Operations to correct astigmatism
Astigmatism surgery is common in refractive surgery, because this defect is sometimes isolated, but most often associated with nearsightedness or farsightedness. Contrary to received, astigmatism occursthat it is isolated or associated with another optical flaw. The results of LASIK or the PKR in correction of astigmatism are excellent, otherwise it uses latest generation lasers and certain options such as the recognition of the iris (Iris recognition), which helps to align the treatment according to the desired axis.
Astigmatism correction is deemed more difficult than that of myopia, quite rightly, because it should be not only to perfectly Center the laser treatment, but also to align it with the axis of astigmatism to be corrected.
To understand the principles of surgical correction of astigmatism, need to know certain characteristics of this optical defect, affecting the means of correction, as well as some of the constraints they impose.
The astigmatic eye has the particularity of not having the same optical power according to the eye meridians (see explanations of astigmatism). Thus, in case of astigmatism, all light rays from a distant source do not converge on the retina as the meridians of the eye do not have the same optical power (astigmatism corresponds strictly to the)lack of stigma !). The variation of optical power according to the meridians is most of the time related to a change in the curvature of the meridians of the cornea (corneal astigmatism, caused by an excess of toricite of the cornea).
The regularization of the corneal curvature laser is a logical solution for astigmatism surgery. LASIK and the PKR to correct the astigmatism with the issuance of a profile of ablation adapted.
In case of cataract, the appearance of an astigmatism of origin "internal" (cristallinienne, because cataract changes the optical properties of the lens) is also possible, he accompanies particularly gladly nuclear forms. This astigmatism is often to 'reverse' direction (non-compliant said astigmatism). In case cataract surgery, this component from astigmatism disappears with the removal of the lens. On the other hand, if a corneal astigmatism is associated with this internal astigmatism, should be possibly fix it by choosing a particular artificial lens called «toric implant» implant
-The formulation of astigmatism (example:-1 x 90 °) has the value of excess (positive sign) or failure (negative sign) to the largest refractive power caused by astigmatism of the concerned eye (it is the number - 1 in the previous example), as well as the axis of the Meridian along which is present this defect (90 °, which is the axis of the vertical Meridian). Astigmatism can be associated to myopia: in this case, we get for example a myopia of - 4 d associated with previous astigmatism a correction formula that is written:-4 (-1 x 90 °)
The 'research-training' section contains a more detailed information on astigmatism.
The correction of astigmatism can be done by various methods including corneal laser surgery, and by the lens implant surgery. The choice of method is a function of many parameters (age, degree of astigmatism, presence of myopia or an associated farsightedness, astigmatism origin - horny or lens-, regularity of astigmatism, the thickness of the cornea, presence of a cataract, etc.):
Equalization of the optical power of the cornea:
It is listed in case fororiginal corneal astigmatism(majority of the case). The excimer laser sculpts the front of the cornea in order to reduce or increase the curvature depending on the meridians (ex: the LASIK technique, the PKR: see)are profiles of ablation for the correction of astigmatism).
Latest refractive laser platforms allow to achieve a cut of flap adapted to the geometry of the LASIK laser correction (ex: suite Wavelight refractive, see) video about the cutting of elliptical flaps).
In more specific case as after a cornea transplant, we can use a different approach and make incisions in the corneal graft to correct astigmatism, without removing corneal tissue as with photoablation techniques. The incisions (say arc because circulars) are carried out with the femtosecond laser.
Replacement of the lens with an artificial lens implant said "o-ring":
The power of the implant is calculated so that the operated eye is more astigmatism (technique used in patients with cataract and an original corneal astigmatism). L' ring implant should be aligned with the axis of astigmatism of the cornea, otherwise induce a under correction in case in malposition or rotation. Cataract surgery allows the correction of the corneal component from astigmatism (corneal astigmatism).
Adding a toric implant in the anterior chamber or the posterior Chamber keeping the lens;
Such an indication is rarer, it concerns patients with anterior Chamber is deep enough, the realization of a LASIK impossible (ex: Keratoconus), and an age too young for surgery of the lens.
In all case, astigmatism is a "oriented" optical defect (axis), the correction of astigmatism requires adequate guidance of treatment, and a correct alignment of the main axes of astigmatism with the corrective device. If the orientation of the treatment is imperfect, theaxis error induces a residual astigmatism.