Topography-Guided and Contoura™ Laser Vision Correction
Topography-guided procedures were originally used only for complicated eyes and retreatments and are based on corneal data derived from either Placido disk topographers or Scheimpflug camera based tomographers. The refractive input is obtained from the manifest refraction, and the customised (or aberration element of the) treatment is derived from the corneal higher order aberrations (HOAs), like spherical aberration and coma. Initially, these procedures were performed in 2 stages, first the topography-guided procedure designed to regularise the cornea and reduce the corneal and hence ocular HOAs and secondly, a refinement procedure to correct the residual refractive error 6 months later. This last correction would typically be with the wavefront-optimised profile. Over time however it became possible to combine the HOA treatment with the low order aberration (LOA) treatment with relatively good refractive predictability, so most treatments performed nowadays are initiated as an intentional once-off procedure. If an enhancement treatment is required, it is certainly not regarded as a failure of the primary treatment, but the key issue is that it was planned as a one-step procedure rather than a two-step procedure. With the success achieved in the cases described above, topography-guided procedures were introduced to primary eye treatments i.e. patients having surgery for the first time and not necessarily patients with complaints. The results were fascinating in that primary topography-guided procedures provided better outcomes for these primary eyes than routine wavefront-optimised procedures for visual quality including night driving. Some of the reasons cited for this are that refinement of the corneal shape should be beneficial for visual quality by reducing HOAs, the refraction can be refined to 0.01 of a dioptre thereby increasing predictability, the tear film has normally been optimised to get good quality data scans, and the treatment is normally centred on the corneal apex rather than pupil centre.
Topography-guided procedures are now becoming more common place for patients without any visual quality issue.
KeywordsTopography Tomography Refractive predictability HOAs LOAs Corneal apex centration Improved contrast sensitivity Topography-guided Contoura™ vision
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