Correlations between corneal hysteresis, intraocular pressure, and corneal central pachymetry

Effect of pathological myopia on biomechanical properties: a study by ocular response analyzer

AIM

To evaluate the ocular response analyzer (ORA) measurements of patients with pathological myopia in comparison with those of emmetropic control subjects, and to investigate the correlation between these ORA measurements and spherical equivalent (SE).

METHODS

Measurements of 53 eyes of 53 subjects with pathological myopia (SE>-6.00 D) were compared with those of 60 eyes of 60 emmetropic controls. Corneal hysteresis (CH), corneal resistance factor (CRF), noncontact tonometer intraocular pressure (IOPg), and corneal-compensated IOP (IOPcc) were obtained for each subject. The refractive error value was determined as SE via a cycloplegic refraction test.

RESULTS

The mean age was 54.1±18.9y (ranging from 5 to 88) in the pathological myopic group and 56.2±19.0y (ranging from 6 to 89) in the control group. There were no significant differences between the groups concerning age and sex. CH and CRF were significantly lower in the pathological myopic group than in the control group (P<0.001, P=0.005, respectively). IOPcc and IOPg were significantly higher in the pathological myopic group than in the control group (P<0.001, P=0.009, respectively). There were significantly positive correlations between CH and SE (r=0.565, P<0.001) and between CRF and SE (r=0.364, P=0.007). There were significantly negative correlations between IOPcc and SE (r=-0.432, P=0.001) and between IOPg and SE (r=-0.401, P=0.003).

CONCLUSION

The present study displayed that pathological myopia affected biomechanical properties measured by ORA. The results of corneal biomechanical properties measured by ORA may need to be appreciated by taking refraction into account. Further, pathological myopia might be related with the increased IOP.

Assessment of the tomographic values in keratoconic eyes after collagen crosslinking procedure

GOAL

This study aimed to investigate the differences in values of K1 and K2 readings, the central corneal thickness (PAH) before the collagen crosslinking procedure (CXL) and 3, 6, 12 months later.

METHODS

64 eyes were evaluated in retrospective cross sectional study. The corneal biomechanical parameters were taken with WaveLight Allegro Oculyzer produced by Alcon before the CXL, 3,6, 12 months after the procedure. The curvature of K1 reading and K2 reading were taken and the central corneal thickness were considered due to the time after CXL.

RESULTS

The value of K1 reading before the treatment was 48.8 diopters (D) (46.65-50.50) and was statistically significant lower comparing to the value of K1 3 months after the collagen CXL procedure 46.30 D (43.57-49.45) (p=0.0006), K1 reading one year post collagen CXL procedure was 47.20 D (44.35-50.07) (p=0.002). The value of K2 reading before the collagen CXL procedure was 52.65 D (47.55-54.72), 3 months after the procedure was 51.4 (45.05-54.0), 6 months later 48.55 D (47.20-50.62), 12 months later 51.30 D (47.22-54.77). There is statistically significant lower value of K2 reading 6 months after the treatment comparing to the values 3 months postoperatively (p=0.014). However there is significantly lower values of K2 reading 12 months postoperatively comparing to preoperative period (p=0.006). The value of central corneal thickness preoperative was 431.0 microns (398.0-446.25), 3 months after collagen CXL procedure was 373.50 microns (363.25-430.75), 6 months later 435.0 microns (360.0-464.75), 12 months after the CXL procedure was 429.50 microns (357.75-496.25). There is statistically significant lower values of central corneal thickness 3 months after collagen CXL treatment comparing to the central corneal thickness preoperative (p<0.005). There is statistically significant lower values of pachymetry 12 months after the CXL procedure comparing to the values 6 months later (p=0.036) and those preoperativelly (p=0.032). There is no statistically significant difference in the values of central corneal thickness in the period from 3 and 6 months postoperatively.

CONCLUSION

After riboflavin-UVA CXL in eyes with keratokonus there was significant decrease in central corneal thickness 3 and 6 months after the procedure and the thickness is almost the same 12 months. However, K2 (Kmax) reading is significantly changed 3 and 6 months later and is followed by changing of K1 reading.

Corneal biomechanics, retinal nerve fiber layer, and optic disc in children

PURPOSE

To evaluate the possible associations between corneal biomechanical parameters, optic disc morphology, and retinal nerve fiber layer (RNFL) thickness in healthy white Spanish children.

METHODS

This cross-sectional study included 100 myopic children and 99 emmetropic children as a control group, ranging in age from 6 to 17 years. The Ocular Response Analyzer was used to measure corneal hysteresis (CH) and corneal resistance factor. The optic disc morphology and RNFL thickness were assessed using posterior segment optical coherence tomography (Cirrus HD-OCT). The axial length was measured using an IOLMaster, whereas the central corneal thickness was measured by anterior segment optical coherence tomography (Visante OCT).

RESULTS

The mean (±SD) age and spherical equivalent were 12.11 (±2.76) years and -3.32 (±2.32) diopters for the myopic group and 11.88 (±2.97) years and +0.34 (±0.41) diopters for the emmetropic group. In a multivariable mixed-model analysis in myopic children, the average RNFL thickness and rim area correlated positively with CH (p = 0.007 and p = 0.001, respectively), whereas the average cup-to-disc area ratio correlated negatively with CH (p = 0.01). We did not observe correlation between RNFL thickness and axial length (p = 0.05). Corneal resistance factor was only positively correlated with the rim area (p = 0.001). The central corneal thickness did not correlate with the optic nerve parameters or with RNFL thickness. These associations were not found in the emmetropic group (p > 0.05 for all).

CONCLUSIONS

The corneal biomechanics characterized with the Ocular Response Analyzer system are correlated with the optic disc profile and RNFL thickness in myopic children. Low CH values may indicate a reduction in the viscous dampening properties of the cornea and the sclera, especially in myopic children.

Fundamentals and Advances in Tonometry

According to the World Health Organization, glaucoma is the leading cause of irreversible blindness worldwide. Although intraocular pressure (IOP) is not considered any more to be a defining feature of the disease, its lowering remains the only treatment option for glaucoma. Therefore, accurate and precise measurement of IOP is the cornerstone of glaucoma. Intraocular pressure is a highly dynamic physiological parameter with individual circadian rhythms. The main limitation of current tonometry methods remains the static and mostly office-based nature of their measurements. This review provides a brief historical overview on tonometry and discusses current tonometry instruments. In recent years, approaches to 24-hour IOP monitoring have been introduced, and there is hope that they may become part of routine clinical management in the future.

Quantitative assessment of responses of the eyeball based on data from the Corvis tonometer

BACKGROUND

The "air-puff" tonometers, include the Corvis, are a type of device for measuring intraocular pressure and biomechanics parameters. The paper attempts to analyse this response and its relationship with other parameters measured in the Corvis tonometer.

METHODS

A number of 13,400 2D images were acquired from the Corvis device and analysed (32 healthy and 16 ill people). A new method has been proposed for the analysis of responses of the eyeball based on morphological transformations and contextual operations.

