Introduction of Two Novel Stiffness Parameters and Interpretation of Air Puff-Induced Biomechanical Deformation Parameters With a Dynamic Scheimpflug Analyzer

PURPOSE

To investigate two new stiffness parameters and their relationships with the dynamic corneal response (DCR) parameters and compare normal and keratoconic eyes.

METHODS

Stiffness parameters are defined as Resultant Pressure at inward applanation (A1) divided by corneal displacement. Stiffness parameter A1 uses displacement between the undeformed cornea and A1 and stiffness parameter highest concavity (HC) uses displacement from A1 to maximum deflection during HC. The spatial and temporal profiles of the Corvis ST (Oculus Optikgeräte, Wetzlar, Germany) air puff were characterized using hot wire anemometry. An adjusted air pressure impinging on the cornea at A1 (adjAP1) and an algorithm to biomechanically correct intraocular pressure based on finite element modelling (bIOP) were used for Resultant Pressure calculation (adjAP1 - bIOP). Linear regression analyses between DCR parameters and stiffness parameters were performed on a retrospective dataset of 180 keratoconic eyes and 482 normal eyes. DCR parameters from a subset of 158 eyes of 158 patients in each group were matched for bIOP and compared using t tests. A P value of less than .05 was considered statistically significant.

RESULTS

All DCR parameters evaluated showed significant differences between normal and keratoconic eyes, except peak distance. Keratoconic eyes had lower stiffness parameter values, thinner pachymetry, shorter applanation lengths, greater absolute values of applanation velocities, earlier A1 times and later second applanation times, greater HC deformation amplitudes and HC deflection amplitudes, and lower HC radius of concave curvature (greater concave curvature). Most DCR parameters showed a significant relationship with both stiffness parameters in both groups.

CONCLUSIONS

Keratoconic eyes demonstrated less resistance to deformation than normal eyes with similar IOP. The stiffness parameters may be useful in future biomechanical studies as potential biomarkers. [J Refract Surg. 2017;33(4):266-273.].

Comparison of Corneal Deformation Parameters in Keratoconic and Normal Eyes Using a Non-contact Tonometer With a Dynamic Ultra-High-Speed Scheimpflug Camera

PURPOSE

To evaluate and compare biomechanical properties in normal and keratoconic eyes using a dynamic ultra-high-speed Scheimpflug camera equipped with a non-contact tonometer (Corvis ST; Oculus Optikgeräte GmbH, Wetzlar, Germany).

METHODS

This retrospective study evaluated 89 eyes (47 normal, 42 keratoconic) and a validation arm of 72 eyes (33 normal, 39 keratoconic) using the Corvis ST. A diagnosis of keratoconus was established by clinical findings confirmed by topography and tomography. Dynamic corneal response parameters collected by the Corvis ST (A1 velocity, deformation amplitude [DA], DA Ratio Max 1mm, and Max Inverse Radius) and a stiffness parameter at first applanation (SP-A1) were incorporated into a novel logistic regression equation (DCR index). Area under the receiver operating curve (AUC) was used to assess the sensitivity and specificity of the DCR index.

RESULTS

DA, DA Ratio Max 1mm, Max Inverse Radius, and SP-A1 were each found to be statistically significantly different between normal and keratoconic eyes (Mann-Whitney test [independent samples]; P = .0077, < .0001, < .0001, and < .0001, respectively; significance level: P < .05). DCR index demonstrated high sensitivity, specificity, and overall correct detection rate (92.9%, 95.7%, and 94.4%, respectively; AUC = 98.5). The sensitivity and overall correct detection rate improved when eyes with Topographical Keratoconus Classification grades (TKC) greater than 0 were reevaluated (from 92.9% to 96.6% and from 94.4% to 96.1%, respectively).

CONCLUSIONS

Combining multiple biomechanical parameters (A1 velocity, DA, DA Ratio Max 1mm, Max Inverse Radius, and SP-A1) into a logistic regression equation allows for high sensitivity and specificity for distinguishing keratoconic from normal eyes. [J Refract Surg. 2017;33(9):625-631.].

Dynamic corneal deformation response and integrated corneal tomography

Measuring corneal biomechanical properties is still challenging. There are several clinical applications for biomechanical measurements, including the detection of mild or early forms of ectatic corneal diseases. This article reviews clinical applications for biomechanical measurements provided by the Corvis ST dynamic non contact tonometer.

Towards the Effective use of Diagnostic Radiology in Surgical Practice: Discussion Paper

This paper takes as an example of the cycle of evaluation the work of the Royal College of Radiologists in connection with the more effective use of preoperative chest X-ray. The work covers a six year period beginning with an observation of practice on a national sample in 1976, progressing to the comparison of existing practice with expectations in 1978. This was followed by the development of guidelines of practice which were disseminated informally, and formally through scientific papers and meetings from 1979. In 1981, practice was again observed in a pilot area to ascertain if the proposed guidelines had produced the desired affect. The results of this second review showed that the intentions of the guidelines had been achieved successfully, i.e. substantial reduction in utilization with considerable potential financial saving without any decrease in the original effectiveness or safety of the intervention. Thus this example shows the closing of the evaluation loop and the successful implementation of change.

Biomechanical Characterization of Subclinical Keratoconus Without Topographic or Tomographic Abnormalities

PURPOSE

To present a case series of patients with subclinical keratoconus with normal topometric (anterior curvature) and tomographic findings in one eye who showed abnormalities detected by Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) in vivo biomechanical assessment.

METHODS

All patients had a complete ophthalmic examination, including the Corvis ST biomechanical measurements, optical tomography, and pachymetry with Pentacam (Oculus Optikgeräte GmbH), and Placido-based topography with either the Nidek (OPD III Nidek, Gamagori, Japan) or CSO platform (Costruzione Strumenti Oftalmici, Florence, Italy). Inclusion criteria were a clinical diagnosis of ectasia in one eye and normal topometric and tomographic findings in the fellow eye (subclinical keratoconus), including a Belin/Ambrósio Enhanced Ectasia total deviation index from the Pentacam with less than 1.6 standard deviations from normative values and a Corvis Biomechanical Index score of greater than 0.5 in both eyes.

RESULTS

Tomographic and topographic analysis was normal in one eye and abnormal in the fellow eye in 12 patients. The biomechanical results with the Corvis Biomechanical Index were shown to be abnormal in both eyes of all patients and aided the diagnosis.

