title: Cyclotorsion during laser in situ keratomileusis.
abstract: OBJECTIVE
  To determine the magnitude of cyclotorsion during excimer laser ablation using a dynamic iris eye tracker.
  
  
  METHODS
  Guy Hugh Chan Refractive Surgery Centre, Department of Ophthalmology, Hong Kong Sanatorium and Hospital, Hong Kong, China.
  
  
  METHODS
  This retrospective study comprised 245 eyes (137 patients) that had laser in situ keratomileusis (LASIK) for myopia with or without astigmatism by the same surgeon. The magnitude of cyclotorsion during LASIK was recorded as the maximum, average, and minimum positions according to the angle of deviation displayed on the excimer laser computer screen. Measurements of cyclotorsion were performed during laser ablation.
  
  
  RESULTS
  The mean total cyclotorsion was 2.181 degrees+/-1.392 (SD) (range 0.0 to 13.3 degrees). The mean average position (AP) was +0.134+/-1.851 degrees (range -7.0 degrees [excyclotorsion] to +12.6 degrees [incyclotorsion]). Forty-five eyes (18.4%) had an AP greater than +/-2 degrees, and 168 eyes (68.6%) deviated from the zero position at the onset of laser ablation. The mean incyclotorsion was 2.136+/-1.440 degrees (78 eyes, 31.8%) and the mean excyclotorsion, 1.772+/-0.809 degrees (78 eyes, 31.8%). Eighty-six eyes (35.1%) had cyclotorsion in both directions (mixed cyclotorsion). The mean cyclotorsion was 2.670+/-1.588 degrees. Eight (3.3%) of the 109 patients having simultaneous bilateral LASIK had bilateral incyclotorsion or bilateral excyclotorsion.
  
  
  CONCLUSIONS
  Cyclotorsion occurs before and during laser ablation. An active rotational eye tracker is fundamental to compensate for cyclotorsion and to enable greater precision in excimer laser ablation delivery.
authors.affiliation: Hong Kong Sanatorium and Hospital, Hong Kong, China. johnchang@hksh.com
authors.forename: John
authors.initials: J
authors.lastname: Chang
mesh_terms: D000293:Adolescent; D000328:Adult; D001251:Astigmatism / Q000150:complications / Q000601:surgery; D003941:Diagnostic Techniques, Ophthalmological; D005128:Eye Diseases / Q000175:diagnosis / Q000209:etiology*; D005133:Eye Movements; D005260:Female; D006801:Humans; D020731:Keratomileusis, Laser In Situ; D054018:Lasers, Excimer / Q000627:therapeutic use*; D008297:Male; D008875:Middle Aged; D009216:Myopia / Q000150:complications / Q000601:surgery; D011187:Posture; D012189:Retrospective Studies; D014102:Torsion Abnormality / Q000175:diagnosis / Q000209:etiology*; D014792:Visual Acuity / Q000502:physiology; D055815:Young Adult