RESULTS

The proposed algorithm enables to determine responses of the eyeball to an air puff coming from the Corvis tonometer. Additionally, responses of the eyeball have been linked to some selected features of corneal deformation. The results include, among others: (1) distinguishability between the left and right eye with an error of 7%; (2) the correlation between the area under the curve in corneal deformation and the response of the eyeball -0.26; (3) the correlation between the highest concavity time and the maximum deformation amplitude of 0.4. All these features are obtained fully automatically and repetitively at a time of 3.8s per patient (Core i7 10GB RAM).

DISCUSSION

It is possible to measure additional parameters of the eye deformation which are not available in the original software of the Corvis tonometer. The use of the proposed methods of image analysis and processing provides results directly from the eye response measurement when measuring intraocular pressure.

A comparison of corneal biomechanical properties in patients with psoriasis and healthy subjects

OBJECTIVE

To evaluate the differences in corneal biomechanical properties between healthy subjects and patients with psoriasis using the ocular response analyzer (ORA).

METHODS

Fifty-nine eyes of 59 psoriasis patients and 66 healthy individuals were included in the study. Corneal biomechanical parameters were obtained using ORA. Ultrasound pachymetry was used to measure central corneal thickness (CCT). The main parameters assessed were corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOPg) measurement and corneal-compensated IOP (IOPcc) through ORA. The dry eye evaluation was performed with tear break-up time (TBUT) and Shirmer test.

RESULTS

The mean CH values in psoriasis and healthy subject eyes were 10.75±2.9 mm Hg, 11.97±3.6 mm Hg, respectively (P=0.047). The mean CRF values were 10.14±3.1 mm Hg and 11.61±3.3 mm Hg, respectively (P=0.012). The mean CCT were 539.1±36 μm and 536.3±28 μm, respectively (P=0.643). Mean TBUT values were 8.2±2.9 sec in psoriasis group and 10.4±3.6 sec in healthy subjects (P<0.001). Shirmer test values were less (8.9±3.8 mm/5 min) in psoriasis than in healthy subjects (13.1±3.6 mm) (P<0.001).

CONCLUSIONS

Psoriasis can influence the corneal biomechanical properties. Patients with psoriasis had lower CH and CRF, but higher IOPg and IOPcc values than healthy controls. These corneal biomechanical changes should be considered when determining IOP values.

Assessment of the Ocular Response Analyzer as an Instrument for Measurement of Intraocular Pressure and Corneal Biomechanics

PURPOSE

The purpose of this study is to provide better understanding of the Ocular Response Analyzer (ORA) and how reliable it is to produce intraocular pressure (IOP) measurements that are free of the effects of corneal stiffness parameters, and stiffness estimates that are independent of IOP.

MATERIALS AND METHODS

A numerical parametric study that closely represents the in-vivo conditions of the human eye and the ORA procedure was conducted to determine the correlation coefficient r(2) between ORA output and the values of true IOP and a number of stiffness parameters, namely corneal thickness, curvature and age. For the purpose of this exercise, the ORA output was put in the form k1P1+k2P2 where k1 and k2 were variables and P1 and P2 were ORA's measured applanation pressures. Two separate clinical datasets involving Moorfields Eye Hospital, London and the University of New South Wales, Sydney participants, respectively, were used to validate the numerical results.

RESULTS

The numerical study results show a strong association between (k1P1 + k2P2) and the true IOP over a wide range of k1 and k2 values apart from a narrow region approximately extending from (k1 = +2, k2 = -2) to (k1 = -2, k2 = +2). On the other hand, (k1· P1 + k2· P2) was found to have a strong association with CCT, R and age (the stiffness parameters) over the same narrow region, beyond which the association was weak. Similar trends were found with the two clinical datasets.

CONCLUSIONS

The results of this study show the potential of the ORA to provide reliable IOP measurements with weak dependence on the cornea's stiffness parameters and the considerably reduced reliability in producing stiffness estimates that are unaffected by IOP values.

Evaluation of corneal deformation analyzed with a Scheimpflug based device

PURPOSE

To evaluate the correlation between corneal biomechanical and morphological data in healthy eyes.

METHODS

A complete clinical eye examination of naïve eyes was followed by tomographic (Pentacam, Oculus, Wetzlar, Germany) and biomechanical (Corvis ST, Oculus, Wetzlar, Germany) evaluation. Linear regression between central corneal thickness (CCT), corneal volume (CV) and anterior corneal curvature measured with Sim'K (SK), versus corneal deformation parameters measured with Corvis ST have been run using SPSS software version 18.0.

RESULTS

Seventy-six eyes of 76 healthy subjects (44 women and 32 men) with a mean age of 36.84 ± 10.74 years and a mean refractive error of -0.55 ± 1.68 D (measured as spherical equivalent) were evaluated. Corneal deformation parameters were weakly correlated with corneal morphological parameters and with spherical equivalent. Although the correlations between deformation amplitude versus SK and between SK versus Velocity of Applanation 2, were higher than the others (R(2) = 0.28 and 0.26 respectively), none of them was statistically significant (p>0.01).

CONCLUSIONS

According with these findings, Corvis ST seems to be able to provide an analysis of corneal deformation independent from corneal morphological characteristics. If these data will be confirmed in further studies, this device could be useful in the management and screening of eyes with corneal diseases.

Automatic method of analysis and measurement of additional parameters of corneal deformation in the Corvis tonometer

INTRODUCTION

The method for measuring intraocular pressure using the Corvis tonometer provides a sequence of images of corneal deformation. Deformations of the cornea are recorded using the ultra-high-speed Scheimpflug camera. This paper presents a new and reproducible method of analysis of corneal deformation images that allows for automatic measurements of new features, namely new three parameters unavailable in the original software.

MATERIAL AND METHOD

The images subjected to processing had a resolution of 200 × 576 × 140 pixels. They were acquired from the Corvis tonometer and simulation. In total 14,000 2D images were analysed. The image analysis method proposed by the author automatically detects the edge of the cornea and sclera fragments. For this purpose, new methods of image analysis and processing proposed by the author as well as those well-known, such as Canny filter, binarization, median filtering etc., have been used. The presented algorithms were implemented in Matlab (version 7.11.0.584-R2010b) with Image Processing toolbox (version 7.1-R2010b) using both known algorithms for image analysis and processing and those proposed by the author.

RESULTS

Owing to the proposed algorithm it is possible to determine three parameters: (1) the degree of the corneal reaction relative to the static position; (2) the corneal length changes; (3) the ratio of amplitude changes to the corneal deformation length. The corneal reaction is smaller by about 30.40% compared to its static position. The change in the corneal length during deformation is very small, approximately 1% of its original length. Parameter (3) enables to determine the applanation points with a correlation of 92% compared to the conventional method for calculating corneal flattening areas. The proposed algorithm provides reproducible results fully automatically within a few seconds/per patient using Core i7 processor.

CONCLUSIONS

Using the proposed algorithm, it is possible to measure new, additional parameters of corneal deformation, which are not available in the original software. The presented analysis method provides three new parameters of the corneal reaction. Detailed clinical studies based on this method will be presented in subsequent papers.

Comparison of ocular biomechanical response parameters in myopic and hyperopic eyes using dynamic bidirectional applanation analysis

PURPOSE

To compare differences in the ocular biomechanical response in myopic and hyperopic eyes.