CONCLUSIONS

Biomechanical analysis showed abnormalities, whereas tomography and topography were normal. Basing on these findings, the authors suggest the use of biomechanics as an additional diagnostic tool. [J Refract Surg. 2017;33(6):399-407.].

Detection of Keratoconus With a New Biomechanical Index

PURPOSE

To evaluate the ability of a new combined biomechanical index called the Corvis Biomechanical Index (CBI) based on corneal thickness profile and deformation parameters to separate normal from keratoconic patients.

METHODS

Six hundred fifty-eight patients (329 eyes in each database) were included in this multicenter retrospective study. Patients from two clinics located on different continents were selected to test the capability of the CBI to separate healthy and keratoconic eyes in more than one ethnic group using the Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany). Logistic regression was employed to determine, based on Database 1 as the development dataset, the optimal combination of parameters to accurately separate normal from keratoconic eyes. The CBI was subsequently independently validated on Database 2.

RESULTS

The CBI included several dynamic corneal response parameters: deformation amplitude ratio at 1 and 2 mm, applanation 1 velocity, standard deviation of deformation amplitude at highest concavity, Ambrósio's Relational Thickness to the horizontal profile, and a novel stiffness parameter. The receiver operating characteristic curve analysis of the training database showed an area under the curve of 0.983. With a cut-off value of 0.5, 98.2% of the cases were correctly classified with 100% specificity and 94.1% sensitivity. In the validation dataset, the same cut-off point correctly classified 98.8% of the cases with 98.4% specificity and 100% sensitivity.

CONCLUSIONS

The CBI was shown to be highly sensitive and specific to separate healthy from keratoconic eyes. The presence of an external validation dataset confirms this finding and suggests the possible use of the CBI in everyday clinical practice to aid in the diagnosis of keratoconus. [J Refract Surg. 2016;32(12):803-810.].

Corneal Biomechanics in Ectatic Diseases: Refractive Surgery Implications

BACKGROUND

Ectasia development occurs due to a chronic corneal biomechanical decompensation or weakness, resulting in stromal thinning and corneal protrusion. This leads to corneal steepening, increase in astigmatism, and irregularity. In corneal refractive surgery, the detection of mild forms of ectasia pre-operatively is essential to avoid post-operative progressive ectasia, which also depends on the impact of the procedure on the cornea.

METHOD

The advent of 3D tomography is proven as a significant advancement to further characterize corneal shape beyond front surface topography, which is still relevant. While screening tests for ectasia had been limited to corneal shape (geometry) assessment, clinical biomechanical assessment has been possible since the introduction of the Ocular Response Analyzer (Reichert Ophthalmic Instruments, Buffalo, USA) in 2005 and the Corvis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) in 2010. Direct clinical biomechanical evaluation is recognized as paramount, especially in detection of mild ectatic cases and characterization of the susceptibility for ectasia progression for any cornea.

CONCLUSIONS

The purpose of this review is to describe the current state of clinical evaluation of corneal biomechanics, focusing on the most recent advances of commercially available instruments and also on future developments, such as Brillouin microscopy.

Influence of Pachymetry and Intraocular Pressure on Dynamic Corneal Response Parameters in Healthy Patients

PURPOSE

To evaluate the influence of pachymetry, age, and intraocular pressure in normal patients and to provide normative values for all dynamic corneal response parameters (DCRs) provided by dynamic Scheimpflug analysis.

METHODS

Seven hundred five healthy patients were included in this multicenter retrospective study. The biomechanical response data were analyzed to obtain normative values with their dependence on corrected and clinically validated intraocular pressure estimates developed using the finite element method (bIOP), central corneal thickness (CCT), and age, and to evaluate the influence of bIOP, CCT, and age.

RESULTS

The results showed that all DCRs were correlated with bIOP except deflection amplitude (DefA) ratio, highest concavity (HC) radius, and inverse concave radius. The analysis of the relationship of DCRs with CCT indicated that HC radius, inverse concave radius, deformation amplitude (DA) ratio, and DefA ratio were correlated with CCT (rho values of 0.343, -0.407, -0.444, and -0.406, respectively). The age group subanalysis revealed that primarily whole eye movement followed by DA ratio and inverse concave radius were the parameters that were most influenced by age. Finally, custom software was created to compare normative values to imported examinations.

CONCLUSIONS

HC radius, inverse concave radius, DA ratio, and DefA ratio were shown to be suitable parameters to evaluate in vivo corneal biomechanics due to their independence from IOP and their correlation with pachymetry and age. The creation of normative values allows the interpretation of an abnormal examination without the need to match every case with another normal patient matched for CCT and IOP. [J Refract Surg. 2016;32(8):550-561.].

Modulating non-native aggregation and electrostatic protein-protein interactions with computationally designed single-point mutations

Non-native protein aggregation is a ubiquitous challenge in the production, storage and administration of protein-based biotherapeutics. This study focuses on altering electrostatic protein-protein interactions as a strategy to modulate aggregation propensity in terms of temperature-dependent aggregation rates, using single-charge variants of human γ-D crystallin. Molecular models were combined to predict amino acid substitutions that would modulate protein-protein interactions with minimal effects on conformational stability. Experimental protein-protein interactions were quantified by the Kirkwood-Buff integrals (G22) from laser scattering, and G22 showed semi-quantitative agreement with model predictions. Experimental initial-rates for aggregation showed that increased (decreased) repulsive interactions led to significantly increased (decreased) aggregation resistance, even based solely on single-point mutations. However, in the case of a particular amino acid (E17), the aggregation mechanism was altered by substitution with R or K, and this greatly mitigated improvements in aggregation resistance. The results illustrate that predictions based on native protein-protein interactions can provide a useful design target for engineering aggregation resistance; however, this approach needs to be balanced with consideration of how mutations can impact aggregation mechanisms.

Biomechanical Modeling of Femtosecond Laser Keyhole endokeratophakia Surgery

PURPOSE

To apply a finite element model to endokeratophakia and evaluate anterior and posterior corneal surface changes.

