NeuroTrack: Optokinetic eyetracking
to prevent cyclotorsion
Preface
 Based on the hype that VISX created around the cyclotorsion issue, WaveLight re-
examined its clinical results for mixed astigmatism and high astigmatism.
 The results were found to be excellent. There were not major over- or under-corrections
appearent.
 This lead us to ask why we did not see the anticipated clinical issues that VISX has been
talking about in our treatments
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Why cyclotorsion is important
 Cyclotorsion may cause decentration of treatment
 Decentrations may cause over- and under-corrections or a shift in axis in astigmatic
treatments
 Decentered ablations may also induce higher order aberrations and negatively impact the
retinal image quality
Error induced from cyclotorsional movements in an eye with 1D of astigmatism at different cyclotorsion angels
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Factors contributing to decentration
1. Saccadic eye movements
2. Improper head alignment
3. Movement from sitting from supine position
4. Pupil shift
5. Eye rolling
6. Technical misalignment of laser beam
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Saccadic Eye movements
 Saccadic eye movements were a major problem in the early years of refractive surgery
 Since the introduction of active, high speed trackers even the fastest saccadic eye
movements can be followed today
 The ALLEGRETTO WAVE features the most advanced eye tracking system in the market
that uses 200-500Hz sampling rates and has a response time of only 4-6ms
 Pupils up to 7mm in diameter can be followed effectively and the system can even track
nystagmus patients
 Most other laser systems in the market today
also feature eyetracking devices
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Improper head alignment
 Improper head alignment may potentially be cause when patients move from a sitting to a
supine position
 In 1995 Smith and Talamo reported no significant difference between seated and supine
position of the body
 Chernyak published a study of 51 eyes that he measured in seated and supine position,
though the highest degree of cyclotorsion that occured was 9.5°, the average amount of
torsion was only 2°
 In 2006 Becker et al. Showed that subjects with and without binocular vision did not
undergo major torsion when moving from seated to supine positioning
Conclusion: the impact of torsion from seated to supine position is insignificant to the
treatment outcome
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Improper head alignment
Improper head alignment is also caused by the patient with movements while under the laser.
There are three ways to deal with the issue of cyclotorsion caused by
improper head alignment:
 Prevent cyclotorsion in patient‘s eye (WaveLight)
 Monitor degree of cyclotorsion in patients eye (WaveLight, Alcon)
 Re-adjust treatment profile to the degree of cyclotorsion of the eye (Alcon, VISX, Zeiss)
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Prevention approach: Optokinesis
 Optokinsis is a neuronal response of the eye to spacial clues of an image. It forces the
eye to stabilize the retinal image to achieve perfect vertical and horizontal orientation
 The so-called vestibular ocular reflex (VOR) automatically rotates the eye back to
compensate for possible head rotation
Images with different degrees of spacial clues that trigger
an optokinetic response (Pensell, Sverkersten, Ygge)
Square shape created by the yellow illumination lights of the
ALLEGRETTO WAVE as seen by the patient and reflection of square
shape on pupil
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Monitoring approach: scleral marking
 Conventional approach: corneal markings corneal markers such as the Geuder markeur
are used to mark the sclera. Head movement is monitored with e.g. the cross line projector
in the ALLEGRETTO WAVE Eye-Q
Horizontal markings on both sides of the sclera
can be used for cyclotorsion control prior and
during refractive treatment
Cross line projector used with the
ALLEGRETTO WAVE Eye-Q
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Monitoring approach: limbus registration
 Limbus registration aligns two limbal ink marks
on the sclera to a horizontal reference line with
two sputnik recticles. This approach is used
prior to treatment and not continued
dynamically during treatment
Limbus registration as used by the ALCON Ladarvision system
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Realignment approach: Iris pattern recognition
 An image of the patient‘s iris is captured. Then
the system segments the image into different
parts and selects the most prominent feature in
each segment. Once the patient is under the
laser, the system reconfirms the position of each
feature and realigns the ablation pattern
accordingly. It is important to realize that the
alignment only happens once prior to beginning
of treatment. After treatment has commenced
there is no dynamic alignment.
Iris pattern recognition as used by the VISX S4.