SETTING

London Vision Clinic, London, United Kingdom, The Ohio State University, Columbus, OH, United States.

DESIGN

Retrospective study.

METHODS

The study population included myopic and hyperopic patients evaluated preoperatively for refractive surgery at the London Vision Clinic between June 2006 and May 2008. Biomechanical response parameters from the dynamic bidirectional applanation device (Ocular Response Analyzer) were analyzed using custom software for signal analysis, including corneal hysteresis (CH) and 10 other output parameters. Hyperopic eyes were compared with myopic eyes first matched for age and pachymetry and then matched for age, pachymetry, and corneal-compensated intraocular pressure (IOPcc). Nonpaired t tests were performed (P<.05) to compare parameters in the 2 groups.

RESULTS

Consecutive patients included 2608 eyes with 1623 myopic eyes and 787 hyperopic eyes that met enrollment criteria. A significant correlation (P<.0001) was shown between CH and age (negative), pachymetry (positive), and IOPcc (negative). The first match included 473 eyes in each group, and the second match included 260 eyes in each group. When matching for age and pachymetry only, certain parameters implied that hyperopic eyes were stiffer, while others implied that myopic eyes were stiffer, but IOPcc was significantly greater in the myopic group. The second match, which also controlled for IOPcc, showed that all biomechanical parameters implied that hyperopic eyes were stiffer.

CONCLUSIONS

Hyperopic eyes demonstrated stiffer response parameters than myopic eyes. Intraocular pressure was demonstrated to be a confounding factor when evaluating ocular biomechanical parameters.

FINANCIAL DISCLOSURES

Proprietary or commercial disclosures are listed after the references.

Deformation response of paired donor corneas to an air puff: intact whole globe versus mounted corneoscleral rim

PURPOSE

To evaluate the influence of ocular shell biomechanical characteristics on corneal deformation response to an air puff.

SETTING

The Ohio State University, Columbus, Ohio, USA.

DESIGN

Experimental study.

METHODS

Twenty-four eyes of 12 human donors were obtained in matched pairs. One eye was secured in a purpose-designed whole globe mount (whole-globe group). The cornea from the fellow eye was placed in a Barron artificial anterior chamber (artificial-chamber group). The corneas were mounted sequentially and connected to a pressure-control system. Deformation data were acquired using the Corvis ST, a dynamic Scheimpflug analyzer. Internal pressure was set to 10, 20, 30, 40, and 50 mm Hg; at least 4 examinations were performed at each pressure.

RESULTS

Statistically significantly higher maximum deformation amplitude was observed in the whole-globe group than in the artificial-chamber group at all pressures. The mean amplitude differences were 1.006 mm ± 0.238 [SD], 0.614 ± 0.137 mm, 0.384 ± 0.099 mm, 0.229 ± 0.087 mm, and 0.133 ± 0.068 mm at 10, 20, 30, 40, and 50 mm Hg, respectively (P<.0001, P<.0001, P<.0001, P<.0001, and P<.0002, respectively). Nonlinear regression of the deformation amplitude differences between pairs showed a significant decrease with increasing pressure (P<.0001, R(2) = 0.8385).

CONCLUSIONS

The deformation response to an air puff was affected by the type of mount used, with a stiffer shell producing a stiffer corneal response and decreasing differences at higher internal pressures. In vivo air-puff examinations may be affected by scleral stiffness in addition to the cornea.

FINANCIAL DISCLOSURES

Dr. Roberts is a consultant to Oculus Optikgeräte GmbH and Ziemer Ophthalmic Systems AG and has received research funding from Carl Zeiss Meditec AG and travel funds from Sooft Italia. No other author has a financial or proprietary interest in any material or method mentioned.

Small incision lenticule extraction (SMILE) history, fundamentals of a new refractive surgery technique and clinical outcomes

This review summarizes the current status of the small incision lenticule extraction (SMILE) procedure. Following the early work by Sekundo et al. and Shah et al., SMILE has become increasingly popular. The accuracy of the creation of the lenticule with the VisuMax femtosecond laser (Carl Zeiss Meditec) has been verified using very high-frequency (VHF) digital ultrasound and optical coherence tomography (OCT). Visual and refractive outcomes have been shown to be similar to those achieved with laser in situ keratomileusis (LASIK), notably in a large population reported by Hjortdal, Vestergaard et al. Safety in terms of the change in corrected distance visual acuity (CDVA) has also been shown to be similar to LASIK. It was expected that there would be less postoperative dry eye after SMILE compared to LASIK because the anterior stroma is disturbed only by the small incision, meaning that the anterior corneal nerves should be less affected. A number of studies have demonstrated a lower reduction and faster recovery of corneal sensation after SMILE than LASIK. Some studies have also used confocal microscopy to demonstrate a lower decrease in subbasal nerve fiber density after SMILE than LASIK. The potential biomechanical advantages of SMILE have been modeled by Reinstein et al. based on the non-linearity of tensile strength through the stroma. Studies have reported a similar change in Ocular Response Analyzer (Reichert) parameters after SMILE and LASIK, however, these have previously been shown to be unreliable as a representation of corneal biomechanics. Retreatment options after SMILE are discussed. Tissue addition applications of the SMILE procedure are also discussed including the potential for cryo-preservation of the lenticule for later reimplantation (Mohamed-Noriega, Angunawela, Lim et al.), and a new procedure referred to as endokeratophakia in which a myopic SMILE lenticule is implanted into a hyperopic patient (Pradhan et al.). Finally, studies reporting microdistortions in Bowman's layer and corneal wound healing responses are also described.

Femtosecond laser-assisted sutureless anterior lamellar keratoplasty for superficial corneal opacities

PURPOSE

To evaluate the visual and refractive outcomes, endothelial cell count (ECC), ocular surface changes, corneal aberrations, and biomechanical profile changes after femtosecond laser-assisted anterior lamellar keratoplasty surgery for superficial corneal scars.

SETTING

Farabi Eye Hospital, Tehran University of Medical Sciences, Tehran, Iran.

DESIGN

Prospective case series.

METHODS

Patients with superficial corneal scars had femtosecond laser-assisted anterior lamellar keratoplasty. Visual and refractive results, ECC, ocular surface changes, corneal aberrations, and biomechanical profiles were assessed preoperatively and for 1 year postoperatively.

RESULTS

Nineteen eyes (19 patients) were evaluated. A significant decline occurred in refractive astigmatism and corneal astigmatism after 1 year. There was a nonsignificant reduction in corneal hysteresis and the corneal resistance factor from preoperatively to 1 year postoperatively. The corneal-compensated intraocular pressure (IOP) and Goldmann-correlated IOP increased during the follow-up; the increase was not significant. A statistically insignificant reduction in the root mean square for trefoil and spherical aberrations occurred between 1 month and 1 year postoperatively (P=.1 and P=.4, respectively). The decreases in primary coma and total higher-order aberrations approached significance (P=.08 and P=.07, respectively). There were no significant changes in the central corneal thickness, ECC, or ocular surface parameters. No intraoperative complications occurred.

