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The multifocalite master

This page is dedicated to the design of a multifocal correction for presbyopia. She outlines of a method of multifocal customized correction based on a non-empirical approach, we have designed in the early 2000s and for which an international patent has been filed Ocular wavefront-correction profiling United States Patent 20060023162 http://www.patentstorm.us/patents/7341345.html

 

In view of simplifying or empirical methods proposed at the time, the interest of a model-based approach precise and rational to induce controlled manner of the multifocalite we became evident.
We initiated the development and validation of a theoretical model that would allow, as the Munnerlyn model for myopia, to establish a profile that is both quantitative, accurate and configurable. Like empirical approaches, this type of model should provide the flexibility required to benefit from improvements through successive adjustments deducted from the responses observed during the course of their application.
In order to lay the groundwork for this "mastered" multifocal, we began by studying the links between pseudo accommodation (depth of field) and optical aberrations. Then, we reformulated the addition concept closely by directly integrating optical aberrations into the realization of a multifocal ocular wavefront.
The Aberrometry allows to establish the precise and quantitative survey of the different "high degree" aberrations, from which a comprehensive analysis of the optical properties of the eye can be carried out, such as the calculation of the maximum visual acuity Theoretical corrected and uncorrected, the prediction of the sensitivity to contrasts of optical origin (curve of transfer modulation), etc...
Used for the study of complex vision disorders and the development of monofocal personalized treatment, Aberrometry is not used for the development of personalized medico-surgical devices intended for Multifocal compensation of the presbyopia. This appeared to us to be the least surprising and encouraged us to develop a method based on the integration of data related to the ocular wavefront.
A multifocal system is traditionally seen as an optical device with multiple foci, at least two when it comes to compensating presbyopia and Ametropia from afar. This design is certainly just but reductive if seen from the aberrométrique angle.
By providing a much more exhaustive and accurate collection of the optical properties of the eye, the Aberrométrique study offers the possibility of simulating the perception of a "multifocal eye" under various lighting or contrast conditions, and supporting it Precise qualitative and quantitative elements (type and rate of optical aberrations, degree of reduction of the sensitivity to contrasts of optical origin). It appeared to us that visual exploration techniques based on the study of the ocular wavefront should allow to establish a true multifocal treatment personalized and controlled.

Multifocalite and ocular Wavefront

Several elements of clinical origin refers to the ocular Wavefront as a key element for the multifocalite.
While the lack of a close, uncorrected vision is a failure for the patient wanting to get rid of the glasses, it is however the degradation of the quality of the vision from afar that is at the origin of the most vehement complaints. This degradation is related to the induction of a high rate of low and high-degree optical aberrations.
The induction of a high level of high-degree optical aberrations may be the cause of a reduction in vision quality while the maximum contrast visual acuity remains unchanged. Some functional complaints (halos, ghost images, glare) observed after refractive surgery or insertion of multifocal implants are often correlated with the induction of high-degree optical aberrations. Coma-type aberration is typically responsible for phantom images, Monocular diplopism, whereas spherical aberration is often implicated in the case of halos or glare.
During sequential multifocal treatment (correction by far and near separately issued), an additional photoablatif treatment centered for close vision can induce an excessive rate of spherical aberration, while decentric it mislead in addition to the Coma. A frequently encountered trap is to consider the corneal vertex as an optical center; The pupil is most often biased in relation to this marker. The pupil being by definition the area borrowed by the incident light to reach the retina, the center of the pupil is considered to compute the optical aberrations of the whole ocular wavefront.
We have carefully studied the Aberrométrique profile of eyes adapted with various types of multifocal lenses. The aberrations found significantly elevated when wearing these lenses were coma-type and spherical aberrations. Some of these aberrations reflect the multifocal character of the lenses tested and is certainly useful in inducing a visual acuity closely.
Spherical aberration, for example, corresponds to the refractive differential between the centre and the edges of the optical zone. Multifocal lenses with addition for near-center vision induce negative spherical aberration, while lenses with addition for near-periphery vision induce positive spherical aberration. We postulated that if some of these spherical aberrations were directly related to the aspherical geometry of these lenses, the other and coma-type aberrations were probably induced by their relative decentering vis-à-vis the pupil Iris. In fact, the multifocal lenses tested were all revolution symmetry and could not induce odd aberrations in the optimal centering case.
Oshika et al calculated the rate of optical aberrations of the cornea of pseudophakes patients. The influence of these aberrations was evaluated by stimulating the point spread function (PSF) and the optical Contrast curve (MTF) from the Wave-front collection. A correlation between pseudo-accommodation and vertical coma-type aberration was found. These authors also showed that a variation in power of 0.5 D (defocusing) had more deleterious effect on the quality of the retinal image when the eye had no high-degree aberrations than when it had a moderate rate of coma .
It was thus apparent to us that the development of a personalized multifocal profile had to take into account, or even predict, to control the rate of optical aberrations necessarily induced

