Multifocal implant: definition
A multifocal implant is an implant to several homes, to correct over a distance of vision after cataract surgery (ex: distance vision, vision for bifocal implants, and intermediate vision plus with trifocals implants which tend to become the benchmark for multifocalite). Multifocal implants are available in order to make operation of cataract patients independence to the correction in glasses for distance vision and the close (bifocal implant) vision, distance vision and intermediate vision (implant to "depth of field"), or the three distances (trifocal implant). They stand so implants called "monofocal", which are equipped with a monofocal lens and allow either to see from a distance, to see up close without glasses (on the) power selected for the implantWe can correct a myopia or hyperopia preoperative, or let a low myopia for reading without glasses).
There are mainly two types of multifocal implants: refractive and Diffractive. They are designed to be implanted bilaterally in most common indications.
Refractive and Diffractive multifocal implant
The insertion of a multifocal implant may be offered to patients who wish to no longer wear glasses in vision from far and near, knowing that the distance of the vision of '' close '' is the distance used by the patient to read a book (usually 30 to 45 cm). The consultation of digital screens (computers and tablets) can be accomplished at a slightly greater distance (60-80 cm). Multifocal implants have an optical design designed to separate the light refracted by the implant to two majority homes: one for distance vision and the other for vision closely, and possibly a third for the intermediate vision (trifocals). They can be of type refractive (depth of field especially implants) or (Diffractive)BI and trifocals, implants to depth of field).
Then, some refractive multifocal implants offer a depth of useful field covering vision from afar, from near and intermediate vision. They are equipped with a perspective where the refractive power variations are spread over different areas (the intermediate vision is provided by the 'junction' between the areas dedicated to the home from far and near). These areas can be concentric or not. THEstudy aberrometrique of the multifocalite of refractive multifocal implants is particularly instructive.
"Diffractive" multifocal implants are Bifocal or trifocals. Bifocal implants generate additional shelter close in addition to the home by far, but not a real home for the intermediate vision, only trifocals implants provide (ex; FineVision, B.j. implant). They are designed as an addition to a monofocal refractive lens of a Diffractive network of type kinoforme. This somewhat technical term refers to a structure that looks somewhat similar to the "steps", and it is this network that makes sharing light between different homes, a little like a deflector for aeration allows you to split a stream of air in several directions in the passenger compartment of a vehicle. All Diffractive implants have at least part of their surface a Diffractive network: Restor implant, implant ATlisa, Tecnis multifocal, Finevision, and Symfony (this last representing a Diffractive implant for distance vision and the intermediate vision, homeless for near vision).
Diffractive implants are designed to exploit certain physical properties of the visible light spectrum: their specificity towards conventional monofocal and multifocal implants refractive lies in the ability granted by a diffractive of structure separate light energy into various distinct homes.
The realization of a diffractive Optics is an elegant solution to the challenge that represents a multifocal lens design to reduce dependence on the optical correction in glasses of the patients operated for cataract.
Multifocal implants: optical effects
It is conventional to state that multifocal implants allowing the simultaneous focus of at least two images on the retina (far and near), and that they impose the patient to "extract" the image useful according to the context. Thus, the ability of the subject to perform this tri-cortical (choice of the image) would be one of the keys to the success of implantation of Multifocal lenses. In reality, this design is completely wrong because too simplistic: the look is only about an object (remote, or near) both!
Thus, there is no "choice of vision" to do for each eye surgery, but the vision in each eye will be net less net (or contrasting) depending on the quality of focus achieved with the implant. Sharing the light in several homes is potentially a reduction of the vision of the contrasts in distance vision and can induce the perception of parasitic light phenomena as the night-time halos goshawks of the light bright. These halos are circular, and don't prevent that rarely drive at night. Some optical designs, like the apodisation, to reduce the perception of luminous halos. Bifocal implant Restor (Alcon) and the trifocal implant FineVision (B.j.) are apodisés; the diffractive pattern is dimmed to the edges of the implant (markets are less and less high as they are close to edges; less light is diffracted to the home closely). When the pupil is dilated, the proportion of light diffracted to the home by far proportionally with the pupillary opening, allowing to reduce the risk of luminous halos.
Similarly, the refractive requirement for these implants is great: the calculation of the power of the implant (for far vision) must be done very precisely. Despite this, a biometric calculation error can occur, and can then be corrected by a corneal refractive surgery as needed (or change your implant, which is more invasive in most situations).
He must inform the patient in preoperative of these contingencies.
Indications of multifocal implants
If these multifocal implants are initially discouraged in patients who perform mainly visually demanding activities such as driving at night, the Visual phenomena that they induce are most often well tolerated.
Multifocal implants represent an attractive alternative to patients before all eager to not wear glasses after surgery to see far and close but also in intermediate vision (screens dashboards, etc), including with trifocals implants. Besides, there is no contraindication to the placement of an implant trifocal when the indication of a bifocal implant has been asked, unless the patient does not wish to take advantage of the intermediate vision without correction.
We get good functional results with multifocal implants provided you follow certain indications as the correction (prior or joint surgery of cataract) a astigmatism corneal more of a diopter adjustment, and a precise biometric calculation. Near useful vision is between 30 and 45 cm. The intermediate vision (trifocal implant) is located between 65 cm and 80 cm. With trifocals implants, there is a mixture of sharp vision between reading vision and far vision.
Reading without glasses is possible but requires most of the time a good lighting (it is much more difficult to read in dim light).
The implants to "depth of field" (ex: Oculentis, refractive implant, or Symfony, Diffractive implant) are characterized by a vision almost smaller than for trifocals implants (the energy allocated to the home for the vision of close is zero or low). They cater to patients who do not necessarily wish to dispense with glasses for reading, but simply to be able to read at distance of intermediate vision a digital screen, or even a text manuscript including the font is not too fine.
The following video is dedicated to the links between aberrations, contrast, depth of field of accommodatifs and multifocal implants:
Between 2008 and 2010, I participated in the design and the filing of the patent of the first implant «» trifocal", designed to improve the quality of the intermediate vision (60 to 75 cm), which is often the weak point of the diffractive bifocal implants. This implant trifocal, known as' Finevision'® (B.j. laboratories) was initially available exclusively to the Rothschild Foundation, and was introduced on the market in the fourth quarter of 2010. There are today in o-ring version (for simultaneous correction of astigmatism).
Another trifocal Diffractive implant (Tri ATLisa) is also available, and was introduced on the European market some months after the implant FineVision.
The following video describes the principles involved and the explanations concerning the design of a Diffractive multifocal implant:
This video explains the basics of the design of a diffractive Optics (Diffractive multifocal implant):
More information here:
Link to the project pages on the site CÉROC : explanations on the principles at stake for the Diffractive optics.