LASIK (Laser Assisted In Situ Keratomileusis) :
LASIK is a type of eye surgery done to correct refractive errors of the eye.
Refracrive error is based on two factors:
1. Anterior-Posterior length of the eyeball (normally 23 mm)
· Increased A-P length of the eyeball = myopia
· Decreased A-P length of the eyeball = hypermetropia
a. Steep corneal curvature > More bending / refraction > Image will form in front of retina > Myopia (refractory power is more)
b. Flat cornea > Decreased bending > Image will form behind retina > Hypermetropia (refractory power is less)
c. If corneal curvature is unequal > 2 images will be formed > Astigmatism
Principles of LASIK:
Reshaping of cornea (alter shape of cornea):
· In myopia > make cornea flat
· In hyper metropia > make cornea more steep
How much correction is possible?
· Myopia > up to 16 D
· Hypermetropia > up to 7 D
· Astigmatism > up to 7 D
Procedure of LASIK:
2. Treat bed with Excimer laser. Ablate Bowmen’s layer and stroma with laser > precisely cur tissue without disturbing surrounding structure.
3. Reposition of flap
For 10µm ablation = correction of 1D myopia.
eg if you need 5 D correction = 50 µm tissue has to be ablated.
Note: Before LASIK, asses corneal thickness by pachymetry.
· Normally centre = 0.56 mm
· Periphery = 1.00 mm
Complication of LASIK:
3. Cellular proliferation in between interface = becomes hazy
4. Overcorrection or under-correction
5. Ectesia (procedure not done in center, but to one side)