Understand the Cornea Understand the Pressure

Understand the Cornea Understand the Pressure
Corneal Biomechanics, Accurate IOP, and CCT in one Simple Instrument

One Device, Five Parameters
IOPG - Goldmann Correlated IOP IOPCC - Corneal Compensated IOP CH - Corneal Hysteresis CRF - Corneal Resistance Factor CCT - Central Corneal Thickness IOPG - Pressure measurement that has been clinically shown to correlate very strongly with GAT IOPCC - Pressure measurement that correlates with GAT on average, but is uninfluenced by corneal properties such as thickness and elasticity CH - the worlds first and only direct measure of corneal biomechanical properties CRF - an indication of the cornea’s overall response, including viscous and elastic resistance CCT - central corneal thickness

ORA Technology Background

Measuring “Pressure” Goldmann Tonometry Principles
The Goldmann Tonometer has long been considered the gold standard for measuring pressure. It is based upon the Imbert-Fick Law (W = P x A) where: - W is the force to applanate - P is Intra Ocular Pressure (IOP) - A is the area applanated

Measuring “Pressure” Goldmann Tonometry Assumptions
- Surface is dry - Volume is perfectly spherical - Surface is infinitely thin and perfectly flexible - Tear-film effect and corneal thickness effect cancel each other out Recognizing that corneal effects and surface tension are factors which influence the measurement; Goldmann selected a tonometer tip size of 3.06mm which he believed would nullify these effects based on a constant central corneal thickness of 525 microns

Measuring “Pressure” Goldmann Tonometry Flaws
- Experimentation done on cadaver eyes - Not representative of live corneas - Variation in corneal thickness is significantly greater than assumed - Variations in corneal biomechanical properties unaccounted for By now nearly everyone recognizes that the current gold standard for measuring pressure, the Goldmann tonometer, has considerable flaws. The GAT, and any tonometer calibrated to “read like” a GAT, is affected by corneal properties including rigidity, thickness, hydration, curvature, and perhaps other factors not yet identified. Dr. Goldmann designed his tonometer to provide accurate measurements in eyes with “average” corneas. But we now know that many corneas vary more significantly from average than previously thought. In addition, the Goldmann tonometer is technique and operator dependant. 2 Different Goldmann operators could arrive at two different measurement results for the same eye. Finally, recent studies have shown that over 50% of all Goldmann tonometers in the field are out of calibrations by 2.5 mmHg or more. Accordingly, Goldmann tonometry cannot compensate for differences in corneal thickness, corneal elasticity, and many other parameters that influence tonometer readings. This applies to all other Goldmann-correlated tonometers!

Non- Contact Tonometers
- Invented by Dr. Bernie Grolman in the 1960’s (American Optical) - To enable OD’s in the USA to perform tonometry - Introduced in 1971 - Uses rapid air pulse technology - Easy to use - Strong Goldmann correlation - Objective: no operator bias - No anesthetic required - No risk of cross-contamination Modern NCT - AT555

NCT Traditional Method of Operation
The NCT I and NCT II used time as the correlate to Goldmann pressure. Therefore, the air pulse had to be linear (constant rate of increase). In order to make the device able to measure a wide range of pressures, the air pulse had to be quite hard. Any eye being measured would receive the same air pulse. So, a 12 mmHg eye would be hit with enough force to applanate a 60 mmHg eye. As a result of the hard puff and the loud noise, many patients were not fond of being measured on early NCTs. Newer versions of the Reichert NCT utilize a variable ramp rate. This enables the device to deliver only the required amount of force necessary to applanate the eye being measured. In addition, the sound of the puff has been almost eliminated.

Traditional NCT vs. GAT Countless clinical studies over the past 35 years have demonstrated that the NCT measurement correlates very strongly with Goldmann pressure values. In fact, the NCT measurement has been shown to be more repeatable because it it not operator or technique dependant. “In conclusion, the current study shows that the XPERT vs GAT Sdiff (1.5 mmHg) is comparable to single GAT instrument repeatability, and far superior to that of two GAT instrument repeatability/reliability.”

Ocular Response Analyzer Method of Operation

Static vs. Dynamic Measurement
Goldmann tonometers make ‘static’ measurements. That is they derive IOP from the force measured during a steady state applanation of the cornea. The Ocular Response Analyzer makes a ‘dynamic’ measurement, monitoring the movement of the cornea in response to a rapid air impulse. The ‘dynamic’ nature of the ORA measurement makes possible the capture of other useful data about the eye.

