Choice of technique (LASIK, PKR, etc.)
The choice between LASIK and PKR)surface laser) concerns mainly the correction of myopia. For the correction of hyperopia, LASIK is a technique of choice: the hypermetropia usually have corneas thick and resistant than short-sighted. Some short-sighted have corneas over thin or irregular, which can lead to reconsider the indication of LASIK for the benefit of the laser surface.
Operation of myopia, hyperopia, and astigmatism: choose which technique?
Choice of technique for the correction of myopia
When a laser for myopia correction is considered, the choice of surgical technique is between:
-techniques with cut-of flap or flap (LASIK)
-techniques without cutting flap (surface laser = PKR and its variants).
Up to 6 diopters of myopia to correct, there is no difference to the effectiveness of these techniques. In the past, there is less risk of sous-correction in LASIK. The choice between LASIK and laser surface for less than 6 d myopia is based on various clinical elements, and anatomical restrictions mainly due to the thickness and the regularity of the cornea (corneal topography study, possibilities of LASIK correction offered by corneal thickness).
Constraints of the cornea
To understand the issues and the indications of the myopia surgery, some reminders are necessary
The cornea is a transparent membrane composed of several layers of tissue. Of these layers, the epithelium represents about 10% of the corneal thickness, while the stroma occupies almost the entire remaining thickness. The epithelium and the stroma are separated by a thin, acellular layer called Bowman's coat. The thickness of the corneal dome is not constant, as the cornea is thinning from the edges (about 650 microns) to the center (530 microns on average).
Constraints of corneal thickness
The profile of ablation laser issued by laser for the correction of myopia involves a maximum tissue ablation, which is going to be carried at the Center of the cornea, next to the constitutionally finest box it. Any myopia in LASIK surgery must respect a minimum residual thickness of the corneaequal to about half of the initial thickness, or about 250 microns.
Indeed, with the technique of LASIK, the photoablation is performed in the stromaunder a component superficial whose thickness is between 100 and 130 microns about. The flap is cut by a femtosecond laser (or the mechanical microkeratome in the centres which are not equipped with femtosecond technology). Femtosecond laser is particularly indicated in case thin cornea, or "narrow" orbital conformation because the flaps cut to the microkeratome have sometimes better than what was desired thickness.
Even if he is rested after realization of the photoablation, the stromal flap no longer involved in the biomechanical stability of the cornea. Thus,. the sum of the thickness of the flap and the depth of ablation laser must be maximally such that, subtracted from the initial thickness of the cornea, a value of Wall residual stromal posterior at least equal to 250 microns can be obtained. In LASIK, there is maintenance of the epithelial layer, which explains the speed of Visual recovery (the central surface of the cornea remains smooth and intact). The sensory nerves in the cornea section largely explains the absence of pain Ocxxx_xxx_5163ionnees. The cutting of the flap, however, involve peripheral circular section of the epithelium: this is why that transient discomfort or difficulty to open the eyes is often felt in the first hours after LASIK.
In a technique known as ' surface ' (PKR), the laser is issued to the» surface of the stroma (layer of Bowman)after peeling of the epithelium located in the center of the cornea. A contact lens is usually asked at the end of the surgery, in which the epithelium pushes in some days.
Prevention of corneal ectasia
The Keratoconus is a condition which corresponds to a progressive deformation of the cornea, caused by a decrease in stiffness of the cornea and which is accompanied by a reduction in the thickness of the cornea in its central region. It causes evolutionary and irregular astigmatism which in severe forms can no longer be corrected in full by glasses glasses. Diagnosis of forms beginner or turned of Keratoconus occurs during adolescence or adulthood due to corneal topography.
Keratoconus is a absolute contraindication to LASIK. Advanced forms of Keratoconus screening is easy, but beginner shapes, called infra clinical, much less...
Indeed, the realization of a LASIK on a cornea damage of Keratoconus is unknown (one speaks of sub clinical form or form not detected fruste) before surgery is the main cause of post-LASIK ectasia. The cutting of the flap and consecutive to the myopic photoablation central thinning are certainly the triggering factor, because they participate in the aggravation of the biomechanical corneal wall weakness.
An important part of our research is devoted to the study of medium screening very beginner forms of Keratoconus, which is not always obvious and requires a fine interpretation of preoperative topographic maps (you can find articles dedicated to this issue for download on this site). We have developed a software testing automated for the prevention of the corneal ectasia that uses topographic data from the study of the faces prior and posterior of the cornea, as well as its thickness: SCORE Analyzer software.
We have indeed acquired the conviction that corneal topography should interest not only the front (anterior) of the cornea, but also its back side (posterior) and allow the creation of a map comprehensive corneal thickness at each point. (ex: Orbscan topography). The measurement of the thickness of the cornea with a ultrasonic method (ultrasonic product) we appears suboptimal insofar as, unlike the product or optical tomography (rear elevation anterior topography), it is not possible to determine with certainty the point of minimum thickness of the cornea. The measure of biomechanical properties of the cornea brings additional elements; the measurement of the hysteresis and corneal resistance carried out by the Ocular Response Analyzer (CH, FIU) instrument can often confirm a doubt when the possibility of a fragility corneal pre existing.
When should you prefer the PKR LASIK?
