The keratometry data to be supplied in this simulation should be, preferentially, the ones provided by the simulated keratometry "Sim K" of the axial topographical map in diopters (D).

When the topographer instead of supplying the "Sim K", just supplies K1 and K2 in the diameters of 3, 5 and 7 mm, we should choose the 3 mm one, once it is the one the most resembles the simulated keratometry.

The topographical map is the main tool in the determination of the surgical strategy. In case this exam was not properly performed, it must be repeated until it is appropriate to an accurate simulation. In case this is overruled, there is a very big risk to determine inadequate Ring Segments for the case.

The manual keratometry may be of great use, in case the map is not reliable.

Patients bearing physical or mental deficiencies, that may create difficulties to get a good topographical map, induce the use of inadequate and mispositioned segments.

The pachymetric value to be introduced in this simulation is that determined in the exact site where one would make the incision. It will be simulated 75% of this value, which would be the diamond blade calibration.

It is important to evaluate also the pachymetry of the entire corneal area through where the segment would be implanted. This may be done through pachymetric maps obtained from equipments such as Pentacan or Orbscan, or even using an ultrasonic pachymeter that should search the whole 5mm diameter zone. The pachymetry along the implantation zone should be at least the double of the segment thickness in order to supply enough stroma to involve it, avoiding histological and metabolic alterations that could jeopardize the surgery results. As an example, a cornea that is 400 micrometers thick would accept segments up to 200 micrometers. If in this simulation is presented, for this hypothetical case, a segment of 250 micra, it would be required a modification to a segment of 200 micra. Certainly, the corneal flattening simulated will be a little lesser than expected, nevertheless, being it considered better an undercorrection than a suffering cornea.

This simulation requires information about how the ectasic area is distributed in the cornea. For that, we use the steepest meridian indicated on the topography ("Sim K" of the axial map) as reference to split the cornea into two hemispheres. Then, you must observe if the ectasic area is equally distributed in both hemispheres or if this area is concentrated in just one.

The best way to analyse the keratoconus morphology and its distribution in relation to the steepest corneal meridian, is to use the "Normalized Scale".

This simulation uses the following classification for the Ectasic Area Distribution:

Almost the totality of the ectasic area (at least 75%) is located in one hemisphere and a small portion is placed in the other.

2/3 of the ectasic area is in one corneal hemisphere and 1/3 is in the other.

The corneal steepest meridian splits the ectasic area into equal part (half by half).

We should select the subjective refraction instead of the one supplied by the auto-refraction. However, we should test the dioptric values supplied by the latter, specially in cases where the refractional exam was difficult or the patient could not determine which was the best lens. This generates an inaccurate result, what is relatively frequent in advanced keratoconus cases.

The simulation uses the dynamic refraction instead of the static, because the pupil dilation exposes the peripheral aberrations and alters the refractional power of the eye. A special attention is required in cases resulting in high myopia, higher than should be expected for a certain eye, due to the possibility to be facing an accommodation spasm. In these cases, a refraction under cycloplegia is required.

The simulator evaluates whether a specific eye has or not a good prospect for a ring surgery, and estimates the satisfaction of the patient with the result of a surgery, indicating it by a 3 stars score (1 star to dissatisfied, 3 to higher satisfaction).

The variables assessed are: K max, BSCVA, Astig value, Astig axis and Astig Coer:

The best results are achieved when the "K Max" does not exceed 60D. Dissatisfaction can also occur if the cornea is too flat, because we know that the ring will produce an even greater flattening, which may impair vision quality.

The astigmatic axis of refraction should be close to the flattest meridian of the cornea. The absence of this typical feature may indicate that the topography or refraction is not reliable or that the corneal deformation is too important (out of therapeutic possibility for ring surgery).