METHODS

Spatial elevation data (Pentacam HR; Oculus, Wetzlar, Germany) were obtained for the front and back corneal surfaces of an eye prior to undergoing an endokeratophakia procedure. These were used to warp a spherical template finite element model of the cornea to create a patient-specific finite element mesh and the initial stress distribution was computed with an iterative approach. The finite element model (Optimeyes; Integrated Scientific Services, Biel, Switzerland) included non-linear elastic characteristics of the stroma. The endokeratophakia procedure was recreated in the model: a donor lenticule (-10.50 diopters [D], 5.75-mm zone, 127-µm thick) was inserted into a lamellar pocket (180-µm deep, 6.25-mm diameter) and two 2-mm small incisions were made at 150° and 330°. Anterior and posterior surfaces, computed by the finite element model, were compared to clinical data to assess accuracy and reliability of finite element modeling.

RESULTS

The postoperative axial curvature produced by the model closely resembled the patient data; average curvature was 48.01 D clinically and 48.23 D in the simulation, and corneal astigmatism was 3.01 D clinically and 2.88 D in the simulation. The posterior best-fit sphere elevation map also matched the patient data, replicating inward bulging of the posterior surface by approximately 40 µm. Stress distribution modeling predicted a stress increase by 159.94% ± 73% in the cap and a stress decrease by 32.41% ± 21% in the stromal bed.

CONCLUSIONS

Finite element modeling of the cornea reproduced the clinically observed anterior and posterior corneal surface changes following an endokeratophakia procedure. This case sets the stage for further study to refine and yield predictive finite element modeling for the evaluation of corneal refractive surgical procedures.

Increased Ocular Pulse Amplitude Associated with Unilateral Dysgenesis of the Orbital Roof

Introduction

Two patients (one with neurofibromatosis type 1) presented with unilateral ocular pulsation.

Methods

A CT scan of the orbits revealed extensive dysgenesis of the orbital roof with herniation of the frontal lobe into the orbit in both cases. PASCAL dynamic contour tonometry was performed.

Results

The ipsilateral ocular pulse amplitude (OPA) was greater than the contralateral side, and the ocular pulse waveform morphology more closely approximated the known intracranial waveform in these patients.

Conclusions

We hypothesize that the greater OPA was due to stronger transmission of the intracranial pressure waveform amplitude and morphology in the absence of the orbital roof.

Ocular pressure waveform reflects ventricular bigeminy and aortic insufficiency

Ocular pulse amplitude (OPA) is defined as the difference between maximum and minimum intraocular pressure (IOP) during a cardiac cycle. Average values of OPA range from 1 to 4 mmHg. The purpose of this investigation is to determine the source of an irregular IOP waveform with elevated OPA in a 48-year-old male. Ocular pressure waveforms had an unusual shape consistent with early ventricular contraction. With a normal IOP, OPA was 9 mmHg, which is extraordinarily high. The subject was examined by a cardiologist and was determined to be in ventricular bigeminy. In addition, he had bounding carotid pulses and echocardiogram confirmed aortic insufficiency. After replacement of the aortic valve, the bigeminy resolved and the ocular pulse waveform became regular in appearance with an OPA of 1.6–2.0 mmHg. The ocular pressure waveform is a direct reflection of hemodynamics. Evaluating this waveform may provide an additional opportunity for screening subjects for cardiovascular anomalies and arrhythmias.

Comparison of biomechanical effects of small-incision lenticule extraction and laser in situ keratomileusis: finite-element analysis

PURPOSE

To theoretically compare the corneal stress distribution of laser in situ keratomileusis (LASIK) with the stress distribution of small-incision lenticule extraction.

SETTING

Cleveland Clinic Cole Institute, Cleveland, and The Ohio State University, Columbus, Ohio, USA.

DESIGN

Computational modeling study.

METHODS

A finite-element anisotropic collagen fiber-dependent model of myopic surgery using patient-specific corneal geometry was constructed for LASIK, small-incision lenticule extraction, and a geometry analog model with unaltered material properties from preoperative but with postoperative geometry including thickness. Surgical parameters, magnitude of myopic correction, LASIK flap thickness, and lenticule depth in small-incision lenticule extraction were varied. Two sets of models, 1 with uniform and 1 with depth-dependent material properties, were constructed.

RESULTS

Stress distribution between small-incision lenticule extraction simulations and the geometry analog model were similar. In contrast, LASIK consistently reduced stress in the flap and increased stress in the residual stromal bed (RSB) compared with the geometry analog model. An increase in flap thickness or lenticule depth resulted in a greater increase in RSB stress in the LASIK model than in the small-incision lenticule extraction model.

CONCLUSIONS

Small-incision lenticule extraction may present less biomechanical risk to the residual bed of susceptible corneas than comparable corrections involving LASIK flaps. Deeper corrections in the stroma may be possible in small-incision lenticule extraction without added risk for ectasia.

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.

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.

Effect of Intraocular Pressure and Anisotropy on the Optical Properties of the Cornea: A Study Using Polarization Sensitive Optical Coherence Tomography

PURPOSE

We hypothesize that because of the anisotropic properties of the cornea, there should be a nonuniform change in birefringence with an increase in intraocular pressure (IOP). In this in vitro study, anisotropic properties, stress distribution within the cornea, and the effect of IOP on changes in stress level were investigated.

DESIGN

Button inflation tests for deformation with polarization sensitive optical coherence tomography were used to demonstrate optical and material anisotropy on ex vivo human corneas.

METHODS

Inflation tests were performed on human donor corneoscleral rims. Using a turntable and hydrostatic column, each corneoscleral rim was subjected to a hydrostatic pressure of 0, 10, 15, and 20 mm Hg. At each pressure step, 4 scans at 0, 45, 90, and 135 degrees were taken by a polarization sensitive optical coherence tomography system, and the birefringence images and normal intensity-based images were recorded; images were later compiled for analysis.

RESULTS

The retardation changed with the axis of orientation (P [T ≤ t] 1-tailed = 0.025) and IOP (P [T ≤ t] 1-tailed = 0.019). Optical thickness of the cornea decreased with increasing IOP.

CONCLUSIONS

The optical properties of the cornea are modified with change in IOP. This is not uniform because of distinct anisotropic properties. Anisotropic properties may unpredictably affect the optical quality of cornea during or after the surgeries. Changes in corneal birefringence can be also used as a tool for measuring the IOP of the eye.

