The early detection of ectatic change is of paramount importance to the refractive surgeon. Anterior curvature and ultrasonic
pachymetry, alone, does not provide enough information to detect early corneal disease. The combination of anterior and posterior
elevation and complete pachymetric data gives the clinician a more complete view of the structure of the cornea and allows
for more effective screening. The OCULUS Pentacam's Scheimpflug Cross-sectional imaging (OCULUS Optikgerate GmbH, Wetzlar,
Germany) provides for a three-dimensional reconstruction of the anterior segment, making possible the assessment of the anterior
and posterior corneal surfaces and the creation of a complete pachymetric map. The Belin / Ambrósio Enhanced Ectasia Display
(BAD display) was designed to utilise the data supplied by the Pentacam's rotating Scheimpflug camera and provide a comprehensive
keratoconus screening display. The display combines the anterior and posterior elevation and pachymetric data into one all-inclusive
display giving the clinician a more complete overview of the corneal shape and allowing for quick and effective screening
of refractive surgery patients.
Figure 1
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The current display (Belin / Ambrósio Enhanced Ectasia Display – software release # 1-16b96) shows both anterior and posterior
elevation data relative to a standard best-fit-sphere (BFS) calculated at a fixed optical zone of 8.0 mm. Fitting a best-fit-sphere
to the central 8.0 mm zone appears best for clinical interpretation and allows for the generation of standardised normal values.
The display also shows anterior and posterior elevation values relative to the 'enhanced reference surface' computed by determining the BFS from the central 8.0 mm zone after excluding all the data from a 3.5 mm optical zone centred
on the thinnest point of the cornea. Not often appreciated is that the BFS will be influenced by any abnormal portion of the
cornea. In the case of keratoconus or ectasia, the cone will have the effect of steepening the BFS. This steepened BFS will
actually minimise the elevation difference between the apex of the cone and the BFS. By eliminating the conical portion of
the cornea from the BFS computation, the 'enhanced reference surface' serves to further accentuate ectatic or conical protrusion,
while having little if any effect on normal corneas. The BAD display then computes the change in elevation values going from
the standard BFS and the 'enhanced BFS.' This change (elevation change between the standard BFS and 'enhanced BFS') has been
shown to be a key differentiator between normal and ectatic corneas.
