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