyperopic laser vision correction

Laser vision correction became a popular method of refractive error treatment. The laser vision correction techniques influence the corneal biomechanical properties including corneal hysteresis and corneal resistance factor. The ocular response analyzer and Corvis ST devices are used in clinical practice to measure the corneal biomechanics. Reasonable laser treatment planning, taking into account the impact on corneal biomechanics, may potentially improve the safety of the refractive procedures. Thicker caps in refractive lenticule extraction and thinner flaps in flap-related procedures promote better corneal biomechanics preservation. The myopic refractive treatment appears to have a greater effect on corneal biomechanics weakening than hyperopic correction.

Clinical Evaluation of Corneal Biomechanics following Laser Refractive Surgery in Myopic Eyes: A Review of the Literature.

The role of corneal biomechanics in laser vision correction (LVC) is currently being raised in the assessment of postoperative corneal ectasia risk. The aim of the paper was to evaluate the changes in corneal biomechanics after LVC procedures based on a systematic review of current studies. The results of a search of the literature in the PubMed, Science Direct, Google Scholar, and Web of Science databases were selected for final consideration according to the PRISMA 2020 flow diagram. Included in our review were 17 prospective clinical studies, with at least 6 months of follow-up time. Corneal biomechanical properties were assessed by Ocular Response Analyzer (ORA), or Corvis ST. The results of the study revealed the highest corneal biomechanics reduction after laser in situ keratomileusis (LASIK) followed by small incision lenticule extraction (SMILE) and surface procedures, such as photorefractive keratectomy (PRK) or laser-assisted sub-epithelial keratectomy (LASEK). In SMILE procedure treatment planning, the use of thicker caps preserves the corneal biomechanics. Similarly, reduction of flap thickness in LASIK surgery maintains the corneal biomechanical strength. Future prospective clinical trials with standardization of the study groups and surgical parameters are needed to confirm the results of the current review.

Goldmann and modified Goldmann tonometry measuring intraocular pressure changes in eyes which underwent myopic laser in situ Keratomileusis and photorefractive keratectomy

PurposeCompare intraocular pressure (IOP) measured by a standard Goldmann applanation tonometer prism (IOPg) and a modified correcting applanation tonometer surface Goldmann prism (IOPc) before and after laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK).MethodsGoldmann tonometry was analyzed in a retrospective, cross-sectional study, using both GAT and modified-GAT prisms pre-operatively and at the 3 month post-operative appointment on 120 eyes (64 patients) who received LASIK (n = 58) or PRK (n = 62). Demographics, central corneal thickness (CCT), manifest refraction and corneal curvature (CC) data was collected at each visit as well as surgical parameters, including maximum ablation depth.ResultsMean paired IOP following LASIK decreased by − 3.28 ± 3.2 mmHg measured by IOPg and − 1.93 ± 3.3 mmHg by IOPc (p ≤ 0.0001). Mean paired IOP following PRK reduced by − 1.92 ± 3.6 mmHg measured by IOPg and − 1.06 ± 3.6 mmHg by IOPc (p ≤ 0.0001). Increased LASIK ablation depth and post-procedural change in CCT trended toward a statistically significant reduction in IOPg (p = 0.07,p = 0.12), but not IOPc (p = 0.18,p = 0.32). PRK ablation depth was not associated with a reduction in IOPg or IOPc.DiscussionThe modified Goldmann (IOPc) prism measured less of an IOP reduction following LASIK and PRK compared to the standard (IOPg) prism, and the IOP reduction with both prisms was associated with the degree of myopic correction.What is already known and the residual queryCorneal refractive surgery generally demonstrates significant postoperative Goldmann IOP reductions. Presumably, this is due to corneal biomechanical changes for which a newer method of Goldmann IOP measurement may be able to compensate.What this study addsA modified, corneal conforming Goldmann prism demonstrates significantly less IOP reduction following myopic LASIK and PRK compared to the standard flat Goldmann prism.How this study might affect research, practice or policyA newer, modified Goldmann prism may help detect glaucoma and OHT at an earlier stage in patients which have undergone LASIK or PRK. The findings corroborate predicted corneal biomechanical changes following the most common corneal refractive procedures.

