LASIK and astigmatism
Correction of astigmatism in LASIK
THEastigmatism can be corrected by the LASIK: this optical defect is found in 90% of the population, and 25% of it has an astigmatism greater than one diopter. Astigmatism can be pure (insulated) or associated with myopia or hyperopia.
LASIK is a quite proper technique to correct the vision of the astigmatic, pure or composed his astigmatism (IE associated with myopia or hyperopia). Astigmatism is a particular optical defect, in the sense that it is "oriented"; It is defined by its magnitude and its axis (ex in the noted expression - 3 x 0 °:-3 is the magnitude in diopter and 0 ° axis in degrees). This peculiarity induces some specifics for the realization of LASIK, including him requires optimal orientation of the laser treatment.
Astigmatism has therefore, in addition to the short-sightedness (hyperopia or), a directional component. Because of this constraint, its correction must be done according to rules responding to common principles regardless of the type of astigmatism (simple or compound), to optimize the results and provide the best possible vision after LASIK.
The correction of astigmatism in LASIK is to increase and/or reduce selectively some meridians of the cornea curvature (see the page dedicated to the) profile of ablation of the for the correction of astigmatism excimer laser).
Rating of astigmatism and correction
Astigmatism is expressed as a figure (magnitude in dioptres) and an axis (in degree). For example: (-3 × 10 °), or (+ 2 × 90 °).
The first number is important, the more the astigmatism. The axis mentioned in degree is that of the meridian of the cornea more flat - or less arched when the magnitude is negative (ex:-3 × 10 °: the less camber axis is 10 °). When the magnitude is positive (+ 2 × 90 °), the axis matches that of the arched Meridian. In all case, it is important to align the treatment according to the direction of astigmatism.
Establishing the origin of astigmatism before LASIK
Astigmatism is mainly induced by excessive corneal toricite; in general, there is a match between the axis of the cylinder refractive and major corneal meridians. In case of dissonance, must ensure the reliability of the performed tests, and choose instead of corneal refractive cylinder axis. The presence of a debutante cataract can induce the presence of an internal astigmatism (often reverse). The presence of a Keratoconus can explain the existence of refractive changes: remove a Keratoconus fruste or starting in the presence of astigmatism oblique or reverse in particular.
An aberrometrique examination must be made to decide in the difficult case (ex: calculation of internal and corneal aberrations).
Terms of the preoperative examination before LASIK for astigmatism
You must respect the following:
-in case strong astigmatism and wide pupillary diameter, the risk of refractive vagueness and regression, as well as with halos is more important than for pure spherical corrections.
-rigid lenses should be removed at least one month before the intervention, and if the corneal topography and/or refraction are unstable, it is better to extend these time limits and review the patient later.
Choice of the profile of ablation (excimer laser) issued in LASIK
The correction of astigmatism aims to restore an identical curvature of all meridians of the cornea. In case of pure or myopic astigmatism composed, to reduce the curvature of the arched meridians. case hypermetropique pure or compound astigmatism, there arch less arched meridians. case of mixed astigmatism, there arch less arched meridians, and reduce the curvature of the arched meridians (bitorique treatment).
LASIK and astigmatism: key points of the operative technique
In LASIK astigmatism treatment must comply with the following points:
-check the programmed values of the refraction in the laser to eliminate any input error, particularly in case of transposition of the refractive formulas sphero-cylindrical expression in cylinder negative than positive cylinder (this concerns especially the astigmatismes compound or mixed).
-ensure the correct alignment of the patient under the laser, and avoid misleading a rotation of the head. A previous marking of the cornea on the horizontal Meridian can correct a cyclotorsion of the globe when the use of a high-performance eye-tracker is recommended. The use of technologies such as iris recognition here is a very interesting application: it allows to align the treatment accurately, even in case of turning movements of the eye when the patient lies under the laser (the cyclotorsion correction).
-Choose a suitable dimensions flap. The hinge should be placed theoretically in the axis of the initially more arched Meridian to offer photoablatif treatment possible stromal surface after cutting. Indeed, the zone of ablation of a cylindrical treatment is oval with a centerline parallel to the Meridian initially as flat. So, in case of direct astigmatism, it is better to get a flap to horizontal hinge superior, as in case of inverse astigmatism, a flap to nasal vertical hinge is an interesting choice. This allows to reduce the risk of fire on the hinge, especially in case of flap of small diameter. The use of the femtosecond laser allows for wide flaps which it is possible to position and size the hinge with precision; some lasers (Alcon Wavelight FS 200) allow even the realization of oval flaps, marrying so the perimeter of the correction issued by the best way excimer laser:
See page dedicated to the profile of ablation for correction of myopic astigmatism