misinterpretations in corneal topography...1 misconceptions and misinterpretations in corneal...
TRANSCRIPT
1
Misconceptions and
Misinterpretations in Corneal
Topography
Biomedical Engineering
Cynthia Roberts, Ph.D.Professor of Ophthalmology and Biomedical Engineering
Martha G. and Milton Staub Chair for Research in OphthalmologyThe Ohio State University
Milan, 2012
ESCRS
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Disclosure
• Travel funds provided by Carl Zeiss Meditec
• Travel funds provided by Sooft Italia
• Consultant to Oculus Optikgerate GmbH
• Consultant to Ziemer Ophthalmic Systems AG
• Equipment provided by Ziemer
• Interest in the Galilei
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Surface Representations�Axial
�Average representation of surface curvature
�Tangential (also Meridional)
�Local representation of surface curvature
�Elevation
�Relative to a reference
�Optical (Snell) Power (also Refractive)
�Image formation
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Misconceptions
• Topography vs Wavefront
• Elevation vs Curvature
• Axial vs Tangential Curvature
• Power vs Curvature
• “Bumps” on a Posterior Power Map?
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Why do I need Corneal
Topography when WavefrontAnalysis Measures the
Entire Eye?
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Because Refractive Surgery Alters the
CORNEA!
100% of Induced Aberrations are from the
CORNEA!
2
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Function vs ShapeWavefront vs Topography
Wavefront tells you the origin and
destination
Shape (topography) tells you the
mechanism
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Wavefront cannot provide the
location of an aberration-producing feature
Example:
COMA
Is the origin of 3rd order coma a central or
peripheral feature?
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Coma is
nonspecific
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Misconceptions
• Topography vs Wavefront
• Elevation vs Curvature
• Axial vs Tangential Curvature
• Power vs Curvature
• “Bumps” on a Posterior Power Map?
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Elevation
• Anterior or Posterior Surface
• Requires a Reference
– Relative height
– Can be compared to a plane
– Can be compared to a sphere
– Can be compared to an asphere
• “Best-fit” sphere is most often chosen
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Why Does BFS Vary in Same Patient?
• Depends on the Region over which the BFS is Fit!– On a normal eye, the smaller the region, the
greater curvature of the BFS since corneas tend to have greater curvature in the center
• If missing data are different from exam to exam, then data used for calculation are different and BFS will also be different
3
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Height relative to a
plane reference
A
DC
B
BFS = 48.10D, 7.02mm BFS = 39.59D, 8.53mm
What conditions?
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Plane Reference
A
DC
B
BFS = 48.10D, 7.02mm BFS = 39.59D, 8.53mm
Best-Fit Sphere
Reference
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Elevation relative to “Best-Fit”Sphere
• Central “High” point
– Steeper than the sphere
• Peripheral “High” point
– Flatter than the sphere
• Central “low” point (negative number)
– Flatter than the sphere -- NOT A CONCAVITY still highest
spot on cornea
• Peripheral “low” point
– Steeper than the sphere
4
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Misconceptions
• Topography vs Wavefront
• Elevation vs Curvature
• Axial vs Tangential Curvature
• Power vs Curvature
• “Bumps” on a Posterior Power Map?
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
C
B
A
ab
cOptical
Axis
Refractive Power
• Incoming light rays are refracted by the first surface
• The angle of refraction is dependent on the incoming angle of incidence
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Power vs Curvature for a Sphere and two Ellipses
Roberts C: "The Accuracy of "Power" Maps to Display Curvature Data in Corneal Topography Systems." Investigative Ophthalmology and Visual Science, 1994, 35(9):3525-3532.
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Power vs Curvature
Optical power Axial diopters Tangential Curvature
Power and Curvature are directly proportional ONLY in the central paraxial region
Spherical
Aberration
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Total Corneal Power derived from
anterior and posterior corneal surfaces
• Ray Tracing through BOTH surfaces
– Snells’ Law Refraction:
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Misconceptions
• Topography vs Wavefront
• Elevation vs Curvature
• Axial vs Tangential Curvature
• Power vs Curvature
• “Bumps” on a Posterior Power Map?
5
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Retrospective Study of Post-Op Shape
• 2,380 myopic LASIK patients with pre-op and 6 month post-op Orbscan Topography
• Technolas 217 excimer laser
– “Optical Zone” sizes from 5.0 to 6.5mm diameter
– Transition zones extended to 9mm diameter
• Hansatome or Automated Corneal Shaper Microkeratome
• Collaborators: Dr. John Chang and the refractive surgery team of the Hong Kong Sanatorium and Hospital
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Average Posterior Difference Maps for Entire Population
• n = 2,380
• All regions show statistically significant
differences from pre-operative state.
Data acquired at the Hong Kong Sanatorium and Hospital
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Average Posterior Difference Maps Myopic Correction < 2 Diopters
• n = 25
• For elevation and tangential
curvature, only the intermediate regions were
significantly different from pre-op
Data acquired at the Hong Kong Sanatorium and Hospital Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Average Posterior Difference Maps Myopic Correction from 2 to 4 D
• n = 321
• All regions except inner elevation and intermediate axial zones show statistically significant differences from pre-operative state.