CONCLUSION

Femtosecond laser-assisted anterior lamellar keratoplasty was an efficient and safe procedure for improving the quality of vision in patients with anterior corneal pathology, and the results remained stable during the 1-year follow-up.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Evaluation of corneal deformation analyzed with Scheimpflug based device in healthy eyes and diseased ones

This study was designed to evaluate the correlation between corneal biomechanical and morphological data in healthy eyes, eyes that underwent myopic photorefractive keratectomy (PRK), keratoconus affected eyes, and keratoconus affected eyes that underwent corneal collagen crosslinking (CCC). Complete clinical eye examination of all eyes was followed by tomographic (Pentacam, Oculus, Wetzlar, Germany) and biomechanical (Corvis ST, Oculus, Wetzlar, Germany) evaluation. Differences among Corvis ST (CST) parameters in the different groups have been performed. Linear regression between central corneal thickness (CCT), intraocular pressure (IOP), and anterior corneal curvature measured with Sim'K (KM), versus corneal deformation parameters measured with Corvis ST in the different groups, has been run using SPSS software version 18.0. We evaluated 64 healthy eyes of 64 patients with a mean refractive error of -0.65 ± 1.68 D (measured as spherical equivalent), 17 eyes of 17 patients that underwent myopic PRK for a mean refractive defect of -4.91 ± 2.05 D (measured as spherical equivalent), 16 eyes of 16 patients affected by keratconus (stage 2-3 of Amsler Classification), and 13 eyes of 13 patients affected by keratoconus that underwent CCC. Our data suggest that corneal curvature would have a greater influence on corneal deformation than CCT; in fact KM values are more strongly associated with more CST parameters both about corneal change in shape and both about the corneal ability to come back at original shape.

Selected parameters of the corneal deformation in the Corvis tonometer

Introduction

Contemporary ophthalmology knows many methods of measuring intraocular pressure, namely the methods of non-contact and impression applanation tonometry. In non-contact applanation tonometers, e.g. the Corvis, the corneal flattening is caused by an air puff. Image registration of the corneal deflection performed by a tonometer enables to determine other interesting biomechanical parameters of the eye, which are not available in the tonometer. The measurement of new selected parameters is presented in this paper.

Material and method

Images with an M × N × I resolution of 200 × 576 × 140 pixels were acquired from the Corvis device in the source recording format *.cst. A total of 13'400 2D images of patients examined routinely in the Clinical Department of Ophthalmology, in District Railway Hospital in Katowice, Poland, were analysed in accordance with the Declaration of Helsinki. A new method has been proposed for the analysis of corneal deflection images in the Corvis tonometer with the use of the Canny edge detection method, mathematical morphology methods and context-free operations.

Results

The resulting image analysis tool allows determination of the response of the cornea and the entire eyeball to an air puff. The paper presents the method that enables the measurement of the amplitude of curvature changes in the frequency range from 150 to 500 Hz and automatic designation of the eyeball movement direction. The analysis of these data resulted in 3 new features of dynamics of the eye reaction to an air puff. Classification of these features enabled to propose 4 classes of deformation. The proposed algorithm allows to obtain reproducible results fully automatically at a time of 5 s per patient using the Core i5 CPU M460 @ 2.5GHz 4GB of RAM.

Conclusions

The paper presents the possibility of using a profiled algorithm of image analysis, proposed by the authors, to measure additional cornea deformation parameters. The new tool enables automatic measurement of the additional new parameters when using the Corvis tonometer. A detailed clinical examination based on this method will be presented in subsequent papers.

Impaired corneal biomechanical properties and the prevalence of keratoconus in mitral valve prolapse

Objective. To investigate the biomechanical characteristics of the cornea in patients with mitral valve prolapse (MVP) and the prevalence of keratoconus (KC) in MVP. Materials and Methods. Fifty-two patients with MVP, 39 patients with KC, and 45 control individuals were recruited in this study. All the participants underwent ophthalmologic examination, corneal analysis with the Sirius system (CSO), and the corneal biomechanical evaluation with Reichert ocular response analyzer (ORA). Results. KC was found in six eyes of four patients (5.7%) and suspect KC in eight eyes of five patients (7.7%) in the MVP group. KC was found in one eye of one patient (1.1%) in the control group (P = 0.035). A significant difference occurred in the mean CH and CRF between the MVP and control groups (P = 0.006 and P = 0.009, resp.). All corneal biomechanical and topographical parameters except IOPcc were significantly different between the KC-MVP groups (P < 0.05). Conclusions. KC prevalence is higher than control individuals in MVP patients and the biomechanical properties of the cornea are altered in patients with MVP. These findings should be considered when the MVP patients are evaluated before refractive surgery.

Corneal biomechanical property measurement with an IOP loading method in keratoconic patients

PURPOSE

To study the biomechanical properties of the corneas of both eyes in patients with evident keratoconus manifestation in one eye.

METHODS

Our study consisted of nine patients with keratoconus and 25 volunteers in the control group. Both eyes of all participants were measured twice with a Pentacam Scheimpflug system: first with the standard Scheimpflug system and subsequently with the original version of the same technique in combination with a new device that can generate experimental artificial intraocular pressure (IOP) elevation. Diagnoses of keratoconus or non-keratoconus were made using the Pentacam Scheimpflug system software (StatSoft, Inc., Tulsa, OK).

RESULTS

The artificially elevated IOP caused bulging of the anterior corneal surface in both eyes of keratoconic patients and a small flattening of the cornea in the eyes of the control group. Corneal ectasia, expressed in terms of diopters, during IOP loading in both keratoconic and nominally unaffected eyes was 4.12 D and 1.37 D, respectively. The changes were statistically significant (p < 0.05). Our dynamic study revealed a distinct weakness of the corneal tissue and an inability of keratoconic and contralateral eyes to resist IOP elevation.

CONCLUSION

The IOP loading method permits evaluation of the biomechanical properties of the cornea across its entire surface. All nine contralateral unaffected eyes in patients with a diagnosis of unilateral keratoconus presented weak biomechanical properties and in fact should be considered as forme fruste keratoconus.

Corneal biomechanical properties in different ocular conditions and new measurement techniques

Several refractive and therapeutic treatments as well as several ocular or systemic diseases might induce changes in the mechanical resistance of the cornea. Furthermore, intraocular pressure measurement, one of the most used clinical tools, is also highly dependent on this characteristic. Corneal biomechanical properties can be measured now in the clinical setting with different instruments. In the present work, we review the potential role of the biomechanical properties of the cornea in different fields of ophthalmology and visual science in light of the definitions of the fundamental properties of matter and the results obtained from the different instruments available. The body of literature published so far provides an insight into how the corneal mechanical properties change in different sight-threatening ocular conditions and after different surgical procedures. The future in this field is very promising with several new technologies being applied to the analysis of the corneal biomechanical properties.

Relationship Between Corneal Hysteresis and Corneal Resistance Factor with Other Ocular Parameters

PURPOSE

To evaluate the relationship between corneal hysteresis (CH) and corneal resistance factor (CRF) with age, central corneal thickness (CCT), corneal curvature (KM), corneal volume (CV), and refractive error in naïve eyes.

