Refractive Defects of the Eye
   In fact, refraction is a path that light rays pass on objects in our visual field until making a visible picture in our eyes. After receiving light by our visual system and processing light rays, they are sent to brain in form of electrical waves that lead to something that we call it seeing.  


 



   Effective factors in refraction and eyeglass prescription 
   Cornea form, eye pupil power of the eye and the eye size are the factors which are responsible for picture transparency of things on the retinal. For those who want to see without any glasses completely, it is necessary for all of these features to be in accordance with each others; otherwise, refractive defects such as myopia, hyperopia, astigmatism and presbyopia will be formed. 


   Different Kinds of Refractive Defects
   Myopia 
   
In this disorder, because of the long length of anterior and posterior diameter of the eye glob, and increased corneal curvature, the pictures of the things are formed in front of retinal and you have a problem seeing things in distant. 
 
   
Hyperopia 
   
this disorder is the result of the shortness of anterior and posterior diameter of the eye glob, and decreased corneal curvature; and individual see things in blur both close and distant things. But close vision is darker than distant vision. This disorder like myopia is often inherited and more aging makes reduction of lens power accordance sever.  


   Presbyopia 
   The eye lens is very flexible in young years and the diameter of that is easily changed. After 40-year old, the lens of the eye will be more rigid and lose its capability of adapting. Consequently, individuals cannot easily study and do his/her short distance businesses. This is presbyopia which is a natural disease in absence of other diseases. But, it might be with other disorders such as myopia, hyperopia, or astigmatism. 

 
  Astigmatism
   
astigmatism is a very frequent disorder and many people are suffering from a degree of that. Sever astigmatism is inherited in some cases. Natural cornea is smooth and circle and its curvature is the same in all axis. Astigmatism usually happens when the surface of cornea is not symmetric. In this position, light rays, in different parts of cornea, are not exactly focused on retina because of refraction. Distant and close images seem darker than natural.  In this position, the cornea is like a baseball ball instead of being global. This position causes the formation of the images on two separate parts of the retina. 


 
 Methods of defective refraction reformation
  • Glasses
  • Contact lens
  • Defective refraction surgery

   Methods of frequent defective Lasik refraction
  • Lasik
  • Femtolasik
  • PRK
  • Lasek
  • Internal eye lens 


   Lasik (Laser-Assisted-In-Situ Keratomileusis)
   Is an incorporation of a microscopic surgery and oxymora laser to reform myopia, hyperopia and astigmatism. 

   Femtolasik
  
I
s one of the modernist methods of reformation of myopia, hyperopia and astigmatism. It is the last innovation of defective refraction reformation in which the flap of cornea with femtosecond laser is cut.  

   PRK (Photorefractive keratectomy)
   Epithelial layer of cornea (the last external layer of cornea) is removed and the surface of cornea is razed by oxymora laser 

   Lasek (Laser assisted sub-epithelial keratomileusis)
   
Lasek is a modified PRK method. The oculist removes the epithelium layer of the cornea with an alcohol solution in this method. Then, oxymora laser is radiated on the cornea and the layer of epithelium is placed on the primary place.       

   Phakic IOL - Phakic Intra Ocular Lens
   
Internal eye lens )intraocular( is for those who are suffering from severe refraction defects or having a thin cornea and their refraction defect is not cured by Lasik surgery.