What is an A constant IOL?
A-constant The constant is a theoretical value that relates the lens power to AL and keratometry, it is not expressed in units and is specific to the design of the IOL and its intended location and orientation within the eye.
What is a constant in biometry?
A-constant, although called a constant is actually highly variable depending upon multiple factors – IOL dependent: type, material, position; surgeon dependent: technique of incision, placement of incision; K and AL measurement adjustments; or even adjustment for the manner of carrying out biometry.
What is SRK T formula?
The SRK T formula is a third generation formula, described in 1990 by John Retzlaff, Kraff and Sanders [6]. This formula combines the benefits of both the theoretical and regression formula and uses the A-constant to calculate the ACD, using the retinal thickness and corneal refractive index.
Which is the best formula for IOL calculation?
In conclusion, calculation of the IOL power can be performed using a great variety of formulas. According to clinical studies, the SRK-T formula is recommended for rather long eyes whereas the Hoffer Q formula is recommended for rather short eyes.
What is K1 and K2 in keratometry?
Keratometric changes with cycloplegia. K1: flat meridian of the anterior corneal surface, K2: steep meridian of the anterior corneal surface.
What is SRK II?
Adjusting the A-constant to different axial length ranges, the SRK II formula is obtained: SRK II: P = A1 – 0.9 K – 2.5 L. (2) The difference between SRK I and SRK II is given by the A1 constant.
What does a flat cornea mean?
Definition. Cornea plana is an abnormally flat shape of the cornea such that the normal protrusion of the cornea from the sclera is missing. The reduced corneal curvature can lead to hyperopia, and a hazy corneal limbus and arcus lipoides may develop at an early age. [ from HPO]
WHAT IS lens power?
The power of a lens is defined as the reciprocal of the focal length. Lens power is measured in dioptres (D). p o w e r o f a l e n s ( D ) = 1 f o c a l l e n g t h ( m ) P = 1 f. Converging (convex ) lenses have positive focal lengths, so they also have positive power values.