Visco-Elastic System An Automotive “Strut” Assembly
- Coil Spring: Static Resistance (Elasticity). strain (deformation) is directly proportional to stress (applied force), independent of the length of time or the rate at which the force is applied. The cornea is similar to an automotive strut assembly in that it exhibits two types of resistance: static and dynamic. Static resistance is present in the coil spring. If you compress the spring, it doesn’t matter how fast you push on it, or how long you hold it down for, the force required will be the same Dynamic resistance is present in the damper. The faster you push it, the more it will resist the applied force - Shock Absorber: Viscous Resistance (Damping). The resistance to an applied force depends primarily on the speed at which the force is applied.

Method of Operation There are 4 main components in the Ocular Response Analyzer. An infrared light emitter, a light intensity detector, a solenoid driven air pump, and a pressure transducer inside the plenum chamber. At the start of the test sequence, the Infrared light shines on the cornea and the intensity of the reflected light is monitored by the detector. Upon alignment with the apex of the cornea, the air pump delivers a collimated stream of air and the cornea begins to flatten

Applanation Signal Plot
Two pressure values are derived from the INWARD and OUTWARD applanation events. One might expect these two pressure values to be the same. However, due to its viscous material characteristics, the cornea resists the dynamic force of the air pulse, causing a delay in the inward and outward applanation events. This results in two different pressure values, which are recorded at the instant of each applanation event. The difference between these two pressure values is corneal hysteresis. The ability to measure this effect provides us with the key to understanding the biomechanical properties of the cornea.

Definitions Hysteresis
The phenomenon was identified, and the term coined, by Sir James Alfred Ewing in 1890. Hysteresis is a property of physical systems that do not instantly follow the forces applied to them, but react slowly, or do not return completely to their original state. Drived from an ancient Greek word meaning ‘coming behind’. It was introduced into scientific vocabulary about 120 years ago by the Scottish physicist, Alfred Ewing. He discovered hysteresis when he was studying magnetic systems, systems that don’t have a material substance but have elasticity and viscosity properties. Nowadays "hysteresis" has multiple meanings as this property has been studied in many fields such as mechanics hysteresis in plastic materials, as ferromagnetic hysteresis in magnetic systems, as soil-moisture hysteresis in hydrology, and as shock analysis in economy. For instance, if you push on a piece of wet sponge it will assume a new shape, and when you remove your hand it will not return to its original shape, or at least not immediately and not entirely. Corneal Hysteresis The difference in the inward and outward pressure values obtained during the dynamic bi-directional applanation process employed in the Ocular Response Analyzer, as a result of viscous damping in the cornea.

Corneal Hysteresis: A New Ocular Parameter

Right/Left Eye Hysteresis
Right and left eye hysteresis values are highly correlated, further demonstrating that the CH measurement is a repeatable, biological, indicator

Hysteresis vs. Corneal Radius
There is no relationship between CH and corneal radius

Hysteresis vs. Corneal Astigmatism
There is very little relationship between CH and corneal astigmatism

CCT vs. CH normal eyes To determine if hysteresis is related to central corneal thickness (CCT), the CCT of 184 normal eyes was measured with a 20 Mhz ultrasound pachymeter. The CCT data were plotted against the Corneal Hysteresis measurement for the same eyes. The correlation is statistically significant, but weak, having an R2 value of 0.19 Therefore: corneal hysteresis is not a function of thickness. Data courtesy Mitsugu Shimmyo, MD

IOPG vs CH Normal Eyes The Corneal Hysteresis parameter plotted against IOPG (traditional Goldmann-correlated) for a population of normal subjects (N=182 eyes), demonstrating that CH is independent of pressure in normal eyes. Therefore: corneal hysteresis is not a function of pressure Data courtesy New England College of Optometry

In/Out Applanation Regressions 32 eyes - 3 pressure levels (ODM induced)
Conclusion: Hysteresis stays constant over a wide range of pressures for the same eyes To ensure that this new measurement is strictly a function of corneal properties and not just another artifact of the pressure, 32 normal eyes were measured at 3 different pressure levels using both a Goldmann tonometer and the ORA. All measurements were made within minutes of each other to minimize diurnal effects. Initial pressure measurements were made on the eyes in their normal state, followed by two artificially elevated pressure levels induced by a modified ophthalmodynamometer. The resulting inward and outward applanation data for each set of measurements were plotted against the observed IOP values. The average corneal hysteresis for the population did not change significantly, regardless of the induced pressure level, further demonstrating the corneal hysteresis is independent of pressure in normal eyes