Three-Year Follow-up of Biomechanical Changes in Very Asymmetric Ectasia

Purpose: The purpose of this study was to assess longitudinal corneal biomechanical changes in fellow eyes with normal topography in patients with very asymmetric ectasia (VAE) for 3 years. Methods: Clinical records of 30 patients with VAE who met the inclusion criteria were retrospectively reviewed. Patients with VAE were defined as having clinical ectasia in 1 eye and normal topography in the fellow eye. Patients were subdivided into 2 groups based on baseline corneal tomography data: fellow eye with normal topography/tomography (fellow eye with NTT) and fellow eye with normal topography and not normal tomography (fellow eye with NT). Corneal biomechanical assessment with high-speed dynamic Scheimpflug-based corneal imaging was performed at baseline and each follow-up visit on each eye. Longitudinal changes in biomechanical parameters were also investigated. Linear mixed-effects models were used to evaluate changes over time in 2 groups. Results: Significant changes in the Corvis Biomechanical Index and the stress–strain index (SSI) (P = 0.023, P = 0.034, respectively) were observed during the 3-year period in fellow eye with NT group, and only SSI showed a decreasing trend over time. A significant association between the SSI and age at the initial biomechanical assessment was observed (P &lt; 0.001). No significant changes during the 3-year period were found in any parameters in fellow eye with NTT group. Conclusions: Based on biomechanical evaluation of the cornea over a 3-year period, possible corneal softening in fellow eye of VAE having normal topography and not normal tomography was observed, highlighting the characteristics of progressive ectatic corneal disorder even in a subclinical status. Future studies with a longer follow-up period are required.

Changes in Stress-Strain Index and Corneal Biomechanics in Granular Corneal Dystrophy.

The aim of this study was to assess stress-strain index (SSI) and corneal biomechanical parameters in eyes with granular corneal dystrophy (GCD). This case-control study included 12 eyes of 12 patients with GCD (mean age 45.2 ± 18.7 years) and 20 eyes of 20 healthy individuals (mean age 54.4 ± 3.8 years). In addition to SSI, dynamic corneal response (DCR) parameters were assessed at the first and second applanation, including length (AL1, AL2), velocity (AV1, AV2), time (AT1, AT2), and deformation amplitude (DA A1, DA A2), and at the highest concavity (HC) phase, including DA, peak distance (PD), radius (HCR), and DA ratio (DAR 1 and 2 mm), by Corvis ST. Central corneal thickness (CCT) and biomechanically corrected intraocular pressure (bIOP) were considered covariates in comparing DCR parameters between the two groups. SSI was statistically significantly lower in eyes with GCD than in normal eyes (p = 0.04). The corneal velocity towards the first applanation was 0.02 m/s faster in the GCD eyes AV1 (0.15 ± 0.02 vs. 0.13 ± 0.02 m/s, p &lt; 0.001) and IR (7.48 ± 1.01 vs. 6.80 ± 1.22 mm, p = 0.003) parameters were significantly higher in the GDC group, while AT1 (7.33 ± 0.66 vs. 7.47 ± 0.36 ms, p = 0.002) and HCR (7.42 ± 0.76 vs. 8.20 ± 1.08 mm, p = 0.014) were significantly lower in the normal group. GCD led to a change in biomechanical properties of the cornea. SSI refers to fewer stiff corneas in GDC than normal.

Comparing corneal biomechanics and intraocular pressure between healthy individuals and glaucoma subtypes: A cross-sectional study.

BackgroundThe utilization of corneal biomechanical features in evaluating glaucoma and its progression has received particular attention. The severity of corneal biomechanical changes can play an essential role in response to medical or surgical treatment. The present study evaluated the biomechanical features of the cornea in glaucoma patients in different subtypes and compared them with the normal condition.MethodsIn this cross-sectional study, glaucoma patients and healthy individuals were referred to the tertiary hospital in 2021. Both eyes underwent a complete ophthalmologic examination, intraocular pressure measurement, and corneal biomechanical parameters using Corvis and ORA devices. Finally, data from both groups were compared.ResultsBased on the ORA evaluation, Lower CRF and CH were seen in glaucoma patients. In the Corvis evaluation, minor differences were observed in glaucoma as increased pachy, radius, and pachy slope, and decreased HC deformation amplitude, HC deflection amplitude, HC deflection area, deflection amplitude max, dArc length max, max inverse radius, and integrated radius. Lower ACD and higher CCT differentiated PACG from others. Lower CCT and higher C/D and WTW indicated NTG. Based on ORA, the highest CRF and CH were related to PACG and the lowest related to PEXG. In contrast, based on Corvis, higher pachy and radius and lower max inverse radius and integrated radius were specified for PACG. PEXG also had the highest values of the last two parameters.ConclusionEvaluation of corneal biomechanical parameters and other indicators can be beneficial in assessing the status and severity of glaucoma and distinguishing between disease subtypes.

Analysis of corneal biomechanical properties 25years after myopic photorefractive keratectomy.