Impact of chamber pressure and material properties on the deformation response of corneal models measured by dynamic ultra-high-speed Scheimpflug imaging

PURPOSE

To study the deformation response of three distinct contact lenses with known structures, which served as corneal models, under different chamber pressures using ultra-high-speed (UHS) Scheimpflug imaging.

METHODS

Three hydrophilic contact lenses were mounted on a sealed water chamber with precisely adjustable pressure: TAN-G5X (41% hydroxyethylmethacrylate/glycolmethacrylate, 550 µm thick), TAN-40 (62% hydroxyethylmethacrylate, 525 µm thick) and TAN-58 (42% methylmethacrylate, 258 µm thick). Each model was tested five times under different pressures (5, 15, 25, 35 and 45 mmHg), using ultra-high-speed Scheimpflug imaging during non-contact tonometry. 140 Scheimpflug images were taken with the UHS camera in each measurement. The deformation amplitude during non-contact tonometry was determined as the highest displacement of the apex at the highest concavity (HC) moment.

RESULTS

At each pressure level, the deformation amplitude was statistically different for each lens tested (p<0.001, ANOVA). Each lens had different deformation amplitudes under different pressure levels (p<0.001; Bonferroni post-hoc test). The thicker lens with less polymer (TAN-G5X) had a higher deformation (less stiff behavior) than the one that was thinner but with more polymer (TAN-40), when measured at the same internal pressure. The thinnest lens with less polymers (TAN-58) had a lower deformation amplitude (stiffer behavior) at higher pressures than the thicker ones with more polymer (TAN-40 and TAN-G5X) at lower pressures.

CONCLUSIONS

UHS Scheimpflug imaging allowed for biomechanical assessment through deformation characterization of corneal models. Biomechanical behavior was more influenced by material composition than by thickness. Chamber pressure had a significant impact on deformation response of each lens.

Corneal curvature gradient map: a new corneal topography map to predict the corneal healing process

PURPOSE

To evaluate a new curvature gradient topography map to predict postoperative corneal remodeling.

METHODS

In this retrospective study, 32 eyes of 16 patients with myopia underwent excimer ablation surgery with a postoperative high curvature gradient. The new curvature gradient map (acquired immediately postoperatively) shows the difference between the curvatures of two points over the distance between them; it was compared to the tangential curvature difference map between 1 and 12 months postoperatively to determine their relationship. Corneas were divided into 12 regions for analysis: four 90°-wide sectors centered on 0°, 90°, 180°, and 270°. There were three subdivisions in each sector: central (radius: 0 to 2.75 mm), paracentral (radius: 2.75 to 3.25 mm), and peripheral (radius: 3.25 to 4.5 mm). Linear regression analysis was performed by region.

RESULTS

The following regions had significant relationships between the initial curvature gradient and curvature difference between 1 and 12 months postoperatively: the paracentral zone of the 90° sector (P = .0145; R(2) = 0.1832) and both the central (P = .0034; R(2) = 0.2522) and paracentral (P = .0452; R(2) = 0.1271) zones of the 270° sector. The greatest average initial tangential curvature was in the 270° sector.

CONCLUSIONS

The initial curvature gradient after surgery predicted change in tangential curvature over the subsequent 12 months in areas where initial tangential curvature was greatest. When the curvature gradient was high, the surface curvature modification remained in progress months after surgery.

Air puff induced corneal vibrations: theoretical simulations and clinical observations

PURPOSE

To investigate air puff induced corneal vibrations and their relationship to the intraocular pressure (IOP), viscoelasticity, mass, and elasticity of the cornea based on theoretical simulations and preliminary clinical observations.

METHODS

To simulate the corneal movement during air puff deformation, a kinematic viscoelastic corneal model was developed involving the factors of corneal mass, damping coefficient, elasticity, and IOP. Different parameter values were taken to investigate how factors would affect the corneal movements. Two clinical ocular instruments, CorVis ST (Oculus Optikgeräte GmbH, Wetzlar, Germany) and the Ocular Response Analyzer (ORA; Reichert, Inc., Buffalo, NY), were employed to observe the corneal dynamical behaviors.

RESULTS

Numerical results showed that during the air puff deformation, there would be vibrations along with the corneal deformation, and the damping viscoelastic response of the cornea had the potential to reduce the vibration amplitude. With consistent IOP, the overall vibration amplitude and inward motion depths were smaller with a stiffer cornea.

CONCLUSIONS

A kinematic viscoelastic model of the cornea is presented to illustrate how the vibrations are associated with factors such as corneal mass, viscoelasticity, and IOP. Also, the predicted corneal vibrations during air puff deformation were confirmed by clinical observation.

Ocular biomechanical metrics by CorVis ST in healthy Brazilian patients

PURPOSE

To evaluate ocular biomechanical metrics given by the CorVis ST (Oculus, Inc., Berlin, Germany) in a population of healthy Brazilian patients.

METHODS

An observational and cross-sectional study involving 1 eye randomly selected from 90 healthy patients. Studied parameters (including deformation amplitude, first applanation time, highest concavity time, second applanation time, first applanation length, second applanation length, curvature radius highest concavity, curvature radius normal, velocity in, and velocity out) derived from the CorVis ST were correlated to central corneal thickness from the Pentacam (Oculus, Inc.). Differences between data on the basis of gender were evaluated.

RESULTS

Mean patient age was 35.80 ± 12.83 years (range: 21.07 to 78.84 years). Mean central corneal thickness was 547.50 ± 32.00 μm (range: 490 to 647 μm) and mean spherical equivalent refraction was -3.29 ± 3.69 diopters (range: -9.50 to +10.37 diopters). Mean deformation amplitude was 1.05 ± 0.08 mm (range: 0.91 to 1.26 mm). Highest concavity time was 18.38 ± 0.93 ms (range: 16.95 to 21.07 ms). Intraocular pressure was 16.43 ± 2.15 mm Hg (range: 11.50 to 21.0 mm Hg). First applanation time was 8.32 ± 0.33 ms (range: 7.53 to 9.12 ms) and second applanation time was 23.80 ± 0.44 ms (range: 22.76 to 24.95 ms). First applanation length (max) was 2.07 ± 0.38 mm (range: 1.20 to 3.10 mm) and second applanation length (max) was 2.37 ± 0.47 mm (range: 1.33 to 4.12 mm). Curvature radius highest concavity was 11.09 ± 2.06 mm (range: 7.58 to 15.98 mm) and curvature radius normal was 7.59 ± 0.67 mm (range: 6.82 to 11.02 mm). Velocity in was 0.21 ± 0.05 m/s (range: 0.16 to 0.72 m/s) and velocity out was -0.33 ± 0.07 m/s (range: -0.72 to -0.20 m/s). Studied parameters were not associated with gender.

