Corneal Applanation Research Articles

Изучение вязкоэластичных свойств роговицы у пациентов с субклиническим кератоконусом

Abstrakt Corneal viscoelastic properties in patients with subclinical keratoconus I.V. Dutchin, E.L. Sorokin, Yu.V. Kutuzova, Ia.E. Pashentsev Fyodorov Eye Microsurgery Federal State Institution, Khabarovsk Branch, Khabarovsk, Russian Federation Far-Eastern State Medical University, Khabarovsk, Russian Federation Purpose. To evaluate corneal viscoelastic properties in subclinical keratoconus (SKC), analysis of its association with corneal thickness, and degree of risk of iatrogenic corneal ectasia. Material and methods. The main group consisted of 21 patients with signs SKC according to Pentacam (Oculus, Germany). The control group consisted of 21 patients without signs of subclinical SKC. In both groups, moderate myopia prevailed; groups were comparable in age and gender. A comparative statistical analysis of parameters of topography system Pentacam and corneal viscoelastic properties using the Corvis ST (Oculus, Germany) was carried out. Results. In the main group, the values: Corneal Applanation Velocity1, the Deformation Amplitude, and the Tomographic and Biomechanical Index (TBI) were statistically significantly higher compared to the control group. The values of the radius and the corneal central thickness, on the contrary, had statistically significantly lower values than in the control group. According to the values of the TBI (from 0 to 0.28), in the control group there was a «no obvious risk» of postoperative ectasia. In the main group, the average value of the TBI (0.43) was much higher than in the control group, which indicates weakened protective frames of cornea and risk of iatrogenic corneal ectasia in case of refractive surgery. There was no relationship between corneal thickness and TBI in the main group: r= –0.34 (p=0.13). Despite the signs of SKC in the main group, according to Pentacam, in 74% of eyes the corneal viscoelastic properties had sufficient rigidity for keratorefractive surgery. Conclusion. When selecting patients for keratorefractive surgery, along with assessing the parameters of corneal topography, it is also necessary to evaluate the corneal viscoelastic properties. Key words: subclinical keratoconus; fruste keratoconus; corneal viscoelastic properties, corneal biomechanical properties; iatrogenic corneal ectasia

A portable applanation tonometer for accurate intraocular pressure measurements

Accurate intraocular pressure (IOP) measurement in a convenient way is critical for the diagnosis and treatment in glaucoma development. In this study, we present a prototype of a portable tonometer by utilizing a new applanation method. The tonometer consists of two components: a capacitive force sensor (CFSen) and a portable displacement platform (PorDP). The capacitance output of CFSen and moving distance of PorDP reflects the force exerted on the eye and the corneal applanate area respectively. A numerical model based on finite element method (FEM) has been developed to analyze the tonometer performance. In addition, the tonometer has been validated on in vitro pig eyes, from which an analytical relationship of IOP, capacitance of CFSen and moving distance of PorDP has been established. Different from the traditional applanation-based tonometer which measures the force and area at single corneal applanate state, our tonometer takes readings at several corneal applanating phases. By considering multiple corneal applanation states, the measurement error of the tonometer is in the range of −0.38 to +0.67 mmHg and the resolution is less than 1 mmHg. We believe this approach provides a technological platform to develop an advanced tonometer for accurate IOP monitoring.

