Nomogram of Gatinel for the correction of presbyopia at the farsighted.
This joint correction of presbyopia and Hyperopia is the issue of this nomogram, and is based on a theoretical model, as well as the Clinical results considered satisfactory.
It aims to overcome the lack of consensus and theoretical bases established for the realization of the «» presby-LASIK« .
All multifocal correction strategies share some aspects with the technique of Monovision where it induces a difference of intentional correction between the eyes. Multifocal approach is distinguished by having a better visual acuity for the non-dominant eye.
The keys to success are based on:
- Obtaining a Good visual acuity in vision from afar for the dominant eye, which also benefits from an improvement of the intermediate vision and gains a few lines in close vision.
- A good visual acuity not corrected in close vision for the non-dominant eye, but, at the difference of the classical monovision, By limiting the reduction of visual acuity by far from this eye, thanks to the multifocal.
For example, thestereoscopic acuity (two-eyed depth perception) is better than classic Monovision. If the multifocalite improves the performance of the dominant eye in near vision, and the eye not dominant in distance vision, there are a range of "recovery" from the vision of both eyes, and most of the patients operated do not spontaneously feel the difference between the refraction of the two eyes.
The originality of the proposed correction is based on the induction of a multifocalite controlled, that is to say the induction of an improvement in vision of far and near, which is not the same between the dominant (preferred intermediate and distance vision) and eye non-dominant (preferred intermediate and near vision).
It was designed to be used with the lasers Wavelight (EX 500). This model allows, thanks to the F - CAT (or Custom Q) mode of customize the corneal Asphericity value target (Q) in the postoperative period. It is particularly suitable for the use of a nomogram of aspheric correction to induce ocular multifocalite, whose headquarters is the cornea. The multifocalite is guided by the change of theasphericity of the corneal profile.
Why and how to change the corneal Asphericity?
The Asphericity is a change in curvature between the corneal vertex (vertex) and the outskirts, along a Meridian. Curvature governs the refractive power of the cornea, and any increase in the Asphericity returns to an increase in the difference in curvature between the Center and the corneal periphery and therefore a variation more important of the refractive power of the cornea
Unlike a related misconception a misunderstanding of the role of optical asphericity of the cornea, the change of the corneal Asphericity (hyperprolaticite) does not provide an increase in performance in near vision in the operated eye.
To the contrary, more prolate the cornea of an Emmetropic eye would make it overall farsighted. Central refraction ' (paraxial) would indeed remain unchanged, but the peripheral flattening of the cornea would be responsible for a reduction of the vergence of the rays of light refracted by the edges of the cornea.
To allow the prolate aspherisation to operate, induce a sufficient central myopic refraction to allow a good visual acuity closely uncorrected. The hyperprolate apshericite then allows a return to the rays refracted by the average emmetropia corneal periphery (see the) page dedicated to the study of the relationship between corneal Asphericity and multifocalite for further explanation on the theoretical foundations of this approach).
Thus, the Central myopisation is achieved at the cost of a central over-correction of the refraction, as in classic monovision, but the quick emmetropisation on the outskirts of the pupil (to improve distance vision") is obtained through the negative aspherisation.
We can rephrase this method by the induction of a corneal Asphericity more prolate than that measured in pre operative (= more negative Q factor), which reduces the effect of the myopisation to the edges of the pupil.
To set this method, the surgeon must enter the parameters of the laser correction to deliver in the so-called "F - Cat" or 'custom Q' of the Wavelight laser.
Effect of the Asphericity target prolate
As explained above, it is essential for a reason in contrast to the usual precepts : effective multifocal surgery by the programming of the 'custom-Q' excimer laser module is to correct the eye non-dominant for near vision. This requires a central myopisation in the optical area scheduled induction. The aspherisation is to add power negative (subtracted from the positive power) to the most peripheral region of the optical zone. This reduction in the power added to the Center is linked to the rapid peripheral flattening of the corneal profile. This gradual reduction and return to the average emmetropia periphery of the optical zone saw that Visual acuity of far greater than a look that would be "monofocalement" evenly myopic in the optical area.
The explanations given here make explicit than we aspherise (more goal is a negative value of the Q factor), it tends to give the distance vision, and not near vision. A theoretical model published in the Journal of Refractive Surgery in 2014 suggested that the optimal value for the variation of the Asphericity (ΔQ) between-0.6 (low peripheral demyopisation) and-0.8 (high demyopisation).
Aspheriser is not enough: interest of the Central myopisation
Once established the desirable value of the change of corneal Asphericity intended to induce a rapid reduction of the refractive power in the pupil, it remains to determine how many diopters need to overcorrect the hyperopic eye intended to benefit from the multifocalite to get an effective vision uncorrected closelykeeping in mind that under the principles established above, the negative aspherisation (quick demyopisation to the periphery) and the sur-correction (Central myopisation) have opposite effects.
It should be superfluous to recall the Need for Central myopia. The rays that pass through the centre of the pupil will thus be focused on the retina if they are emitted by objects located at a low distance from the eye (typically the reading distance). The change of asphéricité corneal induces a demyopia towards the edges of the cornea. Thus, in vision from afar, peripheral rays induce a focused retinal image for the observed targets (thanks to the pupillary zone emmétropisée on the average periphery) and in close vision, it is the central rays that provide a focused image For rays emitted by close targets.
