ASCRS Posterior surface

1
Heidelberg IOL refractive surgery research group
www.lasik-hd.de mirjam.reuland_at_med.uni-heidelberg
.de
Reliability of Posterior Corneal Elevation
Topography for Orbscan II and a Rotating
Scheimpflug Camera
M.S. Reuland, MD A.J. Reuland, MD T.M.
Rabsilber, MD I.J. Limberger, MD G.U. Auffarth,
MD Prof.
Deptm. of Ophthalmology Ruprecht-Karls-University
Heidelberg Chair Prof. Dr. med. H.E.Völcker
The authors have no financial interests in any of
the products mentioned.
2
Purpose
Corneal topography of the anterior and posterior
surface has great potential for the detection of
corneal abnormalities such as keratokonus (1) or
astigmatism.  Todays IOL calculation formulas
use assumed posterior surface values instead of
measured values. Even though the posterior
surface contributes 10 to the total refractive
power of the cornea, reliability studies are
missing (2 - 6).
Our purpose was to compare corneal posterior
surface measurements of two devices regarding
their reliability (precision).
Methods
We conducted a prospective clinical study of 63
eyes of 32 healthy, phakic volunteers with no
corneal pathologies (20 females, 12 males mean
age 31 yrs (23 70 yrs) spherical equivalent -1
D (-6 to 2 D)). We compared posterior surface
representations achieved by the rotating
Scheimpflug camera Pentacam (Oculus, Wetzlar,
Germany fig. 1) and by the scanning slit
topography system Orbscan IIz (Bausch Lomb,
Rochester, NY, USA, fig. 2).
Fig. 1 Rotating Scheimpflug camera Pentacam
Fig. 2 Orbscan IIz scanning slit topography
system
3
Methods
For each eye and device, three consecutive
measurements were made. Elevation measurements
(fig. 3) were performed using comparable system
settings for both devices (in relation to a fixed
sphere of 6.5 mm from the apex no best fit
sphere no floating).
Fig. 3 Posterior surface map of Pentacam (l) and
Orbscan II (r)
We evaluated 16 data points on the generated maps
on a 1 to 4 mm radius from the apex (stars on
maps in fig. 4). Intra-system reliabilities of
the three measurements for each point and device
were calculated. Observed posterior surface
formations were divided into 13 different forms.
1 mm zone
4 mm zone
Fig. 4 Evaluated data points for Pentacam (l)
and Orbscan II (r)
4
Results
Absolute values. The directions in the 3-D map
in fig. 5 correspond to the directions on the
corneal posterior surface map. Pentacam shows on
the mean positive values in the horizontal plane,
Orbscan shows negative values in the vertical
plane. This both corresponds to an astigmatism
with the rule. The absolute values corresponded
to average mean powers of - 5.2 D for Pentacam
and - 6.4 D for Orbscan II.
50µm
0µm
-50µm
Zone
Zone
Fig. 5 3-dimensional representation of the mean
values for all 16 points
5
Results
Reliability. Intra-system reliability was
overall good for all four zones for Pentacam.
The graph in fig. 6 (l) is quite flat. For
Orbscan (r), repeatability was less reliable. On
the horizontal plane, a zone with distinctly
reduced reproducibility showed at the 2 to 3 mm
radius (yellow arrows). For both devices,
limitations could be found in the area of the
upper lid, as was to be expected. The
box-and-whisker plots (fig. 7) confirm a better
repeatability for Pentacam.
Upper lid
Difference between 2 repeated measures
20µm
20µm
0µm
0µm
Zone
Zone
Optimal correspondance
Fig. 6 Intra-system reliability as a 3-D plot
for Pentacam (l) and Orbscan II (r)
Fig. 7 Intra-system reliability as a
box-and-whisker plot for Pentacam (l) and Orbscan
II (r)
6
Results
Comparison of shapes. When comparing the overall
forms, again, a better repeatability was found
for Pentacam (fig. 8) than for Orbscan (fig. 9).
27 of Orbscan measurements showed sometimes
quite distinct differences in shape for repeated
measurements. In direct comparison of Pentacam
with Orbscan maps (fig. 10), only half the maps
showed the same shape for both devices.
No difference in shape 61/63 (97) 2 different
shapes 1/63 (2) 3 different shapes 1/63
(2)
Fig. 8 Patient 1 (OS) 3 repeated Pentacam
measurements
No difference in shape 45/62 (73) 2 different
shapes 15/62 (24) 3 different shapes 2/62
(3)
Fig. 9 Patient 2 (OS) 3 repeated Orbscan
measurements
No difference in shape 29/62 (47) Different
shapes 33/62 (53)
Fig. 10 Patient 3 (OS) Pentacam (l) vs. Orbscan
(r)
7
Discussion
Pentacam
Orbscan
Repeatability is superior for Pentacam in
comparison to Orbscan II. The differences are
based for one on the different measurement
principles of the two devices. For the rotating
Scheimpflug system, the central planes are
measured very precisely due to repeated
representation (fig. 11, l). The scanning slit
system measures the center only twice (fig. 11,
r).
Fig. 11 Cause 1. Repeated representation of the
center, improving reliability for Pentacam.
Pentacam
Orbscan
Secondly, the resolution of the surface outline
is better for Pentacam than for Orbscan, which
facilitates automatic detection (fig. 12).
Thirdly, Orbscan measurements show two light
reflexes. These reflexes hamper correct
determination of corneal posterior surface (fig.
13). The localization of these two reflexes
corresponds to the localization of the
empirically determined problematic spots of the
Orbscan measurements (yellow arrows in fig. 6).
For Pentacam, no such reflexes exist.
Fig. 12 Cause 2. Better surface outline,
facilitating surface detection for Pentacam.
Orbscan II
Fig. 13 Cause 3. Light reflex, hampering
posterior surface detection for Orbscan.
8
Conclusion
Reliability of the posterior surface measurement
is higher for the rotating Scheimpflug camera
than for the scanning slit system. Noticeable
differences exist for the two topographical maps,
which challenge the clinical usability of the
Orbscan posterior surface measurement.
References

