| We discuss the principle of reflection corneal topography and the importance of fixation and alignment in clinical application. We discuss the concept of geometrical topography and visual topography. We study a set of various ellipsoids including prolate, oblate and astigmatic spherical and astigmatic aspheric surfaces using geometric topography approach. We illustrate that astigmatism, regular and irregular can be induced by fixation error and that error may be significant. In particular, with some simulated post-surgical corneas, apparent de-centered ablation may in fact be due to fixation error. Clinical aspects in minimizing fixation and alignment error are discussed. We discuss patient alignment, alignment on corneal vertex and alignment on center of pupil. We illustrate the controlled study cases (specialty surfaces) through corneal power, elevation, difference displays and Zernike analysis of the cornea and prove that Zernike analysis is effective in identifying regular astigmatism and irregular astigmatism. We present the concept of multi-dimensional data acquisition and data alignment, during measurement and during surgery including corneal elevation data, various axes, pupil size and axes at various illumination condition, corneal vertex and visual axis. We also present clinical cases of various patients under different fixation condition to confirm our observation. Proper care should be given to fixation when using the topography to assess the post-op corneal topography. We discuss the importance of consistency of diagnostic measurement and treatment in the context of data alignments of multiple dimensions.
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