ORA and Corneal Specialties

Corneal Biomechanics: A New Area of Clinical Interest
Ocular Response Analyzer is the only instrument capable of measuring the biomechanical properties of the cornea. Clinical data has shown that the Corneal Hysteresis measurement is useful in identifying corneal pathologies and may be valuable in identifying potential LASIK candidates who are at risk of developing ectasia. In consequence, the instrument is attracting interest from corneal specialists and refractive surgeons.

Corneal Biomechanics and Refractive Surgery
“Refractive surgery is not an exact science. Achieving the cornea’s ultimate shape depends on our ability to predict the biomechanical response to surgery.” Cynthia Roberts, Ph.D. Associate Professor of Ophthalmology and Biomechanical Engineering, OSU “The promise of wavefront-guided laser ablation will not be fully realized until researchers gain a more complete understanding of corneal biomechanics.” John Marshall, Ph.D. “Father of the Excimer Laser” “Wavefront by itself is a great tool but we still need to understand corneal biomechanics to reap the whole benefit.” David Williams, Ph.D. Direct of The Center For Visual Science, University of Rochester

Classifying Corneal Pathologies
A comparison of CH values obtained from eyes with known corneal conditions to that of normal-subject measurement values reveals significant differences. It is east to see that the Corneal Hysteresis measurements in the eyes with corneal disorders are, on average, significantly lower than in normal eyes. Low CH demonstrates that these corneas are less capable of absorbing (damping) the energy of the air pulse.. Similar results have been observed in various studies. Clearly these low values are indicative of a compromised cornea. With this in mind, some experts theorize that normal eyes exhibiting significantly lower than average CH may be at risk of developing corneal conditions in the future. Data courtesy Shah, Brandt, Pepose, Castellano

To investigate the biomechanical characteristics of eyes with: - Fuchs’ Corneal Dystrophy (n=14) - Post-Penetrating Keratoplasty (18±10 months postop, n=32) - Corneal Ectasia (n=46) - Advanced Keratoconus (CCT < 490 µm, n=15) - Pellucid Marginalis (n=4) - Early or Forme Fruste Keratoconus (CCT > 490 µm, n=27) - Compared to 3 pachymetry matched control groups Group 1: > 580 µm (n=31) Group 2: between 510 and 580 µm (n=66) Group 3: < 510 µm (n=17) To compare IOP measurements using 3 testing techniques GAT; NCT with ORA; PDCT Data courtesy Jay Pepose, MD - ASCRS 2006

Control Group Differences ± 2.7 ± 3.3 ± 2.3 PDCT mmHg ± 2.7 ± 3.2 ± 2.3 GAT mmHg ± 3.5 ± 3.1 ± 3.7 ORA-g mmHg ± 3.0 ± 3.2 ± 3.5 ORA-cc mmHg ± 0.9 ± 1.5 ± 1.2 ± 20.0 31 Group 3 ± 0.8 ± 1.3 ± 1.4 ± 18.3 66 Group 2 ± 1.1 OPA mmHg ± 1.8 CRF mmHg ± 1.8 CH mmHg ± 20.0 CCT µm 17 N Group 1 Controls = p<0.05 comparing Group 1 or 3 to Group 2, with Student’s t-test Data courtesy Jay Pepose, MD - ASCRS 2006

Data courtesy Jay Pepose, MD - ASCRS 2006

± 0.9 ± 1.5 ± 1.2 ± 20.0 31 Group 3 ± 0.7 ± 1.8 ± 1.7 ± 77.4 46 KCN/ PMD/ FFKCN ± 0.9 ± 1.5 ± 1.4 ± 72.2 15 KCN advanced ± 1.0 ± 2.0 ± 52.5 14 Fuchs’ ± 0.8 ± 1.3 ± 1.4 ± 18.3 66 2 2.6 ± 1.2 ± 1.1 OPA ± 2.1 ± 1.8 CRF ± 1.7 ± 1.8 CH ± 47.1 ± 20.0 CCT µm 32 17 N PKP Group 1 = p<0.05 comparing study group to its respective control group, with Student’s t-test Data courtesy Jay Pepose, MD - ASCRS 2006

