Profiles of ablation (excimer laser)
The profile of ablation determines the volume removed from the cornea by the excimer laser for the correction of the optical defect of vision in LASIK or PKR. Corrections of myopia, hyperopia excimer laser and theastigmatism differ mainly by issued on the cornea ablation profile. We will mainly address profiles of ablation to correct optical defects of vision (myopia, hyperopia, astigmatism). Ondes custom ablation profiles on the collection of the Wavefront (OPD) and/or corneal topography are addressed in a specific section.
Pages are devoted to specific profiles of ablation of some possible: profile of myopic ablation, profile of ablation for simple myopic astigmatism correctionetc.
Even if the issuance of a profile of laser ablation is an act that is automated, it is interesting to analyze its characteristics. Beyond the intellectual satisfaction, this allows to optimize the realization of refractive surgery (choice of the position of the hinge and the dimensions of the flap of LASIK, optimization of the dimensions of the optical zone, a strategy to reduce the amount of photoablate tissue,...). Moreover, understanding the characteristics of ablation profiles allows to better understand the constraints and practical difficulties associated with the different treatments.
Here are a few design elements that it is useful to have in the field of the photoablative excimer laser surgery.
The zone of ablation
The ablation zone is equivalent to the corneal surface that is exposed to the fire of the excimer laser. A conventional ablation profile allows you to change the optical power of the cornea in by changing the curvature. There is a direct relationship between curvature and optical power; corneal power is inversely proportional to the value of the central corneal curvature RADIUS (RADIUS apical). The profile of ablation can be broken down into two parts: a central treatment refractive aiming (optical area) and peripheral treatment for connecting with the untreated peripheral cornea (transition zone). The ablation zone is equivalent to the addition of the optical zone and the transition area.
Area = optical Zone ablation + Transition area
Munnerlyn et al. have enacted in 1988 the General principles of the correction of the sphero-cylindrical ametropia by excimer laser photoablation (1). The cornea is similar to a single spherical refractive surface (models paraxial, where the corneal profile section is circular). The photoablation allow the resection of a corneal (a lenticule of corneal) volume of equal power but of opposite sign to the ametropia to correct (1-3).
Profiles of ablation for a given refractive error correction realize different combinations of three profile types basic to the level of the different meridians corneal depending on whether they will have to fix a pure spherical ametropia, cylindrical pure, or sphero-cylindrical (Figures 3-5 and table 1)
Spherical ametropia: nearsightedness and farsightedness (without astigmatism)
The profile of ablation is the same for each of the meridians of the cornea (regardless of orientation).
Myopia: must décambrer the corneal surface.
Hyperopia: to arch the corneal surface:
Simple astigmatism (not made up):
The profile of ablation is different for each of the meridians of the cornea. It theoretically leads to arch (hypermetropique astigmatism) or décambrer (myopic astigmatism) but in a VARIABLE way all meridians except one.
Compound astigmatism (astigmatism myopic composed, composed hypermetropique astigmatism, mixed astigmatism):
There are mainly two types of issue for these treatments:
The spherical and cylindrical of the ametropia components are processed successively according to the above principles with for each of these components a transition zone and a defined optical area. This type of treatment is the preserve of some lasers excimers full beam or slot balayante.
With the flying spot lasers, the distribution of impacts is performed continuously after calculation of the total profile of ablation. In general, the processing of compound myopic astigmatism involves a removal area whose perimeter is circular. The spherical and cylindrical components are then processed simultaneously, realizing an optical area whose circumference is circular or elliptical (short axis along the meridian of the arched, long axis along the less arched Meridian). Treatment elliptical stricto sensu, as defined by the company, VISX, was an ablation strategy to optimize the dimensions of the treatment according to the difference of correction between the arched corneal Meridian and the less arched corneal Meridian.
Limits of physico-mathematiques models used for the profiling of ablation:
In theory, the ablation profiles are calculated from accurate mathematical models. In practice, the result of using a given profile of removal depends on not only the net difference between the change of curvature intentional printed to the corneal surface, but also the action of biological factors (healing) or biomechanical.
Schematically, regression is more important in view of the maximum depth of ablation area (which is in the Center for myopia, on the outskirts to the hyperopia), and this all the more that there is a steep fitting with a no area dealt with at this level (case to the hyperopia, and astigmatism myopic or hypermetropique for some meridians).
Ablation of type parallel (or plano) is used in procedures with a therapeutic purpose (PKT) mode. It provides an example of a mismatch between the refractive effect expected (no since removal is of constant depth) and that obtained (effect him more often démyopisant). This effect reflects an unexpected decrease of corneal camber who based his prediction on purely physicomathematiques features, including two of the causes are biological (hyperplasia epithelial compensating more important on the edges of the optical zone due to the abrupt fitting) and Biomechanics (voltage of the contact strips by the section of the surface lamellas implementation) (4-6). The realization of a broad transition zone on the outskirts of ablation plano allows to limit the induced flattening.
The schematic representation of the lenses photoablatés CG in these pages has been conducted with the model simple dimensional sort (sphere, ellipsoid) on which the operations of addition, subtraction or intersection (Boolean operations) have been applied (7,8). The scale representation is possible, but the proportions of these lenses have however been exaggerated for didactic purposes.
The study of the characteristics of ablation profiles allows to understand the higher difficulty represented by hypermetropiques and cylindrical, treatments due to geometric constraints that are unique. In these pages, the study of ablation profiles was limited to treatment of sphero-cylindrical anomalies. Profiles based on corneal topography and the OPD change the look of these profiles in detail, without changing the overall characteristics as long as the sphero-cylindrical ametropia correction remains predominant. Knowledge of the overall dimensions of the profile of ablation should allow to optimize the surgery, at best by adjusting the dimensions of the flap and the position of the hinge and minimizing the amount of corneal tissue photoablate, which is in fact one of the aspectss of the concept of personalized treatment.
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(2) McDonnell PJ, H, Clapham TN Moreira, D'Arcy J, Munnerlyn CR. Also keratectomy for astigmatism. Initial clinical results. Arch invest, 1991; 109: 1370-73.
(3) W, Hersh PS Blaker. Theoretical and clinical effect of preoperative corneal curvature we excimer laser also keratectomy for myopia. J Refract Corneal Surg, 1994; 10: 571 - 74.
(4) P, Hamberg-Nyström Fagerholm H, Tengroth B. Wound healing and myopic regression following also keratectomy. ACTA invest. 1994; 72:229 - 34.
(5) Gauthier CA, D, Holden BA Epstein, et al. Epithelial changes following also keratectomy for myopia. J Refract Surg. 1995; 11:113 - 18.
(6) Talamo JH, Helena MC. PTK complications. In: Azar DT, Steinert RF, WJ Starck, editors. Excimer laser phototherapeutic keratectomy. Baltimore: Williams and Wilkins; 1997.p.143 - 56.
(7) Gatinel D, Hoang Xuan T. three-dimensional modelling and descriptive geometry of the profiles of photoablation spherical and cylindrical pure excimer laser. J Fr Ophtalmol, 2002; 25 (3): 247-56
(8) Gatinel D. profiles and volumes of ablation in different types of ametropia. In 'Refractive surgery', Saragoussi JJ, Arne JL, Colin J, Montard M. Masson, 2001; pp245-249