If the cornea is particularly thin (less than 500 microns in the Center), and slightly irregular curvature, and its biomechanical resistance index is lower than normal. a technique of LASIK may be questioned. A technique of photoablation of surface (including referred to as variants PKR, LASEK, or Epi LASIK represent only few differences to the patient in terms of feeling post-operative) would, however, possible, if the correction is Interior to about 6 D.
Insist on the fact that the Beginner Keratoconus is a contraindication formal to the realization of a LASIK technique, and this regardless of the initial thickness of the cornea, and the type of cutting (microkeratome or laser femtosecond). Only corneal topography to ensure screening, supported by a measure of corneal biomechanics. Modern techniques of topography combine study of the anterior curvature, and the collection of the rear elevation and the study of the variations of the corneal thickness (CT): Orbscan, Pentacam, etc.
In summary: choice between LASIK and PKR
Corneas after 'successfully' topographic tests and biomechanical resistance may be operated in LASIK. The cutting of the flap will be made with the femtosecond laser and its thickness will be determined based on the topographic and eye of the patient characteristics. It will be even thinner that the cornea is thin and myopic correction is important. This cut is in all case followed by photoablative corrective step (excimer laser).
When there is a doubt about the ability to undergo a LASIK conducting, a surface photoablation technique (PKR, less invasive but painful 24 hours approximately to the waning of the surgery) will be proposed. The absence of cutting of the stromal flap better preserves the potentially fragile biomechanical corneal wall rigidity.
There is no 'best' technique in general, but there are sometimes a 'better' technique for a particular patient. The choice of the most suitable technique is the responsibility of the surgeon, and is the first step of the 'key' to a successful refractive surgery procedure. Safety is the key parameter to choose between different possible techniques in a given patient. The important thing is the long term, and the 'comfort' aspect should not prevail on practical considerations or "marketing."
Correction of hyperopia and the strong astigmatismes
These corrections require the achievement of ablation profiles whose geometry involves a large amount of tissue photoablation. LASIK is the technique of choice for these corrections. Keratoconus is almost always associated with a certain degree of myopia, the risk to operate a fragile cornea (Keratoconus beginner) is much rarer in the context of a farsightedness. The correction of hyperopia in LASIK corrects distance vision, but also improves vision closely in the presbyter (see the page dedicated to the) correction of hyperopia in the presbyter).
This property can be used to induce an increase of near vision in some subjects by elsewhere free of disturbances in vision of far (so-called normalsighted issues and presbyopes)- Download article about it
Personalized treatment guided by the Wavefront collection, treatment optimized
The vision correction laser using a conventional treatment or conventional allows to correct myopia, hyperopia, and astigmatism, which are responsible for the blurry vision of patients with these visual defects. In some patients, other optical defects called "aberrations of high degree" can also impair vision, particularly night. The ocular wavefront by the aberrometric analysis to detect the optical aberrations of high degree. A high rate of these aberrations can be the indication of a personalized treatment intended to give a vision of better quality than that obtained with standard treatments in certain indications. Indeed, treatment guided by collection of the wave front (custom treatment) are designed specifically to address the very small Visual irregularities specific to each eye and measured using an aberrometer.
This type of treatment offers additional benefits for people whose vision is heavily corrected with glasses or contact lenses. It can even correct irregularities of vision resulting from previous surgeries or injuries. In these rare and special, indications l ' use of information about the fine relief of the cornea can also be used to establish a treatment personalized laser to reshape the cornea (corneal topography-guided treatment).
The preservation of the natural (prolate) asphericity of the cornea can also reduce the risk of nocturnal halos, as well as the choice of a larger treatment area. This approach is intended to minimize the risk of night-time glare and halos, especially for patients whose correction order is high and for those who have large pupils. They are intended to provide the best possible vision, limiting the risk of optical aberrations and maximizing the chances of getting a vision of 12 or even 15/10 after treatment. Treatments called "wavefront optimized" (Alcon Wavelight laser) are designed to best preserve the corneal Asphericity and increase the optical quality of the cornea in post operative with respect to conventional treatments.
Finally, the correction of the strong astigmatismes benefit of the technique of Iris recognitionwhich allows to reduce the risk of "misalignment" (called cyclotorsion) between the axis of astigmatism measured in sitting (the consultation) and lying (during surgery). A mapping of the iris is made during the consultation by an aberrometer. This map is then used to serve as a benchmark for the laser treatment (the picture of the iris of the patient is transferred to the laser, and serves as a benchmark for the issue of the treatment). We have the technologies of active compensation for cyclotorsion during the sequence shot of the excimer laser (Active Compensation, laser Technolas), and the system of correction of the cyclotorsion by limbic, iris recognition and prevention of the pupillary shift (laser Wavelight EX500).
Reprocessing in patients operated in the past.
All of these technologies are available to the Rothschild Foundation (IFS 150 femtosecond laser, femtosecond laser FS200, different types of lasers, excimer such as laser Wavelight EX 500, Technolas Z100, customized and optimized - laser ablation laser ex: wavefront optimized, custom-Q, technology Zyoptix, guided by the Wavefront, of screening anterior and posterior corneal topography, measure the biomechanical resistance of the cornea) treatment guided by corneal topography (topolink), etc...).