Thin Cornea with no ectasia Thin Cornea with Keratoconus CH=11.2 CRF=10.8 CH=8.1 CRF=7.9 Data courtesy Renato Ambrosio, MD - ASCRS 2006

Data courtesy Renato Ambrosio, MD - ASCRS 2006

Pre / Post Lasik Potential clinical applications of the CH measurement in the area of refractive surgery is obvious. Currently, Central Corneal Thickness (CCT) is a primary factor used for screening candidates for refractive surgery. Patients with thinner corneas are considered to be at higher risk for developing post-LASIK corneal ectasia. While rare, this complication is not confined to patients with thin corneas and is a major concern for both doctors and patients. When it occurs it may result in life-long complications for the patient and litigation against the surgeon. The easily identifiable differences in CH between normal and compromised corneas demonstrate how these metrics provides a more complete characterization of the biomechanical state of the cornea than CCT. This observation, coupled with the fact that CH is only weakly correlated with CCT, leads us to believe that the corneal hysteresis measurement will be a useful tool for eliminating potential LASIK patients who are at risk of developing post-LASIK ectasia. Studies investigating this issue are currently ongoing. Clinical data from several studies show a universal reduction in post-LASIK CH. Pre and post LASIK CH. Some experts hypothesize that reduced post-LASIK CH/CRF is not primarily a function of corneal thinning, but rather a result of weakening of the structure related to the flap. This patients pre-lasik CH is lower than the population average post-lasik CH. This patient may be a candidate for ectasia! Data courtesy Dr. David Castellano, MD / Dr. Jay Pepose, MD

Normal vs. Keratoconic Signals
KERATOCONUS In addition to simple differences in the CH values, significant differences are observable in the morphology of the measurement-signal waveforms. While we are currently only using the corneal hysteresis value, clearly there is additional valuable information present in the shape of the measurement signal. NORMAL Data courtesy Mr. Sunil Shah, MD

Normal vs. Fuchs’ Signals
Signals from patients with Fuchs’ Dystrophy behave similarly to the keratoconic signal from the previous slide indicating “weak” corneas display similar results. NORMAL Data courtesy Dr. James Brandt, MD

Pre and Post Lasik Signals
Post LASIK eyes demonstrate a universal reduction in CH due to complex biomechanical changes induced by the LASIK procedure. Notice the significant changes in the shape of the measurement signals post-lasik. The post lasik signals look similar to keratoconus waveforms. PRE-LASIK Data courtesy Dr. David Castellano, MD

Signals are “Corneal Signature”
NORMAL KERATOCONUS The morphological signal that is produced by the Ocular Response Analyzer is a unique “signature” for the eye being measured. Although we are currently only utilizing the CH measurement, it is clear that there is other valuable information contained in these signals. Investigation are ongoing to further support this theory. FUCHS’ POST LASIK

Predicting Ectasia Risk
A comparison of CH values obtained from eyes with known corneal conditions to that of normal-subject measurement values reveals significant differences. It is east to see that the Corneal Hysteresis measurements in the eyes with corneal disorders are, on average, significantly lower than in normal eyes. Low CH demonstrates that these corneas are less capable of absorbing (damping) the energy of the air pulse.. Similar results have been observed in various studies. Clearly these low values are indicative of a compromised cornea. With this in mind, some experts theorize that normal eyes exhibiting significantly lower than average CH may be at risk of developing corneal conditions in the future. Data courtesy Peter Hersh

ORA and Glaucoma

Landmark Studies Many recent studies have concluded, for the first time, that controlling IOP in glaucoma patients and suspects stops or slows the progression of the disease. These studies include: - OHTS - Ocular Hypertension Treatment Study - AGIS - Advanced Glaucoma Intervention Study - CNTGS - Collaborative Normal-Tension Glaucoma Study - CIGTS - Collaborative Initial Glaucoma Treatment Study Many of these studies have also investigated the role of the cornea in the diagnosis and management of glaucoma.

The cornea and glaucoma
Some studies have investigated Corneal thickness as a contaminating factor in measuring IOP Others have investigated Corneal thickness as an independent indicator of glaucoma risk - Could a thin cornea be a surrogate for eyes susceptible to glaucoma damage?

