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Ocular biometry, calculation of implant

Biometrics: definition

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).

IOL master 700

Biometric parameters collected with instrument IOL master 700 (interferometric technology coupled with swept source OCT)

Biometer AL SCAN measurement of the axial length, the keratometry, and the depth of the anterior Chamber.

Representation of the AL SCAN biometer (Nidek), which has a system for measuring the depth of the anterior Chamber by Scheimpflug camera. The biometer's software acquires the value of the axial length, keratometry, and the depth of the anterior Chamber.

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:

ocular biometry, length axial keratometry

Axial length is the distance between the top of the cornea and the retina. Light is focused on the retina, the cornea and the lens must focus the incident rays so that they are focused at a point in the plane of the retina. The depth of the anterior Chamber is anatomically the gap between the top of the cornea and the top of the front of the lens. The optical power of the cornea is estimated through the keratometry.

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.

Biometrics ocular cataract parameters

In case cataract, the lens thickness increases; This reduces the anatomic depth of the anterior Chamber.

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).»

Biometrics eyepiece aphakia secondary hyperopia

After removing the Crystalline lens during the cataract surgery (the crystalline bag that is preserved is not represented here), the eye is aphake and its refractive is usually hypermetrope in the absence of implant placement. Indeed, the vergence of the cornea is insufficient to focus the light emitted by the sources observed on the retina (unless the eye is very long, i.e. with a strong myopia: in some case of extreme myopia, the power of the implant is then close to zero or even negative!).

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".

biomterie ELP

Optical biometrics (IOLMaster 700) with OCT cut swept source made before cataract surgery (top) and after cataract surgery implant (in the middle). The effective position of the implant (ELP) is not known before surgery, but it depends statistically on certain parameters that are used in modern biometrics formulas: anatomical depth of the anterior Chamber, thickness of the Crystalline lens Etc.


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.

Biometry (IOL Master 700, Zeiss) in a patient with a significant Anisometropia: the right eye is more nearsighted, because he is longer a few milimtres. The anterior Chamber is deeper on the side of the longest eye, which portends a greater effective final position (distance horny / implant) on the right side to the left side.

Biometrics (IOL master 700, Zeiss) in a patient with an important anisométropia: the right eye is more nearsighted because it is longer than a few milimeters. Its anterior Chamber is deeper on the side of the longer eye, which foreshadowed a larger final effective position (corneal distance/implant) on the right side than on the left side. Keratometry is relatively similar on both sides. Narrator of the 3rd generation implant calculation formulas use the value of the axial length for predicting the effective position of the implant.

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.

result of the calculation of the power of the implant in mode echo B

Biometrics in B-mode ultrasound to measure the length of the eye and see section the main elements of it to share echo it produces: successively, we observe a peak for the cornea, the front and back of the Crystal faces, and the retina. From the position of these echoes in time, one can estimate the anatomic depth of the anterior Chamber is 3.17 mm, the thickness of the lens of 4.61 mm, and the length of the posterior segment (vitreous cavity) 15.55 mm. The total length of the eyeball (axial length) is 23.33 mm. Using this value and the keratometry (48.62 mm), the calculation using various formulas converges to the value of 15 diopters for the emmetropia (A constant used is of 118.7).


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.

29 responses to "ocular biometrics, implant calculation"

  1. Hello it about 1 year ago and dust I took an a air gun ball, I have more than crystalline or inner support,
    The centre of Ophthalmology of Dijon offers me an implant that I should do: Thank you for your reply, greetings M Rousselot

  2. Dr. Damien Gatinel says:

    To a secondary implant to correct hyperopia induced by the loss of the lens (aphakia)

  3. Hervé says:


    This is a publication really interesting that illuminates on those prior calculations.
    My elderly aunt who presents a former right amblyopia has had cataract surgery
    On this eye with the installation of an implant of 30, 5d, the other eye that presents only a beginning of cataract has been left waiting for more than a year and the ophalmologiste thinks to send it back to the surgeon because it feels that there is too much difference in image size Between the two sides and that she is responsible for the pain and the diplopness that appeared a time after this intervention, the glasses with prism no longer supported. She tells me about it and I wonder how will we calculate the power of the implant of the eye to operate and especially the precautions to be taken to restore a vision quite comfortable as it was the case before.
    Indeed it seems that the calculations are made for one eye at a time, but these are the two together that work and you have to be able to overlay compatible images of the same size I guess, all that in a brain younger!... Thank you for enlightening me a little on this point!

