Ординатура / Офтальмология / Английские материалы / Elevation Based Corneal Tomography 2nd_Belin, Khachikian, Ambrósio_2011

PPI or PI: Pachymetric progression indexes, are calculated for all hemi-meridian over the entire 360 degrees of the cornea, starting from the TP. The average of all meridians is the (PPI Ave) and the meridians with maximal (PPI Max) and minimal (PPI Min) pachymetric increase are noted along with their axes.

Q-value: Also known as asphericity, this is the corneal shape factor of the cornea.

Quality Statement (QS): This is a check on the Pentacam image acquisition quality. Here “OK” means the acquired image was of sufficient quality.

Radii Minimum (RMin): Gives the smallest radius of curvature over the corneal surface.

Reference Surface: A reference shape (sphere, ellipse or toric ellipse) mathematically generated of which corneal elevation is measured.

Rm/Km: Mean central radius, arithmetic average of the flat (Rf) and steep (Rs) radii of curvature of the cornea.

Rper: The mean radii of the peripheral zone between the 7mm and 9mm ring.

Sagittal Curvature: Also known as axial curvature, this is a measurement of curvature at different points on the corneal surface and assumes that the radius of curvature is equal to the distance from the corneal surface to the intersection with the line of sight (or measurement axis).

Scheimpflug Principle: Principle of photography where the lens plane is not parallel to the image plane. This technique is used to correct distortion and allows for accurate crosssectional images of the cornea.

Simulated Keratometry: Keratometry values as measured by a corneal topographer. Often these are reported as always orthogonal regardless of the corneal shape.

Tangential Curvature: Also called the instantaneous radius of curvature or local curvature, measures curvature of a single point by calculating the radius of a sphere which would intersect that point.

Tomography: The recreation of a three-dimensional object by computer imaging utilizing multiple images taken from different vantage points.

Topographical Keratoconus Classification (TKC): Based on anterior corneal data, this classifies keratoconus based on the Amsler/Muckenhirn staging. This classification ignores the degree of thinning and ectatic changes on the posterior surface. This classification would classify as normal eyes with significant posterior changes associated with thinning if the anterior surface was uninvolved and is also prone to false positives in corneas with a displaced apex.

Topography: Topography implies surface contour. This term is commonly applied (and incorrectly) to curvature maps which do not technically have true knowledge of surface topography.

Total Cornea Refractive Power: Ray tracing calculation of the corneal vergence power, considering the front and back elevation data along with corneal thickness.

TP: Thinnest Point or lowest pachymetric value on the map.

True Net Power: Corneal power calculation accounting for both anterior and posterior corneal surfaces and their respective optical performance. This power measurement should not be used in standard IOL calculation formulas.

Zernike Polynomials: In ophthalmology, Zernike polynomials are a mathematical way of representing the wavefront of the eye and help quantify ocular aberrations.