Custom corrections are adapted to some characteristics of the operated eye surgery refractive laser. They are not confined to consideration of the optical formula to correct (example: myopia of-3 D) but are intended to establish a 'correction boss' custom. Taking into account the asphericity of the cornea, and aberrations of high degree of wave front, etc, is made for this purpose. The excimer laser then establishes a profile of ablation says "custom".
Customized ablation profiles
The basic ablation of myopia or hyperopia profile is based on a spherical model simplified the cornea and non-custom profiles are simply "sphero-cylindrical": he correct the spherical ametropia (myopia, hypermetropia) and cylindrical)astigmatism). They are based on the conditions optics paraxiales, and even if they are established on a large diameter optical area, they do not consider changes in corneal curvature of the centre to its edges)Asphericity), but only the curvature para - Central of the cornea (the k measure curvature).
Thus, for myopia given (ex:-2.50 D), a keratometry (Central corneal curvature, example: 43 D) and a planned optical area (example: 6.5 mm), corrective treatment established by the laser is identical regardless of the eye surgery on the same short-sightedness and having the same keratometry. Unlike these sphero-cylindrical conventional ablation profiles, so-called "personalized" ablation profiles are derived from more specific data of the operated eye. Custom treatments are particularly indicated in case of pronounced astigmatism, or when other optical anomalies are present)irregular astigmatismIE high rateaberrations of high degree).
Names of customized ablations
Several denominations exist to designate these treatments: personalized, customized, aberrometriques, etc.
Commercial names vary depending on the laser companies:
"Zyoptix" (Technolas PV, initially Bausch & Lomb), "iLASIK" (AMO), "Wavefront-guided" (Alcon, Nidek), "Wavefront optimized" (Alcon), CATz (Nidek), etc... The competition between the different laser companies (and sometimes between some surgery centers) explains the craze for this type of correction, which is used as a kind of technological standard. In some surgical centers, these strategies are presented as desirable options, but occaseionnant a surcharge paid by the patient, which increases the rate of surgery compared to the often attractive initial price.
The "custom" term is in fact relatively overused, if we realize that many aspects of 'classic' treatment are actually intrinsically custom, by the laser software and/or the surgeon. Indeed, for one optical defect (ex: 3 diopters of myopia), the programming of the excimer laser correction by the surgeon may differ, depending on the diameter of the pupil of the patient, the curvature of the cornea, but also his age, motivation, etc... It is also possible, with the platerformes laser of the latest generation, to achieve an optimal integration of the excimer laser profile thanks to the realization of a flap of elliptical outline.
Moreover, despite their support by the personalized treatment guided by the collection of ocular wave-front (OPD), the reduction in the rate of aberration of high degree is not obtained in routine after refractive surgery. However, the elevation is sufficiently moderate so that there is no Visual complaints felt in most of the case.
Taking into account of the corneal Asphericity is a form of personalization that is intended to prevent the elevation of optical aberrations of high degree, as the positive spherical aberration: in this context, the optical aberrations of high degree measure does not affect issued treatment.
Anterior corneal topography measurement allows to design a custom processing to regularize the surface of the cornea; treatment is then said "guided topo" ("topography guided"). This approach is interesting in case of recovery for shift, because only custom treatment allows to correct certain aberrations of high degree such as a coma, and the trefoil.
The debate on the justification for the systematic realization of a personalized treatment actually moves the type of parameters to include in this customization.
Aspherical ablation profiles:
The geometry of the aspherical profiles aims to preserve a value of corneal Asphericity (Q) physiological in the optical area treated in the postoperative period. These profiles are based on models with corneal theoretical aspheric surfaces prolates, and are essentially designed to get a wide optical functional area to prevent the induction of a spherical aberration rate and the appearance of postoperative nightly halos. The Technolas Z100 laser to establish a treatment that takes into account the initial corneal Asphericity and post surgery you want. The value of the Asphericity (Q value) is also customizable with laser Wavelight EX 500.
Profiles of ablation guided by corneal topography.
These profiles (topo-guided) are intended to address the optical aberrations of high degree of original corneal In addition to the error refractive cylindrical sphero. They assume the use of data from the corneal topography. They are based on a calculation of the difference in elevation between the irregularities of the cornea of the patient, and an ideal surface whose curvature is calculated to be emmetropisante.
Photoablations guided by the Wavefront:
They are established from the collection and analysis of the total Wavefront Ideally performed in conditions mesopiques (twilight, for a body to wide pupillary diameter). They are intended in theory to correct optical aberrations of high and low degree, without distinguish the origin (horny or crystalline). The profile of ablation is deducted from the amount of corneal tissue to photoablater to correct the optical phase shifts present within the wave front. As pointed out, all the studies show a tendency to the reduction in the increase of the aberrations of high degree, but not their reduction, at least in routine.