Central Corneal Thickness
Recently a great deal of attention has been focused on the relationship between central corneal thickness (CCT) and Goldmann-obtained IOP values. Studies have found that corneal thickness influences the accuracy of IOP measurements. - Thicker corneas, on average, tend to overstate GAT IOP values - Thinner corneas, on average, tend to understate GAT IOP values HOWEVER, this is only true ON AVERAGE for large populations - The IOP/CCT relationship is actually quite weak and varies from study to study, making correcting IOP based on CCT impractical

The problem with CCT 184 Normal Eyes
In theory, thicker corneas overstate IOP values and thinner corneas understate IOP values. Many attempts have been made to establish a CCT-based correction algorithm to adjust Goldmann-obtained IOP values. However, the magnitude of the CCT/IOP relationship identified by the various studies is inconsistent. This variation has resulted in numerous algorithms that differ significantly from one another. A typical IOP vs. CCT plot for a population of 182 normal eyes is shown in Figure 16. Among other things, the scatter present in this plot highlights the fallacy that CCT can be used to adjust individual measured IOP values. Analysis of this and other IOP vs. CCT data has shown that adjusting IOP based on CCT could lead not only to errors in the magnitude of the adjustment, but also in the direction of the adjustment (40% of the time!!) Opinion leaders in glaucoma caution practicioners that adjusting IOP based on CCT data is not advisable (Harry Quigley, Jamie Brandt, Ted Garway-Heath, Cindy Roberts, etc) Data courtesy New England Collage of Optometry

The problem with CCT Two corneas, both 0.65 mm One is clear
The other is edematous The first reads high (compared to manometry), the second low Thickness can’t be the whole answer Other corneal factors besides thickness determine response of corneo-scleral shell to force Hydration Connective tissue composition Bio-elasticity Data courtesy Harry Quigley, Wilmer Eye Institute

“Physiology is more important than anatomy”
The problem with CCT “We should not assume that corneal thickness is the parameter of greatest interest in monitoring glaucoma or in determining what features of the eye are important in optic nerve damage”. “Physiology is more important than anatomy” - Harry Quigley, Director of Glaucoma Service, Wilmer Eye Institute “Adjusting IOP by means of a fixed CCT algorithm is almost certainly wrong in the majority of our patients and is attempting to instill a degree of precision, into a relatively flawed instrument (the Goldmann tonometer), that simply is not there” - James Brandt, Director of Glaucoma Services, UC Davis

CH distribution - Normals & Glaucoma
Recently, the Ocular Hypertension Treatment Study (OHTS), and other studies on the subject, have brought to light the importance of Central Corneal Thickness (CCT) in diagnosing and managing glaucoma. These studies have suggested that low CCT (thin cornea) may be an independent risk factor for the development and progression of the disease. Many experts believe that corneal parameters other than CCT may provide clues that will aid in the diagnosis and management of glaucoma. Evidence suggests that the cornea may reflect the condition of the lamina cribrosa. Clinical studies utilizing the Ocular Response Analyzer support this hypothesis. As shown in this slide, compared to normals, glaucomatous subjects have a significantly lower than average Corneal Hysteresis and a much wider range. An interesting observation is that lower-than-average CH is also observed in patients who have been identified as “Normal Tension Glaucoma” (NTG) subjects. Currently individuals who unknowingly have NTG are missed during routine IOP screening. Obviously it would be a tremendous breakthrough if the Corneal Hysteresis parameter proves to be a reliable indicator of this disease condition. Data courtesy New England College of Optometry and Mitsugu Shimmyo, MD

Corneal Properties and Glaucoma Risk

Additional Parameters: P1 and P2 provide independent information
about the eye

Background P1 has a much stronger correlation with CCT than P2, demonstrating the fact that P2 is less influenced by corneal properties

Background P2 changes more significantly post-lasik than P1, further demonstrating the fact that P2 is less influenced by corneal properties Data courtesy Dr. David Castellano, MD / Dr. Jay Pepose, MD

