Generally, when special instruments like phoropters, auto refractors, and wavefront analyzers are used to diagnose both the lower order (i.e., the sphere and cylinder) and the higher order (i.e., spherical aberration, coma and trefoil) aberrations of the eye, the patient is in the sitting position. When the patient is treated under the excimer laser, the patient is in the sleeping position.
When a patient is moved from the sitting position to the sleeping (supine) position, the eye rotates on its axis. This movement of the eye is known as cyclo rotation, or cyclotorsion. For treating errors which are spherical in nature (i.e., the sphere or the spherical aberration), this movement does not matter. However, this rotation does matter for errors which have an angular component (i.e., an axis). This includes the lower order astigmatism (or cylindrical) error, as well as higher order aberrations. If the entire planning and measurement is done when the patient is in the sitting position, and then the eye rotates, then the axis on which the treatment would be done, would not be the correct axis.
Cyclotorsional movement can be from zero to about 15 degrees of rotational movement. When the cyclo rotational movement is small (i.e. < 5 degrees), this does not have a significant impact on the treatment. However, if the cyclo rotational movement is large, then there can be a significant under correction of those errors which have an angular component. How much rotation occurs in a given individual cannot be predicted in advance.
Therefore, Modern LASIK demands that such cyclo rotational movement is compensated for. Essentially, what is done is that the extent (angle) of the rotational movement is determined, and the whole treatment is then rotated to that extent. This requires a flying spot laser-most lasers available today have at least some version of a flying spot.
The technology lies in determining the amount of rotation the eye suffers from as the patient is moved from a sitting to a supine position. This is done by capturing an iris picture while the patient is in the sitting position. Along with this, some blood vessels near the cornea are also photographed.
The iris is like a fingerprint for the eye. It has peculiar features, which do not vary much with time, and which are individual to each person. These crypts and features in the iris are photographed in the sitting position, and then when the patient is made to lie down under the excimer laser, the iris picture is again taken. The iris features are matched against the previously measured features, and the extent of the rotation determined. The whole treatment plan is then rotated appropriately.
This has the added benefit of security. The wrong patient or eye would not be treated, as the iris pattern would be different.
Cyclotorsional compensation can be static (i.e., once at the beginning of the laser treatment) or dynamic (constant compensation). However, since cyclotorsional movement occurs mainly while moving from a sitting position to a supine position, the static compensation is also as good as dynamic compensation.
So, if you have high astigmatism, or high higher order errors, it is a good idea to get treated by a laser system which has cyclotorsion and iris registration.