OCT and OCT-angiography parameters of intraocular blood flow in healthy adults

PURPOSE.To identify the relationship between the parameters of choroidal and retinal blood flow by optical coherence tomography (OCT) and OCT angiography (OCT-A), and the structural, hydrodynamic and systemic hemodynamic parameters in healthy adults.METHODS.The study included 42 male subjects aged 54.43±4.1 years without signs of local or systemic pathologies. The following parameters were evaluated on the RTVue-XR tomograph (USA): structural and microcirculatory parameters of the optic disc (OD) — peripapillary retinal nerve fiber layer (pRLNF) and radial peripapillary capillary plexus (RPCP); the macula — thickness, superficial capillary plexus (SCP), deep vascular complex (DVC), foveolar avascular zone (FAZ); the choroid — thickness, structure. Blood pressure was determined using the Riva–Rocci method; tonometry — by the Maklakov method. The parameters of bidirectional corneal applanation were evaluated, and perfusion and tolerance pressure were calculated.RESULTS.Negative correlations were revealed between intraocular pressure, pRNFL and capillary density of RCP (R= −0.58… −0.73); between choroid thickness, systemic blood flow, and tolerant pressure (R= −0.56… −0.72); between the thickness of the choroid and the volume of focal losses (FLV, %), as well as the OCT-A parameters of FAZ (area, perimeter, and density of foveolar vessels) (R= −0.45… −0.58). Positive correlations were found between perifoveal thickness (including ganglion complex) with the pRNFL (R=0.58–0.71), as well as with the thickness of the choroid (R=0.41–0.65). The confidence level was p≤0.001.CONCLUSION. The relationship of retinal, choroidal, micro hemodynamics parameters by OCT (choroid thickness) and OCT-A (RCC) (HD Angio Disc 4.5 protocol); area, perimeter, and vascular density of FAZ (Angio Retina 3.0 protocol) with the parameters of systemic blood flow in healthy subjects can likely be considered as hemodynamic biomarkers in the diagnosis, monitoring, and evaluation of treatment effectiveness of any intraocular vascular process (including glaucoma).

Keratoconus prognosis study for patients with corneal external mechanical stress mode.

To demonstrate the correlation between excessive eye rubbing and corneal degeneration for Keratoconus patients. Keratoconus (KC) patients who regularly rub their eyes had shown a rapid degeneration rate of their affected corneas. This observation is experimentally and numerical discussed and developed based on clinical data of 8 of KC Patients with a mean age of 26.5 ± 9.4years old, and four healthy individuals with a mean age of 24.33 ± 5years old at the baseline. Corneal topography was used to measure both central corneal thickness (CCT) and its total refractive power. The registered data had been exploited to assess the progression of the disease, and the final results were embedded in a finite element model of human corneas to simulate their response to eye rubbing at different stages of the pathology. Corneal lifetime prognosis using multi-layer perceptron was then established to estimate the number of eye rubbing cycles for each stage of KC. The survey of KC patients who declared stopping eye rubbing had shown a decrease in CCT loss rate, followed by a durable stability. Mechanical stresses numerical simulations had shown different corneal behaviours in term of shape deformity, apical raise and corneal applanation between healthy and KC stages models. Apical rise ranged from 0.122 to 0.389mm for an applied intraocular pressure that equals to 15mmHg. A normal stress of 5kPa provoked a corneal applanation that ranged from 0.27mm in healthy cases to 1.173mm in severe stages of the disease. The application of 2.5kPa biaxial stress had resulted normal and tangential applanations that successively ranged from 0.152 and 0.173mm in healthy corneas to 0.446mm and 0.458mm in severe KC stages. An adopted prognosis algorithm was able to predict the current stage of the disease and to estimate the remaining number of eye rubbing cycles before failure. Eye rubbing was proven to be a considerable contributing factor in KC patient's corneal degeneration. The progression of this pathology could be decreased or halted by stopping eye rubbing at early stages.

Association between long-term orthokeratology responses and corneal biomechanics

Myopia is very prevalent worldwide, especially among Asian populations. Orthokeratology is a proven intervention to reduce myopia progression. The current study investigated association between baseline corneal biomechanics and orthokeratology responses, and changes of corneal biomechanics from long-term orthokeratology. We fitted 59 adult subjects having myopia between −4.00D to −5.00D with overnight orthokeratology. Corneal biomechanics was measured through dynamic bidirectional corneal applanation (in terms of corneal hysteresis, CH and corneal resistance factor, CRF) and corneal indentation (in terms of corneal stiffness, S and tangent modulus, E). Subjects with poor orthokeratology responses had lower E (mean 0.474 MPa) than subjects with good orthokeratology responses (mean 0.536 MPa). Successful orthokeratology for 6 months resulted in reducing CH (reduced by 5.8%) and CRF (reduced by 8.7%). Corneal stiffness was stable, but E showed an increasing trend. Among subjects with successful orthokeratology, a higher baseline S resulted in greater myopia reduction (Pearson correlation coefficient, r = 0.381, p = 0.02).

Corneal Buckling during Applanation and Its Effect on the Air Pressure Curve in Ocular Response Analyzer.

