PRK Laser Energy

Visible light and all other forms of electromagnetic radiation carry energy. Light passes through windows, radio waves pass through buildings and x-rays pass through people, but each of these energy forms can also interact and thus release the energy. Beneficial or harmful effects will occur depending upon the wavelength of the energy source, the strength of the radiation, and what substance interacts or is struck.

Lasers are a method of producing an intense beam of energy with a precise wavelength. The first optical laser appeared in 1960 (1). The early medical lasers (2) produced visible light wavelengths which relied upon the transfer of heat energy to burn or photo coagulate tissue. Later lasers (3) used infrared (IR) wavelengths whose heat and energy was sufficient to either photo vaporize or photo disrupt (explode) tissue. Ultraviolet (UV) lasers were first suggested in 1975 (4) and subsequently a class of lasers known as Excimer lasers has evolved. The argon fluoride (ArF) version emits radiation of 193.3 nm wavelength. This is the laser which has revolutionized refractive surgery because when this laser interacts with tissue it removes only a fraction of the cell with virtually no damage to surrounding cells. A recent Ophthalmology textbook (5) has excellent comprehensive reviews showing collections of pioneering photomicrographs. We hope soon to receive permission to reproduce extraordinary photographs of grooves in a human hair (6), and laser incisions in human cornea (7). The remarkable feature is incredibly smooth incisions with no evidence of heat damage in immediately adjacent tissue. This could be called a cold laser. It turns out that wavelengths in the 200 nm range deliver just the right energy to break intermolecular bonds and simply ablate tissue without collateral damage to immediately adjacent cells. A longer wavelength such as a 248 nm (KrF Excimer) radiation burns a wide path of adjacent tissue in addition to the directly affected tissue. Since longer UV wavelengths (UV-B) are known to increase the occurrence of skin cancer a number of scientific studies have been done to study the possibility of 193 nm (UV-C) radiation causing cancer and each one has shown that 193 nm radiation does not damage DNA (8). Wavelengths shorter than 100 nm enter the X-Ray bands. X-Rays pass through cell and can also cause Cancer. Excimer 193 nm rays strike a cell surface and ablate only 0.25 (9) um of tissue. Since the distance from cell wall to nucleus in a corneal epithelial cell is 1.5 to 3.0 um (10) it is thought that the nuclei are either shielded from the radiation or destroyed with little potential for mutagenesis (cancer production).

The action of 193 nm excimer radiation is even more elegant than ablating 0.25 um of tissue. It turns out that after each laser pulse the remaining cell elements are resealed by the formation of a pseudo membrane or new layer or membrane. It is helpful to think of corneal cells as rather like grapes with a liquid center and surrounding membrane which holds the liquid center in. You can imagine each laser pulse removing 1/10 of the grape and resealing the portion of the grape (cell) not ablated or destroyed! To place the 0.25 um ablation in perspective, some corneal epithelial cells are 18 um tall and the depth of the cornea at center is 500 um.