The Assessment of Corneal Topography

Comparison of Keratometric and Topographic Cylinder and Axis Measurements on Normal Corneas with Low Astigmatism

PURPOSE

To evaluate agreement in measurements of astigmatic axis power and location between keratometry and computer assisted videokeratography (corneal topography) on normal corneas with less than 1.50 D of idiopathic astigmatism.

METHODS

Keratometric readings with the 10 SL/O Zeiss ophthalmometer and corneal topographic maps with the TMS-1 were obtained by two independent examiners on 32 normal corneas. Measurement agreement between the two instruments was evaluated in regard to steep and flat meridian power and location, and in astigmatism magnitude (D).

RESULTS

The limits of agreement (d-2 SD to d+2 SD) between the two instruments were found to be broad for clinical purposes in measuring the steep meridian power (-0.16 to -1.20 D), flat meridian power (0.43 to -1.25 D), and astigmatism (0.60 to -1.12 D). A constant bias of the TMS-1 towards the 10 SL/O Zeiss ophthalmometer was found, in measuring steeper both principal meridians and higher amount of astigmatism. Mean location difference was 19 degrees (+/-190) for the steep meridian and 17 degrees (+/-20 degrees) for the flat meridian.

CONCLUSIONS

Despite the differences seen in measurements between the 10 SL/O ophthalmometer and the TMS-1, these differences may be clinically small enough for the methods to be used interchangeably in measuring only the magnitude of astigmatism on normal corneas. However, the disagreement in astigmatism axes is too great to be ignored.

Three-dimensional representations of corneal deformations from keratotopography data

PURPOSE

To improve the perception of corneal shape from the data measured by a corneal topography system.

METHODS

A computer program to illustrate the deformation of the cornea measured by corneal topography is described. The deviations from a regular corneal curvature are presented in a three-dimensional (3-D) map. Elimination of the basic curvature in the individual cornea allows the small deformations of the cornea to be seen as shapes and not as power maps.

CONCLUSION

The images obtained with the 3-D maps are different from the keratotopographic images. The latter show the deformation of the cornea on a micro-metric scale and are understandable in terms of shape or surgical planning. Using our inborn 3-D perception mechanism provides a better understanding of corneal shape. A shape reconstruction from the keratotopographic image alone is nearly impossible.

Original papers: Does strabismus surgery induce significant changes of corneal topography?

To determine whether strabismus surgery significantly influences corneal curvature, corneal topography was measured in 68 patients before and both one day and three months after strabismus surgery. With few exceptions, early corneal topographic changes were moderate, statistically not significant and nearly disappeared after 3 months. Long term absolute astigmatism increased not significantly by 0.16 D. Our results demonstrate that strabismus surgery can induce transitory but usually no long term changes of corneal topography. Patients should be informed, however, that in rare cases an induced astigmatism may persist.