Diurnal Variation of Ocular Hysteresis, Corneal Thickness, and Intraocular Pressure

Short- and mid-term changes in CORVIS ST parameters in successful, adult orthokeratology patients

CLINICAL RELEVANCE

The changes in various biomechanical and tomographic characteristics of the cornea associated with orthokeratology may allow us to identify potential mid- and long-term structural alterations, resulting in a better understanding of the governing mechanisms of this procedure and in its optimisation.

BACKGROUND

The study aimed at describing short and mid-term changes in CORVIS ST® parameters and indices in orthokeratology (ortho-k), and their diurnal variations.

METHODS

A prospective observational study was designed in which several CORVIS ST® parameters of 75 new adult participants successfully fitted with overnight ortho-k Seefree® (Conóptica - Hecht Contactlinsen) contact lenses were explored. Measurements were conducted in baseline (BL) conditions and in the morning and evening at the one-night (1 NM/1NT), one-week (1WM/1 WT) and 3-month (3 MM/3MT) follow-up visits.

RESULTS

Statistically significant differences were found in DARatio_2 mm, IntRad, ARTh, CBI and TBI following overnight ortho-k, when compared with BL values, with most values reaching stability at 1WM or reverting to BL values at 3 MM. The ARTh and CBI parameters showed some of the most significant temporal variations (both p < 0.001), probably reflecting the encountered differences in central corneal thickness between BL and 1WM (p = 0.010) and between BL and 3 MM (p = 0.016). In general, corneal rigidity was higher in the morning at all follow-up visits, and decreased during the day. No statistically significant changes in adjusted intraocular pressure values were found.

CONCLUSION

Ortho-k in adults may be considered a safe procedure in terms of short and mid-term changes in CORVIS ST® parameters. The observed alterations in most of the parameters provided by the Corvis ST® probably responded to the well-described changes in corneal pachymetry and tomography, rather than to actual alterations in corneal rigidity.

Corneal Hysteresis for the Diagnosis of Glaucoma and Assessment of Progression Risk: A Report by the American Academy of Ophthalmology

PURPOSE

To review the current published literature on the utility of corneal hysteresis (CH) to assist the clinician in the diagnosis of glaucoma or in the assessment of risk for disease progression in existing glaucoma patients.

METHODS

Searches of the peer-reviewed literature in the PubMed database were performed through July 2022. The abstracts of 423 identified articles were examined to exclude reviews and non-English articles. After inclusion and exclusion criteria were applied, 19 articles were selected, and the panel methodologist rated them for level of evidence. Eight articles were rated level I, and 5 articles were rated level II. The 6 articles rated level III were excluded.

RESULTS

Corneal hysteresis is lower in patients with primary open-angle glaucoma, primary angle-closure glaucoma, pseudoexfoliative glaucoma, and pseudoexfoliation syndrome compared with normal subjects. Interpretation of low CH in patients with high intraocular pressure (IOP) or on topical hypotensive medications is complicated by the influence of these parameters on CH measurements. However, CH is also lower in treatment-naïve, normal-tension glaucoma patients compared with normal subjects who have a similar IOP. In addition, lower CH is associated with an increased risk of progression of glaucoma based on visual fields or structural markers in open-angle glaucoma patients, including those with apparently well-controlled IOP.

CONCLUSIONS

Corneal hysteresis is lower in glaucoma patients compared with normal subjects, and lower CH is associated with an increased risk of disease progression. However, a causal relationship remains to be demonstrated. Nevertheless, measurement of CH complements current structural and functional assessments in determining disease risk in glaucoma suspects and patients.

FINANCIAL DISCLOSURE(S)

Proprietary or commercial disclosure may be found after the references.

Diurnal Variation of Corneal Hysteresis in Patients With Untreated Primary Open Angle Glaucoma and Normal Individuals

PRCIS

Corneal hysteresis in both patients with untreated open angle glaucoma and normal individuals demonstrated significant diurnal variation independent of confounding factors and was higher in the nighttime than in the daytime.

PURPOSE

To investigate diurnal variations in corneal hysteresis (CH) in patients with untreated primary open angle glaucoma (POAG) and normal individuals by using an ocular response analyzer.

MATERIALS AND METHODS

This prospective study included 72 eyes of 53 patients with untreated POAG and 53 eyes of 47 normal individuals. Intraocular pressure (IOP) and CH were measured using Goldmann applanation tonometry (GAT) and ocular response analyzer, respectively, every 3 hours from 9:00 to 24:00. Mixed-effects models were used to determine factors associated with CH values and CH amplitude (maximum values minus minimum values) and to examine the diurnal variations in GAT IOP and CH in each group. Significant differences between time points were defined as significant variations.

RESULTS

The diurnal average GAT IOP and CH in patients with POAG were significantly higher and lower than those in normal individuals ( P =0.001, 0.002). In the multivariate analysis, the larger central corneal thickness was associated with larger CH values in POAG and normal eyes (both P <0.001). A larger amplitude of GAT IOP was significantly associated with a larger CH amplitude in POAG and normal eyes ( P =0.010, 0.013). CH, in both groups, showed similar significant diurnal variation and was higher in the nighttime than in the daytime, even after adjusting for confounding factors, while IOP showed an antiphase pattern.

CONCLUSION

CH in both untreated POAG patients and normal participants demonstrated similar diurnal variations, that is, higher at night, independent of confounding factors. These findings suggest that viscoelastic properties of the cornea may fluctuate diurnally independent of IOP.

