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Comparative evaluation of corneal tomography symmetry based on centration: corneal vertex vs corneal geometric center

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First Author: S.Awwad LEBANON

Co Author(s):    M. Yehia   C. Mehanna   L. Asroui              

Abstract Details

Purpose:

To evaluate corneal symmetry output when tomography measurements are centered on the corneal vertex versus the corneal geometric center.

Setting:

Department of Ophthalmology, American University of Beirut Medical Center, Beirut, Lebanon.

Methods:

105 eyes of 105 patients were measured: 66 myopic eyes and 39 hyperopic eyes. Vertex-based measurements were performed first, then the measurements were manually decentered close to the geometric center of the cornea (GCC) after removing the automatic centration option of the Galilei dual placido and Scheimpflug system. Primary outcome measure included the posterior best fit sphere elevation data, with area under the curve (AUC) of the temporal elevation data point subtracted from the nasal one to evaluate horizontal symmetry and superior minus inferior one for vertical symmetry.

Results:

The angle alpha chord magnitude and angle in myopes was 0.42±0.13 mm horizontally and 0.09±0.09 mm vertically (vector magnitude: 0.44±0.13 mm). In hyperopes, it was 0.57±0.44 mm horizontally and 0.15±0.21 mm vertically (vector magnitude: 0.53±0.13 mm). With vertex centration, the absolute difference in AUC for horizontal BFS elevation data was 1.08±0.78 μm², compared to 0.53±0.45 μm² for GCC centration (p<0.01), while for vertical BFS elevation, it was 0.82±0.77μm² vs 0.84±0.81 μm² (p>0.05). For myopes, the horizontal AUC was 0.79±1.00 μm² with vertex vs. 0.50±0.35 μm² with GCC (p<0.05), and the vertical AUC was 0.50±0.45 μm² vs. 0.50±0.38 μm² (p>0.05).

Conclusions:

Tomography measurements based on the geometric center of the cornea display much more symmetry than the vertex-centered ones, notably horizontally, in agreement with angle alpha offset which is often much more pronounced nasally than vertically. Referencing over the corneal geometric center is valuable for diagnostic interpretation, while referencing over corneal vertex is essential for therapeutic planning.

Financial Disclosure:

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