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Myopia: simulations of surgical correction

Nearsightedness surgery should transform a myopic eye in an Emmetropic eye. Myopia is related to excessive with respect to the optical power (vergence) axial length of the couple horny + crystalline. Logically, the most common techniques are to reduce the vergence of the cornea by reshaping laser)LASIK(, PKR) or replace the lens implant which the vergence is less than that of the lens native (the ocular biometry calculation before surgery, allows to establish the power of the implant "emmétropisant").

The indications and clinical aspects of myopia surgery are detailed on other pages of the site. This page is in some aspects more fundamental, the illustrious 'virtual' way on a model eye thanks to the ray tracing.

 

The myopia corneal surgery

It aims to reduce the power or anterior corneal vergence; even if the myopia is axile (it's the case for most of the myopia greater than-4 D), surgical correction of myopia is surgery of the cornea with the laser.

The earlier corneal vergence is equal to the difference in index between air and cornea, divided by the apical curvature radius of the anterior corneal dome. In theory, this power reduction could be accomplished by a decrease in the index of refraction of the cornea, or a decrease in its central curvature. There are no methods can reduce the value of the refractive index of the cornea; corneal techniques have a single common goal; reduce the curvature of the central cornea. History of refractive surgery is full of techniques designed to reduce the curvature of the cornea: radial keratotomy, myopic keratomileusis, etc.

Currently, LASIK and PKR are the headlights of refractive surgery techniques: they are based on the incomparable precision of the excimer laser for precise contouring of the corneal curvature.

Here is an example, illlustre by a ray tracing technique, reduction of myopia on a 'virtual' eye

Reduction of the corneal vergence and correction of myopia (ray tracing)

This is an example for the correction of myopia of-4.25 D. The modeled eye has an 'average' corneal curvature (apical radius of the anterior face of the cornea: 7.8 mm)

Ray tracing of an eye with axile myopia of-4.25 D: from rays of point sources (on the centerline or (7) of it) are focused in front of the retina. It receives a beam of defocalises rays. The following figure shows the distribution of "impacts" of these rays.

 

representation of the single spot diagram of a myopic eye

The representation of the "spot diagram" (diagram of commercials) reveals the dispersion of the rays in the retinal plan: these rays come from a home located in front of the retinal plan. The stigma is very degraded: the distribution of the impacts of the rays is a surface whose RADIUS is 100 times greater than that of the Airy task (eye limited by diffraction)

2D myopei corrected myopic eye

After corneal surgery (LASIK or PKR) myopia, the vergence reduced corneal (of-4.25 diopters) allows from rays from a distant point source to be refracted more backward, in the plan of the retina. The eye now sees ' clear ' the distant sources.

single spot diagram of an initially nearsighted eye after correction of myopia

Reduction scheduled the vergence of the cornea (of 4.25 D, from 43 to 38.75 D D) allows you to move the home originally located in front of the retina to the retina: there is, however, a certain residual dispersion associated with persistent high degree aberrations (ex: positive spherical aberration). The rays are focused on a surface whose RADIUS is double that of the focal spot of an eye without optical aberrations of high and low degree (Airy task). Aspherical and personalized treatments have for main goal to reduce the rate of optical aberrations of high degree myopia corneal surgery-induced.

 

Surgery of the lens for reduction of high myopia

Axile originated from high myopia: it is related to an excessive elongation of the eyeball. Here is an example of affected eye of strong placentation myopia: the distance between the top of the cornea is equal to 30.6 mm.

Eye 2D myopia strong simulation by ray tracing

Modeling of an eye reached axile high myopia. The effective focal length (plan main image to the image focus) East of 22.01 mm, then the axial length of the eye (of the top of the cornea to the retina) is 30.6 mm. After refraction by the cornea and the lens, the rays emitted by a distant source focus well in front of the plan of the retina.

The axial length that should have the eye to be Emmetropic is estimated at 23.8 mm. There is a difference estimated at 6.8 mm. Using a formula linking axial length and myopic We can estimate that the refractive error (myopia) of this eye is 6.8 x 2.70 = 18 D about. Myopia-18(d) cannot be corrected by LASIK.

If the patient concerned is of more than 50 years old, or if the lens is partially clouded (cataract is earlier in patients with high myopia, especially in its nuclear form), then the technique of choice consists of the extraction of the crystalline by phaco emulsification and its replacement with an implant whose power is calculated to correct the myopia of the eye. This pusisance is logically less than a no or low myopic eye.

In our virtual example, power calculated for the implant able to correct myopia after removal of the lens is... zero. Thus, the removal of the lens enough to correct nearsightedness, by sufficiently reducing the vergence of the eye; the vergence of the cornea is enough to focus the incident rays on the retina (a subject not myopic, the retina being closest to the cornea, the cornea would focus the rays backwards, causing a hypermetropisation of the eye - see page to the) aphakia).

However, it is recommended to place an implant in the capsular bag, to maintain a barrier 'physical' between the segments anterior and posterior of the eye. So we choose in this case a power implant 0 diopter which surfaces are parallel.

Model eye implanted with lens power zero

After removal of the lens and an implant of power (or vergence) zero, the eye initially strong myopic is Emmetropic: the total vergence of the operated eye is equal to that of the cornea only. The home to which converge the rays refracted by the cornea is located in the plan of the retina.

This theoretical simulation provides results comparable to situations encountered in clinical practice:

Axile high myopia corrected by cataract surgery; Clinical case

A 51 year old patient consulted for the correction of high myopia of the left eye: the refraction of the patient finds a myopia of-17.5 diotprie (in the plane of glasses). The lens has a beginning of cataracts. The cataract surgery is so programmed.

Biometrics (calculation of the power of the implant eye parameters: keratometry and axial length mainly) suggests that the power of the implant for the emmetropia is + 0.5 D.

biometrics for implant emmétropisant, high myopia

The axial length of the eye is close to 30 mm, for a normal value keratometry: this biometric characteristic explains the strong myopia.

It was decided to replace the lens by an implant of power zero:

blister of the implant placed power zero

Blister of the placed implant, of power anywhere (0D). The MN60MA wording refers to the type of implant, which consists of a lens diameter of 6 mm and haptic intended to stabilize the implant in the capsular bag.

 

 

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