Corneal Behavior during Tonometer Measurement during the Water Drinking Test in Eyes with XEN GelStent in Comparison to Non-Implanted Eyes

Biomechanics of the cornea have significant influences on the non-contact measurement of the intraocular pressure. The corneal behaviour during tonometry is a fundamental factor in estimating its value. The aim of the study was to analyse the behaviour of the cornea during tonometric measurement with the forced change in intraocular pressure during the water drinking test. Ocular Response Analyser (Reichert) was used to the measurement. Besides four basic parameters connected with intraocular pressure (IOPg, IOPcc) and biomechanics (corneal hysteresis CH and corneal resistance factor (CRF), other parameters representing the behaviour of the cornea during a puff of air were analysed. There were 47 eyes included in the study, including 27 eyes with a XEN GelStent implanted and 20 without it. The eyes of people with monocular implementation were the reference group. The values of analysed parameters were compared before and after 10, 25, 40, and 55 min after drinking the water. The intraocular pressure increased by 2.4 mmHg (p < 0.05) for eyes with a XEN stent and 2.2 mmHg for eyes without a stent (p < 0.05) in the tenth minute after drinking of water. This change caused a decreasing of corneal hysteresis (p < 0.05) without significant changes in the corneal resistance factor (p > 0.05). Corneal hysteresis changed similarly in the reference group and the group with a XEN GelStent. The analysis of additional parameters showed a difference in the behaviour of the cornea in eyes with a XEN GelStent in comparison to the corneas of eyes without a stent. This was particularly visible in the analysis of the cornea’s behaviour during the second applanation, when the cornea returns to its baseline state after deformation caused by air puff tonometry.

Effects of Medium-Term Soft Contact Lens Fitting on Dry Eye: Analyses Using Ultra-High Resolution Optical Coherence Tomography and Digital Slit-Lamp Biomicroscopy

Purpose

This study aimed to evaluate the medium-term fit of soft contact lenses (SCLs) and its effects on dry eye by using ultra-high-resolution optical coherence tomography (UHR-OCT) and digital slit-lamp biomicroscopy (DSLB) and identify factors indicating a better contact lens fit.

Methods

A total of 14 participants (7 men, 7 women; mean age, 23.1 years) were recruited. Temporal lag and conjunctival coverage were imaged by UHR-OCT. DSLB was used to assess lens decentration and blink-induced movement. In addition, dry eye indices, including ocular surface disease index (OSDI), tear break-up time (TBUT), and Schirmer I test scores, were measured at baseline and 1 day, 1 week, 1 month, 3 months, and 6 months after wearing.

Results

Good repeatability was found in the assessments of lens fit characteristics by two researchers and in the assessments by one researcher at different follow-up times. There were no significant differences in lens fit characteristics, TBUT and Schirmer I test scores at each checkpoint (P >0.05). However, the OSDI was significantly higher than baseline after wearing (P <0.05). Changes in the temporal lag were positively correlated with changes in TBUT (r =0.544, P =0.044) and negatively correlated with changes in OSDI (r = -0.629, P =0.016). Changes in blink-induced movement were positively correlated with changes in OSDI, Schirmer I test scores, and TBUT (P <0.05).

Conclusions

UHR-OCT and DSLB can be used to comprehensively and quantitatively evaluate SCL fit. Both the contact lenses and ocular surface conditions are almost stable within 6 months of SCL wear. Temporal lag and blink-induced movement have a strong correlation with dry eye parameters in SCL wearers.

Effects of warm compress on tear film, blink pattern and Meibomian gland function in dry eyes after corneal refractive surgery

Background

To assess the effects of warm compress (WC) on tear film lipid layer, blink pattern and Meibomian gland function in patients with dry eye following femtosecond laser small incision lenticule extraction (SMILE) and laser-assisted subepithelial keratomileusis (LASEK).

Methods

We enrolled 37 eyes of 37 participants, each with dry eye for more than 2 years following SMILE (25 eyes) or LASEK (12 eyes). WC was performed using a spontaneously heating eye mask. Tear film break-up time (TBUT), tear film lipid layer thickness (TFLLT), blink pattern, Meibomian secretory function scores (MGS), visual acuity, spherical equivalent (SE), keratometry, central corneal thickness (CCT) and aberration were assessed before and after WC.

Results

After WC, the following mean values all increased relative to baselines: CCT, SE, minimum (Min-), maximum (Max-) and average (Ave-) TFLLT, TBUT, total MGS (TMGS), number of glands secreting any liquid (MGL), and complete blink rate (CBR) (p values ranging from < 0.001 to 0.042). Partial blink frequency (PBF) and partial blink rate (PBR) decreased (p = 0.002 in both cases). The decrease of PBF was higher in SMILE subgroup than in LASEK (p = 0.030). TBUT variation was positively correlated with that of Ave-TFLLT and TMGS (p = 0.046, 0.028, respectively). Max-TFLLT variation was correlated with that of TMGS (p = 0.020).

Conclusions

WC may temporarily increase tear film thickness and stability, decrease partial blink, and partly augment Meibomian gland function in dry eye patients after corneal refractive surgeries. Future studies are required to investigate long term clinical efficacy and safety.

Corneal Biomechanical Changes Caused by Acute Elevation of IOP in Eyes with and without Glaucoma

SIGNIFICANCE

Although corneal biomechanical parameters are well linked with glaucoma, their clinical utility has not yet been fully elucidated. This study was designed to provide unique evidence about the dynamic nature of corneal biomechanical parameters and their potential prognostic ability for glaucoma.

PURPOSE

This study aimed to evaluate the effect of acute intraocular pressure (IOP) elevation on corneal hysteresis (CH) and corneal resistance factor (CRF) and the associations of these biomechanical parameters with glaucomatous disease.

