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.  

 
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 axis of the anterior corneal astigmatism.   Therefore, the total corneal astigmatism may be greater than, or less than, the anterior corneal astigmatism.  The clinical astigmatism of the entire cornea may therefore be different in both axis and magnitude than the anterior corneal astigmatism.  

 

Natural Lens

The natural lens is the third source of astigmatism.  Before surgery, astigmatism in your natural lens might affect your glasses prescription.   This source of preoperative astigmatism disappears when the natural lens is removed during cataract surgery.

 
Summary

Astigmatism present in the anterior cornea, posterior cornea, and natural lens  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 main source of astigmatism is the anterior cornea.  The anterior corneal curvature is measured with your "K Values".  K Values are discussed here.