Profile of ablation: hyperopique simple astigmatism
In the presence of a astigmatism hypermetropique pure, the ablation profile must include the increase of corneal camber global, selective according to the affected meridians.
Hypermetropique astigmatism - for example: plan (+ 1 × 90 °): the meridians are all (except one, the one located at 90 °) insufficiently arched. This lack of camber varies less arched Meridian to the Meridian the camber (the only one whose curvature allows to focus the incoming light in retinal plan).
Arch flat Meridian, respecting the curvature of the less flat Meridian! It is the positive cylinder treatment. The lenticule photablate (only optical zone) is represented in the next Figure. The photoablation is truly selective and savings in the strict sense the less flat Meridian. The non-prime meridians should also be arched in a variable way.
Pure hypermetropique astigmatism: representation of the lenticule photoablaté and profile d ' ablation at the level of the intermediate and main meridians. Profile d ' removal varies for each of the meridians, which must be arched gradually (lack of camber) initially the more rounded Meridian to the Meridian as flat (maximum camber).
(b) transition area
It should allow to level the peripheral Groove induced by the realization of the refractive treatment depth is maximum along the flattest principal meridian (Meridian having undergone an ablation in refractive aiming). Its diameter along this Meridian should logically be the diameter along the Meridian less flat. The circumference of the transition area will also be an ellipse whose big diameter is located along the originally flat Meridian.
The representation of the lenticule total (area optics and transition zone) allows to visualize these constraints.
: Tridimensionelle of the total theoretical lenticule representation (area optics and transition zone) for the theoretical amount of corneal tissue photoablaté for the cylindrical positive treatment. The shape of the lenticule materializing cornea volume consumed for the treatment of l ' hypermetropique pure astigmatism is linked to the profile ' ablation done at the level of the Meridian too flat next to which depth d ' ablation is maximum, whereas it is zero along the opposite Meridian. Profile d ' theoretical ablation was pointed out in dark green at the level of the Meridian initially as flat next to the circular perimeter of the optical area. In order to limit the phenomena of regression, making ' a constant slope transition area implies that its diameter is larger on the side of the initially flat Meridian (Meridian highlighted in light green to the level of the transition area) the internal perimeter of the transition zone is circular (matching the optical edge). Its outer perimeter is overall elliptical. This feature explains the need d ' area d ' exhibition of sufficient size in l ' axis of the initially flat Meridian. In this example (hypermetropique direct pure astigmatism), if a LASIK is performed, the hinge of the stromal flap should ideally be placed higher.