METHODS

Subjects participating in a prospective, longitudinal glaucoma research study had CH and CRF measured before and during ophthalmodynamometry during visits in the years 2011 to 2012. All participants were diagnosed with primary open-angle glaucoma, ocular hypertension, glaucoma suspect, or normal eyes and had a minimum of 3 years of study participation with at least five reliable visual field (VF) tests. Changes in CH, CRF, and IOP induced by ophthalmodynamometry were compared between diagnostic groups and evaluated for relationships with existing and future glaucomatous VF loss.

RESULTS

In 248 eyes of 248 subjects followed up for 7.7 ± 2.3 years, ophthalmodynamometry induced a mean IOP increase from 15.1 to 29.9 mmHg, causing a mean 34 ± 28% increase in CRF and 21 ± 25% decrease in CH. Magnitude of CH change did not differ between diagnostic groups or between eyes that did (n = 20) and did not (n = 95) develop new VF loss during the study period, nor was it related to rate of future VF progression.

CONCLUSIONS

Ophthalmodynamometry-induced IOP elevation resulted in significant acute changes in CH and CRF in this study; this suggests accounting for IOP may be important in clinical interpretation of these parameters. However, because the degree of CH change was not related to glaucoma or its progression, acute changes in CH and CRF do not seem to have a prognostic value for glaucoma.

New applanation tonometer for myopic patients after laser refractive surgery

This study assesses the agreement between intraocular pressure (IOP) measurements taken with the Goldmann applanation tonometer (GAT) and a new experimental applanation tonometer with a convexly shaped apex (CT) after laser myopic refractive surgery. Two different CT radii (CT1 and CT2) were designed with a finite element analyser, and a prospective double masked study on 102 eyes from 102 patients was carried out. A Bland-Altman plot and intra-class correlation coefficient (ICC) were calculated to assess the agreement between GAT measurements and the measurements of both CT1 and CT2 before and after myopic laser assisted in situ keratomileusis (LASIK; n = 73) and photorefractive keratectomy (PRK; n = 29). We evaluated a subset of two subgroups (n = 36 each) for intra and inter-observer (IA/IE) error. From the whole cohort, the best IOP agreement was observed between GATpre and CT1post surgery: 16.09 ± 2.92 vs 16.42 ± 2.87 (p < 0.001); ICC = 0.675 (95% CI: 0.554-0.768). In the analysis of LASIK vs PRK, GATpre and CT1post showed the highest agreement, although LASIK measurements were more accurate than PRK, as the ICC = 0.718 (95% CI: 0.594-0.812) and ICC = 0.578 (95% CI: 0.182-0.795) respectively. Excellent agreement was observed for IA/IE, and there was an ICC > 0.8 (95% CI) in all cases. CT1 proved more accurate in the LASIK subgroup. In conclusion, our new version of GAT could be used with post-surgery LASIK patients as a more accurate measurement device compared to the current reference tonometer.

Diurnal variation of anterior segment parameters handled with Scheimpflug imaging in keratoconus patients

PURPOSE

To define diurnal changes in anterior segment parameters of keratoconus patients by using Scheimpflug imaging.

METHODS

All keratoconus patients had corneal topography measurements 3 times a day (around 09:00 AM, 13:00 PM and 17:00 PM) by the same experienced operator. Three consecutive scans in each measurement session were obtained. The average of three measurements was used for analysis. A repeated measures analysis of variance with one within-subject factor (time of day) was carried out to assess diurnal variation.

RESULTS

This study included 26 eyes of 26 patients with keratoconus. Significant diurnal variation was found in CCT and TCT measurements (P < 0.01, and P < 0.01, respectively). The mean amplitudes of change in CCT and TCT were 4.2 ± 1 µm (95% CI: 1.7-6.8 µm) and 4.1 ± 1.1 µm (95% CI: 1.2-8.0 µm), while the mean of CCT and TCT were 462.4 ± 34.5 µm (95% CI: 448.4-476.6 µm) and 452.9 ± 6.6 µm (95% CI: 439.3-466.5 µm), respectively. The maximum value and the lowest value were observed at 09:00 AM and at 17:00 PM, respectively. Other parameters except CCT and TCT did not show diurnal variation.

CONCLUSION

Statistically significant but clinically insignificant diurnal variation in CCT and TCT measurements of the keratoconus patients were detected over an 8-hour period (09:00 AM-17:00 PM).

Intraocular Pressure Based on Dynamic Bidirectional Applanation and Air-puff Tonometry: A Comparative Study

Aim

The aim of this study is to compare intraocular pressure (IOP) in healthy eyes measured via dynamic bidirectional applanation and air-puff tonometry, and to evaluate the effect of age, gender, the spherical equivalent (SE), mean keratometry (Kmean), corneal hysteresis (CH), and the corneal resistance factor (CRF) on IOP measurements.

Materials and methods

IOP measured in 956 healthy eyes using the ocular response analyzer (ORA) and air-puff tonometer was compared in this cross-sectional retrospective study. Correlations between IOP, and age, gender, SE, Kmean, CH, and CRF were investigated using univariate and multivariate analyses. Bland–Altman plots were used to determine the level of agreement between the IOP values obtained with the two devices: IOP via air-puff tonometer (IOP-air-puff) and corneal-compensated IOP (IOPcc) using ORA (ORA-IOPcc). Linear mixed modeling was used to evaluate the effects of the study parameters on IOP. Intermethod reliability was established by calculating intraclass correlation coefficients (r).