Careful observation of the becoming of farsighted patients and presbyopes operated with different platforms of excimers lasers (Nidek, Technolas Bausch & Lomb, Alcon - Wavelight) combined with mathematical modelling work which he did reference previously led me to adopt to develop a precise nomogram for the correction of presbyopia at the presbyopic farsighted.
Clinical audits are underway to to refine recommendations. Its general principles are explained in the following paragraph.
Principles of the Gatinel nomogram for the correction of hyperopia and presbyopia
In this strategy, it should be in most situations to emmétropiser the eye says 'dominant', and apply the described nomogram more far to the eye "not dominant." This difference allows to combine the benefits of monovision without its main drawback, which is the reduction in the sensation of vision (Stereopsis) embossed, and the sometimes unpleasant impression felt by the patient of a significant difference of view between the two eyes.
Only the correction of the non-dominant eye is intentionally multifocal, although it is possible to create a multifocal bilateral surgery, especially in patients for whom obtaining a vision almost without correction is the issue of surgery, and which do not (or rarely) high Visual distance vision solicitation activities (night driving, golf, etc.). In addition, intentionally monofocal correction of hyperopia from the dominant eye allows more often improve intermediate vision, and significantly reduce the blur in near vision.
In preoperative, allowing about read it with the eye not dominating a text which the police is Parinaud 2 to measure the necessary correction to the distance vision, and the minimum addition.
-The sum of these corrections (far and near) then corresponds to the minimum value to be considered to program the laser for effective correction in vision closely. Minimal addition of + 2 D seems to not induce slot of near vision correction. An equivalent strategy is to aim for a postoperative refraction understood between - 2D and-2.50D. If the non-dominant eye presents a farsightedness from + 2D, the correction program is of + 4.50D to target a central pupillary postoperative refraction of-2.50D.
– What value target for the factor Q of asphericity does introduce in the the laser software for the maintenance of the optical power at the center of the pupil (necessary for vision myopia closely), and a more peripheral emmetropisation? Remember that the answer to this question was given by a precise modeling of the eye surgery starting from the premise that the desirable value of aberration spherical negative is C40 = - 0.3 microns on a 6 mm box (either a variation ΔC40 = - 0.4 microns on 6 mm, because the value of the C40 coefficient is usually close to + 0.1 microns prior to surgery).
The value to be programmed for the postoperative Asphericity is equal to the initial Asphericity less a close ΔQ of-0.6 (the case with a value of asphericity Q equal to-0.25, to set a value of Q = - 0.85)
-the treatment is issued on a 6 mm Optical Area For the non-dominant eye.
Refine the value of this aberration spherical "desirable" would imply to know some eye settings, such as the Central keratometry, pupillary 'medium' in distance vision diameter, the depth of the anterior Chamber, etc. Remember if this was still necessary that the issue of the aspherisation is to induce an aberration value spherical negative including the differential between the central power and the edges correspond approximately to a deviation of 1.50 to 2.50 dioptres (assuming that near vision requires an excess of vergence of the same order at the center of the optical zone: the periphery of the optical zone is then emmétropisée).
In practical terms, to correct a farsighted requiring a correction of + 1.50 D by far, and an addition of + 2.00 D for vision closely (non-dominant eye), must be programmed at least + 3.50 D on an optical zone of 6 mm (myopia target = - 2D), and choose a low Q-factor to a value of-0.6 (ex:-0.85 if the original corneal Asphericity is - 0.25).
Here is a concrete example images where this nomogram was applied successfully.
The negative aspherisation tends to reduce maximum peripheral ablation of a hypermetropique correction (who don't 'dig' the cornea in the Center). This is of course related to geometry "hyperprolate": the profile of the cornea to achieve postoperative being flatter on the outskirts, it is located "above" (at any depth) than a profile less prolate.
The following figure summarizes the objective results Topo aberrometriques (OPD SCAN III) obtained for the right eye and the left eye.
A detail of the maps of vergence (map OPD) allows to check the induction of a true vergence gradient (multifocalite) within the pupil of the right eye. These maps have the interest to directly show the variations in refractive power within the pupillary area (the maps of corneal topography do not directly provide information on the multifocalite actually reached after presbyopia surgery).
Clinical results of this strategy are currently analyzed to improve the nomogram, depending on the degree of correction scheduled and the preoperative characteristics of the operated eye (keratometry, apshericite, etc.). In some case, it is necessary to increase slightly (ex: + 20% to 25%) value corresponding to the addition of the sphere of correction for vision of far and near, especially for widely presbyopic patients (low residual accommodation to nowhere). It is likely that the induction of a negative aspherisation mislead a reduction of the corneal vergence a bit 'too close' to the center of the cornea, which leads to slightly anticipate the reduction of central power. Moreover, the less depth of ablation in periphery induced by the change of the Q-factor (with respect to that which would be reached by a spherical correction) tends to be a bit "under corrective" in practice.