[Supine cyclotorsion and asphericity]

Customised ablation to correct refractive errors is becoming more and more important today. Preoperative examinations are performed in a seated position while laser surgery is carried out in the supine position. It is well known that cyclotorsions occur but until now there have not been any investigations on possible changes in astigmatism, root mean square (RMS) and asphericity.

18 eyes of 14 individuals were measured in both the seated and the supine positions with a mobile videokeratoscope, the Keratron Scout (Optikon).

A change from the seated to the supine position caused cyclotorsion in left eyes from - 9 degrees to 7 degrees (p = 0.56) and from - 8 degrees to 8 degrees (p = 0.53) in right eyes. The average amount of change of the cyclotorsion was 3.6 degrees. The overall power of astigmatism increased to 0.3 D and decreased to 0.2 D compared to astigmatism in a seated position (p = 0.06). The root mean square (RMS) of the corneal wavefront increased to 0.20 microm and decreased to 0.24 microm in comparison to the same parameter in a seated position (p = 0.36). The asphericity average changed from - 0.19 in a seated position to - 0.16 in a supine position (p = 0.04).

Cyclotorsion is statistically not significant when changing from a seated to a supine position as both excyclo- and incyclotorsion occur. Since individual cyclotorsion is clinically relevant, this has been taken into account by the latest laser systems. Changes in astigmatism and statistically significant changes in asphericity may be the reason for suboptimal customised refractive surgery. It must be considered whether the preoperative measurements for laser ablation in the supine position should not also be carried out in the supine position.