6
–4.99 –0.31 x 34. However, Contoura is detecting astig-
matism along an axis of 10 degrees. (Figure 1). Double
checking the Contoura against another topographer, the
Orbscan, shows it too detects corneal astigmatism along
axis of 10 degrees. (Figures 2 and 3). The surgeon now
faces a decision: choose the astigmatic axis based off
manifest or measured? Figure 4 shows that there is a 48
degree difference between these two and an 0.62 D dif-
ference in magnitude. A lot of difference! Why are they so
different? What should be treated?
The answer lies in the influence that the topograph-
ic abnormalities have on how light is being focused. The
manifest is the optical image as it strikes the retina. It is
the sum total of the effects of the smaller topographic
abnormalities and the true corneal astigmatism. Figure 5
shows just the topographic correction for this eye after
removing the cylinder and sphere from treatment. It is
the effect of these smaller topographic abnormalities that
"bend" the manifest cylinder away from the true corneal
measured astigmatism. Once you remove these abnor-
malities, the true corneal cylinder measured by Contoura
is at 10 degrees. This is the cylinder that should be treat-
ed with Contoura topographic LASIK.
By treating all of the corneal abnormalities, vision
sharpness and better than normal manifest results can be
obtained. It seems to be a leap of faith for the surgeon at
first to treat a different axis than manifest refraction but
the optical physics here are real and can and should be
trusted to gain the best possible results.
T
opographic-guided LASIK is available in the United
States as Contoura from Alcon. It is a versatile and
powerful new tool for refractive surgery. However, it
does require refractive surgeons to rethink their treat-
ment calculations and to consider the patient's total refractive
error in a new way. In the past, LASIK surgeons would simply
target the patient's astigmatic axis and magnitude off the
patient's manifest refraction. As surgeons begin to become
more precise and target the small topographic imperfections
of the cornea, they must treat the astigmatic axis and magni-
tude that the advanced Contoura topographer detects. It be-
comes disconcerting when the Contoura astigmatism differs
from the manifest. At this point, the surgeon needs to resist
his or her experience-based knowledge and trust the technol-
ogy, treating measured and not the refraction axis.
This case highlights these issues. The patient is a
23-year-old woman. Pre-LASIK MRx is –5.00 –0.50 x 35
20/15. Wavefront PPR is in good agreement with this at
Case study:
Trust the technology
by Mark Lobanoff, MD
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5: Topographic abnormalities
to be treated in this eye
Figure 6: MRx cylinder axis vs. Vario measured axis
Mark Lobanoff, MD,
is in private practice at North
Suburban Eye Specialists in Coon
Rapids, Minnesota, and at Total
Eye Care in Wyoming, Minnesota.
He can be contacted at
mlobanoff@gmail.com.
