Corneal Biomechanical Properties Research Articles

Central corneal thickness in glaucoma

The purpose of this review is to summarize the role of central corneal thickness (CCT) in the clinical management of a glaucoma patient. The prognostic value of CCT is well recognized in patients with ocular hypertension. However, its predictive value in other glaucoma suspects and patients with established glaucoma is less certain. Tonometry artefacts can result from variations in CCT. However, an adequately validated correction algorithm for Goldmann applanation tonometry measurements does not exist. Newer methods of tonometry are potentially less influenced by CCT but are limited in their clinical use. There may also be biological and genetic associations between corneal thickness and glaucoma. Demographics, environmental factors, glaucoma treatment and the measurement device used have a significant influence on CCT, and should be considered when interpreting the effect of cornea thickness in patients with glaucoma. New measurements of the biomechanical properties of the cornea are likely to be better approximations of the globe biomechanics than CCT, but these require further evaluation. The clinical significance of CCT is well recognized in the context of glaucoma diagnosis and management, though the extent of its importance remains debatable. Corneal biomechanical properties may be more significantly associated with glaucoma than CCT.

Applanation optical coherence elastography: noncontact measurement of intraocular pressure, corneal biomechanical properties, and corneal geometry with a single instrument.

Current clinical tools provide critical information about ocular health such as intraocular pressure (IOP). However, they lack the ability to quantify tissue material properties, which are potent markers for ocular tissue health and integrity. We describe a single instrument to measure the eye-globe IOP, quantify corneal biomechanical properties, and measure corneal geometry with a technique termed applanation optical coherence elastography (Appl-OCE). An ultrafast OCT system enabled visualization of corneal dynamics during noncontact applanation tonometry and direct measurement of micro air-pulse induced elastic wave propagation. Our preliminary results show that the proposed Appl-OCE system can be used to quantify IOP, corneal biomechanical properties, and corneal geometry, which builds a solid foundation for a unique device that can provide a more complete picture of ocular health.

Association between corneal volume change and corneal biomechanical property before and after small incision lenticule extraction in myopia

Background The relationship between corneal central thickness (CCT) and corneal biomechanical property does not reflect the effective amount of ablated tissue because CCT measures only the change in a single point.Corneal volume (CV) is a representative parameter of corneal morphology, and it can fully reflect corneal thickness and tissue distribution. Objective This study was to investigate the correlation between volumetric corneal changes and corneal biomechanical properties after small incision lenticule extraction (SMILE). Methods A prospective series cases-observational study was carried out.This study protocol was approved by Ethic Committee of Tianjin Eye Hospital, and written informed consent was obtained from each patient prior to entering the cohort.Sixty-seven right eyes of 67 myopia or myopic astigmatism patients who received SMILE in Tianjin Eye Hospital from June 2014 to July 2015 were included in this study.Before and 3 months after surgery, the CV at 3, 3-5, 5-7 and 7-10 mm (CV3, CV3-5, CV5-7, CV7-10) was measured by Pentacam anterior segment analysis system, and corneal hysteresis (CH) and corneal resistance factor (CRF) were obtained by ocular response analyzer (ORA). The changes of CV (ΔCV), CH (ΔCH) and CRF (ΔCRF) were calculated, and the correlations between ΔCV and ΔCH or ΔCRF were analyzed. Results CV3, CV3-5 and CV5-7 values after SMILE were significantly lower than those before SMILE (t=36.24, 20.38, 16.17, all at P< 0.001). The CH values before and after SMILE were (10.06±1.11)mmHg and (8.10±1.05)mmHg (1 mmHg=0.133 kPa), and the CRF values before and after SMILE were (10.40±1.38)mmHg and (6.91±1.19)mmHg, respectively, showing significant reduces after SMILE than before SMILE (t=16.71, 27.41, both at P<0.001). Positive correlations were seen between the CV values at different corneal areas and CH value or CRF value.Moderate positive correlations were found between CV3 and CH or CRF (r=0.571, 0.569; both at P<0.001) before surgery, and 3 months after surgery, a weak positive correlation was seen between ΔCV3 and ΔCH (r=0.394, P<0.001) or a moderate positive correlation between ΔCV3 and ΔCRF (r=0.501, P<0.001). Conclusions The CV value is gradually increased from the central cornea to periphery cornea.The CV change is associated with CH and CRF changes after SMILE, and CV3 probably is a useful parameter for the evaluation of corneal biomechanics after refractive surgery. Key words: Biometry; Cornea; Volume; Corneal biomechanics; Corneal hysteresis; Corneal resistance factor; Small incision lenticule extraction