Results

The mean age of the patients was 39.56 ± 14.44 years. The mean IOPair-puff and ORA-IOPcc were 16.72 ± 2.37 mm Hg and 13.75 ± 3.12 mm Hg, respectively (p < 0.001). The mean CH and CRF were 11.14 ± 1.61 mm Hg and 10.53 ± 1.65 mm Hg, respectively. Multivariate analysis showed that both CH and CRF were significantly correlated with IOP-air-puff and ORA-IOPcc (p < 0.001). The 95% limit of agreement for IOP-air-puff and IOPcc was −2.843 to 8.784. There was a significant correlation between IOP-air-puff and ORA-IOPcc (r = 0.443, p < 0.001); the R² value was 0.196.

Conclusion

A low degree of agreement was noted between IOP-air-puff and ORA-IOPcc. The present findings show that air-puff tonometry overestimates IOP, as compared to ORA.

Clinical significance

IOP based on air-puff tonometry must be interpreted in conjunction with other ophthalmologic findings and the same IOP measuring device should be used for follow-up evaluations.

How to cite this article

Bahadir Kilavuzoglu AE, Cosar CB, et al. Intraocular Pressure Based on Dynamic Bidirectional Applanation and Air-puff Tonometry: A Comparative Study. J Curr Glaucoma Pract 2019;13(2):68–73.

Diurnal variation of central corneal thickness and intraocular pressure in eyes with pseudoexfoliation

Purpose:

Pseudoexfoliation syndrome (PXS) is an age related microfibrillopathy characterized by deposition of whitish flaky material over various ocular tissues. PXS eyes are known to have thinner corneas and thus can lead to an underestimation in intraocular pressure measurement. The purpose of this study was to find if there is any variation in central corneal thickness and intraocular pressure in PXS eyes and if there was any relationship between them.

Methods:

A prospective observational study was done on 141 eyes of 85 patients with PXS without glaucoma between November 2015 to April 2017 in the department of Ophthalmology in a tertiary hospital. CCT and IOP were measured by a handheld ultrasonic pachymeter (DGH Technology INC. Pachette 2, USA) and Goldmann applanation tonometer respectively at 4 different times during office hours.

Results:

A significant reduction of about 10 μm in mean CCT and 1.4 mmHg in mean IOP was noted over the 4 sessions which was statistically significant (P < 0.001). A significant correlation exists between IOP and CCT in PXS eyes at all times during the day (P < 0.001).

Conclusion:

The CCT measurements show significant thinning throughout the day, similar drop in IOP was also noted. Our study shows that there is a significant correlation between diurnal variation of CCT and IOP. Hence, it is prudent to measure CCT along with IOP at all times.

Estrogen- and Progesterone-Induced Variation in Corneal Parameters According to Hormonal Status

OBJECTIVES

To determine the effects of estrogen and progesterone on corneal thickness, curvature, and biomechanics in healthy corneas according to hormonal status.

METHODS

The study included four groups of females: group A (menstruating, not pregnant, not lactating, and not menopausal; n=100), group B (pregnant; n=50), group C (lactating; n=50), and group D (menopausal; n=50). Group A was subdivided according to age, as subgroup A15-25 (age 15-25 years) and subgroup A>25 (age >25 years). Blood estradiol and progesterone levels were measured in each participant. All the participants underwent a full ophthalmologic examination, including corneal thickness and corneal topography measurement, and evaluation of corneal biomechanical properties.

RESULTS

The corneal resistance factor and anterior corneal flat keratometry values were significantly higher in group D (P=0.040 and P=0.026, respectively) than in the other three groups. Posterior corneal steep keratometry values were significantly higher in subgroup A>25 during the preovulatory phase than ovulatory and postovulatory phases (P=0.012). In group B, there was a significant negative correlation between gestational week and intraocular pressure (IOP) (r=-0.322, P=0.024). Corneal volume was significantly higher during the early postpartum period than the late postpartum period in group C (P=0.028). Intraocular pressure, Goldman-correlated IOP, and corneal-compensated IOP differed significantly between the groups (P<0.05).

CONCLUSIONS

Blood levels of estrogen and progesterone were associated with variations in IOP, but estrogen and progesterone did not have a consistent effect on topographic parameters or biomechanical properties in healthy corneas.

Changes in Corneal Biomechanics and Intraocular Pressure Following Cataract Surgery

PURPOSE

To investigate the effects of cataract surgery on corneal biomechanics and intraocular pressure (IOP) measured with the updated Corvis ST tonometer (CST).

DESIGN

Prospective, interventional case series study.

METHODS

This study included 39 eyes of 39 cataract patients. CST measurements were performed at presurgery (Pre) as well as 1 week (1W), 1 month (1M), and 3 months (3M) postsurgery. The following CST parameters were recorded: deformation amplitude max (DA max), DA ratio max 1 mm and 2 mm, integrated radius, stiffness parameter at applanation 1 (SP A1), Ambrosio relational thickness to the horizontal profile (ARTh), Corvis biomechanical index (CBI), central corneal thickness (CCT), noncorrected intraocular pressure (IOPnct), and biomechanically corrected IOP (bIOP). IOP was also measured with Goldmann applanation tonometry and the noncontact tonometer CT-90A. All measurements were compared at each period using the linear mixed model, with and without adjustment for bIOP and CCT.

RESULTS

All IOP measurements decreased over time (P < .01). CCT was increased at 1W and 3M (P < .01), whereas ARTh was decreased at 1W and 1M (P < .01), but returned to its Pre level at 3M. DA max and Integrated radius were increased at 3M (P < .01), whereas SP A1 was decreased at 3M (P < .01). CBI was increased at 1W (P < .01), but returned to its Pre level at 1M.