Data acquired at the Hong Kong Sanatorium and Hospital
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Average Posterior Difference Maps Myopic Correction from 4 to 6 D
• n = 635
• All regions
show statistically significant
differences from pre-operative
state.
Data acquired at the Hong Kong Sanatorium and Hospital Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Average Posterior Difference Maps Myopic Correction from 6 to 8 D
• n = 622
• All regions show statistically significant
differences from pre-operative state.
Data acquired at the Hong Kong Sanatorium and Hospital
6
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Average Posterior Difference Maps Myopic Correction from 8 to 10 D
• n = 465
• All regions show statistically
significant differences from pre-operative
state.
Data acquired at the Hong Kong Sanatorium and Hospital Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Average Posterior Difference Maps Myopic Correction for > 10 Diopters
• n = 311
• All regions except outer pachymetry zone show statistically significant differences from pre-operative state.
Data acquired at the Hong Kong Sanatorium and Hospital
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
• 3 models of increased posterior curvature were compared with actual patient posterior topography
• 2,380 patients with pre-op and 6 month post-op Orbscan topography (Hong Kong Sanatorium and Hospital)
• Pre-op subtracted from corresponding post-op, and all difference maps averaged for each of 3 fitting protocols
Interpretation of Increased Posterior
Curvature after Refractive Surgery
• Gryzbowski, et. al., JCRS, January 2005 Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Central Decompensation?
Post-op Posterior
Pre-op
Posterior Surface
• Gryzbowski, et. al., JCRS, January 2005
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Forward Vault?
Post-op Posterior
Pre-op
Posterior Surface
• Gryzbowski, et. al., JCRS, January 2005 Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Inward Peripheral Movement?
Post-op Posterior
Surface
Pre-op
Posterior Surface
• Gryzbowski, et. al., JCRS, January 2005
7
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
It depends on the fitting protocol used!!
How would these surfaces appear if
fit and subtracted?
• Gryzbowski, et. al., JCRS, January 2005 Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Pre and Post-Op Surfaces Fit over Entire Region of Interest
• Most common default of the Orbscan
• Surfaces fit above and below each other
• Gryzbowski, et. al., JCRS, January 2005
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Pre and Post-Op Surfaces Fit over Entire Region of Interest
• Average difference map of 2380 LASIK patients
• Similar pattern to all models, as predicted!
• Gryzbowski, et. al., JCRS, January 2005 Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Apex Fit of Pre and Post-Op Surfaces
• Post-op surface fits “below” pre-op for all
models
• Gryzbowski, et. al., JCRS, January 2005
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Apex Fit of Pre and Post-Op Surfaces
• Average difference map of 2380 LASIK patients
• Similar to all!
• Gryzbowski, et. al., JCRS, January 2005 Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Peripheral Fit of Pre and Post-
Op Surfaces
• If forward movement, post-op should fit “above” pre-op
• If backward movement, post-op should fit “below” pre-op in periphery
• Gryzbowski, et. al., JCRS, January 2005
8
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Peripheral Fit of Pre and Post-
Op Surfaces
• Average difference map of 2380 LASIK patients
• Consistent with backwardmovement model!
• Gryzbowski, et. al., JCRS, January 2005 Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Posterior Surface
Patient Data is consistent with Peripheral
Inward Movement, NOT outward central movement!!
Stable Remodeling!!!!!!!!!!!!!!!!
• Gryzbowski, et. al., JCRS, January 2005
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Misconceptions
• Topography vs Wavefront
• Elevation vs Curvature
• Axial vs Tangential Curvature
• Power vs Curvature
• “Bumps” on a Posterior Power Map?
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Curvature
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Axial vs Tangential: Normal Eye
Axial Tangential
Figure courtesy of F. Carones, M.D. Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Keratoconus
Axial Tangential
Figure courtesy of F. Carones, M.D.
9
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
PRK
Axial Tangential
Figure courtesy of F. Carones, M.D. Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Decentered Ablation?
Axial Tangential
Figure courtesy of F. Carones, M.D.
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
??
Axial Tangential
Figure courtesy of F. Carones, M.D. Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Contact Lens Warpage
Axial Tangential
Figure courtesy of F. Carones, M.D.
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Axial (A1 and A2) vs
Tangential (C1 and C2)
C1
A1
A2
C2
Reference
Axis
S2
S1
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
The location of the greatest curvature is
more stable on a Tangential map, as the
Center of the Map is moved
Figure Courtesy of Renzo Mattioli of Optikon
Tangential: Axial:
10
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Axial vs Tangential Curvature
• Advantages
– Axial: global indication of corneal shape
– Tangential: details of corneal shape
• Disadvantages
– Axial: miss important details
– Tangential: noisy
NEITHER ONE IS POWER!!
Cynthia J. Roberts, Ph.D.
Biomedical Engineering
Summary
• Topography is shape and Wavefront is function
• High resolution is required to calculate curvature from elevation
• Tangential Curvature represents local shape and Axial Curvature represents global shape.
• Refractive or Optical Power represents image formation and curvature labeled in diopters is NOT power.
• A red spot on the posterior surface does NOT always mean ectasia.
• The terms “Oblate” and “Prolate” are not meaningful on a post-LASIK cornea
• BE CAREFUL when you read the literature!