  4. Dr. Damien Gatinel says:

    A power greater than 30 d implant corresponds to what is usually placed in the eyes strongly hypermetropia (low axial length). Initially, the other eye also presents a farsightedness which, if it is corrected in glasses, actually causes a difference in size of the image projected onto the retina between the eyes: we're talking aboutaneisoconie (annoying difference between the perceived size of an object by the right eye and the left eye). The eye corrected (all or part) of its hypemretropie thanks to cataract surgery and the placement of an implant of high-power receives an image smaller (or less) than the other eye, surgery and still strongly farsighted and who needs a 'convex glass' magnifying the image on the retina. It is therefore interesting to operate the left eye. The calculation of the power of the implant to ask on this side is specific. It can take account of the residual correction in glasses may be required to improve Visual acuity on the right. Once both eyes operated cataract and close to the emmetropia (through the replacement of the lens by a more powerful implant), there will be more noticeable difference between the images seen by the eyes (disappearance of the aneisoconie)

  5. Hervé says:

    Thanks for these interesting details. Actually the 2nd eye surgery (with an implant of 22, 5 d) seems less suffering, like the right also.
    However in the stabilization period following the operation, the patient can see even a little near/mid distance, but the pains come back quickly and she waits for glasses to see from Pres and a half distance... It is hoped that the left eye that had to have the top in relation to the right eye amblyopic, can take it back. Meanwhile she seems to suffer from glare and wears tinted glasses (ordinary sunglasses)... Would there be a more appropriate modality of eyeglasses in these new conditions? Knowing that we have just discovered a new parameter, a paresis of verticality!... This seems to exclude progressive lenses already... Can there be glasses with great depth of field, more indicated?

  6. NATASHA says:

    Good evening

    Thanks for this article, I would like if possible a precision on the anterior implant for implants of room calculation and it is possible to convert a calculation of posterior Chamber directly implant to have one previous implant of room. Thank you for your answer

  7. Dr. Damien Gatinel says:

    An implant of anterior Chamber is located in a plan that is closest to that of the cornea than an implant of posterior Chamber. For a same target refraction (for example the emmetropia, absence of distance vision correction), an implant of anterior Chamber will have a reduced power with respect to that of an implant of posterior Chamber. By default, the cataract surgery is done today with implantation of posterior Chamber, except specific situation (absence of capsular support later to receive the implant). The biometres have an option allowing to calculate the power of an implant of anterior chamber (by reviewing its position and taking into account the applicable case specifications).

  8. Joëlle says:

    Hello doctor, I want to know if an echo biometry made a year ago is still valid for a cataract operation next month, or is it better to do it again for more precision (myopia-10). Thanks in advance for your answer. Kind regards.

  9. Dr. Damien Gatinel says:

    To most eyes having completed their growth, biometry does not change in a few months. In the context of a strong myopia (elongation of the eyeball marked and possibly progressive), it may be interesting to a biometry to control. Priori there should not be many changes if your correction has not changed.

  10. Jean-Pierre says:

    My child was operated at 2 months of a unilateral cataract. His eye operated was slightly microphtalme before the Crystalline lens was removed (15.4 mm vs. 16.5 mm). Since it wears a Contact lens with very strong correction (D: + 20). He is now 15 months old and his eye operated is now larger than the other (22.1 mm vs. 20.9 mm).
    Can this strong increase in axial length be an inherent myopia in the port of Contact lens? (Adapting the eye to a very close view) or should we look for another reason otherwise (glaucoma...). Thank you in advance for your return. Very cordially.