The paper presents, for the first time, corneal buckling, during the air puff applanation, recorded with use of Ocular Response Analyzer (ORA), when the cornea is deeper deformed after its applanation. Precise numerical analysis of the air pressure curve from the raw data, distinct local disturbances of the curve, which appear almost exactly at the time of the first and the second applanations. Thirty measurements taken on six eyes show clear dependencies between times of both applanations and appearances of local wave disturbances on the air pressure curve as well as between the amplitude of pressure wave disturbances and the respective height of applanation curve. These findings can be interpreted as a result of very fast corneal buckling, that produces the air pressure wave, propagating from the cornea towards the device. The quantitative dependencies measured and described in this study, enable to characterize the individual buckling during respective applanations. Due to these individual characterizations and dependencies it is possible to understand and describe better the ultrafast corneal applanation process. Such phenomena could likely be employed to increase the accuracy of measured parameters by ORA or for identifying new types of biomechanical properties of the cornea.

Effect of Intravitreal Bevacizumab Injection on Corneal in vivo Biomechanics: A Pilot Study.

Purpose:To evaluate the effect of intravitreal bevacizumab (IVB) injection on corneal biomechanical parameters as measured by the ocular response analyzer (ORA) and Corneal Visualization Scheimpflug Technology (CorVis).Methods:In this prospective pilot study, ORA and CorVis parameters were recorded before and after a three-month course of IVB injection therapy in 16 patients in the injected and the contralateral non-injected control eyes. The changes in the recorded parameters in each group and the differences between the two groups were evaluated and compared.Results:None of the changes in ORA parameters were statistically significant in the injected and non-injected groups before and three months after injection, except for corneal resistance factor (CRF) in injected eyes (paired t-test, P = 0.039). The differences in corneal hysteresis (CH) and CRF were not statistically significant between the two groups (P = 0.441 and 0.236, respectively), but significant differences were noted between corneal compensated IOP (IOPcc) and Goldmann-correlated IOP (IOPg) (P = 0.045 and 0.047, respectively). None of the changes in CorVis parameters were statistically significant in the groups before and at the end of study, except for the time of first corneal applanation (TAp1 ms) in the injected group (P = 0.040, paired t-test). Differences in TAp1, length of the second corneal applanation (LAp2 mm), velocity of the second corneal applanation (VAp2 m/s), intraocular pressure (IOP), and central corneal thickness (CCT) also showed borderline significance between the two groups.Conclusion:In this pilot study IVB injection could change CRF, IOPcc, IOPg, and TAP1 as measured by ORA and CorVis.

Corneal Biomechanics Using a Scheimpflug-Based Noncontact Device in Normal-Tension Glaucoma and Healthy Controls.

To determine if a novel biomechanical parameter, corneal applanation velocity, as measured by the Corvis ST, is associated with a diagnosis of normal-tension glaucoma (NTG). Prospective, cross-sectional study. Study and control subjects were recruited from the ophthalmology clinic of a university teaching hospital in Hong Kong over an 8-week period in 2013. A total of 80 eyes with NTG diagnosis and 155 healthy eyes randomly selected to be in the control group were included in the final analysis. All subjects underwent corneal biomechanical testing with the Oculus Corvis ST non-contact tonometer. Logistic regression analysis adjusted for age and central corneal thickness was conducted to assess the relationship between inward and outward applanation velocity and the risk of NTG. Secondary outcome variables included corneal applanation time, length, amplitude, and highest concavity. Inward applanation velocity was faster in the NTG eyes (0.15 ± 0.02 m/s) than in the control eyes (0.14 ± 0.02 m/s) (P = 0.016). The odds ratio for a 0.01 m/s increase in inward applanation velocity when comparing NTG eyes with control eyes adjusted for age and central corneal thickness was 1.15 (95% confidence interval, 1.03-1.30) (P = 0.016). There was no evidence that outward applanation velocity or any secondary corneal biomechanical variable differed between the NTG and control eyes. Normal-tension glaucoma eyes demonstrated a small, statistically significant faster corneal inward applanation velocity than normal control eyes.

Intraocular Pressure Course During the Femtosecond Laser-Assisted Cataract Surgery in Porcine Cadaver Eyes.