CONCLUSIONS

IOP and Corneal biomechanical properties are changed after cataract surgery. In particular, SP A1 decreases while DA max and integrated radius increase, even at 3M, suggesting a less stiff cornea.

Characterization of Soft Contact Lens Edge Fitting during Daily Wear Using Ultrahigh-Resolution Optical Coherence Tomography

Purpose

To determine conjunctival overlap over the edge of soft contact lens and to visualize the peripheral postlens tear film (PoLTF) underneath soft contact lenses using ultrahigh-resolution optical coherence tomography (UHR-OCT).

Methods

Twenty participants (4 males and 16 females, 23.0 ± 3.7 years) were fitted with two different types of soft contact lenses randomly. The limbus with lens was imaged with the UHR-OCT at the horizontal meridian every two hours up to 6 hours during lens wear. The conjunctival overlap was ranked as the percentage of the edge covered by the conjunctiva. The frequency of occurrence for visualized peripheral PoLTF was determined.

Results

The average conjunctival overlaps at insertion were 49% and 73% for galyfilcon A and balafilcon A lenses and increased significantly to 84% and 90% by 6 hours of lens wear (P < 0.001). Lenses with rounded edges had more conjunctival overlap than the lenses with angled edges (P=0.014). There were significant decreases for PoLTF on the conjunctiva (P=0.014) and peripheral cornea (P=0.004) over the study period compared to insertion. The percentage of subjects with PoLTF on the conjunctiva (32.5%) and peripheral cornea (36%) were greater in subjects wearing balafilcon A lenses (P=0.017).

Conclusions

Increased conjunctival overlap over the lens edges and reduced PoLTF underneath the peripheral region of soft contact lenses were shown during lens daily wear. The lens edge configuration may play a role in conjunctival response and peripheral PoLTF.

Corneal hysteresis and corneal resistance factor in pellucid marginal degeneration

Purpose

To evaluate and compare corneal hysteresis (CH) and corneal resistance factor (CRF) in pellucid marginal degeneration (PMD), keratoconus (KCN), and normal eyes using the Ocular Response Analyzer (ORA).

Methods

In this retrospective study, corneal biomechanical parameters were measured in patients with PMD (n = 102) and KCN (n = 202) and normal subjects (n = 208) using the ORA. Data, including full patient history as well as the results of refraction, slit-lamp biomicroscopy, Pentacam HR (Oculus), and ORA (Reichert; Buffalo, New York, USA), were collected from medical records. Also, the data of only one eye per individual were selected for the analysis. The inclusion criteria for PMD and KCN groups were a reliable diagnosis of these ectatic disorders based on the clinical and corneal tomographic findings. CH, CRF, CH–CRF, intraocular pressure (IOP) measurements were assessed for each subject. Data were analyzed with SPSS and MedCalc using the ANOVA, Pearson Correlation, and receiver operating characteristic (ROC) curve analysis.

Results

The mean CH was 8.91 mmHg ± 1.05 [standard deviation (SD)], 8.43 ± 0.78, and 10.89 ± 1.08 in the PMD, KCN, and normal group, respectively. Also, the mean CRF was 8.21 ± 1.35, 7.19 ± 1.11, and 10.69 ± 1.41 in the PMD, KCN, and normal group, respectively. ANOVA showed differences in the mean CH, CRF, and CH–CRF between three groups (P < 0.001). Also, ROC curve analysis showed the cut-off points ≤9.5, ≤9.5, and >1.3 mmHg for CH, CRF, and CH–CRF in the PMD group, respectively. For biomechanical parameters in PMD eyes, CRF had the highest sensitivity (75.49%) while the greatest area under the ROC curve (AUC) was seen for CH (0.903). Moreover, central corneal thickness (CCT) showed no correlation with CH (P = 0.30, r = −0.104) or CRF (P = 0.75, r = 0.033) in the PMD group.

Conclusions

This study presented the values of corneal biomechanics for PMD using the ORA. The results of the ORA were markedly different between PMD, KCN, and normal eyes.

Characterization of Soft Contact Lens Fitting Using Ultra-Long Scan Depth Optical Coherence Tomography

Objectives

To evaluate the centration and movement of soft contact lenses and to verify the repeatability of two repeated measurements of the lens centration and movement using ultra-long scan depth optical coherence tomography (UL-OCT).

Methods

A 1-day Acuvue® Define™ lens was tested on both eyes of 10 subjects (5 males and 5 females; mean age, 31.6 years). The centration and blink-induced movement of the contact lens were measured using UL-OCT at 5 min and 30 min after insertion. The measurements were repeated once at each checkpoint.

Results

Good repeatability was found in the lens centration and movement between the two repeated measurements at either checkpoint. The values of the lens movement were 0.457 ± 0.248 mm and 0.402 ± 0.229 mm at 5 min and decreased to 0.197 ± 0.065 mm and 0.211 ± 0.110 mm at 30 min after insertion for the right and left eyes, respectively (P < 0.05).

Conclusions

The custom-built UL-OCT presented good repeatability of centration and movement in Define lenses at 5 min and 30 min after insertion. Most of the lenses were centered temporal and inferior to the cornea during the first 30 min wearing period. Compared with 5 min after insertion, the lens was centered better and exhibited less movement at 30 min.

High intercorneal symmetry in corneal biomechanical metrics

Backgroud

To evaluate the symmetry of corneal biomechanical metrics, measured using an ocular response analyzer (ORA) and self-built corneal inflation test platform, in bilateral rabbit corneas and to investigate their relationship with physical intraocular pressure (IOPp).