Effect of topical antiinflammatory drugs on mechanical behavior of rabbit cornea.

BackgroundA variety of antiinflammatory therapies are employed to promote corneal wound healing. The effects of steroidal and nonsteroidal antiinflammatory drugs on the biomechanical properties of rabbit cornea were investigated over time using tensile tests.MethodsFull-thickness incisions were made and used to analyze the effects of dexamethasone sodium phosphate 0.1% and diclofenac sodium 0.1% on corneal biomechanical properties during wound healing at 7, 14 and 21 days after surgery.ResultsThe full-thickness incision deeply modified all of the mechanical properties. At 3 weeks after incision, regardless of the drug therapy, the tensile modulus was about 70% of the value for the intact cornea.ConclusionsTopical treatment with dexamethasone was particularly effective during the first week after surgery; the second week after surgery, a similar result was observed in the corneas treated with diclofenac. Low doses of steroidal and nonsteroidal antiinflammatory drugs would seem to have the potential to improve biomechanical properties only during the early stage of the healing process of the cornea.

Analysis on corneal deformation and corneal biomechanical changes after small incision lenticule extraction

Objective: To investigate biomechanical effects of central corneal thickness (CCT), and to explore the possible safe range for the tissue removal and percentage of tissue ablation (PTA) of small incision lenticule extraction (SMILE). Methods: Clinical observational study. One hundred and eighty-seven cases (365 eyes) undergoing SMILE surgery were enrolled. Corneal hysteresis (CH) and corneal resistance factor (CRF) were measured before and after surgery by the ocular response analyzer (ORA). Linear regression of CH, CRF and CCT, and percentage of tissue ablation were calculated. The fifth order polynomial functions provided the closest fit to the data of ΔCH, ΔCRF and the thickness of tissue removal. Results: CH and CRF decreased by 20.7% and 33.0%, respectively, after SMILE, and statistically significant differences were found between preoperative and postoperative values (P<0.05). CH and CRF showed a positive correlation to CCT (r=0.483, P<0.05. r=0.507, P<0.05). The fifth order polynomial functions showed that ΔCH and ΔCRF increased with the increasing tissue removal. The variety rate of ΔCH and ΔCRF was lower after 50 μm to 140 μm of tissue removal, but was higher when the tissue removal was less than 50 μm and more than 140 μm. The variation trend was similar for PTA. Conclusions: Corneal thickness has an effect on the corneal biomechanical properties, which decrease with the increase of tissue removal. It seems that the tissue removal of less than 140 μm and the PTA of less than 25% are safer in SMILE, and further investigations are needed. (Chin J Ophthalmol, 2017, 53:11-17).

Optical coherence elastography for evaluating customized riboflavin/UV-A corneal collagen crosslinking.