Gaining additional Useful Information
Clinical data analysis demonstrated that p1 and p2 respond independently to various factors (CCT, LASIK, IOP reduction, etc) Therefore, an “optimum combination” of the two independent parameters may yield the best IOP and Corneal Parameter, resulting in: Reduced or eliminated ORA IOP change after LASIK Reduced or eliminated Corneal Parameter change after pressure reduction Increased correlation of Corneal Parameter and CCT Reduced or eliminated correlation of ORA IOP and CCT Reduced or eliminated correlation of ORA IOP and Corneal Parameter Reduced (slightly) correlation of ORA IOP and GAT Higher correlation of Corneal Parameter with GAT than CCT with GAT Reduced or eliminated anomalous low IOP for keratoconus, fuch’s patients

IOPcc Corneal Compensated IOP

Define & Describe IOPCC Corneal-Compensated Intraocular Pressure
- An Intraocular Pressure measurement that is less affected by corneal properties than other methods of tonometery, such as Goldmann (GAT). IOPCC has essentially zero correlation with CCT in normal eyes and stays relatively constant post-LASIK. - Developed using clinical data and a proprietary algorithm.

Method for finding “invariant” pressure
Use linear combination of P1 & P2 - avoids potential coupling of IOP & CH Vary ratio of P1 & P2 to minimize difference of pre-post LASIK IOP Upon achieving desired post-LASIK results, verify that: Correlation with Goldmann is still strong Correlation of IOP with CCT in various data sets is minimal Correlation of IOP with CH in various data sets is minimal Optimum formula: IOPcc = P2 - (0.43*P1)

IOPG vs. CCT - 184 normal eyes
In theory, thicker corneas overstate IOP values and thinner corneas understate IOP values. Many attempts have been made to establish a CCT-based correction algorithm to adjust Goldmann-obtained IOP values. However, the magnitude of the CCT/IOP relationship identified by the various studies is inconsistent. This variation has resulted in numerous algorithms that differ significantly from one another. A typical IOP vs. CCT plot for a population of 182 normal eyes is shown in Figure 16. Among other things, the scatter present in this plot highlights the fallacy that CCT can be used to adjust individual measured IOP values. Analysis of this and other IOP vs. CCT data has shown that adjusting IOP based on CCT could lead not only to errors in the magnitude of the adjustment, but also in the direction of the adjustment (40% of the time!!) Opinion leaders in glaucoma caution practicioners that adjusting IOP based on CCT data is not advisable (Harry Quigley, Jamie Brandt, Ted Garway-Heath, Cindy Roberts, etc) Data courtesy New England Collage of Tonometry

IOPCC vs CCT 184 Normals IOPCC Versus CCT in a population of 184 normal eyes demonstrates that IOPCC is unaffected by central corneal thickness. Data courtesy New England Collage of Optometry

Thin, Average, and Thick Cornea Groups
IOPcc vs. GAT and DCT IOP Thin, Average, and Thick Cornea Groups Data courtesy Jay Pepose, MD - ASCRS 2006

28 eyes Pre/Post LASIK IOPCC
Traditional IOP drops dramatically post LASIK (2-6mmHg or more). IOPCC stays constant on average. 26% IOP drop 3% IOP drop Data courtesy Dr. David Castellano, MD / Dr. Jay Pepose, MD

IOPCC is higher than traditional IOP in “NTG” subjects
24 “NTG” eyes Observing the IOPCC of a population of Normal Tension Glaucoma (NTG) subjects, reveals that the pressure in these eyes is significantly higher than indicated by the traditional Goldmann-correlated IOP measurement. In this population, IOPCC is, on average, more than 2.25 mmHg higher than IOPG. IOPCC is higher than traditional IOP in “NTG” subjects Data courtesy Mitsugu Shimmyo, MD

Is IOPcc Better than GAT?
IOPcc correlates strongly with GAT on the average HOWEVER, IOPcc has the following advantages over GAT Not affected by CCT Not affected by corneal biomechanical properties (rigidity) As such, it is more accurate in KC, Fuchs’, OHT, NTG eyes In addition, it has less measured IOP reduction post-LASIK No operator bias

Is IOPcc Better than GAT?

CRF Corneal Resistance Factor

Define & Describe CRF Corneal Resistance Factor
An indicator of the overall “resistance” of the cornea, including both the viscous and elastic properties. It is significantly correlated with Central Corneal Thickness (CCT) and GAT, as one might expect, but not with IOPCC. CRF appears to be an indicator of the overall “resistance” of the cornea, and is significantly correlated with Central Corneal Thickness (CCT) and GAT, as one might expect, but not with IOPCC.