  11. Dr. Damien Gatinel says:

    I have no experience regarding the influence of the lens correction after congenital cataract. An excess of intraocular pressure can actually explain an increase in axial length, but it is more likely to be it here of the consequences of complex regulatory mechanisms governing the growth of the eyeball. The specialized team following your child certainly is able to learn on this point.

  12. Laurent says:

    I have a patient with a high myopia and an important kératocone that I have to operate cataracts. I find it hard to determine the power of the LIO. This varies between-13 and-4.50.
    It has a 29.05/ACD épith: 4.03/K1 54.64 (75)/K2 57.81 (163) (Lenstar), keratometry and axis is slightly different to Pentacam.
    What formulas do you recommend for calculating the implant? What kératométries?
    Given the vagueness of calculation, is it advisable to reduce its astigmatism with a toric? If not which axis (that of the topographer, the Lenstar or the ARK Nidek: all 3 different)?
    I almost told myself not to implant it and to redo a post-operative biometrics, but it may change not much after the surgery....
    Thanks for your advice

  13. Dr. Damien Gatinel says:

    The answer to this kind of biometric dilemma is difficult. Beyond the keratometry issue, it is important to use a formula that does not use the keratometry to predict the actual position of the implant (ex: formula Haigis, avoid the SRK - T formula). Concerning the choice of the keratometry, I generally use the average value in the 1.5 mm Central, which provides for example the Pentacam topographer. The choice of a toric implant is indicated if astigmatism (its regular component) is important and that the patient has no rigid lens correction (or does not have it), otherwise then to "unmask" astigmatism of toric implant (because of the neutralization or part of anterior corneal astigmatism by the rigid lens). For the choice of the axis, I use the "Fourier transform" card also provided by the Pentacam, where you can view the axis of the cylinder in the central region of the cornea. Another option, with the OPD scan, is to select the "Toric IOL display" and choose the suggested axis. In all case, the importance of corneal deformation suggests refractive uncertainty and incomplete correction of astigmatism, need to warn the patient. Finally, in my experience, the o-rings implants have slightly more likely to suffer postoperative rotations, and I sometimes inserts a ring of capsular bag after installation and alignment of the toric implant to stabilize it.

  14. colette buffet says:

    I saw a surgeon ophthalmologist for cataract surgery and offers to correct my nearsightedness by implant at the same time.
    What information does the surgeon need to provide for the calculation of the implant (biometrics)?
    Thank you

  15. Dr. Damien Gatinel says:

    The surgeon must indicate the degree of myopia in post operative. For example,-2.50 D (for playback), or-1.50 D (for intermediate vision). In general, the biometries made to let a low residual myopia cover these refractions and each is given the theoretical power of the implant to achieve them.

  16. Akram says:

    Hello doctor,
    I have a ectopia cristallinienne with the left eye doubled with a posterior capsular catarcte.
    My ophthalmologist prescribed an implant calculation.
    My question is: Is it possible to have a multifocal implant (Type Technis Symphony) Das This kind of situation?
    Thanks in advance

  17. Dr. Damien Gatinel says:

    Priori multifocal implants themselves to surgery resolved, without risk of displacement of the implant (that let fear the ectopy), so initially a monofocal implant seems more appropriate.

  18. John P says:

    Hello Mr. Ganeshan,
    I am an optician and work in a refractive surgery clinic, particularly responsible for the calculation of implants for cataract surgery.
    We usually use, among other things, Barrett's formulas, and have found its limits in the case of Keratocône (note in phase "2 kissing birds") or more generally in the case or posterior astigmatism is more Arched horizontally.
    Indeed, Barrett considers the adjustments based on the Baylor and Koch'reporting algorithm, adjusting the steep K based on the value of the posterior astigmatism, admitting that the latter is more vertically arched.