UV-induced collagen cross-linking is a promising treatment for keratoconus that stiffens corneal tissue and prevents further degeneration. Since keratoconus is generally localized, the efficacy of collagen cross-linking (CXL) treatments could be improved by stiffening only the weakened parts of the cornea. Here, we demonstrate that optical coherence elastography (OCE) can spatially resolve transverse variations in corneal stiffness. A short duration ( ? 1 ?? ms ) focused air-pulse induced low amplitude ( ? 10 ?? ? m ) deformations in the samples that were detected using a phase-stabilized optical coherence tomography system. A two-dimensional map of material stiffness was generated by measuring the damped natural frequency (DNF) of the air-pulse induced response at various transverse locations of a heterogeneous phantom mimicking a customized CXL treatment. After validation on the phantoms, similar OCE measurements were made on spatially selective CXL-treated in situ rabbit corneas. The results showed that this technique was able to clearly distinguish the untreated and CXL-treated regions of the cornea, where CXL increased the DNF of the cornea by ? 51 % . Due to the noncontact nature and minimal excitation force, this technique may be valuable for in vivo assessments of corneal biomechanical properties.

Corneal hysteresis values in normal Egyptian population

IntroductionKnowledge of the cornea's biomechanical properties is important for a wide variety of applications within ophthalmology. Corneal biomechanical variations are known to affect the accuracy of intraocular pressure measurements and may be used to identify early corneal disease and may assist in predicting refractive outcomes following corneal refractive surgery. It has also been suggested that corneal biomechanical properties may reflect globe biomechanics, and thus give an indication of the susceptibility of developing glaucomatous damage.Aim of the workThe purpose of this study was to explore the relationships between Ocular Response Analyzer-generated corneal biomechanical characteristics and age in a sample of Egyptian population.Materials and methodsCorneal hysteresis (CH) and corneal resistance factor (CRF) were measured in 195 Egyptians of different age groups.ResultsThe mean CH value was 10.25 ± 0.12 mmHg (range, 6.5–14.4), and the mean CRF was 10.25 ± 0.15 mmHg (range, 4.9–14.2). The CH value was lower in older eyes, and the difference between the youngest age group (19–40 years) and the oldest age group (40–71 years) was statistically significant (t-test = 0.01). The mean CH in the youngest age group was 11.1 ± 0.14, and that in the oldest age group was 9.8 ± 0.21. The mean CRF in the youngest age group was 10.9 ± 0.18, and that in the oldest age group was 10.1 ± 0.19.ConclusionThis study of corneal biomechanics in normal Egyptian eyes describes the interactions between age and Ocular Response Analyzer metrics. The study reveals that age is significantly associated with CH and CRF. This may help in diagnosis and treatment.

The Association between Corneal Biomechanical Properties and Initial Visual Field Defect Pattern in Normal Tension Glaucoma

The Association between Corneal Biomechanical Properties and Initial Visual Field Defect Pattern in Normal Tension Glaucoma

Corneal properties and glaucoma: a review of the literature and meta-analysis.

Studies have suggested that corneal biomechanical properties influence intraocular pressure (IOP) measurements, namely central corneal thickness (CCT) and corneal hysteresis (CH). The present study aimed to investigate the associations of CH and CCT with glaucoma development. We performed a review of the literature and meta-analysis of observational studies (2006-2016) that included both adult glaucoma patients and controls and reported CCT and CH as outcomes. Nineteen studies were conside red eligible, and the mean difference (MD) between groups (patient and control) for both variables was used for statistical analyses. A total of 1,213 glaucoma and 1,055 healthy eyes were studied. Quan titative analysis suggested that CH was significantly lower in the glaucoma group than in the control group (MD=-1.54 mmHg, 95% CI [-1.68, -1.41], P<0.0001). Additionally, CCT was significantly lower in the glaucoma group than in the control group (MD=-8.49 µm, 95% CI [-11.36, -5.62], P<0.001). Corneal properties appear to differ between glaucoma patients and healthy controls. Our results emphasize the importance of corneal biomechanical properties in IOP interpretation and should support further studies on the influence of CH and CCT in glaucoma screening and diagnosis.

