1. Lasik decentration with the VISX ActiveTrak™ System
Mark Edmund Johnston MD FRCSC
Authors have no financial interest.

2. Direction of Decentration (Coma) pre-op enhancement
A subset of patients presenting for enhancement following LASIK with the VISX ActiveTrak system had superior de-centration.
The purpose of this study was to document this clinical impression and understand the mechanism.

3. Post -op Lasik with mild superior decentration and myopic cylinder
Decentration resulted in under-correction of sphere and induction of astigmatism; among the HOAs, decentration mainly induced coma.
Decentrations 1 mm led to an under-correction of sphere and cylinder of 0.5 D.
The Effect of Optical Zone Decentration on Lower- and Higher- Order Aberrations after Photorefractive Keratectomy in a Cat Model
Jens Buhren, Geunyoung Yoon, Shawn Kenner, Scott MacRae,and Krystel Huxlin
Investigative Ophthalmology & Visual Science, December 2007, Vol. 48, No. 12,
Post -op Lasik with mild superior decentration and myopic cylinder

4. We looked at induced astigmatism as a marker of decentration using our database, the Refractive Surgical Consultant™.
The postoperative cylinder centroid is reported in Diopters (D) and axis as 0 to 180 Degrees.
Corrections above (high) and below (low) 6.0 diopters spherical equivalent were analyzed separately.

5. The tracking cameras are always to the left and inferior.

6. The ActiveTrak system output is found next to the video output feed on the VISX laser and can be recorded and monitored during surgery.
A video is attached as a separate file

7. N=188, SD 0.21 N=36, SD=0.34 0-6 diopters SE 6-10 Diopters SE
Centroid shift: A retrospective analysis was done on all myopic patients (0-10D) with less than 1.5 D cylinder treated with IntraLase™ during a one year period. This difference was highly statistically significant (P=0.0001)
0-6 diopters SE
6-10 Diopters SE
N=188, SD 0.21
N=36, SD=0.34

8. OD: Early vs. late deviation 0.125 mm 0.250 mm
Observation and recording of the ActiveTrak system showed that as the ablation progressed there was a trend during longer ablations for the centration crossbar to migrate superiorly. (See attached video)

9. Model eye using a 90D aspheric lens:
Corneal apex on -axis :
the model pupil is centered
Corneal apex off -axis: The model pupil appears to be displaced superiorly

10. Conclusion
While in general the ActiveTrak system performs well, with higher ablations there is a trend toward an upward decentration of the ablation. This is away from the camera and may be related to prismatic effect of the cornea or the changing index of refraction of the corneal surface as the ablation progresses.