CONCLUSIONS

Eight of 11 ocular biomechanical metrics given by the CorVis ST were associated with central corneal thickness, but the influence of central corneal thickness on these measurements was low.

Supersonic shear wave elastography for the in vivo evaluation of transepithelial corneal collagen cross-linking

PURPOSE

To assess corneal stiffening with supersonic shear wave imaging (SSI) technology in an experimental model of iontophoresis-assisted transepithelial corneal collagen cross-linking (I-CXL).

METHODS

Six rabbits underwent full, central I-CXL in one eye. The contralateral eye served as control. In vivo iontophoresis was used for 10 minutes to perform transepithelial delivery of riboflavin prior to UV-A irradiation. Accelerated UV-A protocol was applied for 9 minutes with a 10-mW/cm(2) irradiance. Animals were killed and both treated and control corneas were then immediately mounted on a corneal artificial anterior chamber and internal pressure was varied from 15 to 50 mm Hg in 5-mm Hg increments. Swelling was evaluated via central corneal thickness measurements. Ex vivo inflation tests were monitored using SSI technology that provides real-time mapping of the corneal elasticity.

RESULTS

Corneal yellowing of the central 9-mm diameter area was clearly visible in the iontophoresis area of all treated eyes. Elasticity versus internal pressure revealed significant differences of the change in elasticity coefficient with pressure between I-CXL-treated and control corneas with a mean slope that was 27.1 and 16.9 kPa/mm Hg, respectively (P = 0.029). Differences in elasticity at individual pressure levels between groups were statistically significant above 40 mm Hg (P < 0.05).

CONCLUSIONS

Intraocular pressure variations were the most important limitations for in vivo stiffness monitoring with SSI because stiffness is a function of internal pressure. Supersonic shear wave imaging succeeded in comparing corneas that underwent I-CXL by performing ex vivo inflation tests where pressure was controlled. Iontophoresis-assisted transepithelial corneal collagen cross-linking corneas exhibited increased resistance to pressure rise, indicating stiffening. In vivo I-CXL and ex vivo SSI is an interesting model to evaluate the sole effect of photopolymerization occurring in the CXL process close to physiological conditions.

Expanding the cone location and magnitude index to include corneal thickness and posterior surface information for the detection of keratoconus

PURPOSE

To extend the capabilities of the Cone Location and Magnitude Index algorithm to include a combination of topographic information from the anterior and posterior corneal surfaces and corneal thickness measurements to further improve our ability to correctly identify keratoconus using this new index: ConeLocationMagnitudeIndex_X.

DESIGN

Retrospective case-control study.

METHODS

Three independent data sets were analyzed: 1 development and 2 validation. The AnteriorCornealPower index was calculated to stratify the keratoconus data from mild to severe. The ConeLocationMagnitudeIndex algorithm was applied to all tomography data collected using a dual Scheimpflug-Placido-based tomographer. The ConeLocationMagnitudeIndex_X formula, resulting from analysis of the Development set, was used to determine the logistic regression model that best separates keratoconus from normal and was applied to all data sets to calculate PercentProbabilityKeratoconus_X. The sensitivity/specificity of PercentProbabilityKeratoconus_X was compared with the original PercentProbabilityKeratoconus, which only uses anterior axial data.

RESULTS

The AnteriorCornealPower severity distribution for the combined data sets are 136 mild, 12 moderate, and 7 severe. The logistic regression model generated for ConeLocationMagnitudeIndex_X produces complete separation for the Development set. Validation Set 1 has 1 false-negative and Validation Set 2 has 1 false-positive. The overall sensitivity/specificity results for the logistic model produced using the ConeLocationMagnitudeIndex_X algorithm are 99.4% and 99.6%, respectively. The overall sensitivity/specificity results for using the original ConeLocationMagnitudeIndex algorithm are 89.2% and 98.8%, respectively.

CONCLUSIONS

ConeLocationMagnitudeIndex_X provides a robust index that can detect the presence or absence of a keratoconic pattern in corneal tomography maps with improved sensitivity/specificity from the original anterior surface-only ConeLocationMagnitudeIndex algorithm.

Comparison of keratometric changes after myopic ablation: ray tracing versus simulated keratometry

PURPOSE

To compare the changes in corneal power (ΔK) induced by myopic ablations estimated by Placido-disk and total corneal power (TCP) ray tracing methods to the refractive change (ΔSE).

METHODS

Manifest refraction, simulated keratometry from a Placido-disk based system, and TCP from a dual-Scheimpflug analyzer were obtained from 35 patients (58 eyes) before and 3 months after a myopic ablation. The change in the manifest refraction that occurred after surgery was then vertex distance corrected and compared to the changes observed in corneal power with the various systems analyzed.

RESULTS

The Placido-based simulated keratometry overestimated the corneal power after the procedure by 0.50 ± 0.53 diopters (D) when compared to refractive change in the corneal plane induced by the laser surgery (ΔSE-ΔK). The ray tracing method showed the opposite trend, with the TCP simulated keratometry showing an underestimation of the corneal power of -0.25 ± 0.48 D. The Placido system showed a direct correlation between the overestimation of the corneal power and the level of myopia, whereas the ray tracing method showed an underestimation, which was more pronounced in higher levels of myopia.

CONCLUSIONS

Ray tracing methods underestimate corneal power as opposed to the overestimation in corneal power after refractive surgery, directly related to the level of myopia, observed in Placido-based systems. They have the potential to overcome the errors in calculations based on anterior curvature alone. However, ray tracing methods need to be validated and optimized before it can be used routinely in IOL calculation.