Comparison between corneal biomechanics and HbA1c levels in healthy individuals and type 2 diabetic patients

Objectives The aim of this study was to evaluate the corneal biomechanical properties using the ocular response analyzer (ORA) in type 2 diabetic patients. Background The ORA is a device developed in recent years and reveals the biomechanical properties of the cornea. It reflects certain biomechanical properties of the cornea, such as corneal hysteresis and corneal resistance factor. Patients and methods A total of 150 eyes of 75 patients were enrolled in a cross-sectional prospective comparative study from May 2015 to May 2016. Patients were divided into three equal groups. ORA and HbA1c evaluation were carried out for all patients. Results The corneal hysteresis of the study groups ranged from 5.5 to 9.8 mmHg for the control group (group A), and it ranged from 6.00 to 11.4 mmHg for the diabetic group (group B), whereas it ranged from 9.5 to 15.20 in the diabetic group (group C). The corneal resistance factor of the study groups ranged from 5.8 to 10.00 mmHg for the control group (group A), from 6.2 to 12.00 mmHg for the diabetic group (group B), and from 8.90 to 16.10 mmHg for the diabetic group (group C). The Goldmann-correlated intraocular pressure of our study groups ranged from 10 to 30 mmHg for the control group (group A), from 10 to 22 mmHg for the diabetic group (group B), and from 10.5 to 26.00 mmHg for the diabetic group (group C). The corneal-compensated intraocular pressure of the study groups ranged from 10.8 to 25 mmHg for the control group (group A), from 11.2 to 30 mmHg for the diabetic group (group B), and from 10.00 to 28.00 mmHg for the diabetic group (group C). Conclusion The corneal biomechanics in diabetic groups are significantly higher than those in the control group.

Refinement of Intraocular Pressure Measurements made by Ocular Response Analyzer following Laser in Situ Keratomileusis using M2 90 Moria Microkeratome for Egyptian Myopic and Astigmatic Patients.

IntroductionThe Goldmann applanation tonometer readings were noted to be markedly reduced after Lasik surgery using a thin flap technique which is widely used nowadays, to correct a wide range of myopia and astigmatism. The Ocular Response Analyzer (ORA) is considered one of the important tools for refinement of Intraocular Pressure (IOP) measurements. The aim of this study is to evaluate and refine ORA measurements for IOP changes post- M2 90 Moria microkeratome Lasik procedure for Egyptian myopic and astigmatic patients trying to aid in glaucoma diagnosis and management.MethodsThirty-five eyes of nineteen Egyptian patients with myopia or myopic astigmatism who had undergone Lasik procedure using M2 90 Moria microkeratome were included in this consecutive case series study. All cases were subjected to full ophthalmological examination including uncorrected and best corrected visual acuity, refraction, slit lamp examination, Scheimpflug imaging and ORA (Reichert Ophthalmic Instruments, Inc., Buffalo, NY, USA). The refractive errors were measured using an auto refractometer and were confirmed by trial. For the ocular response analyzer, the Goldmann-correlated IOP measurement (IOPg) which simulates IOP measured by Goldmann tonometer and the Corneal-Compensated Intraocular Pressure (IOPcc) that takes corneal biomechanical properties into consideration were reported as well as the Corneal Hysteresis (CH) and the Corneal Resistance Factor (CRF). Cases were examined preoperatively and again about one month after Lasik, after stabilization of eye condition. The collected data were tabulated and analysed with the suitable statistical methods. The mean values and standard deviation were calculated for quantitative data. Comparison tests (t-test) and correlation tests (Pearson) were also performed.ResultsIn our study, involving M2 90 Moria microkeratome Lasik procedure, a highly significant post-Lasik reduction in IOPg (t-test = 8.62 (p<0.01), and a statistically significant reduction in IOPcc (t-test = 3.37 (p<0.05) by ocular response analyzer. The average post-Lasik reduction in IOP in mmHg was 4.84±2.82 for IOPg and 2.23±3.51for IOPcc. A statistically significant correlation (p<0.05) existed between post-Lasik reduction of IOPg and ablation depth as well as preoperative spherical equivalent. A non-significant correlation existed between post-Lasik reduction of IOPcc and both of ablation depth and preoperative spherical equivalent.ConclusionsFollowing Lasik procedure using M2 90 Moria microkeratome, the IOPcc values by ORA were closer to the preoperative values than the IOPg values and the reduction of IOPcc values was not affected by ablation depth (i.e. preoperative spherical equivalent). Therefore, it is advisable to use the IOPcc values by ORA when evaluating post-Lasik patients for glaucoma diagnosis and management, but with the addition of the average value for post-Lasik reduction which in the study was 2.23±3.51mmHg.

