Ocular biometry, calculation of implant
Ocular biometry is a test that allows the collection of certain dimensions of the eye as its axial length, in order to calculate the power of the implant intended to replace the Crystalline lens during the cataract surgery . Remember that for the retinal image (and thus vision) to be clear spontaneously (without correction of glasses or lenses), it is necessary that the cornea and the implant focus the incident light emitted by the light source observed in the plane of the retina.
When cataracts are performed, the optical power (vergence) of the cornea is not altered. We withdraw the Crystalline lens , which is replaced by an implant: ocular biometrics is intended to collect data to calculate (to predict) the power of the implant (vergence) which, combined with that of the cornea, will allow the eye to be corrected for the NET vision distance without desired glasses. The ocular biometrics thus groups together the collection of biometric data, and généralmeent also the prediction of the optical power of the implant intended to obtain the postoperative refraction of the eye operated.
(see: PEARL-DGS calculation formula, proposed by Dr Debellemanière, Gatinel and Saad and based on the integration of biometric data by an artificial intelligence algorithm: https://www.pearldgscalculator.com/
In addition to the length of the eye (axial length) which is generally measured by one interferometry techniqueocular biometry requires the measurement of the optical power of the cornea (keratometry) to predict the power of the implant based on the desired correction (final refraction) of the operated eye. (see:)calculation of the power of the implant in one eye model simplified). The prediction of this power, that example will allow the eye to see net from a distance without glasses (emmetropisation), appeals to a so-called formula formula for calculating the power of the implant (There are several types of formulas for the calculation of the implant, and some are more precise than others according to the morphology of the eye considered). In case ofastigmatism Corneal (An internal astigmatism disappears with the removal of the Crystalline lens during the surgery), it is possible to envisage the installation of an implant called " o-ring", and this requires an additional calculation (to determine the axis and degree of correction of astigmatism brought by the implant).
Why should we do a biometry before cataract surgery?
The installation of a Crystalline lens artificial life is rigorous during the cataract surgery , where the Crystalline lens is removed (its envelope, the capsular bag, is preserved and serves as an anatomical support for the implant). the Crystalline lens is a lens whose optical power is (before the onset of cataract) usually adjacent to 22 dioptres: coupled to the cornea, the Crystalline lens acts as a convergent lens that allows to focus the light rays in the retina plane. Removing it leaves the eye in a State called "aphakie" (aphake eye). Due to the loss of the vergence (optical power) of the Crystalline lens , the aphake eye has an important power defect (high hyperopia).
It is not easy to measure the optical power of the Crystalline lens In Vivo; In addition, when the patient exhibits an amétropia in preoperative (myopia, hypermetropy, etc.), the replacement of the Crystalline lens by an implant that would have the same power is not interesting. Indeed, it would leave the eye operated and implanted (eye says "pseudo phake") with the same optical defect as before the operation.
Most cataract patients wish to stay or become Emmetropic (benefit from a good vision of far uncorrected after the intervention). Some patients, initially myopic, wish to stay in order to be able to read without glasses (if they are very myopic, a partial reduction of myopia is indicated to leave a residual myopia close to-2.50 to 3 D).
Biometrics is designed to calculate the power of the implant Crystalline lens that will allow the eye to achieve the desired refractive status (emmetropy, or slight nearsightedness). The vergence of the cornea then added to that of the implant allow the light emitted by a distant source to be focused on the retina (emmetropy) or slightly forward (slight myopia).
The advent of multifocal implants imposes a precise Biometrics: multifocal implants must allow optimal correction of the vision from afar, so that the addition they possess for the near vision is effective and that the patient can become in post-operative glasses.
Ocular biometry: parameters of the calculation of the power of the implant
Biometrics allows to measure at least two essential parameters:
-the axial length : it separates the top of the cornea (vertex) from the fetal retinal.
-the keratometry: the measurement of the curvature of the central cornea (around the vertex) allows to predict the optical power of the corneal (Coral vergence).
In most of the case, as in it - even is performed through a contactless optical biometer non-contact, which uses the properties of interferometry to measure the length of the eye (ex: IOL Master, Zeiss). This instrument is also fitted with a keratometer, and a system of the anatomic depth of the anterior Chamber of the eye (this parameter is used in some formulas).
It is important to note that any inaccuracy of measurement of either of these two parameters will have an impact on the calculation of the power of the implant.
The following figure shows schematically these parameters:
In case of advanced cataract, axial length is not changed, as well as the keratometry. However, the depth of the anterior Chamber may be reduced, because the volume of the lens increases as it loses its transparency.
Once the lens has been removed during cataract surgery, the eye is «» aphake«: its refraction is hypermetropique: the vergence of the cornea alone is not sufficient to focus light on the retina (except in very important axial length case: very high myopia).»
