Astigmatism is a common vision condition that causes blurred vision. Astigmatism is an error
of refraction in which incident parallel rays can't be focused to a single point .It could be
corneal or lenticular astigmatism or both . Regarding Corneal astigmatism it is due to
difference in the corneal curvature in the different meridian of both the anterior and
posterior surface .
The are 2 methods of studing corneal curvature :
Corneal topography which depend on placcido disc to study anterior corneal surface .
Corneal tomography is more precise in detecting anterior and posterior corneal curvature
depending on Scheimpflug principle which uses a rotating Scheimpflug camera .
Pentacam is a device which depend on Scheimpflug technique .
The importance of studing the posterior corneal curvature includes :
• The posterior corneal curvature is a more sensitive indicator of early keratoconus
detection especially posterior corneal elevation.
Previously, The change in the posterior corneal surface plays a more subtle role than the
anterior corneal surface in optical performance because of the smaller difference in
refractive indices between the cornea and aqueous, and it has been thought that the magnitude
of PCA in the normal population is clinically negligible.
Posterior corneal astigmatism (PCA) is important for better predictability of toric
intraocular lens calculation to avoid under- or overestimation of toricity in intraocular
lens (IOL) power calculations.
But recently it has been demonstrated how neglecting PCA will lead to overcorrection of
astigmatism in eyes with with-the-rule astigmatism (WTR) and undercorrection in eyes with
against-the-rule astigmatism (ATR).(3) In our study we aim to Determine the contribution of
posterior corneal astigmatism to total corneal astigmatism and the error in estimating total
corneal astigmatism from anterior corneal measurements only using Scheimpflug analyzer
(Pentacam ).
Our patients will undergo Scheimpflug imaging using pentacam .
From the corneal parameters displayed in pentacam we can calculate :
Anterior corneal astigmatism by substracting Ksteep -Kflat on the front surface
Posterior corneal astigmatism by substracting Ksteep-Kflat on the back surface.
Total corneal astigmatism is calculated by addition of anterior and posterior corneal
astigmatism using vector analysis by using ASSORT vector calculator.*
The percentage of contribution of posterior corneal astigmatism to magnitude of Total
corneal astigmatism is then calculated .
ASSORT vector calculator:
Is a total planning and outcomes analysis software for astigmatism using Alpins method
,cheltman ,Victoria ,Australia .