Corneal Biomechanical Properties After Penetrating Keratoplasty or Deep Anterior Lamellar Keratoplasty Using the Ocular Response Analyzer: A Meta-Analysis.

To evaluate the corneal biomechanical properties of patients who have undergone penetrating keratoplasty (PK) or deep anterior lamellar keratoplasty (DALK) using the ocular response analyzer. Stata 13.0 SE was used for this meta-analysis. Studies in the literature that focused on corneal hysteresis (CH) or corneal resistance factor (CRF) after PK or DALK were retrieved by searching PubMed, Embase, Ovid, and Cochrane databases. We present the results as weighted mean difference (WMD) with a corresponding 95% confidence interval (CI). Eight studies with a total of 750 eyes were included in the post-PK versus control group, and 4 studies with a total of 218 eyes were included in the post-DALK versus control group. The pooled results showed that CH and CRF were significantly reduced (P < 0.00001) for patients who have undergone PK (WMD = -1.16, 95% CI: -1.73 to -0.60 and WMD = -1.00, 95% CI: -1.61 to -0.40). No significant differences were found in both CH and CRF for patients who have undergone DALK (WMD = -0.27, 95% CI: -0.64 to -0.09 and WMD = -0.15, 95% CI: -0.53 to 0.23). This meta-analysis suggested that both CH and CRF had better recovery after corneal transplantation with DALK than PK.

Assessing the effects of riboflavin/UV-A crosslinking on porcine corneal mechanical anisotropy with optical coherence elastography.

In this work we utilize optical coherence elastography (OCE) to assess the effects of UV-A/riboflavin corneal collagen crosslinking (CXL) on the mechanical anisotropy of in situ porcine corneas at various intraocular pressures (IOP). There was a distinct meridian of increased Young's modulus in all samples, and the mechanical anisotropy increased as a function of IOP and also after CXL. The presented noncontact OCE technique was able to quantify the Young's modulus and elastic anisotropy of the cornea and their changes as a function of IOP and CXL, opening new avenues of research for evaluating the effects of CXL on corneal biomechanical properties.

Changes in intraocular pressure values measured with noncontact tonometer (NCT), ocular response analyzer (ORA) and corvis scheimpflug technology tonometer (CST) in the early phase after small incision lenticule extraction (SMILE).