    What adjustment do you recommend in these case?

    Thank you for your return

  19. Dr. Damien Gatinel says:

    It is difficult to give you a proven method in this context where subjective refraction is generally fluctuating because of the importance of irregular astigmatism, which effectively and jointly affects anterior and posterior cornea. In this type of situation, the posterior astigmatism can be considered this time "parallel" geometrically (toricité identical, but astigmatism induced from opposite sign from an optical point of view due to the inverse index gradient). It is then preferable to be slightly under-corrective (while Baylor's Nomogram recommends being rather overcorrected for anterior reverse astigmatism).

  20. PETITJEAN says:

    My daughter is now 18 years old. It was operated by a bilateral congenital cataract in 2004 and 2006 (Necker Pr) with the installation of multifocal implants. However, she always needed to wear progressive glasses, the vision of closeness has long remained unstable. Today, it seems that she has less need of the addition of the progressive for the vision closely, and her vision from afar is better (myopia more corrected to get better acuity) but big worries: she sees no net in her intermediate vision. Several attempts: Eyezen glass with small added comfort, then progressive + 2, then + 2.75... nothing makes the intermediate vision remains blurred. Do you think it is a technical problem and a correction adjustment? Or can it come from a malfunction of the implant itself? I believe that a change of implant is also not recommended because of the adherences and presents risks... thank you for your reply.

  21. Dr. Damien Gatinel says:

    The intermediate vision was not ensured by the first multifocal implants (generally bifocal for vision from afar and near). However, it should be possible to correct the intermediate vision (70/80 cm) with well-fitted glasses. Make a specific point in consultation with your ophthalmologist and optician.

  22. Sylvie says:

    I was operated last week from cataract
    My operated eye sees very well what was planned (monofocal implant) and the other sees well from afar.
    On the other hand the combination of the two is not comfortable I have a feeling of blur and especially glare
    The rocker had been decided because before the operation it had already established itself naturally
    My ophthalmologist and my surgeon tell me that if it is not perfect now it is unlikely that it evolves positively and that if I want to be reoperated to put a vision implant from afar it must be done immediately
    I consulted another ophthalmologist and an optician who advised me to wait and were reassuring about the future evolution
    Do you think that it can still improve naturally, have you had similar case? What would be your recommendation?

  23. Dr. Damien Gatinel says:

    My recommendation is to wait for an improvement to happen, thanks to a phenomenon of adaptation. If this was not the case, it would always be possible, a priori, to perform corneal refractive surgery in the future to modify the correction of one eye (or both) – in LASIK or PKR for example.

  24. Nicole Harrisson says:

    To perform biometrics in anticipation of cataract surgery, should the ophthalmo dilate the Crystalline lens with drops like during an exam?

  25. Dr. Damien Gatinel says:

    Dilation is not necessary or recommended to perform ocular biometrics for implant calculation in cataract surgery .

  26. Catherine's collar says:

    Hello I will soon be operated by the catarcte. I'm a strong myopic-20 and-19.Il is going to ask me an implant of-4 and-5. This is a good prognostic.

  27. Dr. Damien Gatinel says:

    The power of implants that need to be posed does not allow, without access to other biometric information, to predict what the eventual residual correction will be after surgery. In General, the results of the cataract surgery in patients are excellent, as the simultaneous correction of cataract and strong myopia provides a dramatic improvement in vision.

  28. Corroyez Brigitte says:

    I've been cataract lately.
    For the right eye it was handed to me a map d implant identification.
    On the other hand for the left eye there is no way to get it, and two months after the intervention I always feel a discomfort (a veil) in the outer corner of the eye.
    The surgeon tells me that this map is not at all useful.
    Is that normal?
    Thank you in advance for your reply.

  29. Dr. Damien Gatinel says:

    Although the map is not available, you can simply ask your surgeon that it communicates to you type and the power of the implant posed during the cataract surgery of the left eye.

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