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Corneal Biomechanics

Biomechanical properties clinical evaluation of the corneal with theOcular Response Analyzer (ORA).

Interest and principles of mesure.de corneal hysteresis.

While many instruments allow to study geometric properties and optical (corneal topography) or histo-morphological (confocal microscopy study), the estimate of the biomechanical properties of the cornea was long confined to research until the introduction of the Ocular Response Analyzer® (Reichert, Buffalo, NY). This instrument provides the clinician with a noninvasive estimate of corneal hysteresis, and this physical setting is used to increase the accuracy of the measurement of intraocular pressure.
We use the Ocular Response Analyzer (ORA) in a variety of clinical contexts.

It is important to understand the principles and parameters measured.


What's the point to evaluate the biomechanical properties of the cornea in clinical practice?

Management of glaucoma and corneal refractive surgery is the main field of the study of corneal biomechanics.

(1) glaucoma

As part of the screening and follow-up of glaucomatous patients, it is important to better measure eye pressure, and understand the entangled mechanical factors that may vary the figures obtained with the classical tonometers.
Ocular hypertension (HTO) is the most significant risk factor for glaucoma, and remains the only element whose treatment decreases the incidence and progression of glaucoma. The most commonly used method of measuring intra-ocular pressure (IOP) is that of the Goldmann tonometer, mounted on a slit lamp, based on the law of Fick. This law assumes that the internal pressure exerted on a thin-walled sphere is equal to the pressure necessary to flatten a small area of its wall. However, it has been shown that the corneal thickness has an influence on the measurement of IOP. Moreover, the consideration of the pachymetry (thickness) corneal is not sufficient to predict the mechanical characteristics of the corneal wall. The law of Fick is valid for perfectly elastic walls (zero hysteresis). Finally, this law applies to spheres of homogeneous resistance. However, the sclera and the cornea have varying thickness and stiffness depending on the subjects. Thus, the Goldmann Tonometer has been calibrated for a thickness of 525 µm and neglects the corneal viscosity. The reliability of the measurement with this type of ocular tonometry is therefore subject to a certain vagueness, all the more so because the cornea of the subject goes out of the simplified model proposed by Goldman.

(2) refractive surgery

The change of the optical power of the cornea caused by radial keratotomy (KR) was based on the induction of a modification of the biomechanical structure of the cornea (peripheral deep corneal incisions), without change in volume (absence of tissue subtraction). These incisions weakened biomechanically the periphery of the cornea, which the Center "membership" and is applatissait.
The techniques of photoablation are built on an inverse postulate and base the modification of the refractive power on a subtractive corneal remodeling (sculpture by the Excimer Laser), neglecting the possible biomechanical reaction.
It is now shown that some of the "unpredicted" effects of ablation profiles are based on changes in the geometry of the corneal wall of biomechanical origin. The low predictability and refractive fluctuations of the KR could be attributed to the inter-and intra-individual variations of parameters influencing the biomechanical state of the cornea (intra-ocular pressure, variable corneal rigidity of a patient at The other, ect...).
LASIK-induced corneal ectasia is a very rare complication. It could be related to the decompensation of a precarious corneal biomechanical state (Keratoconus fruste). The objective measurement of the preoperative biomechanical state could increase the sensitivity of the detection of these "biomechanically hazardous" corneas.

The corneal hysteresis as interest is not limited to the diagnosis of glaucoma and the screening of corneas at risk in refractive surgery. It could also allow to assess the biomechanical consequences of 'local' eye conditions (malfunction meibomien and dry eye, dystrophy corneal degenerations, ect...) or secondary to General conditions (atopy, diabetes, collagenases, diseases of systems, ect...)... Finally, the assessment of corneal rigidity could quantify the beneficial effect of therapeutic procedures such as the insertion of intra-corneens rings or cross-linking of the corneal collagen (cross linking - CXL). Since the rise of the practice of the CXL, biomechanical measures show no real change of corneal Biomechanics (lack of elevation of the hysteresis, or hysteresis), what needs to reconsider the interest of this technique.

What is that the hysteresis?

The phenomenon has been described by Sir James Alfred Ewing in 1890. The hysteresis (or hysteresis) is a present property in some physical systems characterized by the character differed in time response to a force that is applied to them. These systems react 'gently' and do not instantly return to their original form as they absorb a part of the incidental mechanical energy they dispel another form (heat). Viscous systems have a high hysteresis.

What is than the visco elasticity of the cornea?

The pressure to the pressionnelle measure corresponds to a mechanical stress to the cornea (force exerted on the entire surface offered by the corneal wall). The collection of data on the 'management' of this mechanical stress by the cornea provides information about its biomechanical properties.
The mechanical behaviour of the corneal tissue responds to the laws of physics and can be modelled as a system with visco-elastic behaviour. Elasticity and viscosity give the corneal tissue two distinct behavioral characteristics:

-elastic behavior; a perfectly elastic system can store energy before you return it almost entirely. A metal spring is an example of elastic system. After compression (even extended), the energy is stored (reversible molecular deformations) then returned almost instantly. In certain conditions of tension, an elastic system tends to present oscillations during the return of energy.

-viscous behavior; a viscous system a resistance that grows not proportional to intensity exercised deformation force. There is a dissipation of mechanical energy incident (in the form of thermal energy), which is a return deferred to state of origin.

How does the Ocular Response Analyzer?