BackgroundCorneal biomechanical properties are always compromised after corneal refractive surgeries thus leading to underestimated intraocular pressure (IOP) that complicates the management of IOP. We investigated the changes in postoperative baseline of IOP values measured with noncontact tonometer (NCT), ocular response analyzer (ORA) and corvis scheimpflug technology (CST) in the early phase after small incision lenticule extraction (SMILE).MethodsTwenty-two eyes (−6.76 ± 1.39D) of 22 moderate and high myopes, (28.36 ± 7.14 years, 12 male and 10 female) were involved in this prospective study. IOP values were measured using a non-contact tomometer (NCT-IOP), an ocular response analyzer (corneal-compensated IOP, IOPcc and Goldmann-correlated IOP, IOPg) and a Corvis scheimpflug technology tonometer (CST-IOP) preoperatively, at 20 min and 24 h, postoperatively. Repeated measures analysis of variance (RM-ANOVA), Pearson’s correlation analysis and multiple linear regression models (stepwise) were performed. Cut-off P values were 0.05.ResultsExcept for IOPcc, NCT-IOP, IOPg, and CST-IOP values significantly decreased after SMILE procedure (All P values <0.05). ΔCCT, as well as ΔMRSE and ΔKm, did not significantly correlated with ΔNCT-IOP, ΔIOPcc, ΔIOPg or ΔCST-IOP, (all P values >0.05). Multiple linear regression models (stepwise) showed that the practical post-operative IOP value was the main predictor of the theoretical post-operative NCT-IOP, IOPcc and IOPg values (all P values <0.001). The postoperative applanation time 1 (AT1) value (B = 8.079, t = 4.866, P < 0.001), preoperative central corneal thickness (CCT) value (B = 0.035, t = 2.732, P = 0.014) and postoperative peak distance (PD) value (B = 0.515, t = 2.176, P = 0.043) were the main predictors of the theoretical post-operative CST-IOP value.ConclusionsIOP values are underestimated when assessed after SMILE by using NCT-IOP, IOPg and CST-IOP. The practical postoperative IOPcc value and theoretical post-operative CST-IOP value may be more preferable for IOP assessment in the early phase after SMILE.Trial registrationCurrent Controlled Trials ChiCTRONRC13003114. Retrospectively registered 17 March 2013

Optical coherence elastography assessment of corneal viscoelasticity with a modified Rayleigh-Lamb wave model

The biomechanical properties of the cornea play a critical role in forming vision. Diseases such as keratoconus can structurally degenerate the cornea causing a pathological loss in visual acuity. UV-A/riboflavin corneal collagen crosslinking (CXL) is a clinically available treatment to stiffen the cornea and restore its healthy shape and function. However, current CXL techniques do not account for pre-existing biomechanical properties of the cornea nor the effects of the CXL treatment itself. In addition to the inherent corneal structure, the intraocular pressure (IOP) can also dramatically affect the measured biomechanical properties of the cornea. In this work, we present the details and development of a modified Rayleigh-Lamb frequency equation model for quantifying corneal biomechanical properties. After comparison with finite element modeling, the model was utilized to quantify the viscoelasticity of in situ porcine corneas in the whole eye-globe configuration before and after CXL based on noncontact optical coherence elastography measurements. Moreover, the viscoelasticity of the untreated and CXL-treated eyes was quantified at various IOPs. The results showed that the stiffness of the cornea increased after CXL and that corneal stiffness is close to linear as a function of IOP. These results show that the modified Rayleigh-Lamb wave model can provide an accurate assessment of corneal viscoelasticity, which could be used for customized CXL therapies.

The Effect of Dehydration and Fasting on Corneal Biomechanical Properties and Intraocular Pressure.

To evaluate the changes in corneal biomechanical properties and intraocular pressure (IOP) during fasting period in healthy subjects. Seventy-two eyes of 72 fasting subjects (study group), and 62 eyes of 62 nonfasting subjects (control group) were enrolled in this prospective study undertaken at a single university hospital. All subjects underwent complete ophthalmologic examination including ocular biomechanical evaluation with ocular response analyzer. Ocular response analyzer measurement was performed on the right eyes of the subjects between 5.00 and 6.00 PM after approximately 14 hr of fasting for the study group and after a nonfasting period for the control group. The corneal hysteresis, corneal resistance factor, mean corneal compensated IOP (IOPcc), and Goldmann-correlated IOP (IOPg) values were measured with a patented dynamic bidirectional applanation process. Goldmann-correlated IOP and IOPcc measurements in the study group and the control group were found as 13.8±2.8 mm Hg, 14.6±2.6 mm Hg and 16.3±2.2 mm Hg, 15.7±2.4 mm Hg, respectively. There was statistically significant difference within the two groups in IOPg and IOPcc (P<0.001). In addition, corneal hysteresis and corneal resistance factor significantly decreased in study group compared with control group (P<0.001, P=0.012, respectively). Prolonged fasting causes a significant decrease in IOPg, IOPcc, corneal hysteresis, and corneal resistance factor in healthy subjects, altering the biomechanical properties of the cornea.

