Lasik is most effective when treating myopia from -1.00 dioptres to -8.00 dioptres (i.e. 100 to 800 degrees). Treatment above this power is associated with less predictable results and higher complication rates. This is because beyond -8.00 dioptres of correction, a greater amount of cornea tissue must be sculpted and this will begin to thin the cornea to its safety limits. Current treatment guidelines require that at least 250 microns or a quarter of a millimetre thickness of cornea must be preserved beneath the flap if cornea complications are to be avoided. For this reason, your doctor will measure your cornea thickness prior to the surgery to determine the safe limits of myopia correction.
A power of -12.00 dioptres (1,200 degrees )is usually taken as the upper limit of safe Lasik treatment by most doctors. Above this power, many eye surgeons will instead advise intra-ocular lens implantation as the procedure of choice. Here, a small wound is made at the side of the cornea and an artificial lens, made of plastic, silicone or acrylic material, implanted permanently inside the eye.
The artificial lens is placed either in front of the natural crystalline lens or in its place. Visual recovery is rapid and painless with the eye seeing well the next day.
Of all myopic people, about 90% have corrections less than -6.00 diopters. The list below shows the categories of severity for myopia. The diopters under -3.00 is mild myopia. Moderate Myopia range from -3.00 to -6.00 diopters. Severe Myopia ranges from -6.00 to -9.00 diopters. Extreme Myopia should be above -9.00 diopters. Almost everyone feels that their myopia is severe, because of how dramatically dependent they are on glasses and contact lenses. However, only one in ten myopic people are actually in the severe and extreme categories. LASIK can correct myopia as high as -16.00 diopters.