Experimental design and instability analysis of coaxial electrospray process for microencapsulation of drugs and imaging agents

Recent developments in multimodal imaging and image-guided therapy requires multilayered microparticles that encapsulate several imaging and therapeutic agents in the same carrier. However, commonly used microencapsulation processes have multiple limitations such as low encapsulation efficiency and loss of bioactivity for the encapsulated biological cargos. To overcome these limitations, we have carried out both experimental and theoretical studies on coaxial electrospray of multilayered microparticles. On the experimental side, an improved coaxial electrospray setup has been developed. A customized coaxial needle assembly combined with two ring electrodes has been used to enhance the stability of the cone and widen the process parameter range of the stable cone-jet mode. With this assembly, we have obtained poly(lactide-co-glycolide) microparticles with fine morphology and uniform size distribution. On the theoretical side, an instability analysis of the coaxial electrified jet has been performed based on the experimental parameters. The effects of process parameters on the formation of different unstable modes have been studied. The reported experimental and theoretical research represents a significant step toward quantitative control and optimization of the coaxial electrospray process for microencapsulation of multiple drugs and imaging agents in multimodal imaging and image-guided therapy.

Tuning the mechanical properties of self-assembled mixed-peptide tubes

In this study, nano- and microscale fibrillar and tubular structures formed by mixing two aromatic peptides known to self-assemble separately, (diphenylalanine and di-D-2-napthylalanine) have been investigated. The morphology, mechanical strength and thermal stability of the tubular structures formed have been studied. The tubes are shown to consist of both peptides with some degree of nanoscale phase separation. The ability of the mixed peptides to form structures, which display variable mechanical properties dependent on the percentage composition of the peptides is presented. Such materials with tuneable properties will be required for a range of applications in nanotechnology and biotechnology.

Corneal biomechanics and biomaterials

The ability to clearly observe one's environment in the visible spectrum provides a tremendous evolutionary advantage in most of the world's habitats. The complex optical processing system that has evolved in higher vertebrate animals gathers, focuses, detects, transduces, and interprets incoming visible light. The cornea resides at the front end of this imaging system, where it provides a clear optical aperture, substantial refractive power, and the structural stability required to protect the fragile intraocular components. Nature has resolved these simultaneous design requirements through an exceedingly clever manipulation of common extracellular-matrix structural materials (e.g., collagen and proteoglycans). In this review, we (a) examine the biophysical and optical roles of the cornea, (b) discuss increasingly popular approaches to altering its natural refractive properties with an emphasis on biomechanics, and (c) investigate the fast-rising science of corneal replacement via synthetic biomaterials. We close by considering relevant open problems that would benefit from the increased attention of bioengineers.

Biomechanical and Refractive Results of Transepithelial Cross-linking Treatment in Keratoconic Eyes

Purpose

To analyze change in visual acuity (VA), refractive outcomes, corneal compensated intraocular pressure (IOPcc), corneal hysteresis (CH) and cornea resistance factor (CRF) after transepithelial cross-linking (CXL) treatment.

Setting

Kudret Eye Hospital, Ankara, Turkey

Materials and methods

A total of 32 eyes of 22 patients diagnosed with keratoconus were included in this retrospective study. Changes in VA, spheric and cylinderic refraction for all eyes were analyzed before and 3 months after transepithelial CXL. In addition, a subset of 14 eyes had data acquired using the ocular response analyzer (ORA), and for these eyes, IOPcc, CH and CRF were recorded as well as a measurement of the amplitude of the first peak in the infrared signal (peak 1), used to identify the inward applanation event. The ORA parameters were also compared before and 3 months after treatment.

Results

Uncorrected and best corrected VA increased 1.76 and 1.61 Snellen lines (p < 0.05); spheric and cylinderic refractions improved 0.74 and 0.43 D (p > 0.05) respectively. Mean changes in IOPcc, CRF and CH were not significantly different (p > 0.05). However, mean peak 1 signal value increased significantly (p < 0.05).

Conclusion

Transepithelial CXL is effective in VA improvement in the short-term with an increase in the peak 1 signal value which is consistent with an increase in stiffness of cornea, whereas IOPcc, CRF and CH values remained the same.

How to cite this article

Klllç A, Roberts CJ. Biomechanical and Refractive Results of Transepithelial Cross-linking Treatment in Keratoconic Eyes. Int J Kerat Ect Cor Dis 2012;1(2):75-78.

Comparison of Placido, Scheimpflug and Combined Dual Scheimpflug-Placido Technologies in Evaluating Anterior and Posterior CLMI, SimK's as well as Kmax, in Keratoconic and Postrefractive Surgery Ectasia

Purpose

To calculate and compare cone location and magnitude index (CLMI), Kmax and other corneal measures derived from three different technologies, Placido, Scheimpflug, and a combination dual Scheimpflug-Placido device, from the same group of eyes with keratoconus and postrefractive surgery corneal ectasia.

Methods

Keratoconus (n = 26) eyes of (n = 19) subjects and postrefractive surgery ectasia (n = 5) eyes of (n = 5) subjects were selected to have measurements performed using the Keratron Scout, Pentacam HR and Galilei Dual Scheimpflug Analyzer. Device-generated SimK's and device-specific CLMI and Kmax indices as well as map data, were exported from each device. Index values for multiple exams were averaged. The map data were processed using The Ohio State University Corneal Topography Tool (OSUCTT) to calculate CLMI parameters, Kmax and SimK values using consistent algorithms on all three devices. Maps were averaged before calculation for multiple examinations. Repeated measures analysis of variance and post- hoc analysis were used to identify differences between devices.

Results

The anterior axial CLMI calculated from the Keratron data was significantly higher than CLMI for the Galilei (p = 0.0443) or Pentacam (p < 0.0004) with keratoconus, 12.23 compared with 11.20 and 11.00 diopters, respectively. Kmax was also significantly higher in the Keratron than the Galilei (p = 0.0063) or the Pentacam (p < 0.0002). Galilei and Pentacam were not significantly different from each other in either CLMI (p = 0.6287) or Kmax (p = 0.2115). The anterior CLMI values for the postrefractive surgery ectasia eyes were not significantly different between devices. Posterior CLMI values were calculated from the Galilei and Pentacam data and were −2.60 and −2.46 diopters (p = 0.1173) for keratoconus and −2.66 and −3.04 diopters (p = 0.2242) for postrefractive surgery ectasia.