Method for finding “CRF” corneal resistance factor
Use linear combination of P1 & P2 - avoids potential coupling of IOP & CH Vary ratio of P1 & P2 to: Maximize correlation of CH and CCT in various populations Minimize CH change after pressure reduction / increase Maximize correlation of CH and GAT Ensure CH remains significate indicator of corneal conditions such as Keratoconus, fuch’s, etc Ensure significant CH change post-LASIK remains Optimum corneal parameter: CRF = P1-(0.7*P2)

Correlation of CRF and CCT
The relationship of CRF and CCT is maximized at the same constant (k) in 3 totally unique populations, validating the selection of 0.7 as the CRF constant.

Correlation of CH and CRF vs. CCT 339 Normal Eyes
This demonstrates that CRF is more strongly correlated with CCT than CH is with CCT. This is because CH is a measure of viscous damping (energy absorption) whereas CRF is a better indicator of the overall resistance of the cornea.

Correlation of CH & CRF vs. IOPG (“GAT”)
- CH has a very small correlation with traditional IOP values because it is simply a measure of the viscous damping in the cornea - CRF is strongly correlated with traditional IOP values because it is an indicator of the overall resistance of the cornea. This demonstrates that GAT is seriously affected by corneal properties.

CRF - Normals, Keratoconus, Fuchs’
CRF is a better indicator of KC than CH Data courtesy Shah, Brandt, Pepose, Castellano

CRF distribution - Normals & Glaucoma
. Data courtesy New England College of Optometry and Mitsugu Shimmyo, MD

How do CH and CRF Differ Correlation of CH, CRF & IOPg
A population of 590 glaucomatous patients CH goes down with seriously elevated pressures in Glaucomatous eyes, presumably as a result of corneal remodeling due to the glaucomatous damage - CRF goes up with seriously elevated pressures due to apparent stiffening of the cornea in response to the pressure. Data courtesy Dr. Mitsugu Shimmyo, MD

IOPCC vs CRF 339 Normals IOPCC Versus CRF in a population of 339 normal eyes demonstrates that IOPCC is unaffected by visco-elastic properties of the cornea.

Are CH and CRF Related to the “Modulus of Elasticity”?
NO! Researchers have attempted to identify the young's modulus of the cornea - but the reported values in the literature, vary by four orders of magnitude! The cornea is a system, not an isotropic material such as steel or rubber. Attempting to identify the youngs modulus is a gross over-simplification of a complex subject.

Interpreting ORA Measurement Results

Measurement Signal Components
Applanation Events P1 P2 Raw Signal Filtered Signal Air Pulse

Identifying Normal Signals “Rules of thumb”
X X X X X - Watch for: - Clean, smooth signals - Similar amplitude peaks - Repeatable values - Consistent measurements in both eyes

Identifying Normal Signals
IOPcc and IOPg are close and in normal range CH and CRF are close and in normal range Filtered peaks “line up” under raw peaks Similar signal amplitude Raw signal has clean points Raw signal is fairly smooth Baseline signal is “flat” and nearly same amplitude on both sides

Identifying Normal Signals

Identifying Keratoconus “Rules of thumb”
X X X X X X - Watch for: - Low amplitude peaks - less repeatable signals than normal subject - “noisy” signals - Often present in one eye and not the other.

Identifying Keratoconus Signals
IOPcc Higher than IOPG Low CH Low CRF Thin CCT Low amplitude peaks Sharp, thin peaks P2 raw signal “bounce” More “noisy” raw signal Noisy signals cause less repeatable values

Identifying Keratoconus Signals

Questionable Keratoconus Signal
Measurement may yield unreliable results Raw signal is too “lumpy”

Identifying Severe Keratoconus “Rules of Thumb”
??????????? - Look at the signal, the numbers may be unreliable - Thin CCT - Very low amplitude peaks - practically a flat line - General signal shape is very repeatable - Often present in one eye and not the other

Severe Keratoconus Signal Measurement values will be unreliable
CH and CRF are unreliable due to signal amplitude Forget about the CH, no question this Keratocouns!!

