One of the key measurements in the preoperative evaluation of Lasik eye surgery is the patient’s corneal thickness. The cornea is the clear dome which forms the front of your eye. It functions as a lens and accounts for about 60% of the focusing power of your eye. Lasik surgery works by using a laser to reshape the cornea and alter its focusing power.
Lasik uses a very particular type of laser called an excimer laser. This laser produces light of a specific wavelength which has very unique properties when it contacts human tissue. Essentially, it causes the tissue to vaporize in a very precise manor with very little thermal effect. In other words, it can very accurately remove tissue without causing any burning so there is no collateral tissue damage. By using a controlled, computerized optical system, this laser light can be direct to the surface of the cornea to reshape it with incredible precision.
Lasik requires that some amount of cornea tissue be removed in order to change its shape. Over the years, eye surgeons have developed generally accepted principles as to reliable safety margins for how much tissue can be removed and how much must remain. It is generally believed that if too much tissue is removed from the cornea that its structural integrity can be compromised and it will be weakened. Any weakness of the cornea can lead to distortions of its shape which are unpredictable and irregular. The weakness may also be progressive causing something called ectasia where a part of the cornea pushes outward.
Everyone’s corneal thickness is different and it is an essential part of the preoperative Lasik evaluation to measure it. The most common way to measure the corneal thickness is by ultrasound but there are also a number of newer optical instruments that can make similar measurements. An average corneal thickness is usually in the neighborhood of 550 microns but there is a relatively wide variation with some being thicker and some thinner. In some cases, an unusually thin cornea may be an indication of weakness or something called keratoconus. These people should be screened out prior to having Lasik because their corneas are already inherently weak. This diagnosis is made using measures of corneal thickness, corneal shape or topography, and other clinical signs which can often be seen by a trained ophthalmologist at the microscope. Anybody determined by their eye surgeon to have keratoconus should be excluded from having Lasik surgery based on this information alone.
When performing Lasik, the surgeon can only remove tissue to a certain depth. Most surgeons will want to leave a base of at least 250 to 300 microns of untouched cornea for safety. Lasik requires a certain amount of tissue be used to create a surface flap which is lifted out of the way before the excimer laser removes tissue and reshapes the cornea optically. The thickness of the Lasik flap is critical because the thinner the flap, the more Lasik treatment can be performed before reaching the minimum untouched base requirement of 250 to 300 microns. Again, the feeling is that if the depth of treatment goes significantly beyond that point, that the structural integrity of the cornea may be compromised.
With older technology, a bladed instrument called a microkeratome is used to create the Lasik flap. A typical flap thickness using this instrumentation would be approximately 160 microns. So for a patient with a 550 micron cornea, after creation of the flap, the remaining tissue would be 550-160 or 390 microns in thickness. If the surgeon wishes to leave an untouched base of 300 microns, that would leave 90 microns of tissue to be removed and reshaped for the optical effect. Generally, for each diopter of nearsightedness, it requires the removal of approximately 12-15 microns of tissue. So treatment may only be possible for up to -6 to -8 diopters in this patient.
New Lasik technology is completely blade-free and uses another laser (a femtosecond laser) to create the flap instead of a bladed microkeratome. These femtosecond lasers can accurately make Lasik flaps that are much thinner, around 100 microns in thickness rather than 160 microns. This additional 60 microns leaves more tissue for the optical treatment allowing 5 or 6 diopters of additional possible treatment in an average person. So when using the femtosecond laser systems, higher Lasik treatments can often be performed even in patients with somewhat thinner corneas. It’s this important corneal thickness measure as an indicator of risk for corneal weakness which helped to make these blade-free Lasik systems so popular. Because the femtosecond laser can create significantly thinner flaps with such precision, Lasik surgeons have a much greater sense of safety with regard to treating more nearsighted individuals.
