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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

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).

Graphical representation

Here is an example of schematic representation of a lens removed to the correction of mixed astigmatism (optical area only).

lens astigmatism joint profile of ablation

Mixed astigmatism: strategy called positive cylinder: (-1) (+ 3 x 90 °). Next to the main meridians, the positive profile of cylindrical ablation is highlighted in green, and the negative spherical ablation profile is highlighted in red.

Indeed, treatment said negative cylinder is actually the sum of positive cylinder and sphere negative treatment.

formula cylinder cylinder negative positive relationship laser correction

Decomposition of pure negative cylindrical processing: pure negative cylindrical treatment can be decomposed into a sequential therapy combining a negative spherical and cylindrical positive 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).

Hypermetropique compound astigmatism cylinder positive negative cylinder

Schematic representation the profiles of ablation CGI carried out according to the main meridians for treatment of compound hypermetropique astigmatism: comparison between strategies in positive and negative cylinder. Cylinder negative strategy is not logical if we want to minimize the amount of corneal tissue photoablate, as she is to treat the cylindrical component of the refractive error by inducing at first an equal mistake to + 3D (less remote field of the retina is ' pushed' back by negative cylindrical treatment). This strategy involves the issuance of a photoablation at the center of the cornea. Strategy using the positive cylinder, the furthest focus is brought forward, then the residual sphere after cylindrical treatment equal to + 1 d is processed. This strategy saves the center of the cornea and induces a lesser depth of ablation on the outskirts of the optical zone.

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.

transition strategy positive cylinder area negative mixed astigmatism

Representation in CG to areas of transition for mixed astigmatism: comparison between strategies and positive cylinder negative cylinder. Constant transition area slope, negative cylinder strategy involves a larger, and more expensive transition in corneal tissue area.

References:

(1) azar DT, Primack JD. Theoretical analysis of ablation depths and profiles in laser in-situ keratomileusis for compound and mixed astigatism. J Cataract Refract Surg 2000; 26:1123 - 1136

(2) Gatinel D, Hoang Xuan T, Azar D. Three - dimensional representation and qualitative comparison of the amount of tissue ablation for the treatment of mixed and compound astigmatism. J Cataract Refract Surg, 2002, 28 (11): 2026-34

(3) Ibrahim O. Laser in situ keratomileusis for hyperopia and hyperopic astigmatism. J Cataract Refract Surg 2000; 26:1223 - 36

(4) Vinciguerra P, Sborgia M, Epstein D, Azzolini M, MacRae S. also keratectomy to correct myopic or hyperopic astigmatism with a cross-cylinder removal. J Refract Surg 1999; 15:S183 - S185

(5) P, Camesasca FI Vinciguerra. Cross cylinder removal. Pp 319-327. In customized corneal ablation. The quest for supervision. MC Rae SM, Krueger RR, Applegate RA. Slack incorporated. Thorofare, NJ

(6) Munnerlyn CR, Koons SJ, Marshall J.  Also keratectomy: a technical for laser refractive surgery.  J Cataract Refract Surg.  1988; 14:46-52.

(7) Gallinaro C, Toulemont PJ, Cochener B, Colin J. Excimer laser keratectomy to also correct astigmatism. J Cataract Refract Surg 1996; 22:557 - 563

(8) shah S, Smith RJ, Peiger S, Chateterjee A. effect of year smooth optical area we outcome of photoastigmatic refractive keratectomy. J Refract Surg 1999; 15:188 - 191

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