What Causes Astigmatism?

There are three sources of astigmatism in the eye before cataract surgery:  the anterior cornea, posterior cornea, and natural lens.  
Anterior Cornea- the primary source of astigmatism

The primary source of astigmatism is the anterior cornea. When the anterior cornea is steeper in one dimension than the other, astigmatism results.  This occurs because the steeper dimension of the cornea bends light more than the less steep dimension.  

 

In one dimension, images are focused relatively closer in.  In the opposite dimension, they are focused further out.  Because of this, there is no distance at which an astigmatic optical system is completely in focus.    For diagrams of this click here.

 
Posterior Cornea

The second source of astigmatism is the posterior cornea.  The posterior cornea also can have a different curvature in one dimension than the other.  The axis of astigmatism of the posterior cornea may be the same as, oblique to, or opposite the anterior corneal astigmatism.   Therefore, the total corneal astigmatism may be greater than, or less than, the anterior corneal astigmatism, and may occur at a different axis than the axis of the anterior corneal astigmatism.  

 

Natural Lens

The natural lens is the third source of astigmatism.   The astigmatism in the natural lens disappears when the natural lens is removed during cataract surgery.

 
Summary

The anterior cornea, posterior cornea, and natural lens astigmatism determine the total refractive astigmatism of the eye before surgery.   

 

The natural lens is removed during cataract surgery.  This leaves only two sources of astigmatism inside the eye after surgery: the anterior and posterior cornea.  The anterior corneal curvature is measured with your "K Values".  K Values are discussed here.  

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