Femtosecond laser-assisted cataract surgery (FLACS) is an increasingly common procedure. Most laser systems require corneal applanation and thereby increase intraocular pressure (IOP). The purpose of the present study was to evaluate the IOP changes that occur during the FLACS procedure performed using the Catalys femtosecond laser system. IOP was measured by direct cannulation of the vitreous body of porcine cadaver eyes (N = 20). By inserting a second cannula connected to a water column, all the eyes were set to a baseline IOP of 20 mm Hg. The eyes were lifted by custom-made stands to achieve the appropriate height and position under the Catalys system. The standard FLACS procedure was performed using varying fragmentation times to assess the influence of tissue fragmentation times on IOP peaks. We identified significant IOP elevations from baseline IOP levels during all steps of the FLACS procedure (baseline: 20.28 ± 1.32 mm Hg; vacuum: 34.48 ± 4.21 mm Hg; capture: 47.90 ± 13.02 mm Hg; lock: 48.41 ± 9.04 mm Hg; analysis: 47.15 ± 5.97 mm Hg; capsulotomy: 45.74 ± 6.52 mm Hg; fragmentation: 48.41 ± 6.80 mm Hg; end: 17.81 ± 1.61 mm Hg; all P < 0.001). Furthermore, the tissue fragmentation time had a significant effect on the peak IOP values detected (R = 0.62, P = 0.04, n = 9). The present study reveals significant IOP increases during FLACS procedures carried out using the Catalys system.

Impact of Myopia on Corneal Biomechanics in Glaucoma and Nonglaucoma Patients.

PurposeWe evaluated the impact of myopia on corneal biomechanical properties in primary open-angle glaucoma (POAG) and nonglaucoma patients, and the effect of modification of glaucoma on myopic eyes.MethodsThis cross-sectional study included 66 POAG eyes (33 myopia, 33 nonmyopia) and 66 normal eyes (33 myopia, 33 nonmyopia). Seven corneal biomechanical parameters were measured by ultra-high-speed Scheimpflug imaging, including corneal deformation amplitude (CDA), inward/outward corneal applanation length (ICA, OCA), inward/outward corneal velocity (ICV, OCV), radius, and peak distance (PD).ResultsMean age (SD) of the 65 male (49%) and 67 female (51%) patients was 59 (9.82) years. Myopia was associated with significantly higher CDA (adjusted effect = 0.104, P = 0.001) and lower OCV (adjusted effect = −0.105, P < 0.001) in the POAG group. Within the nonglaucoma group, myopic eyes had a significantly lower OCV (adjusted effect = −0.086, P < 0.001) and higher CDA (adjusted effect = 0.079, P = 0.001). All parameters except PD suggested that glaucoma modified the effect of myopia on corneal biomechanics. Percentage differences in the adjusted myopic effect between POAG and nonglaucoma patients was 31.65, 27.27, 31.65, 50.00, 22.09, and 60.49 for CDA, ICA, OCA, ICV, OCV, and radius, respectively.ConclusionsMyopia had a significant impact on corneal biomechanical properties in the POAG and nonglaucoma groups. The differences in corneal biomechanical parameters suggest that myopia is correlated with significantly lower ocular rigidity. POAG may enhance the effects of myopia on most of these parameters.

ANALYSIS OF RESULTS OF BIDIRECTIONAL CORNEAL APPLICATION AND ELECTRON TONOGRAPHY IN PATIENTS AFTER VITRECTOMY

Purpose: to analysis of the results of bidirectional corneal applanation and electron tonography in patients after vitrectomy and to tormulate practical recommendations on the research results. Patients and methods . 96 patients (96 eyes) with epiretinal fibrosis (37%) and idiopathic macular orifice (63%) were examined. Bidirectional corneal applanation (ORA) and electron tonography were used to determine the intraocular pressure. The main criteria for selection in the study groups were absence of traction retina detachment, the implementation of transciliary vitrectomy without use of silicone oil tamponade, no need for intra- and postoperative panretinal lacer coagulation were. All subjects had emmetropic refraction. Patients didn’t reserved instillations of corticosteroids, because corticosteroids lead to iatrogenic effects on the hydrodynamic parameters in the postoperative period. Results. According to electronic tonography in the postoperative period, an increase in IOP by 10%, intraocular fluid production by 53% was noted. According to the ORA, the increase in corneal-compensated pressure was 20%. A statistically significant decrease in corneal hysteresis was also detected in 7%. According to both methods of IOP, similar trends in its changes after removal of the vitreous body were obtained. Vitrectomy is accompanied not only by an increase in IOP, but also by a change in the biomechanical properties of the fibrous tunic, which can affect the accuracy of ophthalmotonometry. The accuracy of determining the level of IOP by electron tonography is strongly deepens on the presence of edema and the shape of the cornea, as well as biomechanical changes in the fibrous membrane of the eye after vitrectomy. Conclusion. Bidirectional corneal applanation is a non-contact, safe method. It can be used to accurately measure IOP after vitrectomy. This method is objective, independent of the error of the operator, fast and easy to use. We consider that surgery must use IOP screening up to 6 months after the operation and refusal to use steroid drugs in the postoperative period because of the tendency to changes in hydrodynamics after vitrectomy.

