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Alignment effects in videokeratography of keratoconus
1School of Optometry, University of California, Berkeley 94720-2020, USA.
Summary
Corneal apex decentration in keratoconus significantly impacts videokeratography accuracy. Standard alignment leads to errors, especially with sagittal power maps, potentially masking corneal toricity.
Area of Science:
- Ophthalmology
- Optometry
- Biomedical Optics
Background:
- Keratoconus is a progressive corneal ectasia characterized by apical thinning and protrusion.
- Videokeratography is a crucial diagnostic tool for assessing corneal topography.
- Accurate alignment in videokeratography is essential for reliable diagnostic data.
Purpose of the Study:
- To investigate the impact of corneal apex decentration on videokeratography measurements in keratoconus patients.
- To quantify the errors introduced by standard videokeratograph alignment versus apex-aligned measurements.
Main Methods:
- Evaluated 28 eyes from 17 keratoconus subjects.
- Measured corneal positions at the vertex normal, apex, and 1-2 mm above the apex.
- Controlled for corneal apex decentration by adjusting fixation relative to the videokeratograph axis.
Main Results:
- Decentration of the corneal apex led to significant differences in measured radius compared to apex alignment.
- Corneal toricity measurements were notably greater with apex alignment than with decentered positions.
- Standard (sagittal) power maps showed greater power errors with decentered apex compared to instantaneous (tangential) maps.
Conclusions:
- Corneal apex decentration introduces significant errors in videokeratography when using standard alignment.
- Sagittal power maps overestimate apex position and underestimate apex power in decentered keratoconus.
- Standard videokeratography alignment may fail to detect significant toricity in many keratoconus cases.