Even if the extraction of the clouded lens restores the transparency of the eye circles, it is necessary for optical reasons (refraction) to replace the lens with a lens which the vergence (power) should be calculated according to the following biometric parameters:
-desired refraction: emmetropia (from afar clear vision without glasses) or slight myopia (almost clear vision without glasses)
-axial length (over the eye is long, lesser is the power of the implant, and vice versa: short-sighted receive implants of lesser power than the hypermetropia)
-keratometry: more the vergence of the cornea is high (low central curvature), lesser is the power of the implant (and vice versa)
– position of the implant in the eye after surgery, which is also called " effective lens position« .
Among these variables is a which cannot be measured before the intervention : this is the actual position of l' implant in the eye ("effective lens position", ELP) ; This position is not the same as that of the Crystalline lens withdrawn, and it depends on many parameters, that retrospective statistical studies, ideally performed on a large volume of data ("big data") can help frame. It is partly because of this that some biometric formulas use artificial intelligence: by nourishing models of "machine learning" (supervised learning) by a sufficient number of observations (typically examinations Biometrics that have been used for cataract operations known as the type of implant and the final refractive result), we can "learn from past biometric errors" and reduce them by regression techniques, network Convolutional or forest of random trees.
In addition to the initial anatomical parameters such as the depth of the anterior chamber or the thickness of the Crystalline lens , some of the characteristics of the implant may affect the position (its distance from the cornea). For this reason, a linear adjustment is used according to the type of implant being installed which thus has a "constant A"; This constant, the origin of which is linked to the first empirical forms of biometric computation, corresponds more or less directly to the value separating the average error found for a given implant on a significant series of patients for whom one aimed at the emmétropy. It is statistically determined by a linear regression technique, after collecting a sufficient number of observations, and allows to "refocus the distribution of refractive errors to zero".
This uncertainty on the final position of the implant stem issues of biometric calculation formulas that have been successively proposed, in order to better predict the actual position of the implant. It is for this that the biometric calculation is based partly on speculation; It is based on the choice of a final position of the implant in the eye to stop the calculation of a diopter power to provide the desired refraction.
The following review with an interferometric biometer (IOLMaster 700, Zeiss) allows to observe important biometric differences between the two eyes of a patient who has a difference in refraction between the two eyes (the right eye is myopic to-16 D; the left eye is myopic to-8 D). The axial length of the eye is much higher than that of the left eye. This elongation also seems to affect the depth of the anterior Chamber of the eye. However, the thickness of the lens is substantially identical between the eyes, as well as the keratometry. The SRK T formula does not directly take into account the depth of the anterior chamber (which was not measured with the first biometres). But designing the prediction of the actual position of the implant will be performed even more away from the cornea of the eye is long.
Rather than "calculating" the power of the implant, it might be preferable to use the vocacle " predictionbecause the fact of not being able to predict the exact position of the implant in the eye induces some uncertainty the final result (However, this uncertainty is limited to one to two dioptres at most in most case eyes without an ophthalmologic history).
Optical power of implants
It is expressed in dioptria (ex: 22 D) and corresponds to the vergence of the implant (in the ocular circles); for multifocal implants, this is the vergence of the hearth intended to focus the rays from distant sources (vision from afar). For the o-implants, this is usually the "average" power (around which the variation of power required for the correction of astigmatism is distributed). There is no direct relationship between the implant power and the pre-operative bezel correction. On average, Emmetropic eyes benefit from implants with a power of close to 22 D. Longer myopic eyes usually receive lower power implants, and shorter hypermetropic eyes, higher power implants. The power of the implants varies by 0.5 D steps. On average, a modification of 1 diopter of the power of an implant causes a change of 0.7 D in the glass bezel correction (this is related to the variation in the distance between the plane of the bezel glass and that of the implant).
Example of biometry by ultrasonic measurement
Biometric data can be collected using a bi-dimensional ultrasound measurement (B scan): this type of collection is preferred or complements the optical measurement (interferometric) in some indications, especially in strong myopic, or when There is a reduction in the transparency of pronounced optical media (corneal pillowcase, highly evolved cataract, etc.). Before the advent of optical biometrics, the ultra-sound axial biometrics (in mode A) was used to determine the axial length of the eye; This technique was much less accurate because manual and operator-dependent. The B-mode ultrasound is used to collect anatomical data, which makes it possible to better estimate the length of the eye examined.
To measure the length of the eye, based on the time taken by the ultrasonic waves through the eye; the speed of the ultrasonic waves varies according to the crossed middle (it is superior in the lens). On average, it is close to 1550 meters per second, but it varies (higher in the lens).
The following figure shows an example of the ultrasound cutting thus obtained from the measured eye. From the collection of the axial length, and the keratometry measurement (accomplished through another instrument: keratometer, topographer), a power of implant "emmetropisante" can be obtained.
In conclusion, the biometric ocular examination is indispensable before cataract surgery with implant installation. It allows to predict the power of the implant intended to provide the eye, in addition to the restitution of the transparency of the media, a refraction adapted to the patient's wishes. This prediction is vitiated by a certain vagueness, that the most recent formulas aim to reduce to the maximum.