The Utility of Diaton Tonometer Measurements in Patients With Ocular Hypertension, Glaucoma, and Glaucoma Tube Shunts: A Preliminary Study for its Potential Use in Keratoprosthesis Patients.

Glaucoma is common in patients with the Boston type 1 keratoprosthesis (KPro). Because of the inability to perform corneal applanation in KPro patients, digital palpation is currently the mainstay for assessment of intraocular pressure (IOP). The purpose of this study was to determine whether the Diaton tonometer, which estimates IOP through transpalpebral scleral indentation, can reasonably detect high IOP when compared with Goldmann applanation tonometry (GAT) in patients with ocular hypertension, glaucoma, or glaucoma tube shunts. If reliable in these patients, it may be helpful in patients with KPros. We prospectively measured IOP using GAT and Diaton transpalpebral tonometry (DTT) on the upper and lower eyelids in 87 eyes of 57 adult participants with ocular hypertension and glaucoma, several of whom had glaucoma tube shunts. The correlation coefficient between GAT and DTT on the upper eyelid was 0.64 (P<0.0001). For detecting an IOP of >21 mm Hg as measured by GAT, DTT on the upper eyelid had a sensitivity and specificity of 65% and 81%, respectively. The positive and negative predictive values were 74% and 73%, respectively. Bland-Altman analysis demonstrated limits of agreement of 2.0±19.5 mm Hg for GAT versus DTT on the upper eyelid. The presence of a tube shunt did not significantly affect the results. Diaton tonometry has a large margin of error compared with GAT in patients with ocular hypertension, glaucoma, and glaucoma tube shunts. However, given the current lack of an objective and quantitative method of IOP measurement in KPro patients, Diaton tonometry may be useful in this population as a supplement to digital palpation.

Comparison of Diaton transpalpebral tonometer with applanation tonometry in keratoconus.

To investigate the added value of using a Diaton transpalpebral tonometer (DT) to measure IOP in keratoconus. Most type of tonometers use corneal applanation or biomechanical resistance to measure intraocular pressure (IOP); however, these factors can be altered by keratoconus. Specifically, we examined whether DT can detect false-negative low Goldmann applanation tonometry (AT) measurements. Patients with keratoconus were recruited from our tertiary academic treatment center. Measurements included AT and DT (in random order) and Scheimpflug imaging. An age- and gender-matched group of control subjects with no history of corneal disease or glaucoma was also recruited. In total, 130 eyes from 66 participants were assessed. In the keratoconus group, mean AT was 11.0 ± 2.6, mean DT 11.2±5.5 (P=0.729), and the two measures were correlated significantly (P=0.006, R=0.323). However, a Bland-Altman plot revealed a wide distribution and poor agreement between both measurements. Previous corneal crosslinking, corneal pachymetry, and Krumeich classification had no effect on measured IOP. Measurements obtained using a Diaton tonometer are not affected by corneal biomechanics; however, its poor agreement with Goldmann AT values calls into question the added value of using a Diaton tonometer to measure IOP in keratoconus.

Intraocular pressure measurement inside the anterior chamber: a new technical solution and results