Methods

Twenty fresh enucleated eyes from ten rabbits were used for ex vivo whole ocular globe inflation. IOP was increased from 7.5 to 37.5 mmHg with 7.5 mmHg steps and biomechanical metrics were acquired using the ORA. At least 3 examinations were performed at each pressure stage. Two biomechanical metrics, corneal hysteresis (CH) and corneal resistance factor (CRF) were recorded and analyzed as a function of IOPp. Corneal specimens were then excised from the intact ocular globe and tested under inflation conditions up to 45.7 mmHg posterior pressure. The experimental pressure-deformation data was analyzed using an inverse modeling procedure to derive the stress-strain behavior of the cornea.

Results

A comparison of corneal shape parameters showed no statistically significant difference (P > 0.05) between bilateral eyes. Similarly, there were no statistically significant differences in values of CH, CRF and corneal stiffness (as measured by the tangent modulus, Et) between bilateral eyes (CH: F = 0.94, P = 0.54; CRF: F = 4.42, P = 0.35; Et: F = 3.15, P = 0.12) at different pressure levels. IOPp was highly correlated with CRF while the relationship with CH was less pronounced.

Conclusions

An obvious interocular symmetry in biomechanical metrics is found in this research. IOP has been shown to have important influences on the value of CRF provided by ORA.

Optical coherence tomography to evaluate the interaction of different edge designs of four different silicone hydrogel lenses with the ocular surface

Purpose

To evaluate the lens edge interaction with the ocular surface with different edge designs using optical coherence tomography and to examine the effect of lens power on the lens edge interactions.

Methods

Four types of silicone hydrogel lenses with different edge designs (round-, semi-round-, chisel-, and knife-edged) at six different powers (+5.0, +3.0, +1.0, −1.0, −3.0, and −5.0 diopters) were fitted to both eyes of 20 healthy volunteers. Optical coherence tomography images were taken at the corneal center and at the limbus within 15–30 minutes after insertion. The images were evaluated with respect to two parameters: conjunctival indentation exerted by the lens edge; and the tear film gaps between the posterior surface of the lens and the ocular surface. The amount of conjunctival indentation was measured with the distortion angle of the conjunctiva at the lens edge.

Results

The degree of conjunctival indentation was highest with the chisel-edged design followed by the semi-round design (P<0.0001). Knife- and round-edged lenses exerted similar levels of conjunctival indentation that was significantly lower compared to chisel-edged lens (P<0.001). For each one of the tested lens edge designs, no significant difference was observed in the conjunctival indentation with respect to lens power. The chisel-edged lens produced the highest amount of conjunctival indentation for each one of the six lens powers (P<0.0001). Post-lens tear film gaps at the limbus were observed at most in the round-edge design (P=0.001).

Conclusion

The fitting properties of contact lenses may be influenced by their edge design but not by their lens power.

Corneal biomechanics: a review

Biomechanics is often defined as 'mechanics applied to biology'. Due to the variety and complexity of the behaviour of biological structures and materials, biomechanics is better defined as the development, extension and application of mechanics for a better understanding of physiology and physiopathology and consequently for a better diagnosis and treatment of disease and injury. Different methods for the characterisation of corneal biomechanics are reviewed in detail, including those that are currently commercially available (Ocular Response Analyzer and CorVis ST). The clinical applicability of the parameters provided by these devices are discussed, especially in the fields of glaucoma, detection of ectatic disorders and orthokeratology. Likewise, other methods are also reviewed, such as Brillouin microscopy or dynamic optical coherence tomography and others with potential application to clinical practice but not validated for in vivo measurements, such as ultrasonic elastography. Advantages and disadvantages of all these techniques are described. Finally, the concept of biomechanical modelling is revised as well as the requirements for developing biomechanical models, with special emphasis on finite element modelling.

Analysis of seasonal changes in residual refraction 1-year after corneal laser refractive surgery: a retrospective study

PURPOSE

To analyze the effect of seasonal changes in residual refraction 1-year after corneal refractive surgery using the SCHWIND AMARIS laser system.

METHODS

5740 consecutive treatments have been retrospectively reviewed. For all eyes, aspheric treatments were planned with the Custom Ablation Manager software and the ablations were performed with the SCHWIND AMARIS system (SCHWIND eye-tech-solutions). Seasonal outcomes were evaluated in terms of residual refraction stratified per treatment month, as well as stratified per year season. Student's T test comparing stratified values with global ones was used for the statistical analysis.

RESULTS

Treatments performed in April, June, August, September, and October showed relative undercorrections of the spherical equivalent (SE) (-0.09D), whereas treatments performed in January, February, and March showed relative overcorrections of the SE (+0.13D). Similarly, treatments performed in spring and summer showed relative undercorrections of the SE (-0.04D), whereas treatments performed in winter showed relative overcorrections of the SE (+0.10D).

CONCLUSIONS

Seasonal differences in refractive outcomes were observed among a large scale population. The effect of these environmental variables on refractive outcomes warrants further evaluation.

Diurnal variation in central corneal thickness and intraocular pressure in eyes with pseudoexfoliation syndrome without glaucoma

Aim:

The aim was to ascertain if any differences exist in diurnal central corneal thickness (CCT) and intra-ocular pressure (IOP) between eyes with pseudoexfoliation (PXF) syndrome without glaucoma and eyes with no ocular pathology. A secondary aim was to determine whether there was a significant relationship between CCT and IOP.

Settings and Design:

This study was a prospective design conducted within a hospital setting.