Material Properties from Air Puff Corneal Deformation by Numerical Simulations on Model Corneas

ObjectiveTo validate a new method for reconstructing corneal biomechanical properties from air puff corneal deformation images using hydrogel polymer model corneas and porcine corneas.MethodsAir puff deformation imaging was performed on model eyes with artificial corneas made out of three different hydrogel materials with three different thicknesses and on porcine eyes, at constant intraocular pressure of 15 mmHg. The cornea air puff deformation was modeled using finite elements, and hyperelastic material parameters were determined through inverse modeling, minimizing the difference between the simulated and the measured central deformation amplitude and central-peripheral deformation ratio parameters. Uniaxial tensile tests were performed on the model cornea materials as well as on corneal strips, and the results were compared to stress-strain simulations assuming the reconstructed material parameters.ResultsThe measured and simulated spatial and temporal profiles of the air puff deformation tests were in good agreement (< 7% average discrepancy). The simulated stress-strain curves of the studied hydrogel corneal materials fitted well the experimental stress-strain curves from uniaxial extensiometry, particularly in the 0–0.4 range. Equivalent Young´s moduli of the reconstructed material properties from air-puff were 0.31, 0.58 and 0.48 MPa for the three polymer materials respectively which differed < 1% from those obtained from extensiometry. The simulations of the same material but different thickness resulted in similar reconstructed material properties. The air-puff reconstructed average equivalent Young´s modulus of the porcine corneas was 1.3 MPa, within 18% of that obtained from extensiometry.ConclusionsAir puff corneal deformation imaging with inverse finite element modeling can retrieve material properties of model hydrogel polymer corneas and real corneas, which are in good correspondence with those obtained from uniaxial extensiometry, suggesting that this is a promising technique to retrieve quantitative corneal biomechanical properties.

Topographic, tomographic and biomechanical corneal changes during pregnancy in patients with keratoconus: a cohort study

This study aimed at evaluating the effect of pregnancy on topographic, tomographic and biomechanical parameters of patients with keratoconus (KC) in comparison with non-pregnant patients with KC. In a cohort study, patients with KC, whose disease was stable for at least 2years, were evaluated before pregnancy, at their third trimester of pregnancy (34th week of pregnancy) and 6months after pregnancy. As the control group, an equivalent number of age- and severity-matched non-pregnant female patients with KC were evaluated at the corresponding times. All subjects were evaluated with respect to central and thinnest corneal thickness (CCT and TCT), keratometry values, and maximum anterior and posterior elevation measurements (AE and PE) by Pentacam. Furthermore, corneal biomechanical properties including corneal hysteresis (CH) and corneal resistance factor (CRF) were measured using the ocular response analyser. Twenty-two eyes of 11 patients were equally enrolled in each group. The results revealed that there was a statistically significant difference between the means of all measurements during the course of study (p<0.001, repeated measures analysis of variance). The values of CCT, TCT, CH, CRF, IOPg and IOPcc were significantly decreased; however, keratometry values, AE, PE and refraction measurements were significantly increased during the study period (p<0.001). In contrast, no significant differences were observed regarding the study variables in the control group (p>0.05). According to the Amsler-Krumeich classification, severity of KC increased during the pregnancy and postpregnancy periods (p=0.038). The obtained results revealed that KC progressed during the pregnancy period and continued to the post-partum period, indicating that pregnancy may be a risk factor for KC progression. Patients with KC, who intend to become pregnant, may consider corneal cross-linking treatment in an attempt to stop KC progression.

Effect of femtosecond and microkeratome flaps creation on the cornea biomechanics during laser in situ keratomileusis: one year follow-up.

