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18 Corneal tomographic analysis is essential before
cataract surgery - 4 steps in screening candidates
for premium IOLs
The Pentacam® is a Scheimpflug based corneal tomographer. We have been able to develop a
program which makes it easy to perform the 4 steps of the screening procedure as described above.
Similar to the manual keratometer, it is very important to confirm the reproducibility of the data
before attempting an interpretation. Especial attention should be paid to patients of very advanced
age, patients with a narrow palpebral fissure, or patients with poor fixation during measurement.
If QS (quality specification) in the map is indicated with red color, the data should not be used.
In cases of doubt the measurements should be repeated until reproducible topographic maps are
obtained for each eye.
Figure 165, Figure 166, Figure 167 are the examples of the output. This display consists of 3
topographic maps, one Scheimpflug image, and one data box. The upper left is the axial power map,
which is based on anterior surface data with a keratometric refractive index of 1.3375. This map is
used to determine any abnormality in corneal shape. In the upper center is the map of Total Corneal
Refractive Power (TCRP). TCRP (n=1.376 for cornea, n=1.336 for aqueous) is calculated from the
anterior and posterior refractive power of the cornea while considering corneal thickness and Snells
law of refraction. It can be for IOL power calculation for patients with corneal shape abnormality
such as in post-LASIK, post-PTK, and post-keratoplasty. In the upper right is the corneal pachymetric
map. This map can be used to check the thickness at the main incision and at the side ports. The
Scheimpflug image can be used to show the cataract to the patients and also to check the ACD.
The data box shows tomographic indices including total corneal refractive power, corneal irregular
astigmatism (total higher-order aberration), corneal SA, and corneal cylinder. In addition, simulated
K readings, ACD, pupil diameter, corneal thickness and others are available.
Total HOA (0.180 μm), SA (0.307 μm), front-back ratio (80.5 %), and cylinder (-0.5 D) are within
normal ranges. Either multifocal aspherical IOL or monofocal aspherical IOL will be fine in terms of
optical quality of the cornea, although the anterior chamber is shallow (1.74 mm).
Figure 165: Cataract Pre-OP Display normal cornea with a shallow anterior chamber
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