• Reuland, A. J., T. M. Rabsilber, M. P. Holzer, I.
  J. Limberger and G. U. Auffarth (2004). Early
  detection of keratoconus using the Orbscan II
  corneal topography system. American academy of
  ophthalmology, New Orleans, USA.
• Edmund, C. (1994). "Posterior corneal curvature
  and its influence on corneal dioptric power."
  Acta Ophthalmol 72(6) 715-20.
• Hamilton, D. R. and D. R. Hardten (2003).
  "Cataract surgery in patients with prior
  refractive surgery." Curr Opin Ophthalmol 14(1)
  44-53.
• Langenbucher, A., F. Torres, A. Behrens, E.
  Suarez, et al. (2004). "Consideration of the
  posterior corneal curvature for assessment of
  corneal power after myopic LASIK." Acta
  Ophthalmol Scand 82(3 Pt 1) 264-9.
• Leyland, M. (2004). "Validation of Orbscan II
  posterior corneal curvature measurement for
  intraocular lens power calculation." Eye 18(4)
  357-60.
• Prisant, O., T. Hoang-Xuan, C. Proano, E.
  Hernandez, et al. (2002). "Vector summation of
  anterior and posterior corneal topographical
  astigmatism." J Cataract Refract Surg 28(9)
  1636-43.

9
Heidelberg intraocular lenses and refractive
surgery research group
G.U. Auffarth, MD Prof. M.P. Holzer, MD A.
Hunold, MD I.J. Limberger, MD Y. Nishi, MD T.M.
Rabsilber, MD A.J. Reuland, MD M.S. Reuland,
MD M. Sanchez, MD D. Vucic, MD
www.lasik-hd.de mirjam.reuland_at_med.uni-heidelberg
.de