Conclusion

The small cone Placido measured dioptric values that were greater than the pure Scheimpflug system, but the difference was only about 1 diopter, which is not relevant clinically in evaluating and managing ectasia. The combined dual Scheimpflug-Placido system produced measured dioptric values between the other two technologies. The anterior CLMI calculations accurately predicted keratoconus with all three devices. The posterior CLMI in ectasia may be a potentially valuable calculation in demonstrating asymmetric steepening.

How to cite this article

Mauger TF, Mahmoud AM, Roberts CJ, Chheda LV, Kuennen RA, Hendershot AJ, Lembach RG. Comparison of Placido, Scheimpflug and Combined Dual Scheimpflug-Placido Technologies in Evaluating Anterior and Posterior CLMI, SimK's as well as Kmax, in Keratoconic and Postrefractive Surgery Ectasia. Int J Keratoco Ectatic Corneal Dis 2012;1(1):44-52.

• C Roberts is a Consultant for Oculus Optikgerate GmbH and Ziemer Ophthalmic Systems AG, and has an interest in the GALILEI.

• A Mahmoud has an interest in the GALILEI.

• T Mauger, L Chheda, R Kuennen, A Hendershot, and R Lembach have no financial interests.

Change in biomechanical parameters after flap creation using the Intralase femtosecond laser and subsequent excimer laser ablation

PURPOSE

This study aims to investigate the effect of both flap creation and laser ablation on corneal hysteresis (CH) and corneal resistance factor (CRF), as well as the inward applanation signal amplitude produced by the ocular response analyzer (ORA), immediately following each step of the LASIK procedure using the Intralase femtosecond laser for flap creation.

METHODS

A total of 66 eyes of 35 subjects who underwent Intra-LASIK surgery were prospectively enrolled in this study. Changes in the CH, CRF, and amplitude of the first applanation peak (Peak 1) of the infrared signal were measured preoperatively, immediately after flap elevation and repositioning, as well as subsequent to laser ablation in all eyes. Repeated measures analysis of variance (ANOVA) was performed with a post-test of contrast variable profiles to investigate changes following each step of the procedure.

RESULTS

The repeated measures ANOVA indicated a significant difference (p < 0.001) among the three time points for CH, CRF, and Peak 1. There was no significant change in CH (p = 0.20) or CRF (p = 0.57) after flap creation, but there was significant decrease in these parameters following laser ablation (p < 0.0001), when compared to values obtained pre-operatively. There was a significant decrease in Peak 1 both after flap creation (p < 0.0001) and after subsequent ablation (p < 0.0001), when compared to pre-operative ORA signal peak amplitudes.

CONCLUSION

There is a well documented change in corneal biomechanical properties following a LASIK procedure. This study reveals that this change may be predominantly due to laser ablation. However, flap creation with the Intralase laser does produce a biomechanical consequence consistent with reduction of corneal stiffness, as measured by the reduced amplitude of Peak 1. CH and CRF do not fully characterize this change.

Intraocular pressure measurements and biomechanical properties of the cornea in eyes after penetrating keratoplasty

PURPOSE

To compare intraocular pressure (IOP) measurements obtained using the Goldmann applanation tonometer (GAT; Haag-Streit), the Tono-Pen XL (Reichert, Inc), and the ocular response analyzer (ORA; Reichert Ophthalmic Instruments), and to determine the influence of corneal factors on IOP measurements in eyes that had undergone penetrating keratoplasty (PK).

DESIGN

Consecutive, prospective study.

METHODS

Study population. Sixty-one eyes that underwent PK were enrolled in this study. Intervention. IOP was measured using the GAT, Tono-Pen, and ORA. Corneal hysteresis and corneal resistance factor as provided by the ORA were recorded. Central corneal thickness was measured using an ultrasound pachymeter. Main outcome measures. IOP and corneal biomechanical factors.

RESULTS

IOP measurements were obtained in an average of 65 months (range, 6 to 209 months) after PK surgery. ORA-derived IOP measurements (corneal-compensated IOP and Goldmann-correlated IOP) and Tono-Pen XL IOP all correlated in a significant manner to GAT IOP measurements. Corneal-compensated IOP and Tono-Pen XL IOP values were higher than GAT IOP (P < .001 and P = .001, respectively), whereas Goldmann-correlated IOP readings did not differ from GAT IOP readings (P = .054). Central corneal thickness did not correlate to any tonometry technique. In a regression analysis, corneal hysteresis and corneal resistance factor were found to play a role in IOP prediction.

CONCLUSIONS

Central corneal thickness may be of less importance than corneal hysteresis and corneal resistance factor in IOP determination in eyes that have undergone PK, perhaps because of the lower modulus of elasticity in these eyes. GAT IOP seems to be lower than other tonometry techniques in eyes that have undergone PK.

Demonstration of ignition radiation temperatures in indirect-drive inertial confinement fusion hohlraums

We demonstrate the hohlraum radiation temperature and symmetry required for ignition-scale inertial confinement fusion capsule implosions. Cryogenic gas-filled hohlraums with 2.2 mm-diameter capsules are heated with unprecedented laser energies of 1.2 MJ delivered by 192 ultraviolet laser beams on the National Ignition Facility. Laser backscatter measurements show that these hohlraums absorb 87% to 91% of the incident laser power resulting in peak radiation temperatures of T(RAD)=300 eV and a symmetric implosion to a 100 μm diameter hot core.

Endocannabinoid modulation of hyperaemia evoked by physiologically relevant stimuli in the rat primary somatosensory cortex

BACKGROUND AND PURPOSE

In vitro studies demonstrate that cannabinoid CB(1) receptors subserve activity-dependent suppression of inhibition in the neocortex. To examine this mechanism in vivo, we assessed the effects of local changes in CB(1) receptor activity on somatosensory cortex neuronal activation by whisker movement in rats.

EXPERIMENTAL APPROACH

Laser Doppler flowmetry and c-Fos immunohistochemistry were used to measure changes in local blood flow and neuronal activation, respectively. All drugs were applied directly to the cranium above the whisker barrel fields of the primary somatosensory cortex.

KEY RESULTS

The CB(1) receptor agonist WIN55212-2 potentiated the hyperaemia induced by whisker movement and this potentiation was occluded by bicuculline. The CB(1) receptor antagonists, rimonabant and AM251, inhibited hyperaemic responses to whisker movement; indicating that activation of endogenous CB(1) receptors increased during whisker movement. Whisker movement-induced expression of c-Fos protein in neurons of the whisker barrel cortex was inhibited by rimonabant. Movement of the whiskers increased the 2-arachidonoylglycerol content in the contralateral, compared to the ipsilateral, sensory cortex.