To develop a new manometric device for intravital measurement of intraocular pressure (IOP) in the anterior chamber and to assess tonometric data reliability, including post-radial keratotomy (RK) measurements. The experiment was conducted in 2 isolated cadaver eyes, while the clinical study enrolled 20 patients (21 eyes) scheduled for cataract phacoemulsification surgery. Of them, 10 patients (10 eyes) with immature cataract and mild to moderate myopia constituted the control group. The study group consisted of the other 10 patients (11 eyes) with immature cataract, who had undergone RK more than 15 years earlier. The following tonometry methods were used: dynamic bi-directional corneal applanation (ORA, Reichert, USA), dynamic contour tonometry (Pascal tonometer, Zeimer, Switzerland), and rebound tonometry (ICare Pro, Tiolat, Finland). An original device was developed for intravital manometric measurements. Manometric data obtained during the experiment matched the preset pressure in the anterior eye chamber. The median manometry results in the control and study groups were similar and equaled 21.5 and 21.0 mmHg, respectively. Preoperative tonometry readings ranged from 14.9 to 16.5 mmHg in the control group and from 19.7 to 23.3 mmHg - in the study group (with the exception of midperipheral rebound tonometry that showed 15.8 mmHg). The developed device can well be used in experimental research. Midperipheral rebound tonometry was found to be the most informative method for post-RK IOP assessment. Manometry results in the study group mismatched tonometry readings in the controls, which might be due to the specifics of equipment calibration and requires further investigation.

Variability in biomechanical properties of the fibrous tunic of the eye in a healthy population

Studying biomechanical properties of the fibrous tunic of the eye meets current clinical demands. Previous studies have proved bidirectional corneal applanation suitable for this purpose. The so called corneal elasticity coefficient is more informative in terms of corneal elastance than the two other parameters measured during bidirectional corneal applanation, namely, corneal resistance factor and corneal hysteresis. All three are known to have wide normal ranges. The aim of the present study was to determine the range of corneal resistance factor, corneal hysteresis, and corneal elasticity coefficient in a normal population. This work summarizes data from 2205 patients (4410 eyes) with no ophthalmic pathology who got their ocular biomechanics examined with an Ocular Response Analyzer. Normal ranges of biomechanical parameters of the fibrous tunic have been determined. Considerable variation has been demonstrated in central corneal thickness (from 448 to 685 µm, with the mean of 563 ± 37 µm). Moreover, the corneal resistance factor, corneal hysteresis, and corneal elasticity coefficient have been shown to have almost equal means and to decrease with age.

Corneal biomechanical properties after LASIK, ReLEx flex, and ReLEx smile by Scheimpflug-based dynamic tonometry

To evaluate corneal biomechanical properties after LASIK, ReLEx flex, and the flap-free procedure ReLEx smile by Scheimpflug-based dynamic tonometry (Corvis ST) and non-contact differential tonometry (Ocular Response Analyzer, ORA). Patients treated for high myopia (-10.5 to -5.5 diopters, spherical equivalent refraction) more than one year previously at Aarhus University Hospital were included. Treatments comprised LASIK (35 eyes), ReLEx flex (31 eyes), and ReLEx smile (29 eyes). A control group included 31 healthy eyes. Cornea-compensated IOP (IOPcc), corneal hysteresis (CH), and corneal resistance factor (CRF) were measured with ORA. Corneal applanation and deformation were registered with Corvis ST during an air-pulse. Multiple linear regression analysis showed that CH and CRF were significantly lower after all keratorefractive procedures compared to healthy controls (p < 0.05). No significant differences were observed in CH or CRF between the keratorefractive groups. Corvis ST showed no differences in radius at highest concavity (HC radius), time until first applanation (A1 Time), time until second applanation (A2 Time), and deflection length at highest concavity (HC deflection length) between groups. LASIK treated eyes had significantly shorter time until highest concavity than eyes treated with ReLEx smile (HC Time, p = 0.01). The A1 deflection length was significantly shorter in the keratorefractive groups compared to the healthy controls (p < 0.05). Keratorefrative procedures alter the corneal biomechanical properties with regard to corneal hysteresis and corneal resistant factor. The flap-based LASIK and ReLEx flex and the flap-free ReLEx smile result in similar reduction in corneal biomechanics when evaluated by Corvis ST and ORA.