Materials and Methods:

The experimental group consisted of seven participants with bilateral PXF (14 eyes) and the control group comprised of 15 participants (30 eyes). Testing included CCT and IOP measured at four different times on one given day (8.00 a.m.; 11 a.m.; 2 p.m. and 5 p.m.).

Statistical Analysis:

The data were analyzed with the generalized linear latent mixed model.

Results:

PXF eyes displayed a significantly thinner overall mean CCT (520 μm) compared to controls (530 μm). Furthermore, a significant reduction in CCT and IOP occurred in the PXF group from 8 a.m. to 5 p.m. The mean overall IOP in PXF eyes was significantly lower than the control group. A significant association between IOP and CCT was also found in PXF eyes.

Conclusions:

Displaying a significantly thinner mean CCT highlights the importance of measuring CCT in an ophthalmic clinical setting as to avoid falsely underestimated IOP measurements in such a high-risk glaucoma population. Furthermore, a statistically significant correlation between IOP and CCT in PXF eyes suggests that the reduction in CCT that occurred in PXF eyes between 8 a.m. and 5 p.m. may be partly responsible for the reduction in IOP measurements.

Corneal Biomechanical Properties and Anterior Segment Parameters in Forme Fruste Keratoconus

Purpose.

To evaluate the sensitivity and specificity of corneal biomechanical metrics, anterior segment data, and a combination model in differentiating forme fruste keratoconus (FFK) from normal corneas.

Methods.

A total of 50 FFK eyes were identified by calculation of the KISA index and recruited FFK group. Results were compared with 50 normal eyes (NG group) randomly selected from 50 patients. The following parameters were evaluated for their diagnostic capacity by evaluation of their receiver operating characteristic curves (ROC): corneal hysteresis (CH), corneal resistance factor (CRF), corneal astigmatism (Cyl), anterior chamber depth (ACD), corneal volume (CV) at 3 mm (CV3) and at 5 mm (CV5), maximum posterior elevation value (PEL), central corneal thickness (CCT), thinnest corneal thickness (TCT) and its coordinates (TCT x, TCT y), the ratio TCT/CCT, pachymetric progression indexes (PPImin, PPIavg, and PPImax), and Ambrósio's relational thickness (ARTmin, ARTavg, and ARTmax). Logistic regression was attempted for identification of a combined diagnostic model.

Results.

Significant differences were detected in all studied parameters except the Cyl, ACD, TCT x, and CV. Among individual parameters, the highest predictive accuracy was for ARTavg (area under the curve [AUC] 95.4%, sensitivity 90%, specificity 88.9%) and TCT (AUC 95.3%, sensitivity 90.9%, specificity 89%). Sufficient predictive accuracy (AUC 99.4%, sensitivity 98.8%, specificity 94.6%) was identified in a diagnostic model that combined the CRF, ARTavg, and PEL parameters.

Conclusions.

None of the individual parameters provide sufficient diagnostic capacity in FFK. However, diagnostic models that combine biomechanical and tomographic data seem to provide high accuracy in differentiating FFK from normal corneas.

Micrometer-scale contact lens movements imaged by ultrahigh-resolution optical coherence tomography

PURPOSE

To dynamically evaluate contact lens movement and ocular surface shape using ultrahigh-resolution and ultralong-scan-depth optical coherence tomography (OCT).

DESIGN

Clinical research study of a laboratory technique.

METHODS

Four different types of soft contact lenses were tested on the left eye of 10 subjects (6 male and 4 female). Lens edges at primary gaze and temporal and nasal gazes were imaged by ultrahigh-resolution OCT. Excursion lag was obtained as the distance between the lens edge at primary gaze and immediately after the eye was quickly turned either nasally or temporally. The inferior lens edges were imaged continuously to track vertical movements during blinking. Ultralong-scan-depth OCT provided quantifiable images of the ocular surface, and the contour was acquired using custom software.

RESULTS

Excursion lag at the horizontal meridian was 366 ± 134 μm at temporal gaze and 320 ± 137 μm at nasal gaze (P > .05). The lens uplift at the vertical meridian was 342 ± 155 μm after blinking. There were significant differences in horizontal lags and vertical movements among different lenses (P < .05). Horizontal lags were correlated with radii of curvatures and sagittal heights at 6-mm and 14-mm horizontal meridian (P < .05). The blink-induced lens uplift first lowered by 104 ± 8 μm, and then lifted 342 ± 155 μm after the blink.

CONCLUSIONS

Ultrahigh-resolution and ultralong-scan-depth OCT can assess micrometer-scale lens movements and ocular surface contours. Both lens design and ocular surface shape affected lens movements.

Characterization of soft contact lens edge fitting using ultra-high resolution and ultra-long scan depth optical coherence tomography

PURPOSE

To characterize the edge fitting of soft contact lenses using ultra-high resolution optical coherence tomography (UHR-OCT) and ultra-long scan depth optical coherence tomography (UL-OCT).

METHODS

A total of 20 participants (11 men, 9 women; mean age, 32.3 years) were recruited. Four different types of soft contact lenses were randomly fitted to both eyes of each subject on two separate visits. After 30 minutes, the horizontal meridians of the corneal center, midperiphery, and limbus were imaged by UHR-OCT. UL-OCT imaged each lens in vitro and the ocular surface of a physical model eye.

RESULTS

Angle-edged lenses had significantly less conjunctival buildup than did round-edged lenses (P = 0.008). Limbal post-lens tear film gaps were present in 42% of the eyes, with the round-edged lenses having the most at 68%. Similarly, post-lens tear film gaps at the corneal mid-periphery were present in 47% of all eyes, with the round-edged lens having the most at 75%. Mismatches between the lens and the ocular surface were simulated based on UL-OCT images of the in vitro lenses and the model eye. The existence of tear film gaps and touching points were predicted in the simulation.

