Profile of ablation: compound astigmatismes
The correction of theastigmatism compound depends on the type of astigmatism, and its association with myopia or a farsightedness.
Astigmatismes compounds, or sphero-cylindrical ametropia composed, include:
-l' compound myopic astigmatism (AMC): ex - 2(-3×90°): all corneal meridians are "short-sighted" (their degree of myopia vary between two extremes: in the present example, between - 2D - the meriden 90 ° and - 5 D error - error of the opposite Meridian, 0 °)
-l' hypermetropique compound astigmatism (AHC): ex: + 2 (+ 1 × 0 °): all corneal meridians are "farsighted" (again, variation between the two extreme meridians)
-l' mixed astigmatism (AM)::+3(-5×0°) ex: some meridians are "short-sighted" (share and other axis 90 °) and some meridians are "farsighted" (share and other axis 0 °)!
To understand the profiling of ablation conventional, can be used a sequential approach to the treatment of compound astigmatism. The spherical and cylindrical components are separately considered in establishing the profile of ablation. It can be dealt with separately (successively for systems to full beam or balayante slot) or jointly in the case of the systems to scan by spots (once the total sequence has been established, the issuance of spots can be done in a "random" order).
Expression of compound astigmatismes
The astigmatismes myopiques and hypermetropiques compounds can be expressed and treated using different expressions sphero-cylindrical (1-3):
-expression called "positive cylinder": example:-3 (+ 2 × 90 °) (AMC), + 4(+3×0°) (HAC), - 1(+3×0°) (AM)
-expression called "negative cylinder": example:-1(-2×0°) (AMC), + 7(-3×90°) (AHC) + 2(-3×90°) (AM)
-expression in 'crossed cylinders' where astigmatism is broken down into two cylinders of the same magnitude but opposite signs, the sphere then corresponding to the spherical equivalent:
example (+ 1 × 90 °)(0°-1)-2 (AMC), (+ 1, 5 × 0 °)(-1,5×90°) + 5, 5 (AHC) (+ 1, 5 × 0)(-1,5×90°) + 0 °, 5 (AM)
A fourth expression called bitorique can be used for mixed astigmatism: (+ 2 × 0 °)(-1×90°). In this expression, the cylinders do not necessarily have the same magnitude, in which there would be no case spherical equivalent.
A same ametropia can be designated by different expressions sphero-cylindrical, depending on whether it will be expressed in positive, negative cylinder, folded, or in bitorique formula. However, for a same ametropia and when all other parameters are equal (diameter of the optical zones and transition), not all to the same amount of tissue photoablaté in the case of the astigmatismes mixed and hypermetropique sequential treatments corresponding to these various expressions lead compound.
The most economical expression for photoablate fabric is one that includes the largest magnitude of positive cylinder (1,2).
Here is an example of schematic representation of a lens removed to the correction of mixed astigmatism (optical area only).
Indeed, treatment said negative cylinder is actually the sum of positive cylinder and sphere negative treatment.
Thus, when the laser treats (-2 × 0 °), everything happens as if the laser treated successively (+ 2 × 90 °) and (-2)! Associate in the same sequence of therapeutic treatment in negative cylinder and a positive sphere makes no sense if you want to minimize the depth of ablation. Indeed, the combination of all or part of the negative spherical component included in the negative cylindrical treatment and positive sphere results in the removal of a lens in the optical area-parallel faced (and so powerless theoretical perspective).
For example, if hypermetropique astigmatism composed + 1 (+ 2 × 90 °) is treated in cylindrical mode negative + 3 (-2 × 180 °), this will result in the removal of a lenticule parallel-faced. In effect, the treatment + 3(-2×180°) can break down as follows: (+ 3) (-2) (+ 2 × 90 °). The association of treatment (+ 3) (-2) may be referred to as 'redundant' as his equal to + 1 treatment refractive effect will consume a much higher corneal tissue volume (lens parallel face-to-face whose thickness is equal to the maximum thickness of a refractive treatment of (-2) or (2).
Similarly, it is possible to demonstrate using a similar decomposition method and positive cylinder bitorique treatment strategies lead to identical volumes of tissue photoablaté for the treatment of mixed astigmatism.
It is legitimate to ask why the negative cylindrical treatment may have been used or proposed by some lasers for the treatment of mixed or hypermetropiques astigmatismes. The existence of diaphragm-delivery systems and parallel masks making it relatively easy for negative cylindrical treatment, as well as programming in line with the refractive expression in negative cylinder may explain this fact. Remember that in the United States, all the lasers on the market have not yet simultaneously approval from the Food and Drug Administration (FDA) to use the negative positive cylindrical treatment with cylindrical treatment. This may seem surprising considering that treatment of astigmatism is implicitly based on the achievement of a positive cylindrical treatment! It is worth noting that the expression of a compound myopic astigmatism will not positive spherical component regardless of the chosen refraction formula. Each of this one will be equivalent in terms of volume photoablate.
In practice, these theoretical considerations are today dispensable, provided you use a laser equipped with a 'modern' software, which converts systematically home form in the laser in the more efficient treatment strategy.
The choice of the dimensions of the transitional areas is possible on some lasers. Ideally, these dimensions should depend on the treated composed ametropia and the chosen formula. In general, they must correspond to an overall elliptical outline which the centerline is aligned on the originally flat Meridian.
Crossed cylinders (cross-cylinder) processing flows was a removal of tissue above the formula in positive cylinder when the spherical equivalent of the treated ametropia is positive. In fact, this method was proposed by Vinciguerra et al. in order to compensate for the imperfections that have some lasers to treat the cylindrical ametropia (over-correction at the level of the non-prime meridians). The superposition of these positive and negative cylindrical treatments would allow to compensate for these imperfections, and to regularize the corneal surface, prior to the treatment of the spherical equivalent. The realization of a therapeutic type procedures ablation, hidden within this strategy, may explain the results reported by these authors.
Finally, it is important to note that any redundant ablation at the level of the optical zone causes an additional ablation at the level of the transition area. Indeed, it will take photoablater an additional volume of corneal tissue to level a deeper peripheral Groove. Ametropia sphero-cylindrical equal, more the chosen strategy is consuming in corneal tissue next to the optical box, the realization of the transition zone is consuming corneal tissue.
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