METHODS

105 healthy subjects (58 male and 47 female) were included in this study. The ages ranged from 19 to 82 years (mean 43.1 ± 15.4 years) and refraction between -11 D and +6 D (mean -0.79 ± 2.95 D). CH and CRF obtained with the Ocular Response Analyzer (ORA) were correlated with age, refractive error, Goldmann Applanation Tonometry (GAT), and with CCT, KM, CV obtained with the Pentacam, and with Corneal-Compensated Intraocular Pressure (IOPcc) and Goldmann-correlated intraocular pressure measurement (IOPg) obtained with ORA. A multivariable mixed effect model was used to evaluate associations among these parameters.

RESULTS

CH ranged from 6.9 to 14.6 mmHg (mean 10.26 ± 1.49 mmHg); CRF ranged from 5.8 to 17 mmHg (mean 10.38 ± 1.64 mmHg). Multivariate analysis showed a statistically significant correlation between CH with CCT (p < 0.001), and KM (p < 0.001), and between CRF with CCT (p < 0.001) and GAT (p < 0.001).

CONCLUSIONS

Our findings support the hypothesis that CH and CRF are related to the corneal shape and thickness, and show a decrease of CH with age.

Discriminant value of custom ocular response analyzer waveform derivatives in keratoconus

PURPOSE

To evaluate the performance of corneal hysteresis (CH), corneal resistance factor, and 16 investigator-derived Ocular Response Analyzer (ORA) variables in distinguishing keratoconus (KC) from the nondiseased state.

DESIGN

Retrospective case series.

PARTICIPANTS

Fifty-four eyes of 27 unaffected patients and 49 eyes of 25 KC patients from the Instituto de Olhos, Rio de Janeiro, Brazil.

METHODS

Sixteen candidate variables were derived from exported ORA signals to characterize putative indicators of biomechanical behavior. Area under the receiver operating characteristic curve (AUC) and the Z statistic were used to compare diagnostic performance.

MAIN OUTCOME MEASURES

Discriminant value of standard and derived ORA variables as measured by AUC.

RESULTS

Fifteen of 16 candidate variables performed significantly better than chance (AUC, >0.5) at discriminating KC. Diagnostic performance was greatest for a custom variable related to the depth of deformation as defined by the minimum applanation signal intensity during corneal deformation (concavity(min); mean AUC ± standard error, 0.985 ± 0.002) and a new measure incorporating the pressure-deformation relationship of the entire response cycle (hysteresis loop area, 0.967 ± 0.002). Z statistics assessing the discriminative value of each of the top 5 variables demonstrated superiority to CH (AUC, 0.862 ± 0.002). Concavity(min) had the best overall predictive accuracy (cutoff value, 50.37; 94.9% sensitivity, 91.7% specificity, and 93.2% test accuracy), and the top 4 variables demonstrated the most consistent relationships to KC severity.

CONCLUSIONS

Investigator-derived ORA variables related to the depth of deformation and the pressure-deformation relationship demonstrated very high test accuracy for detecting the presence of KC. Beyond their diagnostic value, the candidate variables described in this report provide mechanistic insight into the nature of the ORA signal and the characteristic changes in corneal dynamics associated with KC.

Characterization of the thickness of different corneal zones in glaucoma: effect on dynamic contour, Goldmann and rebound tonometries

PURPOSE

To characterize five models of corneal thickness circular zoning in a sample of healthy controls and a sample of patients with primary open-angle glaucoma (POAG) and to determine their effect on Goldmann (GAT), dynamic contour (DCT) and rebound tonometers (RT).

METHODS

The study participants were 122 controls and 129 cases. Five corneal thickness zoning models (A, B, C, D and E) were constructed. The partitioning pattern consisted of a circle centred at the corneal apex and several concentric rings, until the limbus; the contours of each ring followed the geometry of the corneal contour of each participant. In Model A, the central circle was 1 mm in diameter and five concentric rings were established. Mean was obtained for each zone for both samples and compared between them using a t-test. The effect on the tonometers of central cornel thickness (CCT) and mean thickness of the zones generated was determined through several linear regression models (one per tonometer and per sample).

RESULTS

According to a t-test, cases and controls differ in zones I [mean difference (MD): 17.93 μm], V (MD: 25.52 μm) and VI (MD: 31.78 μm) of model A (higher values in the cases sample). RT was affected by CCT (controls: B = 0.089; cases: B = 0.081). DCT was affected by zone IV of model A (controls: B = -0.029; cases: B = -0.012). GAT was affected by CCT (controls: B = 0.043; cases: B = 0.025) and zone III of model A (controls: B = -0.045; cases: B = -0.033).

CONCLUSION

Our results highlight the importance of the thickness of other regions of the cornea different from its main centre in discriminating between healthy controls and patients with POAG and in IOP measurements made using DCT, GAT and RT.

Analysis of waveform-derived ORA parameters in early forms of keratoconus and normal corneas

PURPOSE

To evaluate the Ocular Response Analyzer (ORA; Reichert Ophthalmic Instruments, Depew, NY) performance in differentiating grades I and II keratoconus from normal corneas using 41 parameters individually and to assess the effect of analyzing all parameters together.

METHODS

This study compared the mean value of 41 ORA parameters in grades I and II keratoconus with healthy age-matched control eyes. Only eyes with a central corneal thickness between 500 and 600 μm were included. The area under the receiver operating characteristic curve was calculated for each of the 41 parameters independently and for all of the parameters together.

RESULTS

This study included 136 eyes with normal corneas and 68 eyes with grades I and II keratoconus. When analyzed individually, four ORA parameters (p1area, p1area1, p2area, and p2area1) had an area under the curve greater than 0.900 for discriminating between both groups. The p2area was the parameter that achieved the largest area under the curve individually (0.931). The area under the curve increased to 0.978 when analyzing all parameters together.

CONCLUSION

Alternative ORA parameters are better for differentiating grades I and II keratoconus from normal corneas than the four parameters originally available for ophthalmologists (corneal hysteresis, Goldmann-correlated intraocular pressure, corneal-compensated intraocular pressure, and corneal resistance factor). Although the ORA did not achieve 100% accuracy, the discrimination between these two groups was optimized by combining all parameters.

Evaluation of corneal biomechanical properties following penetrating keratoplasty using ocular response analyzer

Purpose:

To evaluate corneal biomechanical properties in eyes that has undergone penetrating keratoplasty (PK).

Materials and Methods:

Retrospective observational study in a tertiary care centre. Data recorded included ocular response analyzer (ORA) values of normal and post-keratoplasty eyes [corneal hysteresis (CH), corneal resistance factor (CRF), Goldmann-correlated intraocular pressure (IOPg), and cornea-compensated intraocular pressure (IOPcc)], corneal topography, and central corneal thickness (CCT). Wilcoxon signed rank test was used to analyze the difference in ORA parameter between post-PK eyes and normal eyes. Correlation between parameters was evaluated with Spearman's rho correlation.

Results:

The ORA study of 100 eyes of 50 normal subjects and 54 post-keratoplasty eyes of 51 patients showed CH of 8.340 ± 1.85 and 9.923 ± 1.558, CRF of 8.846 ± 2.39 and 9.577 ± 1.631 in post-PK eyes and normal eyes, respectively. CH and CRF did not correlate with post-keratoplasty astigmatism (P =0.311 and 0.276, respectively) while a significant correlation was observed with IOPg (P =0.004) and IOPcc (P < 0.001).