Identifying Forme Fruste KC “Rules of Thumb”
X X X X X X - Watch for: - Rule out past history of refractive surgery - lower amplitude peaks - Rapid P2 raw signal falloff with small “ricochet bounce” - Suspicious topography - “noisy” signals, but cleaner than advanced KC and more repeatable - Family history, frequent eye-rubbing, trouble wearing contacts

“Sub-Clinical” Keratoconus Signal
Signal looks nearly normal but low CH and CRF IOPcc Higher than IOPG CH just below normal range CRF just below normal range Mild P2 raw signal “bounce”

Identifying Refractive Surgey “Rules of Thumb”
X X X X X X - Watch for: - low amplitude peaks (cleaner in LASIK than PRK) - “Sharp / thin” raw signals (especially in LASIK) - Rapid P2 raw signal falloff with pronounced “ricochet bounce” - less repeatable signals than normal subject (Especially in PRK) - “noisy” signals (Especially in PRK)

Pre / Post-LASIK Signals
PRE-LASIK IOPg, IOPcc close and in normal range CH, CRF close and in normal range Normal Signal POST-LASIK IOPcc higher than IOPg, closer to normal CH, CRF low CCT lower Thin, sharp peaks Reduced signal amplitude Some “noise” P2 “bounce”

Pre / Post-LASIK Signals
PRE-LASIK POST-LASIK

TAKE MULTIPLE READINGS!!
Pre / Post-LASIK Signals Example of less reliable, but still useful, signals PRE-LASIK CH, CRF may be higher in reality P1, not ideal POST-LASIK IOP probably higher in reality CH may be lower in reality But the CRF is reduced! Neither signal is “ideal” but the post-lasik difference is still clear. P1, not ideal TAKE MULTIPLE READINGS!!

Note how well IOPcc works!
Pre / Post PRK Signals PRE-PRK 2 Months POST-PRK IOPg, IOPcc close and in normal range CH, CRF close and in normal range But CH, CRF stay low Signal improves with time Normal Signal 2 wks POST-PRK CH, CRF reduced PRK signals are noisy Note how well IOPcc works!

Identifying Ectasia “Rules of Thumb”
X X X X X - Watch for: - Has had LASIK, PRK, other surface ablation procedure - Very low amplitude, noisy, “messy” signals - Signal quality does not improve over time - Suspicious topography - Often present in one eye and not the other

Identifying Ectasia Signals
IOPcc Higher than IOPG Low CH Low CRF Thin CCT Low amplitude peaks Sharp, thin peaks Lots of noise P2 raw signal “bounce”

Identifying POAG “Rules of Thumb”
X X X X X X X X X X - Watch for: - IOPcc higher than IOPg - Noisy signals - Family history, race, age, CDR, Diabetes status, Visual fields results, optic nerve status

Identifying POAG Signals Uncontrolled Subject, moderately high IOP
Signals are high amplitude, noisy IOPg, IOPcc both elevated Low CH CRF higher than CH

Identifying POAG Signals Subject on meds, but progressing
IOP is in normal range but IOPcc Higher than IOPG Low CH Low CRF Signal is smoother than high IOP signals

Identifying POAG Signals Subject on meds and stable
IOP is well controlled CH, CRF in normal range Signal is smooth

Identifying POAG Signals Uncontrolled Subject - Blind
IOPg, IOPcc both elevated Low CH High CRF Signals are low amplitude, lumpy, and noisy

Identifying OHT “Rules of Thumb”
X X X X X - Watch for: - IOPcc lower than IOPg - Smooth signal - Family history, race, age, CDR, Diabetes status, Visual fields results, optic nerve status

Identifying OHT Signals Subject is a “false positive”
IOPg much higher than IOPcc CH, CRF are very high Signal is smooth

Rules-of-thumb Spotting NTG/LTG
X X X X X X X - Watch for: - IOPcc higher than IOPg, but may still be in “normal” range - Low amplitude signals, some noise - Family history, race, age, CDR, Diabetes status, Visual fields results, optic nerve status

Identifying NTG Signals
IOPcc Higher than IOPG Low CH Low CRF Low amplitude peaks Thin CCT

Rules-of-thumb Spotting “unusual” eyes / corneas
- Atypical measurement signals that are: - less repeatable than normal - highly variable numeric measurement values - Investigate previous ocular history for surgery, disease, trauma, etc - What to do: - take a series of measurement - look for the “best signals” possible - try to get two that look similar and yield similar results - delete clearly “bad” signals and use average values of good ones

Unusual Signals “Bizarre” signals are often very repeatable
Just the fact that they are “different” is telling us something about the cornea / eye

Understand the Cornea Understand the Pressure

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Understand the Cornea Understand the Pressure

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