Changes of Corneal Biomechanics With Keratoconus

To perform advanced analysis of the corneal deformation response to air pressure in keratoconics compared with age- and sex-matched controls. The ocular response analyzer was used to measure the air pressure-corneal deformation relationship of 37 patients with keratoconus and 37 age (mean 36 ± 10 years)- and sex-matched controls with healthy corneas. Four repeat air pressure-corneal deformation profiles were averaged, and 42 separate parameters relating to each element of the profiles were extracted. Corneal topography and pachymetry were performed with the Orbscan II. The severity of the keratoconus was graded based on a single metric derived from anterior corneal curvatures, difference in astigmatism in each meridian, anterior best-fit sphere, and posterior best-fit sphere. Most of the biomechanical characteristics of keratoconic eyes were significantly different from normal eyes (P < 0.001), especially during the initial corneal applanation. With increasing keratoconus severity, the cornea was thinner (r = -0.407, P < 0.001), the speed of corneal concave deformation past applanation was quicker (dive; r2 = -0.314, P = 0.01), and the tear film index was lower (r = -0.319, P = 0.01). The variance in keratoconus severity could be accounted for by the corneal curvature and central corneal thickness (r = 0.80) with biomechanical characteristics contributing an additional 4% (total r = 0.84). The area under the receiver operating characteristic curve was 0.919 ± 0.025 for keratometry alone, 0.965 ± 0.014 with the addition of pachymetry, and 0.972 ± 0.012 combined with ocular response analyzer biomechanical parameters. Characteristics of the air pressure-corneal deformation profile are more affected by keratoconus than the traditionally extracted corneal hysteresis and corneal resistance factors. These biomechanical metrics slightly improved the detection and severity prediction of keratoconus above traditional keratometric and pachymetric assessment of corneal shape.

The effect of riboflavin–ultraviolet A-induced collagen cross-linking on intraocular pressure measurement: an experimental study

PurposeTo investigate the effect of corneal collagen cross-linking (CCL) on tonopen measurements of intraocular pressure (IOP).MethodsCCL with 0.1% riboflavin solution and 30 min of ultraviolet A radiation was performed on...

Investigation of Femtosecond Laser–Enabled Keratoplasty Wound Geometry Using Optical Coherence Tomography

To measure the wound geometry after femtosecond laser-enabled keratoplasty (FLEK) using optical coherence tomography (OCT). Prospective nonrandomized clinical study and laboratory study. Patients who were candidates for penetrating keratoplasty at an academic referral center. Wound architecture was measured and analyzed by OCT in 8 eyes of 8 consecutive patients who underwent FLEK. Femtosecond laser lamellar cuts were performed on 3 eye bank corneas, and the wound diameters were measured by OCT. Sutures were completely removed on average at 1 month per decade of age after surgery. No cases of wound dehiscence were noted, and graft-host tissue apposition appeared excellent on OCT. The mean spectacle-corrected visual acuity 1 month after suture removal was 20/35, and mean astigmatism was 5.0 diopters by manifest refraction and 8.7 diopters by computerized topography. The measured graft diameter was smaller than the laser setting by 2.5% (P = 0.007). The cut diameters of the eye bank corneas were also slightly smaller than the laser setting. FLEK offers benefits of rapid wound healing and predictable wound geometry. The measured wound geometry agreed well with laser setting other than a small systematic deviation that could be explained by the mechanics of corneal applanation during the laser cut. OCT is a useful tool to measure the actual graft diameter after FLEK.

On repeated corneal applanation with the Goldmann and two non‐contact tonometers

Background: Several authors believe it is necessary to randomise the order in which contact and non‐contact tonometers are used in comparison studies. This study was carried out to investigate the effect of repeated applanation on the measured intraocular pressure.Methods: One set of measurements per session was made on each of three sessions (one session per day) with the Goldmann and two non‐contact tonometers (Topcon CT80 and Keeler Pulsair EasyEye), in a pre‐determined order, on one randomly selected eye of 120 subjects randomised to one of two groups. For session one, only the non‐contact tonometers were used to assess the intraocular pressure of both groups. For session two, either non‐contact or Goldmann tonometry was performed first and this order was reversed for session 3. Average intraocular pressures were compared between sessions to determine the presence or absence of effects on the intraocular pressure caused by prior repeated applanation with the Goldmann or either one (or both) of the non‐contact tonometers.Results: Prior applanation with a non‐contact tonometer did not cause a significant (p > 0.05) reduction of the mean pressure measured with either non‐contact tonometer. The mean pressure was slightly but significantly (p < 0.05) reduced (for both non‐contact tonometers in both subject groups) when non‐contact tonometry was performed after Goldmann tonometry. There was no significant difference (p > 0.05) between the pressures measured with the Goldmann tonometer prior to and subsequent to non‐contact tonometry, in both subject groups.Conclusion: A small but statistically significant reduction in the intraocular pressure was found following applanation with the Goldmann tonometer but not with either one of two non‐contact tonometers.