Despre biometria oculara

Ocular biometry

Biometria oculară este o investigație esențială pentru evaluarea sănătății ochilor și pentru obținerea unor măsurători anatomice precise. Această metodă avansată implică analiza detaliată a structurii oculare și a caracteristicilor individuale, oferind informații valoroase despre sănătatea oculară și potențialele afecțiuni. Beneficiile biometriei oculare includ detectarea precoce a bolilor oculare, precum glaucomul și cataracta, ajustarea optimă a implanturilor intraoculare și personalizarea tratamentelor. Această tehnică de investigare revoluționară asigură o abordare individualizată și precisă a sănătății oculare.

1. Introduction

Operatia de cataracta implica extragerea cataractei si introducerea unui implant de cristalin artificial, care are diferite valori dioptrice. Alegerea puterii implantului de cristalin se face in functie de caracteristicile oculare individuale si de obiectivele refractive dorite postoperator. De exemplu, pacientul doreste sa mai poarte ochelari postoperator, doreste doar ochelari pentru distanta sau scris/citit sau nu mai doreste deloc sa poarte ochelari postoperator.

Biometrics is the method of applying mathematics to biology. The term was originally used by Whewell in the 1800s to calculate life expectancy.

Cataract surgery is currently the most common surgical technique performed worldwide. Since the introduction of phacoemulsification by Kelman in 1967, surgical technique and the design and quality of lens implants have improved considerably. Small, sutureless incisions and the use of foldable intraocular lens implants have reduced the incidence of complications and surgically induced astigmatism. In addition, the use of premium intraocular lenses (aspheric, toric, multifocal or a combination) allows the patient to become completely independent of glasses. Improved surgical treatment results in increased patient satisfaction. The key issue is achieving the desired refractive result. Essential to this end are precise measurements of the eye and the selection of the optimal formula for calculating the value of the lens implant.

2. What is biometrics?

This ocular investigation (biometry) involves anatomical measurements of the eye, axial length, keratometry and anterior chamber depth. These biometric measurements are essential for selecting the correct lens implant strength to achieve the desired refractive outcome after cataract surgery. Ocular biometry is as important as cataract surgery. A well performed surgery, if not coupled with a correct biometric measurement, can cause patient dissatisfaction.

3. How many types of biometrics are there?

Two types of biometrics are currently available: ultrasound (direct contact or immersion) and optical biometrics. Due to certain disadvantages of ultrasound biometrics, optical biometrics has become the procedure of choice in ocular biometrics.

Ultrasound biometry is an invasive procedure that requires direct contact with the patient's eye (cornea) and the use of anaesthetics, both of which can be uncomfortable for the patient. Furthermore, this method of measurement may involve errors due to excessive compression of the ultrasound probe on the cornea. Immersion avoids compression of the cornea, but has less control over alignment, and therefore may result in erroneous results. Ultrasound biometry also requires adjusting the ultrasound speed when different environments and/or optical conditions are present, e.g. pseudophakic eyes (which already have lens implants) and silicone oil.

The techniques underlying optical biometry are partial coherence interferometry and swept-source OCT (optical coherence tomography). In partial coherence interferometry a signal is produced as a result of interference between the light reflected from the tear film and that reflected from the retinal pigment epithelium.

One reason for the increased accuracy of optical biometrics is the reduced dependence of biometric measurements on the experience of the examiner. In performing this investigation it is only necessary to align the device to the patient's eye, while the rest of the process is automated.

The only limitation of optical biometers has been the reduced ability to measure axial length and anterior chamber depth in eyes with densely opacified environments, such as mature and hypermature cataracts.

4. Calculation formula and results

Emmetropia (lack of eye sight) is the ideal outcome sought in patients undergoing cataract surgery. However, in certain circumstances, target refractions of different values, myopia, hyperopia may establish.

The axial length is defined as the distance from the anterior surface of the cornea to the retinal pigment epithelium (a layer in the ocular wall). It can be obtained using optical or ultrasound methods. A combination of these measurements prevents errors in axial length measurements. It is perhaps the most important parameter in most modern lens implant calculation formulas. It can influence the strength of the lens implant by almost 2.5 to 3 times, more in shorter eyes than in longer ones. It can be measured using ultrasonography (contact or immersion) or optical methods. Ultrasonography uses mechanical waves to calculate the time it takes for a pulse to travel from the cornea to the retina. Sound travels at different speeds depending on the medium, more in the lens and cornea (1641 m/s) and less in the aqueous and vitreous layers (1532 m/s).

Constant A, although called a constant, is actually highly variable depending on several factors: type, material and position of the lens implant, incision technique, incision placement;

GAIN, used in contact biometrics, is defined as the electronic amplification factor of the sound waves received by the transducer, measured in decibels (dB). The normal setting is around 70% -75%. But it can be increased where large echoes are inadequate (hard cataracts, dense ocular opacities, high myopia). It is low when artefacts are seen near the retinal tips (silicone filled eyes and in pseudophakia)(8,9).

The formula for calculating lens implant strength has evolved over time. They can be classified on the basis of their derivation (theoretical, regression analysis or combination) or according to their evolution over generations. By derivation, theoretical formulas are determined by applying geometrical optics to schematic and reduced eyes using various constants. Later; regression analysis was derived using the actual postoperative results of implant power as a function of corneal power and axial length variables. Sanders, Retzlaff and Kraff developed the SRK formula which has been widely used. Although it has now been replaced by newer generation formulas, it is still useful for understanding the relationship of variables and constant A with lens implant power .

Corneal power (K) is considered the second most important factor in determining the strength of the lens implant. Changes in K values; change the strength of the implant by a ratio of almost 1:1. It can be measured either by keratometry or topography, optical biometers perform these measurements directly.

Today's technology is able to combine all these biometric values and indicate the correct amount of lens implant needed for each cataract patient.

5. Biometric device

La Centrul Oftalmologic Profesor Dr. Munteanu beneficienm de toate tipurile de biometrie, de la clasica biometrie cu ultrasunete la ultima generatie de biometre bazate pe tehnologia de ultima ora swept-source OCT (tomografia in coerenta optica). Astfel ca putem oferi servicii de specialitate, indifferent de statusul ocular sau duritatea cataractei.

Alcon's ARGOS OCT Guided Biometer is an innovative solution designed to improve the surgical experience and help achieve better patient outcomes.

The ARGOS® biometric is faster than other biometers and is the market leader. With a biometric data capture time of less than 1 second and a higher scanning speed, 50% faster than other devices, the ARGOS® biometer enables increased efficiency and accuracy of eye measurements.

Although non-contact-optical biometry is considered to have a disadvantage in taking measurements in hard cataracts, studies with the ARGOS® biometer show significantly higher acquisition rates in grade 4+ (very hard) cataracts compared to other manufacturers' biometers, while significantly reducing the need for time-consuming and more uncomfortable ultrasound biometry procedures for the patient.

The ARGOS Biometer uses OCT-guided keratometry, angle-to-angle, cornea-to-retinal, real-time measurements, allowing the user to monitor and guide the patient's gaze so that the patient's alignment and fixation is correct, helping to get accurate measurements every time.

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Articles - 27th December 2022

About eye biometrics