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Pellucid marginal degeneration layer12/21/2023 Differences in steepness between the two corneas of a given patient.Įxpected topography: inferior steepening on anterior axial map and corresponding thinning on pachymetry map.Large difference between the power of the corneal apex and periphery.Topographic diagnosis of keratoconus is suggested by: K Max (Front): Steepest point over anterior corneal surfaceĭ) Values used in IOL calculations (out of scope of this article) 5 Clinical Uses.Thinnest Location: Thinnest point over anterior corneal surface.Pachy apex: Corneal thickness at the apex.The same variables described for the front of the cornea are used to characterize the back of the cornea.Ĭ) Pupil center: Calculated by finding the center point based on edge detection on the iris then the distance is calculated in mm R min: Smallest radius of curvature in entire field measurement.R per: Average radius of curvature between the 6mm and 9mm zone center.Axis: The meridian that requires no cylinder power to correct astigmatism.More negative values may suggest keratoconus or hyperopic correction whereas positive values may suggest myopic correction.Q-val: Describes the corneal shape factor, or eccentricity of the cornea.“OK,” “Data gaps,” “Fix,” “Model”) may alert the technician to retake the exam due to suspect quality R f, R s, R m: Radii corresponding with K 1, K 2, and K m, respectively.Red corresponds with the steep meridian whereas blue corresponds with the flat meridian. K 1, K 2, K m: The two major meridians ( K 1, K 2), determined using the 3mm ring, are 90 degrees from each other.Useful for identifying forme fruste keratoconusĮxpected topography: Progressive flattening from center to the periphery by 2-4D, with the nasal area flattening more than the temporal area.4) Posterior elevation map (bottom right).Warmer colors indicate where the cornea is elevated above the best fit sphere cooler colors indicate where the cornea is depressed below the best fit sphere.Useful for assessing regularity of astigmatism, location of astigmatism and surgical planning for AK, toric planning.Coolcolors = thick (think “in the cold wear thicker layers”).Warmcolors = thin (think “in the heat wear thinner layers”).Displays distribution of corneal thicknesses across the entire measured area.2) Corneal thickness, aka pachymetry map (bottom left).Warmcolors = steep (think “ steeping warm tea”).Useful for assessing irregularity of astigmatism and planning suture removal after PK.On our representative Pentacam images below, you will see four different types of maps. These range from warm colors ( red, orange, yellow), to neutrals ( green) to cool colors ( blue, purple).
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