To compare the corneal biomechanical outcomes at one year after laser in situ keratomileusis (LASIK) with the flaps created by Ziemer and Moria M2 microkeratome with 110 head and -20 blade. Totally 100 eyes of 50 consecutive patients were enrolled in this prospective study and divided into two groups for corneal flaps created by ZiemerFemto LDV and Moria M2 microkeratome with 110 head and -20 blade. Corneal biomechanical properties including cornea resistance factor (CRF) and cornea hysteresis (CH) were measured before and 1, 3, 6, 12mo after surgery by ocular response analyzer. Central cornea thickness and corneal flap thickness were measured by optical coherence tomography. The ablation depth (P=0.693), residual corneal thickness (P=0.453), and postoperative corneal curvature (P=0.264) were not significant different between Ziemer group and Moria 110-20 group after surgery. The residual stromal bed thickness, corneal flap thickness, CH and CRF at 12mo after surgery were significant different between Ziemer group and Moria 110-20 group (P<0.01);Ziemer group gained better corneal biomechanical results. The CRF and CH increased gradually from 1 to 12mo after surgery in Ziemer group, increased from 1 to 6mo but decreased from 6 to 12mo in Moria 110-20 group. Both CRF and CH at one year after surgery increased with the increasing of residual cornea thickness; pre-LASIK CRF, CRF also increased with residual stromal bed thickness, while CH decreased with the increasing of pre-LASIK intraocular pressure and cornea flap thickness (P<0.01). In one year follow-up, femtosecond laser can provide better cornea flaps with stable cornea biomechanics than mechanical microkeratome.

Corneal biomechanical properties in keratoconic and normal eyes

To compare the corneal biomechanical properties of keratoconic and normal eyes, and to explore the changing characteristics in keratoconic eyes. Case-control study. Thirty-two keratoconic subjects(41 eyes)and 41 subjects(41 eyes)with normal corneas were enrolled in this study. Scheimpflug noncontact tonometry(Corvis ST)was performed to measure the length of Appl 1(1st A length), velocity of Appl 1(Vin), length of Appl 2(2nd A length), velocity of Appl 2(Vout), deformation amplitude(DA), peak distance, and radius. Comparison of the biomechanical property values between the keratoconic and normal eyes was performed using One-Way ANOVA. Pearson or Spearman correlations were used to evaluate the relationship between corneal biomechanical properties and corneal morphology parameters. The areas under ROC curves of the biomechanical properties of Corvis ST were calculated. In the keratoconic group, the values of Vin, Vout, and DA were(0.201±0.268),(0.463±0.121), and(1.146±0.113)mm, respectively, which were obviously greater than the normal group[(0.151±0.017),(0.418±0.060), and(1.146 ± 0.113)mm, respectively; F=6.028, 20.724, 10.606, P=0.016, 0.000, 0.001]. The radius in the keratoconic eyes was(5.898 ± 0.976)mm, apparently shorter than the normal eyes[(7.012 ± 0.728)mm; F=5.277, P=0.023]. In the keratoconic eyes, there were significant correlations between the 1st A length, Vout, DA, radius and the thickness of the thinnest cornea(r=0.330,-0.490,-0.482, 0.676, P=0.035, 0.001, 0.001, 0.000). There were significant correlations between the DA, radius and the anterior surface refractive power Km(r=0.751,-0.528, P=0.044, 0.019). The Vin, Vout, DA, and radius had statistically significant correlations with the maximum posterior surface elevation(r=0.475,-0.552,-0.399, 0.273, P=0.021, 0.015, 0.001, 0.000). The areas under ROC curves of the radius, DA, Vout, and Vin were all greater than 0.9. There were obvious changes of corneal biomechanical properties in keratoconic eyes compared with normal eyes. The radius showed a better sensitivity for the change of corneal biomechanical properties and a major reference value for the diagnosis of keratoconus. (Chin J Ophthalmol, 2016, 52:669-673).