Transcript Limitations of current topographies, clinical problems

Limitations of current
topographies, clinical problems,
and new development
Ming Wang, M.D.,Ph.D.
Director, Vanderbilt Laser Sight Center
http://www.net-serv.com/drmingwang
Corneal topography – an
indispensable integral part of
any refractive surgical
treatment
Limitations of wavefront
analysis
• Wavefront is a total visual axis Z-axis “head-
on” shot (including both cornea and lens);
• No information regarding the LOCATION of
aberration (cornea? lens?);
• No information outside entrance pupil;
• Accommodation dependent, rather than fixed
spatial physical dimension-dependent as in
corneal topography.
Corneal topography will
always be needed
• To determine corneal contribution to
refractive error;
• To gather information outside
entrance pupil;
• Needed for planning of any corneabased refractive treatment.
Limitations of current corneal
topographies
• Lack of stereoposis, relying on one
camera/one shot, giving rise to overdependency on ONE set of data and low
degree of data degeneracy
• “Free-standing” point measurement therefore not
possible as it depends on nearby point relationship
for mathematical construction. Can thus cause
accumulation of errors;
• Not possible to discard bad points due to optical
image artifact as one only has one image shot for
each corneal point.
Limitations of current
topographies
Prone to errors of measurement arising
from optical artifact due to one angle/one
shot and low data degeneracy
• Errors of measurement from abnormal
light scattering: dry eye surfaces;
• Errors from abnormal light reflection:
surface scar;
• Errors from abnormal light transmission:
corneal edema, corneal stromal opacities.
Clinical cases in which existing
topo systems break down
Case 1
• S/p PTK, with KED;
• Topo showed erroneous reading of the
posterior surface and pachymetry, while
anterior elevation and anterior curvature
are OK:
Case 1 conclusion
• SLE showed KED at 6:00 and 9:00
o’clock;
• Blockage of light REFLECTION from
corneal endothelium causes erroneous
reading of posterior and pachymetry
data (both depends on posterior data).
Clinical cases in which existing
topo systems break down
Case 2
• S/p LASIK with epithelial ingrowth;
• Topo showed abnormal “thinning” in
the area of epithelial ingrowth:
Case 2 con’t
• The “thinned” portion at 12:00 o’clock
was artifactual, due to the BLOCKAGE
of light transmission by epithelial
ingrowth, affecting DATA
COLLECTION.
Case 2 conclusion
• Conclusion: Erroneous topo reading can
be caused by
• Prevention of light REFLECTION back
from endothelium, as in KED;
• Blockage of light TRANSMISSION, as in
epithelial ingrowth or scar;
• SCARTERING of light by irregular surface,
as in DES.
Clinical cases in which existing
topo systems break down
Case 3
• A patient who has no KC hx and
normal vision shows “FFKC” in
topography:
Case 3 con’t
• But there are no other signs or symptoms
of FFKC. Topography repeated: normal!
Case 3 conclusion
• What is going on?
• The repeated topo was done after
instillation of artificial tears
• Conclusion:
• Dry surface can lead to erroneous topo
reading due to light SCATTERING.
Clinical cases in which existing
topo systems break down
Case 4
• Reis-Buckler dystrophy;
• Clinical photos:
OD
OS
Case 4 con’t
• Topo shows 280um “thinned” area
centrally, despite of nomral ultrasound
pachy reading of 550um:
Case 4 conclusion
• Cornea indeed 280um thin?
• Ultrasound pachy: 550um.
• Conclusion:
• Corneal opacity, such as dystrophy or
scar, can block light TRANSMISSION
and affect topo DATA COLLECTION.
Clinical cases in which existing
topo systems break down
Case 5
• 56 y/o woman with corneal scar os;
• Ultrasound os showed 450um
centrally, and 520um inferiorly;
• Topo showed:
Case 5 con’t
• Topo showed inferior corneal
thickness of only 312um;
• While ultrasound showed 520um;
• The reason for the difference?
Case 5 conclusion
• The difference is due to artifactual reading of
topo pachymetry, in the setting of corneal scar;
• Corneal scar causes abnormal light
REFLECTION and block light
TRANSMISSION;
• Existing topographers are sensitively
dependent on optical information, data are
subject to surface reflection/transmission
abnormalities.
Clinical cases in which existing
topo systems break down
Case 6
• 43 y/o came for LASIK screen;
• All w/u normal, except topo showed:
Case 6 con’t
• Topo curvature map showed central
steepening;
• No signs or symptoms of KC;
• Dx?
Case 6 con’t
• Repeat topo:
Case 6 conclusion
• Repeat topo is normal. It was done after
copious lubrication;
• Conclusion: Dry eyes can cause
artifactual topo maps in existing
topographers due to irregular surface
SCATTERING and reflection.
Clinical cases in which existing
topo systems break down
Case 7
• Repeated topo in a post LASIK pt
showed an “void” area os:
Case 7 con’t
• Explanation for the lack of data points in
the temporal area of the cornea os?
• Let’s look at clinical photo:
Case 7 conclusion
• Clinical photo shows an area of temporal
corneal scar, blocking light transmission,;
• Existing topo systems can show “no data”
points, when light transmission/reflection
is abnormal.
Clinical cases in which existing
topo systems break down
Case 8
• LASIK screen, normal exam except DES,
but topo showed dramatic thinned cornea
of 268um centrally:
Case 8 con’t
• Repeat topo with more tears:
Case 8 con’t
• Repeat topo was very different from
the initial one;
• Etiology?
• What to do?
Case 8 con’t
• Copious lubrication and closure of eyes
for 30 minutes, in this severe DES eye;
• Repeat topo again:
Case 8 conclusion
• Conclusion: Artifactual topo reading can
occur due to severe DES (causing
abnormal light REFLECTION).
Clinical cases in which existing
topo systems break down
Case 9
• 2nd opinion, s/p LASIK, with epi
ingrowth os;
• Topo shows:
Case 9 conclusion
• “Non-data” points in topo are due to
blockage of TRANSMISSION of light.
Answers to improve the current
topographies
• Stereo topography
• Introducing stereopsis to corneal
topography;
• Multi-angle simultaneous shots;
• High degree of data degeneracy.
Stereo topography: a new
generation corneal topography
(Astramax)
• Stereo view: from different angels (3),
simultaneous shots, thus if necessary,
one can eliminate some erroneous data
points from shot from a particular
angle due to optical artifact.
Stereo topography: a new
generation corneal topography
(Astramax)
• More data points (35,000 points in 0.2
seconds): high degrees of internal data
degeneracy, thus enable
• Elimination of erroneous data points;
• Construction of “free-standing” individual
data point, free from mathematical
relationship from a nearby prior point (thus
free from accumulated error).
AstraMax: a new generation
Other features
• Polar grid: less likely to be subjected to
erroneous measurements;
• Scotopic pupil size measurement;
• Limbus-to-limbus map: for larger zone
ablation to preserve prolateness.
Astramax, a new generation
stereo topography, represents a
technological advance in
corneal topography, an
indispensable tool of refractive
surgery