Towards a multifocalite controlled

-Use of the aberrations of high degree to induce the multifocalite!

We studied the relationship between optical aberrations of high degree to understand their effects with respect to the multifocalite. We then discovered that it was possible to mathematically predict the average change in focal length induced by an aberration given, and for a given pupillary diameter.

aberration spherical defocus

The spherical aberration of the Z40 Zernike polynomial induced the defocus on a central area obtained in this example by concentric reduction (see the small patterns into medallions). Our algorithm allows the calculation of the defocus may be present on any sub-pupil for each listed in the classification of Zernike aberration. In this example, the maximum defocus is reached when the pupillary diameter is reduced by 30%. All aberrations are not equal in terms of defocus-induced, and pupillary reduction percentage optimal. Some aberrations lead of defocus on money not concentric pupils.

This focal variation is not restricted to the aberrations of degree 2 (myopia and) astigmatism change the spherical equivalent), but affects all of the optical imperfections of the ocular system and therefore the aberrations of high degree.
The perimeter of the zones dedicated to these different focal lengths can be determined from the pupillary game of a given patient, for a given activity in a given light environment. For example, it is possible to study the variations of the diameter and the centring of the pupil in different light conditions (photosting, Mesopic, scotopic) and working distance (33 cm, 70 cm, infinity, etc...).
Rather than "play" with the only myopia Defocus alone (diopter correction accomplished on the area dedicated to vision from afar, then addition to area close, etc.,...) The originality of our approach is to use a selection of high aberrations from the outset degree to build the multifocal correction. The collection of these data (pupillary game, patient's wishes) makes it possible to establish a list of constraints.
The optimum combination is achieved through a special algorithm that uses mathematical optimization functions from pre-determined constraints. The algorithm "selects" the type and rate of accuracy of high-degree aberrations necessary for the construction of the wavefront to induce a given multifocal (depending in particular on the required addition and selected pupil domains).

multifocalite aberrations of high degree

Construction of a multifocal wave front from the pupillary game data, we built coins pupils. Knowing each aberration-induced 'degree of multifocalite', we can then generate one or more multifocal eye wave front, to induce exactly the degree of addition pre defined on the domain souhaité(ex: 1.5D, sur une pupil de 2 mm)

We have also included in the parameters taken into account in the development of the multifocal profile the quantitative elements of quality of vision such as the RMS of the Wavefront error, Strehl criterion, or the area under the line of the MTF modulation transfer function. As a result, the rate and the consequences on the quality of vision of the combination of optical aberrations introduced by a given correction is known prior to issuance of the multifocal treatment!
Our method offers increased flexibility, since the correction takes the form of a theoretical ocular wavefront resulting from the integration of a correction for vision from afar and close vision, or even one or more intermediate visions. Concerning the Presby-LASIKintegration of the data corresponding to the correction for vision of far and near in one profile of ablation is a tangible progress, eliminating the risk of overlapping of optical zones and transition induced by sequential treatment.
It is possible to propose for the same patient one or more multifocal correction by weighting differently vision from afar and close vision, etc.,... For example, different magnitudes for the addition of close can be tested, in order to determine a safety margin for the preservation of vision quality from afar.

