Pachymetric evaluation prior to laser in situ keratomileusis

To determine whether deviations in the localization of the cornea's thinnest point or the magnitude and localization of posterior corneal ectasia is associated with deviations in the spherical equivalent, the astigmatism, or the magnitude of an anterior corneal ectasia and whether corneas at risk for iatrogenic keratectasia can be identified without a pachymetry map of the cornea.

University hospital eye clinic.

Three hundred eight eyes of 156 healthy volunteers with various refractive errors were examined with Orbscan II and autorefractometer-keratometer. The corneal thickness was registered at the fixation point, at the geometrical center, and at the thinnest point of the cornea. Keratometry and refraction were determined for all subjects.

The thinnest point of the cornea was predominantly located in the inferotemporal quadrant, and was significantly thinner than the fixation point (539.6 +/- 35.8 microm and 548.0 +/- 35.4 microm, respectively, P<.001). Interestingly, the larger this difference was, the longer the distance between these points. No relationship was found between the refractive or external surface measurements and the internal surface measurements.

The absence of a clear relationship between the shape of the anterior corneal surface or the refractive error, and the shape of the posterior corneal surface, necessitates a thorough pachymetric evaluation of the cornea before a laser in situ keratomileusis procedure, with special attention to the inferotemporal area.