CONCLUSIONS AND IMPLICATIONS

These results support the hypothesis that endocannabinoid signalling is recruited during physiologically relevant activation of the sensory cortex. These data support the hypothesis that the primary effect of CB(1) receptor activation within the activated whisker barrel cortex is to inhibit GABA release, resulting in disinhibition of neuronal activation. These studies provide physiological data involving endocannabinoid signalling in activity-dependent regulation of neuronal activation and provide a mechanistic basis for the effects of cannabis use on sensory processing in humans.

Evaluation of a contact lens-embedded sensor for intraocular pressure measurement

PURPOSE

To evaluate a novel contact lens-embedded pressure sensor for continuous measurement of intraocular pressure (IOP).

METHODS

Repeated measurements of IOP and ocular pulse amplitude (OPA) were recorded in 12 eyes of 12 subjects in sitting and supine positions using 3 configurations of the dynamic contour tonometer: slit-lamp mounted (DCT), hand-held (HH), and contact lens-embedded sensor (CL). The IOP and OPA for each condition were compared using repeated measures ANOVA and the 95% limits of agreement were calculated.

RESULTS

The sitting IOP (mean and 95% CI) for each configuration was DCT: 16.3 mm Hg (15.6 to 17.1 mm Hg), HH: 16.6 mm Hg (15.6 to 17.6 mm Hg), and CL: 15.7 mm Hg (15 to 16.3 mm Hg). The sitting OPA for each configuration was DCT: 2.4 mm Hg (2.1 to 2.6 mm Hg), HH: 2.4 mm Hg (2.1 to 2.7 mm Hg), and CL: 2.1 mm Hg (1.8 to 2.3 mm Hg). Supine IOP and OPA measurements with the CL and HH sensors were both greater than their corresponding sitting measurements, but were significantly less with the CL sensor than the HH sensor. The mean difference and 95% Limits of Agreement were smallest for the DCT and CL sensor comparisons (0.7+/-3.9 mm Hg) and widest for the CL and HH sensors (-1.9+/-7.25 mm Hg); these wider limits were attributed to greater HH measurement variability.

CONCLUSIONS

The CL sensor was comparable to HH and DCT sensors with sitting subjects and is a viable method for measuring IOP and OPA. Supine measurements of IOP and OPA were greater than sitting conditions and were comparatively lower with the CL sensor. HH measurements were more variable than CL measurements and this influenced the Limits of Agreement for both sitting and supine conditions.

Multifunctional microbubbles for image-guided antivascular endothelial growth factor therapy

We synthesize multifunctional microbubbles (MBs) for targeted delivery of antivascular endothelial growth factor (antiVEGF) therapy with multimodal imaging guidance. Poly-lactic-co-glycolic acid (PLGA) MBs encapsulating Texas Red dye are fabricated by a modified double-emulsion process. Simultaneous ultrasound and fluorescence imaging are achieved using Texas Red encapsulated MBs. The MBs are conjugated with Avastin, an antiVEGF antibody for treating neovascular age-related macular degeneration (AMD). The conjugation efficiency is characterized by enzyme-linked immunosorbent assay (ELISA). The efficiency for targeted binding of Avastin-conjugated MBs is characterized by microscopic imaging. Our work demonstrates the technical potential of using multifunctional MBs for targeted delivery of antiVEGF therapy in the treatment of exudative AMD.

Strabismus following endoscopic orbital decompression for thyroid eye disease

Endoscopic orbital decompression may be used to treat disfiguring proptosis or sight threatening optic nerve compression in patients with thyroid eye disease. Strabismus is common in thyroid eye disease and frequently follows decompression surgery. We retrospectively reviewed patients undergoing endoscopic decompression for thyroid eye disease, by a single surgeon, from 1994 to 2000. Twenty-three patients (21 female, 2 male) were identified with a mean age of 47.5 years. At presentation, 21 patients had proptosis, 8 optic nerve compression (2 without proptosis) and 11 strabismus (9 complained of diplopia) with a mean BSV score of 24.5 before decompression. Forty orbits were decompressed with a mean decrease in proptosis of 3.3 mm. Following decompression, the mean BSV score was 25, and 17 patients had manifest strabismus in primary gaze (3 at near only) of whom 10 had pre-existing strabismus. Five patients had new diplopia (22%). Eleven patients ultimately required strabismus surgery of whom 8 had manifest strabismus before decompression. Following strabismus surgery, the mean BSV score was 37. The final BSV score for those not requiring strabismus surgery was 29. Mean follow-up was 28 months. Endoscopic orbital decompression can effectively treat disfiguring proptosis. Diplopia is a common complication, but pre-existing diplopia may improve.

A viscoelastic biomechanical model of the cornea describing the effect of viscosity and elasticity on hysteresis

PURPOSE

To develop a method for evaluating viscosity and elasticity of the cornea and to examine the effect that both properties have on hysteresis.

METHODS

A three-component spring and dashpot model was created in Simulink in Matlab to represent the purely elastic and viscoelastic behavior of the cornea during a measurement using device called an ocular response analyzer (ORA). Values for elasticity and viscosity were varied while sinusoidal stress was applied to the model. The simulated stresses were used to determine how hysteresis is affected by the individual components of elasticity, viscosity, and maximum stress. To validate the model, high-speed photography was used to measure induced strain in a corneal phantom during ORA measurement. This measured strain was compared with the strains simulated by the model.

RESULTS

When the spring in the viscoelastic portion of the model was stiffened, hysteresis decreased. When the spring in the purely elastic element was stiffened, hysteresis increased. If both springs were stiffened together, hysteresis peaked strongly as a function of the viscosity of the viscoelastic element. Below the peak value, lower elasticity was associated with higher hysteresis. Above the peak value, higher elasticity was associated with higher hysteresis. In addition, hysteresis increased as the air maximum pressure was increased. Measurements from phantom corresponded to predictions from the model.

CONCLUSIONS

A viscoelastic model is presented to illustrate how changing viscosity and elasticity may affect hysteresis. Low hysteresis can be associated with either high elasticity or low elasticity, depending on the viscosity, a finding consistent with clinical reports.