Conclusion:

Biomechanical profiles were significantly decreased in post-keratoplasty eyes with significant correlation with higher IOP as compared with that in normal eyes.

The effect of prostaglandin analogs on the biomechanical properties and central thickness of the cornea of patients with open-angle glaucoma: a 3-year study on 108 eyes

Purpose

To evaluate the effect of prostaglandin analogs (PGAs) on the biomechanical properties (corneal hysteresis [CH], corneal resistance factor [CRF]) and central corneal thickness (CCT) of patients with open-angle glaucoma.

Methods

A total of 108 eyes were prospectively included for repeated measurements of intraocular pressure (IOP) with Goldmann applanation tonometry (GAT) and ocular response analyzer (ORA), CCT, followed by CH and CRF measurements by the ORA, during the same visit. Of these, 66 were treated with latanoprost, and 42 were treated with latanoprost and timolol. IOP, CH, CRF, and CCT were measured before treatment and at 6-month intervals.

Results

It appears that under local PGA treatment, IOP values decreased and CH and CCT significantly increased, whereas CRF did not. CCT slightly but significantly increased at all time points of the study (0.50–3.00 μm and 1.50–5.50 μm), and we observed a constant significant increase in CH (0.4–0.7 mmHg and 0.65–0.95 mmHg). Concerning the correlation of GAT IOP with CCT and CH, it was found that at the time points 0, 1, and 2 there were statistically significant correlations.

Conclusion

The changes of CCT and CH under PGA treatment in clinical practice may influence IOP measurements and patient follow-up significantly. This should be investigated further to confirm the relationship between corneal properties and treatment of open-angle glaucoma.

The biomechanical properties of the cornea and anterior segment parameters

BACKGROUND

To investigate the biomechanical properties of the cornea measured with the Ocular Response Analyzer (ORA) and their association with the anterior segment parameters representing the geometric dimensions including the corneal volume and anterior chamber volume.

METHODS

A retrospective review of 1020 patients who visited the BGN Eye Clinic was done. The mean radius of the corneal curvature, corneal astigmatism, corneal volume, anterior chamber depth, and anterior chamber volume were measured with an anterior segment tomographer. The central corneal thickness (CCT) was measured with an ultrasonic pachymeter. The corneal diameter was measured with an Orbscan as White to White. Cornea hysteresis (CH), corneal resistance factor (CRF), Goldmann correlated intraocular pressure (IOPg), and cornea-compensated IOP (IOPcc) were measured with an ORA. Multiple linear regression models were constructed with CH and CRF as the dependent variables and age, gender, and the anterior segment parameters as the covariates.

RESULTS

958 eyes from 958 patients (mean age 26.7 years; male 43.4%) were included in this study after excluding some eyes according to the exclusion criteria. The mean CH and CRF were 10.1 and 9.9 mmHg, respectively. The mean IOPg and IOPcc were 14.8 and 15.8 mmHg. The multivariate analysis showed that CH was negatively associated with the mean radius of the cornea curvature (regression coefficient = -0.481, p = 0.023) and positively associated with CCT (regression coefficient = 0.015, p < 0.001) and corneal volume (regression coefficient =0.059, p = 0.014). The association between CH and the corneal diameter, anterior chamber depth, and anterior chamber volume were not statistically significant. The evaluation of CRF showed that CRF was negatively associated with the mean radius of the cornea curvature (regression coefficient = -0.540, p = 0.013), and positively associated with CCT (β = 0.026, p < 0.001). The association between CRF and the corneal diameter, corneal volume, anterior chamber depth, and anterior chamber volume were not statistically significant.

CONCLUSION

The CH was shown to be positively associated with the corneal volume and the association between CH and the anterior chamber volume were not significant. The associations of CRF with the corneal volume or anterior chamber volume were not significant.

Moderate keratoconus with thick corneas

PURPOSE

To describe two patients with moderate keratoconus and a corneal thickness exceeding 600 μm at the thinnest point.

METHODS

Case report.

RESULTS

In the first case, the steepest keratometric power was 51.50 diopters (D) in the right eye and 53.4 in the left eye and the thickness at the thinnest point was 658 and 625 μm, respectively. In the second case, the steepest keratometric power was 46.70 D in the right eye and 49.60 D in the left eye and the thickness at the thinnest point was 618 and 608 μm, respectively.

CONCLUSIONS

Keratoconus may develop despite a very thick cornea, reinforcing the idea that biomechanical changes can signify an important factor in the development and progression of this pathology.

Combining ocular response analyzer metrics for corneal biomechanical diagnosis

PURPOSE

To evaluate corneal biomechanical properties in non-keratoconic myopic eyes and to identify descriptors for improving the specificity of the Ocular Response Analyzer (ORA) (Reichert Ophthalmic Instruments, Depew, NY) testing for subclinical keratoconus detection.

METHODS

Observational case series of 52 non-keratoconic non-myopic eyes and 97 non-keratoconic myopic eyes (spherical equivalent < -5 diopters [D]) in dataset 1 and 87 non-keratoconic eyes and 73 eyes with subclinical keratoconus in dataset 2. Examination included corneal topography, tomography, and biomechanical testing with the ORA. Receiver operating characteristic curves and logistic regression were used to identify optimal combinations of biomechanical indices for keratoconus detection. Main outcome measures were corneal thickness-corrected hysteresis (DifCH) and resistance factor (DifCRF), the difference between these two (CH-CRF), and the diagnostic performance of their combinations.

RESULTS

Compared to non-keratoconic non-myopic eyes, non-keratoconic myopic eyes with flat corneas (average corneal power < 44.0 D) had reduced DifCH (mean ± standard deviation, 0.11 ± 1.27 vs -0.79 ± 1.50, P < .01) and DifCRF (0.24 ± 1.16 vs -0.70 ± 1.59, P < .01) values, whereas non-keratoconic myopic eyes with steep corneas showed no difference. Keratoconic eyes exhibited lower DifCH and DifCRF values than non-keratoconic myopic eyes. Combinations of DifCH, DifCRF, and CH-CRF had increased specificity (> 80%) for subclinical keratoconus compared to the DifCRF index alone (71%).

CONCLUSIONS

In biomechanical keratoconus screening, some non-keratoconic myopic eyes show altered ocular biomechanical properties and are identified as false-positive cases. The low specificity of DifCRF when dealing with these non-keratoconic eyes could be improved by considering additional biomechanical descriptors such as DifCH and CH-CRF, which seem to be indicative of the aforementioned biomechanical profile.

Corneal biomechanics in iatrogenic ectasia and keratoconus: A review of the literature

The Ocular Response Analyzer (ORA) (Reichert Ophthalmic Instruments, Buffalo, NY) allows direct measurement of corneal biomechanical properties. Since its introduction, many studies have sought to elucidate the clinical applications of corneal hysteresis (CH) and corneal resistance factor (CRF). More recently, detailed corneal deformation signal waveform analysis (WA) has potentially expanded the diagnostic capabilities of the ORA. In this review, the role of CH, CRF, and WA are examined in keratoconus (KC) and iatrogenic ectasia (IE). The PubMed database was searched electronically for peer-reviewed literature in July 2012 and August 2012 without date restrictions. The search strategy included medical subject heading (MeSH) and natural language terms to retrieve references on corneal biomechanics, CH, CRF, corneal deformation signal WA, IE, and KC. The evidence suggests that while CH and CRF are poor screening tools when used alone, increased sensitivity and specificity of KC and IE screening result when these parameters are combined with tomography and topography. Recent advances in WA are promising, but little is currently understood about its biomechanical and clinical relevance. Future studies should seek to refine the screening protocols for KC and IE as well as define the clinical applicability of WA parameters.