To evaluate corneal biomechanical changes using Corvis ST in patients treated with photorefractive keratectomy (PRK) 25years ago. In this study, 32 post-PRK and 38 normal eyes underwent Corvis ST (CST) assessments. The measured CST factors were: time of highest concavity (HC), time of applanation 1 (AT1), time of applanation 2 (AT2), length of applanation 1 (AL1), length of applanation 2 (AL2), velocity of applanation 1 (AV1), velocity of applanation 2 (AV2), deformation amplitude (DA), peak distance (PD), integrated radius (IR), Ambrosio relational thickness horizontal (ARTh), stiffness parameter at first applanation (SP-A1), DA ratio (2mm), Belin/Ambrosio enhanced ectasia display (BAD) and corneal biomechanical index (CBI). The mean [± standard deviation (SD)] age was 51.4 ± 7.36years in PRK, 51.4 ± 3.62 in control group. PRK was performed 24.69 ± 1.78years ago. ARTh, SP-A1, AT1, AL1, and AL2 were lower in PRK. PD, AT2, DA ratio (2mm), and IR were statistically higher in PRK (P < 0.01). In PRK and control group the mean value of CBI was 0.91 ± 0.11 and 0.50 ± 0.27 (P < 0.001), and mean value of BAD was 3.34 ± 1.53 and 1.1 ± 0.70 (P < 0.001). In PRK 71.9% of eyes were classed "high risk CBI plus diseased BAD" and 25% remained in the "high risk CBI and normal BAD" group. In this study, most of the post-PRK eyes which were clinically and topographically normal were classified as "high risk CBI plus diseased BAD" and had significantly worse CBI and BAD values than the control group. This leads to the conclusion that CBI and BAD alone are not appropriate to evaluate post-PRK ectasia.

Short and long term corneal biomechanical analysis after overnight orthokeratology.

To investigate the short and long term corneal biomechanical changes after overnight orthokeratology (OK) and compare them with those occurring in subjects not wearing contact lenses. Retrospective case control study enrolling 54 subjects that were divided into three groups 18 subjects each: control group (CG), short term (15 nights) OK (STOK) group, and long term (more than 1y of OK wear) OK (LTOK) group. Corneal biomechanics were characterized using the CorVis® ST system (Oculus), recording parameters such as time [first/second applanation time (AT1, AT2)], speed [velocity of corneal apex at the first/second applanation time (AV1, AV2)], and amplitude of deformation (AD1, AD2) in the first and second corneal flattening, corneal stiffness (SPA1), biomechanically corrected intraocular pressure (bIOP) and corneal (CBI) and tomographic biomechanical indices (TBI). Significantly lower AD1 and standard deviate on of Ambrosio's relational average thickness related to the horizontal profile (ARTh) values were found in the OK groups compared to CG (P<0.05). Likewise, significantly higher values of CBI were found in STOK and LTOK groups compared to CG (P<0.01). No significant differences between groups were found in integrated radius index (P=0.24), strain stress index (P=0.22), tomographic biomechanical index (P=0.91) and corneal stiffness parameter (SPA1, P=0.97). Significant inverse correlations were found between corneal thickness and CBI in STOK (r=-0.90, P<0.01) and LTOK groups (r=-0.71, P<0.01). OK does not seem to alter significantly the corneal biomechanical properties, but special care should be taken when analyzing biomechanical parameters influenced by corneal thickness such as amplitude of deformation, ARTh or CBI, because they change significantly after treatment but mainly due to the reduction and pachymetric progression induced by the corneal molding secondary to OK treatment.

Changes in Corneal Morphology and Biomechanics in Cases of Small Incision Lenticule Extraction with Prophylactic Accelerated Collagen Cross-Linking.

Purpose To study the corneal morphology and biomechanics in cases of small incision lenticule extraction with prophylactic accelerated collagen cross-linking (SMILE Xtra). Methods This study was a retrospective study. 28 eyes of 14 patients with moderate-high risk of postoperative ectasia according to the Randleman scoring system underwent SMILE Xtra procedure. Outcome data were recorded including uncorrected distance visual acuity (UDVA), manifest refraction spherical equivalent (MRSE), surface regularity index (SRI), surface asymmetry index (SAI), simulated keratometry (SimK), posterior axial curvature (PAC), anterior and posterior corneal elevations (ACE and PCE), central corneal thickness (CCT), corneal resistance factor (CRF), corneal hysteresis (CH), and cornea-compensated intraocular pressure (IOPcc). The follow-up period was 12 months. Results There were 28, 26, 22, 12, and 10 eyes enrolled at postoperative 1st day and 1st, 3rd, 6th, and 12th months, respectively. The UDVA improved from 1.27 ± 0.18 logMAR preoperatively to -0.06 ± 0.04 logMAR postoperatively (P < 0.05). The MRSE improved from -5.05 ± 1.15 D preoperatively to -0.14 ± 0.30 D postoperatively (P < 0.05). SAI, SimK, PAC, PCE, and CCT all changed significantly at 1st month postoperatively (P < 0.05) and stabilized during the remainder of the follow-up (P > 0.05). There was no significant change in SRI or ACE before and after surgery (P > 0.05). CRF, CH, and IOPcc all decreased significantly at 1st month postoperatively (P < 0.05) and remained stable afterwards (P > 0.05). Conclusions The changes in the corneal morphology and biomechanics remained stable after SMILE Xtra, and there was no sign of postoperative ectasia or refractive regression. Combined with the improvement of visual and refractive results, SMILE Xtra may be a promising method for corneal refractive surgeries in patients at risk.