The A-scan is a method of measurement of the ocular axial
length (L). It is coupled with corneal curvature keratometry
(K) readings so that one can measure the required power
(P) of an intraocular lens (IOL) used for cataract surgery.
The Sanders, Retzlaff and Kraft (SRK) formula calculates the
required P of the IOL as P = A − 2.5L − 0.9K where A is the
specific IOL constant. The power of the lens determines the
refraction of the image onto the retina and thus determines the
visual acuity of the eye. The axial length of the eye is measured
by the time a signal takes to be reflected back to the source,
like ultrasound or light. Distance is calculated by the velocity
formula, depending on the speed of the signal. In Mater Dei
hospital, two methods are used to determine the axial length of
the eye prior to cataract surgery, namely an ultrasound method
and an optical method.peer-reviewe