Agreement of flicker chronoscopy for structural glaucomatous progression detection and factors associated with progression

PURPOSE

To evaluate agreement of flicker chronoscopy for structural glaucomatous progression detection and factors associated with progression.

DESIGN

Retrospective cohort study.

METHODS

Two glaucoma fellowship-trained ophthalmologists, masked to temporal sequence, independently graded serial flicker chronoscopy images from 1 eye of a cohort of glaucoma patients for features of structural progression. Agreement between graders was determined, as was accuracy for determining the temporal order of images. After adjudication, simple and multiple logistic models were constructed to determine baseline variables associated with increased odds of progression.

RESULTS

Fifty of 103 included eyes/patients (48.5%) had at least 1 sign of structural progression. Temporal sequence was incorrectly determined in 14 of 206 cases (6.4%). Interobserver agreements for identifying baseline photographs (κ = 0.9), global progression (κ = 0.7), parapapillary atrophy (PPA) progression (κ = 0.7), disc hemorrhages (κ = 0.7), neuroretinal rim loss (κ = 0.5), and retinal nerve fiber layer (RNFL) loss (κ = 0.2) were calculated. Age was significantly associated with global (1.8; 1.3-2.6, P < .001) (odds ratio; 95% confidence interval, significance) and PPA progression (1.7; 1.2-2.4, P = .002). Lower corneal hysteresis was associated with global progression (0.78; 0.56-0.99, P = .049) and RNFL loss (0.5; 0.3-0.9, P = .02). Goldmann-correlated intraocular pressure (1.0, 0.7-1.4, P = .9), visual field mean deviation (1.0, 0.9-1.0, P = .2), and central corneal thickness (0.9, 0.8-1.0, P = .1) were not significantly associated with progression. On multivariable analysis, only age was associated with global progression (1.8; 1.2-2.5, P = .002).

CONCLUSION

Flicker chronoscopy demonstrated acceptable interobserver agreement in structural progression detection. Corneal hysteresis and age were both associated with progression, but age was the only significant factor on multivariable analysis.

Scanning acoustic microscopy for mapping the microelastic properties of human corneal tissue

PURPOSE

To assess the feasibility of applying scanning acoustic microscopy (SAM) on UV cross-linked corneal tissue for mapping and analyzing its biomechanical properties.

MATERIALS AND METHODS

Five corneal pairs (10 corneas) were used. In each pair, one cornea was cross-linked (epithelium removed, riboflavin application for 45 min and UVA irradiation for 30 min) and the contralateral control cornea was epithelial debrided and treated only with riboflavin for 45 min. Histological sections were prepared and their mechanical properties were examined using SAM. A line profile technique and 2D analysis was used to analyze the mechanical properties of the corneas. Then the corneal paraformaldehyde and unfixed sections were examined histologically using hematoxylin and eosin (H&E) staining.

RESULTS

In the frozen fresh corneal tissue, the speed of sound of the treated corneas was 1672.5 ± 36.9 ms(-1), while it was 1584.2 ± 25.9 ms(-1) in the untreated corneas. In the paraformaldehyde fixed corneal tissue, the speed of sound of the treated corneas was 1863.0 ± 12.7 ms(-1), while it was 1739.5 ± 30.4 ms(-1) in the untreated corneas. The images obtained from the SAM technique corresponded well with the histological images obtained with H&E staining.

CONCLUSION

SAM is a novel tool for examining corneal tissue with a high spatial resolution, providing both histological and mechanical data.

Association between corneal biomechanical properties and optic nerve head morphology in newly diagnosed glaucoma patients

BACKGROUND

  To investigate the association between corneal biomechanics and optic nerve head morphology in newly diagnosed primary open-angle glaucoma patients.

DESIGN

  Hospital based prospective study.

PARTICIPANTS

  Forty-two untreated newly diagnosed primary open-angle glaucoma patients.

METHODS

  Patients underwent corneal hysteresis measurement using the Ocular Response Analyzer and confocal scanning laser ophthalmoscopy for optic nerve head topography evaluation. One eye was selected randomly for analysis. Data collected included age, race, gender, intraocular pressure and central corneal thickness.

MAIN OUTCOME MEASURES

  Multiple regression analysis (controlling for baseline intraocular pressure and disc area) was used to investigate factors associated with the following optic nerve head topographic parameters: linear cup-to-disc ratio and mean cup depth.

RESULTS

  Mean age of participants was 66.7 ± 11.8 years. Corneal hysteresis was the only factor significantly associated with both mean cup depth (correlation coefficient [r] = -0.34, P = 0.03) and cup-to-disc ratio (r = -0.41, P = 0.01). Central corneal thickness was significantly associated with mean cup depth (r = -0.35, P = 0.02), but not with cup-to-disc ratio (r = -0.25, P = 0.13). Although a trend towards a positive association between age and cup-to-disc ratio was identified (r = 0.26, P = 0.08), age was not significantly associated with mean cup depth (r = 0.06, P = 0.72). When comparing fellow eyes of patients with bilateral glaucoma, the eye with higher corneal hysteresis had smaller cup-to-disc ratio in 75% of the cases.

CONCLUSIONS

  In untreated newly diagnosed primary open-angle glaucoma patients, those with thinner corneas and mainly lower corneal hysteresis values had a larger cup-to-disc ratio and deeper cup, independently of intraocular pressure values and disc size.

Variation in corneal hysteresis and central corneal thickness among black, hispanic and white subjects

PURPOSE

To determine whether differences in corneal hysteresis (CH) and central corneal thickness (CCT) between black, Hispanic and white subjects exist independently of one another.

METHODS

Retrospective, cross-sectional data were reviewed for 807 eyes of 410 patients consecutively evaluated for glaucoma. Included patients had open angles, at least one reliable 24-2 perimetric examination and no evidence of nonglaucomatous vision loss. Patients underwent CH measurement with the ocular response analyzer followed by CCT measurement and full ocular examination. Patients were asked to self-classify their race or ethnicity. Statistical analyses were performed to identify characteristics that varied between black, Hispanic and white subjects and to explain this variation.

RESULTS

Of the 270 patients (511 eyes) included, 84 were black, 96 Hispanic and 90 white. There were no significant differences in diagnosis, sex, age, intraocular pressure or glaucoma severity between races/ethnicities (p ≥ 0.16). Blacks were found to have lower CCT (529.3 μm) and CH (8.7 mmHg) compared to Hispanics (544.7 μm, p = 0.008; 9.4 mmHg, p = 0.007) and whites (549.9 μm, p < 0.001; 9.8 mmHg, p < 0.001). On multivariable analysis, inter-racial/ethnic differences in CCT were not found to exist independent of CH (p ≥ 0.10), whereas the significant intergroup variation in CH remained after adjustment for CCT and other covariates (p ≤ 0.005).