The instrument emits a jet of continuous calibrated air directed to the corneal dome. This pressure increases by summation over time and exerts a force of increasing intensity at each point of the corneal surface exposed to the air flow. This force will cause a deformation of the cornea. The pressure exerted by the air flow is monitored at very short intervals by the instrument throughout the review. The unfortunately is detected by the measure of the intensity of infrared light reflected from the cornea. This light is emitted according to an oblique impact, and its reflection (also skew in the opposite direction) to a photosensitive sensor is all the more important that the corneal curvature is low. The unfortunately corresponds to a peak of reflected light intensity, because this at this moment the corneal surface acts like a mirror. The unfortunately pressure corresponds to the pressure measured when the infrared peak.

Ora storyboards

The originality of the Ocular Response Analyzer is its ability to perform during a simple review no not one, but two consecutive unfortunately measures: the first during the initial corneal deformation resulting from the increase in pressure, the second at the time, or the cornea returns to its original shape condition.
When the first (infra-red) first peak is detected, the air jet emission is abruptly interrupted. The air pressure exerted on the corneal wall does not fall however immediately, but continues to increase by inertia for a few milliseconds, before reaching a maximum, and then gradually decreasing to the initial equilibrium state. The look of the pressure curve obtained throughout the examination is Gaussian (it marries a bit the allure of a "bell").
The height of this bell curve is proportional to the intraocular pressure value: We should remember that the air flow is interrupted by the instrument only when the first operation occurs. The higher the intraocular pressure, the greater the equivalent air pressure to be provided in the vicinity of the cornea is important to obtain the first level of inflow. At this point, the upward part of the pressure curve and the height of the "bell" will be even higher as the intraocular pressure is high.
After the first decompression, the corneal dome undergoes a pressure higher than the intraocular pressure for a few moments, and the central corneal profile becomes slightly concave forward. The proportion of infrared light decreases then abruptly. The second detection occurs during the pressure decay and is detected by the second peak of reflected infrared light.

How is calculated the quantitative indexes provided by the instrument?

The device allows to measure two consecutive aplanations (P1 and P2) pressure, expressed in mmHg. Schematically, P1 is measured so that the sudden cornea and withstands positive pressure of air stream calibrated, while P2 is measured when the cornea returns to its state of equilibrium, in decreasing positive pressure phase.
Thus, from the values of P1 and P2, the Ora software offers different indexes:

-L' hysteresis (CH for Corneal Hysteresis) is equal to the difference between P1 and P2 (P1 - P2). The hysteresis value is proportional to the degree of viscosity of the cornea, and inversely proportional to its degree of elasticity.

Ora corneal hysteresis

-L' estimate of the (Corneal Resistance Factor = FIU) corneal resistance factor
P1-KxP2, with K = 0.7. The value of K was determined from clinical studies and statistical correlation models. This formula gives a favorable weight for P1 vis-à-vis P2.

-The uncompensated eye pressure (IOPg) is equal to the average arithmetic mean between P1 and P2, be (P1 + P2) / 2.
-The ocular pressure compensated for the viscoelastic properties of the cornea (IOPCC) is equal to a linear combination between P1 and P2, and whose value is particularly sensitive to P2, i.e. to the pressure value measured in the second The same
IOPCC = P2-0. KxP1, with K = 0.43. This value of K was determined from clinical studies and statistical regression models. The CRF is thus better correlated with the central corneal thickness.

How to analyze the signals provided by the ORA?

Three curves are shown for each measure on the same chart which the x axis is a time scale...
The green Gaussian curve corresponds to the pressure exerted against the corneal dome.
The red curve represents the infra-red light intensity detected during the measurement. The blue curve is obtained by a mathematical smoothing of the red bent to reduce the "noise".
The intersection of the peaks of the smoothed curve (in blue) with the pressure curve (in green) makes it possible to spot the two successive pressure pressures.
Our preliminary experience with ORA has enabled us to identify the morphological characteristics frequently found in various clinical contexts such as chronic glaucoma, keratoconus, refractive surgery surgical Suites Corneal, ect...
A preliminary statistical study identified an average value of 11.02 (+/-1.22) MmHg for CH in "normal" patients. The definition of "normality" is here cautious and even if any patient with an ocular hypertonia, a suspicion of keratoconus fruste, ect... was excluded from the analysis, the patients with low amétropes were mainly represented In this sample. The CH appeared weakly correlated with the value of the central corneal thickness in our study.
The value of the CH appears to be statistically significantly reduced for the eyes with Keratoconus (while the intraocular pressure is normal). The cornea affected by keratoconus thus presents an augmented elastic component, which means that its ability to absorb the incident energy is less than a keratoconus-free cornea.
We observed a steady but variable intensity decrease (about 2 mmHg on average) of the CH value after refractive surgery by LASIK. This reduction appears to be prolonged over time (up to one year of recoil) and without clinical consequences. The values obtained for the CH after LASIK are lower than the preoperative values but remain statistically higher than those of the group of patients with keratoconus. It should be recalled that the corrected measurement (IOPCC) of ocular pressure by the ORA takes into account the value of the second adjustment and thus takes better account of the biomechanical state of the cornea.

Several preliminary studies suggest that in extended ocular hypertonia case, the value of the CH reduced remains despite the normalization of eye pressure. The reduction of the CH could translate an alteration in the properties of the corneal tissue subjected to a prolonged hypertension. It is also lawful to apply that a reduction of the corneal hysteresis could make the bed of glaucoma (increased capacity of distension of intra-oculaires tissue involving not only the cornea but the sclera and the blade riddled). In this context, the CH could be considered a marker prognostic glaucomateuse affection.
Finally, the analysis of morphological characteristics resulting from the collection in intensity of the infra-red signal opens up new diagnostic perspectives. These characteristics involve the height and width of the peaks, the more or less smooth appearance of the infrared signal curve, ect... Their study is the subject of various works in our department, in collaboration with Dr. Dave Luce, inventor of the instrument.

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