Transcript Lasik decentration with the VISX ActiveTrak™ System

Lasik decentration with the VISX
ActiveTrak™ System
Mark Edmund Johnston MD FRCSC
[email protected]
Authors have no financial interest.
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.
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, 5806- 5814
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 undercorrection of sphere and cylinder of 0.5
D.
• 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.
• The tracking cameras
are always to the left
and inferior.
• 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
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
N=188, SD 0.21
6-10 Diopters SE
N=36, SD=0.34
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)
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
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.