Another possibility of customization offered by our method is to encourage the expression of some aberration in the final resulting wave front by a technique of selective weighting. This is important when we realize that mechanisms of neural compensation can weigh the subjective effect of an aberration on Visual function.
Knowledge of the rate of induced aberrations simulates the effect of the multifocal correction envisaged by the calculation of objective optical-quality metrics such as the Point-spreading function and the Modulation transfer curve. The realization of convoluées images (simulating the deformation of optotypes), phase blades with the corrections generated, or an adaptive optic technique can also be used to study the subjective perception of a patient given to the Proposed multifocal.
These techniques allow to study the theoretical effect of the correction before its realization. This is all the more important as candidates for multifocal often ignore the visual constraints, except when they experience the wearing of multifocal contact lenses. For example, the use of adaptive optics could simulate different types of correction, and subjectively determine how to use different induced aberrations to optimize depth of field and Induced optical aberrations.

-Steps to the obention of a controlled multifocalite

Important dimensions to the pre-surgery.
Patients who express unrealistic expectations must be challenged. The ability to sort cortical is a crucial element for the success of the technique. The adaptation of a multifocal lens can be proposed before surgery to study the effect induced by multifocalness in a candidate patient. A successful adaptation with these lenses is an encouraging prognostic element. The patient should be warned that his vision will evolve after performing the surgical act because of the scar-remodeling phenomena and the start-up of new vision interpretation mechanisms.
The study of variations in pupillary diameter is a crucial step, as patients with a large diameter in scotopic environment may be particularly exposed to the perception of vision quality disorders after the intervention. The pupillary shift in near vision must be measured, in order to define an optimal subdomain for the addition of close. The existence of a weak and low-reactive pupillary diameter could limit the tolerance of multifocal (selection of a low optical gradient at the cornea).
The frequent practice of night driving and/or activities in scotopic atmosphere represents a contraindication for the realization of a multifocal surgery technique. In all case, the patient must be warned of the risk of degradation of his night vision quality.
Our method has not yet been the subject of extensive clinical studies. This deficiency is inherent in the conditions that presided in its realization; Our initial ambition was to compensate for the lack of a specific model for the controlled induction of multifocality.
However, we have received very encouraging indirect confirmations regarding the relevance of our approach. In particular, we have carefully studied the results of our theoretical simulations carried out from clinical data collected from real patients (dynamic pupillary, degree of addition to be filled). The patterns we generated were very similar in quality (type of induced aberrations) and quantity (RMS rate) to those measured in clinical practice in subjects with an effective and well-tolerated multifocal (natural or obtained after Surgery or port of multifocal lenses). In addition, some simulations induce a small degree of oblique or inverse astigmatism for a pupillary diameter, the beneficial effect on multifocalness has been reported.
Finally, the design of a multifocal wavefront independent of an organic support (cornea, Crystalline lens, etc...) can be induced by different vectors: Laser ablation profile (obtained by "subtraction" corneal of the optical path separating the Pre-operative optical aberrations of the ideal multifocal wavefront), lens, customized implant in situ (Cahloun lens), etc...

Refractive multifocal techniques for presbyopia as presbyLASIK compensation must satisfy a subtle compromise between increased pseudoaccommodation and maintains a quality of vision acceptable for the patient. This compromise depends on various other factors such as brain plasticity of the patient (cortical sorting efficiency) but remains mainly under the influence of the generated optical aberrations.
Aberrometry is now widely used for understanding and establishing a possible treatment of vision quality disorders. It provides a more precise analysis and is advantageously substituted for a purely "focal" conception of multifocal. The inclusion of parameters such as pupillary dynamics and the control of induced optical aberrations is an indispensable step to improve the reproducibility and results of the techniques of induction of multifocal for the correction of the Presbyopia.

References

(1) Anschutz T. Laser correction of hyperopia and persbyopia. Int invest blink, 1994; 34 (4): 107-137
2) Vinciguerra P, Nizzola GM, Bailo G, Nizzola F, Encaria A, Epstein D excimer laser photorefractive keratectomy for presbyopia: 24-month follow-up in three eyes. J Refract Surg. 1998; 14 (1): 31-7.

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