CONCLUSIONS

Variation in CCT between races/ethnicities does not exist independent of CH. However, significant intergroup variation in CH is present independent of CCT. This finding suggests that CH may be a preferable measurement to evaluate intergroup differences in corneal properties and their relationship to open-angle glaucoma.

Relative importance of factors affecting corneal hysteresis measurement

PURPOSE

To evaluate the relative influences of several demographic, ocular, and systemic parameters on corneal hysteresis (CH).

METHODS

This is a prospective, observational, cross-sectional study using subjects recruited from consecutive Albuquerque VAMC eye clinic patients. We classified eligible subjects as primary open-angle glaucoma (POAG), ocular hypertension, glaucoma suspect, or normal. We used the Ocular Response Analyzer, Pascal Dynamic Contour Tonometer, and Goldmann applanation tonometer to obtain intraocular pressure (IOP), CH, corneal resistance factor, and ocular pulse amplitude values. We also obtained corneal curvature, central corneal thickness (CCT), axial length, retinal nerve fiber layer thickness, clinical cup/disc ratio (CDR) estimates, and standard automated perimetry metrics (mean defect, pattern standard deviation). We gathered glycosylated hemoglobin (A1C) data through chart review. Multivariate regression analyses were used to determine independent relationships between CH and the other parameters.

RESULTS

Three hundred seventeen eyes in 317 subjects were studied (116 POAG, 87 ocular hypertension, 47 glaucoma suspect, and 67 normal). In univariate regression analysis, CH varied directly with CCT (β = 0.39, p < 0.001), corneal curvature (β = 0.16, p = 0.01), corneal resistance factor (β = 0.57, p < 0.001), A1C (β = 0.15, p = 0.01), mean defect (β = 0.29, p < 0.001), and retinal nerve fiber layer (β = 0.31, p < 0.001). Factors inversely related to CH were age (β = -0.22, p < 0.001), IOP (β = -0.29, p < 0.001), ocular pulse amplitude (β = -0.11, p = 0.04), CDR (β = -0.34, p < 0.001), and pattern standard deviation (β = -0.29, p < 0.001). CH was lower in POAG compared with the other diagnostic groups. In multivariate analysis, CH was independently associated with age, IOP, CCT, A1C, glaucoma diagnosis, and CDR. Of these factors, CCT and IOP demonstrated twice as much influence on CH compared with the other four factors.

CONCLUSIONS

Although this study identified six separate variables that independently influence CH values, the overall r value indicates that these variables together only explain 40% of CH variability. These results suggest that other significant sources of variability exist and deserve investigation.

Combining corneal hysteresis with central corneal thickness and intraocular pressure for glaucoma risk assessment

PURPOSE

To determine whether adjusting corneal hysteresis (CH) values for central corneal thickness (CCT) and intraocular pressure (IOP) improves its capability to differentiate primary open-angle glaucoma (POAG) from ocular hypertension (OH).

METHODS

This prospective, observational, cross-sectional study included 169 eyes of 169 subjects with a diagnosis of POAG (n=81) or OH (n=88). We utilized the Ocular Response Analyzer (ORA), Pascal Dynamic Contour Tonometer (DCT), Goldmann applanation tonometer (GAT), and ORA ultrasound pachymeter to obtain CH, IOP, and CCT values. Correlational, regression, and t-test analyses were conducted before and after the sample was divided into low, intermediate, and thick CCT subgroups.

RESULTS

In the full sample, CH and CCT were moderately correlated (r=0.44, P<0.001). Although both were related to diagnosis in univariate regression analysis, only CH was independently related to glaucoma diagnosis in multivariate analysis. After the sample was divided into CCT tertiles, CH was significantly lower in POAG vs OH eyes within all three CCT subgroups, and CH was the only multivariate variable that differentiated POAG from OH in each CCT subgroup. Moreover, the relationship between CH and diagnosis was more robust within the CCT subgroups compared with the full sample, suggesting that integrating CCT into CH interpretation is beneficial. Adjusting CH for IOP did not aid diagnostic precision in this study.

CONCLUSION

Our findings suggest that combining CH and CCT for glaucoma risk assessment improves diagnostic capability compared to using either factor alone. Conversely, adjusting CH for IOP provided no clear clinical benefit in this study.

Corneal deformation signal waveform analysis in keratoconic versus post-femtosecond laser in situ keratomileusis eyes after statistical correction for potentially confounding factors

PURPOSE

To evaluate and compare corneal biomechanical waveform parameters between keratoconic and post-femtosecond laser in situ keratomileusis (LASIK).

SETTING

Jules Stein Eye Institute, University of California, Los Angeles, California, USA.

DESIGN

Comparative case series.

METHODS

The Ocular Response Analyzer was used to obtain the corneal hysteresis (CH), corneal resistance factor (CRF), and 39 biomechanical waveform parameters in manifest keratoconic eyes and post-femtosecond LASIK eyes. Univariate tests were used to assess the difference in each parameter between the 2 groups of eyes. After controlling for central corneal thickness (CCT) and age, a logistic regression model was used to select the parameters most useful in distinguishing between the 2 groups.

RESULTS

After statistically controlling for the differences in CCT and age, 7 parameters were found to be the most useful in distinguishing between groups: aplhf (high frequency noise in the region between peaks [P1 and P2]; P<.0001), w2 (width of P2 at base; P=.001), dslop1 (down-slope of P1 of wave; P<.0001), aindex (degree of "non-monotonicity" of rising and falling edges of first peak of wave, P=.0007), uslope1 (upslope of the P1 of wave; P=.001), CH (P=.035), and P1area (area under P1 of wave; P=.006). The area under the receiver operating characteristic curve for the model using these parameters was 0.932.

CONCLUSIONS

Differences in multiple biomechanical waveform parameters between the keratoconus and post-LASIK groups suggests that waveform analysis may be useful to differentiate between healthy and diseased biomechanical conditions.

Measuring corneal hysteresis: threshold estimation of the waveform score from the Ocular Response Analyzer

Background

The aim of this study was to determine a threshold waveform score (WS) for the best score value (BSV) in the Ocular Response Analyzer (ORA).

Methods

Retrospective study. One hundred and thirty-three healthy adults were recruited. Measurements were done with the ORA 2.04.

Results

Two hundred and sixty-six eyes were analyzed. Mean age was 56.49 ± 15.97 years. The mean waveform score of the BSV was 7.39 ± 1.32. The waveform scores ranged from 2.8 to 9.7. Kolmogorov–Smirnov test for normality was significant (p ≤ 0.0001).

Linear regression showed a significant positive correlation between IOPg (measured with the ORA) and IOP measured with Goldmann applanation tonometry (p ≤ 0.0001), as well as significant negative correlation between the difference IOPg–IOP Goldmann and waveform score of the BSV values. Threshold estimation considering 95 % confidence interval was 7.23. Meanwhile, threshold estimation considering the difference IOPg–IOP Goldmann, for 3 mmHg, was 6.7.

Conclusions

When using the ORA device, we recommend that clinicians try to obtain several waveform score measurements of 7 or above. Waveform scores lower than 7 may render less reliable results.