Asphericity of implants
Intra ocular implants have experienced many changes over the past decades. Aspherical optics for the improvement of the quality of the retinal image is one of the recent developments in the Decade. It benefits the monofocal and Multifocal lenses.
I participated in the design of optics to a monofocal lens implant (artificial)Micro AY, B.j.) :
The objective of this work was to establish the optimal Asphericity misleading view of the implant to reduce spherical aberration of the eye using a ray tracing program enabled to achieve a realistic virtual eye and test different geometries of implant.
Asphericity and spherical aberration of the human eye
The spherical aberration is an aberration that reduced the stigma through a refraction excessive (positive spherical aberration) or insufficient (negative spherical aberration) rays located at distance from the optical axis. It is related to a non optimal curvature of the edges of a lens. The human cornea is slightly aspherical, but flattening to the edges of the front of the cornea (prolate geometry) is generally insufficient to do not induce a residual rate of positive spherical aberration. The average coefficient of asphericity of the cornea is - 0.2, and the induced positive spherical aberration is close to 0.2 microns RMS for a 6 mm pupil. The natural lens has an aspheric geometry, and a variable refractive index (decreasing from Center to edges), which helps reduce the spherical aberration of the eye by neutralizing some of the positive spherical aberration of the cornea (the lens generates a spherical aberration negative close in absolute value of half of the spherical aberration positive of the cornea).
The first generations of implants were biconvex spherical geometry, for reasons of machining, and no optical quality. Their first goal was to compensate for the reduction of the ocular vergence caused by the removal of the lens (which the vergence is close to 20 diopters). Their spherical geometry assured them an optical power adjustable, depending on the curvature of the front and rear optics, as well as a certain tolerance to shift and tilt, which were sometimes put at the first cataract surgeries, including manual (prior to phaco emulsification) extra capsular extraction technique and.
However, these implants produce positive spherical aberration, which degrades the quality of the retinal image for large pupillary diameter (greater than 4 mm). Advances of micro machining have allowed to consider the realization of aspherical optics profiles, with the issue of reducing residual positive spherical aberration of the eye after implantation, but that a possibly deleterious effect is the induction of coma in case shift.
Aspherical optics and calculation of the asphericity of the implant
Specificity of the human eye
Improving the quality of the retinal image is the vision Central Foveal (1 Central). It is so no need, unlike systems imagers as cameras, to optimize the optical quality of the image on a broad field. The eye is devoid of optical axis (horny and Crystal have axes of symmetry non-aligned) and the Visual axis is generally shifted from 3 ° to 7 ° towards a "centerline". The eye has a spectral sensitivity peak located in the yellow-green (555 nm).
Purpose of the aspherisation of the implant
The reduction of the spherical aberration of the eye is to focus the rays refracted by the outskirts of the dilated pupil. In the case of obtaining a 'perfect' geometric stigma, it is likely that the optical quality of the eye would be improved, but its depth of field is reduced. It is therefore interesting to evaluate the effect of the reduction of spherical aberration on the retinal contrast around the area of "best home" (trough focus MTF) for each simulation.
Variable and metric used for optimization
In addition to the calculation of the optical aberrations of high and low degree (spherical aberration, coma, etc.), the use of ray tracing allows to use several "visually speaking" clues optical quality: diagram intersection of the rays in the sagittal and southern plan (ray intercept), calculation of the MTF (Modulation Transfer Function), diagram of the spots in the level of the retinal image, etc. These indices are more relevant than the aberrations statement, because it is difficult to infer the quality of the image from a statement of terms of aberrations that can be partially offset.
Theoretical model œil
Initially, we built an eye (without lens) aphake including corneal surfaces are modeled by Conic sections, which can be varied (medium corneal keratometry) apical curvature and the Asphericity (modulation of the rate of spherical aberration). For a cornea and a given axial length, it is possible to calculate a "emmetropisante" in the sense of paraxial implant power. The spherical geometry of the implant concerned here is generally equiconvexe. the radii of curvature, front and rear are almost identical. More rays are large, more powerful implant decreases and vice versa. We know also the Central thickness corresponding to each power, and of course, the refractive index of the material used for optics of the implant (close to 1.5).
For reasons of efficiency (related to geometry) and simplicity, it is best to aspheriser the back side of the implant rather the front if we want to reduce the positive spherical aberration. This induces a slight asymmetry in the profile of Optics (which is equiconvexe).
Thanks to the calculation of ray tracing, determined an optimal Asphericity unsurprisingly negative (near-9.5 Q) for a 22 D implant. The virtual eye consisted of a 'standard' cornea, which the apical curvature (Central keratometry) had an average radius of 7.8 mm and a coefficient of asphericity of-0.25.
Improving the quality of the retinal image after aspherisation of the optics of the implant is evident in the simulations (see below). The calculation of the optimal Asphericity must be repeated for each power implant (from 10 to 30 diopters), and can also be done to eyes which vary the axial length and corneal power. This allows to consider the achievement of optics the Asphericity is "custom" (because the - light - following trends: 1) corneal power, the higher the rate of positive spherical aberration induced by corneal:2) more the Dioptric power of the implant is high, and more the aspherisation of the back side must be high). The benefit of the aspherisation is constant, regardless of the couple "keratometry and axial length. The calculation of the optimal Asphericity led to a new range of aspheric implants; These features have also been taken for the realization of the first implant trifocal aspheric Optics: Finevision (Dec).
The following images allow to compare the gain for the quality of the retinal image to the average eye implanted by a spherical implant 22D: left, the simulations are spherical implant reference, right one replaced this implant with an aspherical implant of the same power (22D).