CONCLUSIONS

The soft contact lens edge fitting was characterized by the conjunctival buildup and tear film gaps. Different types of contact lenses presented different levels of conjunctival buildup as well as different frequencies of tear film gaps. The findings by UHR-OCT were predicted in the simulation by UL-OCT. The application of these new technologies may open new ways of designing lenses and evaluating their fit.

Ocular response analyzer measurements in keratoconus with normal central corneal thickness compared with matched normal control eyes

PURPOSE

To compare corneal hysteresis (CH) and corneal resistance factor (CRF) in eyes with keratoconus with a central corneal thickness (CCT) ≥ 520 μm with CH and CRF in matched controls, and to estimate the sensitivity and specificity of these parameters for discriminating between the two groups.

METHODS

This prospective, comparative case series comprised 19 eyes of 19 patients with keratoconus with CCT ≥ 520 μm and 19 eyes of 19 healthy sex-, age-, and CCT-matched patients who underwent a complete clinical eye examination, corneal topography, tomography, and biomechanical evaluation. The receiver operating characteristic (ROC) curve was used to identify cutoff points that maximized the sensitivity and specificity for discriminating between groups.

RESULTS

Central corneal thickness was 543.1 ± 13.9 μm (range: 520 to 568 μm) in the keratoconus group and 545 ± 12.5 μm (range: 527 to 575 μm) in the control group (P=.6017). Corneal hysteresis was 9.22 ± 1.44 mmHg (range: 6.2 to 11.35 mmHg) in the keratoconus group and 10.58 ± 1.91 mmHg (range: 7.34 to 13.53 mmHg) in the control group (P=.0075). Corneal resistance factor was 8.62 ± 1.52 mmHg (range: 5.60 to 11.20 mmHg) in the keratoconus group and 10.30 ± 1.92 mmHg (range: 6.95 to 14.12 mmHg) in the control group (P=.0049). The ROC curve analyses showed a poor overall predictive accuracy of CH (cutoff, 9.90 mmHg; sensitivity, 78.9%; specificity, 63.2%; test accuracy, 71.05%) and CRF (cutoff, 8.90 mmHg; sensitivity, 68.4%; specificity, 78.9%; test accuracy, 73.65%) for detecting keratoconus in the eyes studied.

CONCLUSIONS

Corneal hysteresis and CRF were statistically lower in the keratoconus group compared with the control group. Given the large overlap, both CH and CRF had low sensitivity and specificity for discriminating between groups.

Ultrahigh-resolution measurement by optical coherence tomography of dynamic tear film changes on contact lenses

Purpose. To determine the dynamic pre- and postlens tear film (PLTF and PoLTF) thicknesses by using optical coherence tomography (OCT). Methods. Ultrahigh-resolution OCT was used to image the tear film of 22 subjects before and after contact lens wear. A soft lens with 1 drop of artificial tears on its concave surface was inserted onto one randomly selected eye. OCT images were taken before insertion, immediately afterward, and every 2 minutes for 10 minutes. For the contralateral eye, the lens inserted was not prewetted on the concave surface. OCT images were taken before insertion, immediately afterward, and at 3 minutes. Then another drop was instilled, and images were taken immediately afterward and every 2 minutes for 10 minutes. Images were processed by custom software to yield tear film thickness. Results. The thickness of precorneal tear film (PCTF) was 1.9 +/- 0.9 mum. The PoLTF was visualized clearly in all cases immediately after lens insertion, with 1 drop on the lens concave surface. Through the first 6 minutes after insertion, the PoLTF was greater than the PCTF. The PLTF (n = 12) and PoLTF (n = 9) were visualized immediately after lens insertion. After 3 minutes, the PLTF in most subjects and PoLTF in all subjects were invisible. The thickness of the PLTF increased after the instillation of artificial tears, whereas the PoLTF did not at any checkpoint for 10 minutes. Conclusions. Ultrahigh-resolution OCT is a promising tool for measuring the tear film. The PoLTF did not increase after instillation of artificial tears.

[Biomechanical condition of the cornea as a new indicator for pathological and structural changes]

AIM

Several methods permit the measurement of geometric parameters of the cornea, but until now biomechanical conditions of the cornea have been ignored (e.g. in refractive corneal surgery). Besides the geometric condition, biomechanical properties of the cornea have been shown to influence applanation measurement of intra-ocular pressure (IOP) and epidemiological studies have identified corneal thickness as an independent risk factor for the development and progression of glaucoma. The aim of this investigation was to characterize the biomechanical properties of the cornea using the ocular response analyzer (ORA).

METHODS

The ocular response analyzer (ORA) is a new method available for non-contact measurement of the biomechanical properties of the cornea. We evaluated the reproducibility of measurements, the difference between static and dynamic factors and the impact of independent factors (e.g. IOP, age, CCT, swelling of the cornea) on 2,500 measurements of corneal hysteresis (CH) and corneal resistance factor (CRF).

RESULTS

In a large sample size we observed changes in CH and CRF after refractive surgery procedures (LASIK, UV-A cross-linking, keratoplasty) and in other corneal disorders (keratoconus, corneal dystrophies).

CONCLUSIONS

CRF and CH changes may reflect structural changes of the cornea. Thus, the ORA provides valuable information for a better understanding and characterization of the biomechanical condition of the cornea, especially with regard to diseases such as keratoconus and glaucoma.