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Pupil shift
 Pupil shift can occur at different pupil sizes. The pupil center moves to a different location.
 Pupil shift can potentially occur when a diagnostic measurement for custom treatments is
conducted at a different pupil size than treatment and
 If the diagnostic device is using manual or a different pupil centration method than the laser
The ALLEGRETTO WAVE uses the same eyetracking technology in the wavefront analyzer
as in the laser system. In addition, it is recommended to simply treat the patient at the same
pupil size that was used for the diagnostic measurement. Since 90% of treatments are done
with the wavefront optimized algorithm, this only applies to a very small amount of treatments:
 Wavefront-guided: measure and treat at large pupil
 Topography-guided: measure and treat with small pupil
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Eye rolling
 All eyetrackers available today can only function when the patient cooperates and
continously fixates on a target light.
 When the patient loses fixation, eye rolling occurs, that cannot be compesated for
 The shorter the treatment time, the smaller the chance of loss of fixation ability of the
patient: the ALLEGRETTO WAVE is today the fastest laser available
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Technical misalignment of laser beam / tracker
 A technical misalignment of the laser beam or the eye tracker can cause substantial
decentration
 The ALLEGRETTO WAVE uses two methods to ensure accurate alignment of laser beam:
- Eyetracker target test: conducted on every treatment day to ensure eyetracker is properly aligned
- Fixation test: conduction once before every treatment
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Technical Summary
Decentration is caused by several factors
1. Saccadic eye movements
2. Improper head alignment
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3. Movement from sitting from supine position
4. Pupil shift
5. Eye rolling
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6. Technical misalignment of laser beam
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The ALLEGRETTO WAVE successfully adresses all issues in every procedure, delivering a
maximum of accuracy for best clinical results
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REFERENCES
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1. Tjon-Fo-Sang MJ, de Faber JT, Kingma C, Beekhius WH.

Cyclotorsion: a possible cause of residual astigmatism in
Williams DR. Surgeon offsets and dynamic eye movements in
refractive surgery. J Cataract RefractSur 2002;28:599-602.
laser refractive surgery. J Cataract Refract Sur 2005; 31:2058-66
2. Mrochen M, Kammerer M, Mierdel P, Seiler T. Increased
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of subclinical decentration. J Cataract Refract Sur 2001; 27:262-
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9. Bueeler M, Mrochen M, Seiler T. Maximum permissible
tosional misalignment in aberration-sensing and wavefront-
4. Chernyak DA. Cyclotorsional eye motion occurring between
guided corneal ablation. J Cataract and Refr Surgery 2004;
wavefront measurement and refractive surgery. J Cataract
30:17-25
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10. Bueeler M, Mrochen M, Seiler T. Maximum permissible lateral
5. Becker R, Krzizok T, Wassill H. Influence of body position on
misalignment in aberration-sensing and wavefront-guided corneal
ocular cycloposition in patients with binocular vision. Klin
correction. J Cataract and Refr Surgery 2003; 29:257-63
Monatsbl Augenheilkd 2006; 223:48-51
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supine patient. J Cataract Refract Sur 1995; 21:402-3
Refract Sur 2004; 30:633-8
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measurements into corneal ablations: an overview of related
topics. Journal of Refr Surgery 2004; 20:550-4
269.
3. Smith EM Jr, Talamo JH. Cyclotorsion in the seated and
8. Mrochen M, Bueeler M, Iseli HP, Hafezi F, Seiler T.
Transferring wavefront
higher order aberrations after laser refractive surgery: a problem
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7. Porter J, Yoon G, MacRae S, Pan G, Twietmeyer T, Cox IG,
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technology. Eyeworld 2004; 1
6. Donnenfeld E. The pupil is a moving target: centration,
repeatability and registration. Journal of Refr Surgery 2004;
11. International clinical experience with iris registration
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12. Schneider D. Iris registration. Experience with iris registration
and non-iris registration wavefront-guided lasik outcomes.
20:593-6 0574-8 123
Eyeworld 2006; 3
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13. Visual spatial clues enhance ocular torsion response during
visual tilt T. Pansell, U. Sverkersten, J. Ygge Exp Brain Res
(2006) 175:567–574 DOI 10.1007/s00221-00623
Merci