|
Welcome to the Medical Section of www.lasik1.com.
|
|
|
|
|
|
 |
Laser Types |
|
|
Laser Manufacturers |
|
|
|
|
|
|
|
|
Laser Refractive Surgery
The Magic of LASIK The magic of LASIK is a surgical precision unprecedented in human history combined with an approach (the flap) allowing rapid healing and visual rehabilitation. Excimer Laser power coupled with today's computers allows one laser pulse to remove as little as one quarter (0.25 nm) of a micrometer (or micron) of corneal tissue. This is exquisite control! In LASIK the focusing power of glasses or contact lenses is sculpted directly into the corneal bed (beneath the flap) to permanently alter the focusing power of the front window of the eye. The new and special excimer laser actually cleaves individual molecular bonds to remove tissue with no damage to surrounding tissue. Computer programs control the sculpting to ensure the highest possible accuracy and success of the intended refractive change. Once the laser treatment is completed the surface flap is repositioned. LASIK 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. TECHNICAL CONSIDERATIONSOPTICAL MODIFICATION Everyone familiar with optics will understand that the refractive effect of sculpting a concave or convex lens into the corneal bed can be precisely calculated with the appropriate formula (11). A higher refraction will require a more curved and thus deeper sculpting. The diameter of sculpting determines, by a factor of its square, the depth of ablation; a larger diameter curve of the same radius will be deeper. Ablation (and centration) diameter is important because if the edge of the modified corneal lens overlaps the pupil or light axis then the patient may experience glare, light sensitivity or other symptoms. Since the depth of ablation is related to the time and degree of healing we have a "catch 22" circle of causes and effects, any of which can influence the patients refractive outcome. An individuals best combination is best chosen by the surgeon after carefully weighing all relevant factors. Table of Ablation Depth vs. Diopters & Diameter of Optical Zone
The calculations for treating hyperopia and astigmatism are similar. The correction for hyperopia is peripheral (leaving the central cornea untouched), and astigmatism is corrected by removing extra tissue in a specific axis of myopia or hyperopia.
Aesculap™- Meditec GMBH Bausch & Lomb Technolas 217 Excimer Laser with PLANOSCAN LaserSight Technologies, Inc.
Nidek, Inc. Novatec LASER SYSTEMS INC SCHWIND - ESIRIS ESIRIS - This is an Argon Fluoride 193 nm excimer scanning system. It has a 1mm spot diameter with a pulse rate of 200 Hz and Gaussian beam profile which allows for smooth surfaces (no grooves and ridges) when treatments in critical overlapping zones are made. The active high speed eye tracking at the rate of 300 Hz is designed to follow not only the laser beam, but every single saccade of the eye. It allows for easy centering onto the desired ablation area. The scanning spot laser ensures that energy is dispersed extremely evenly during the ablation process. An integrated measuring device continusouly controls the level of energy of the laser beam. This system offers customized treatment possibilities in refractive surgery such as correction of aberrations up to a theoretical visual acuity of 20/6.
VisX WaveLight Laser Technologies AG Allegretto - This is an Argon Fluoride 193 nm excimer scanning spot system with active eye tracking. With the use of scanning spot lasers, an even distribution of energy is produced during ablation. A pulse rate of 200 Hz makes for quick procedure times. The active eye tracker allows precise active tracking of the laser beam, and therefore an accurate placement of each shot in case of rapid and saccadic eye movements. The Gaussian beam profile enables smooth transitions (without grooves and ridges) when operating in critical overlap zones. Allegretto is an "open system" in the sense of being independent of masks, lenses and diaphragms. At the touch of a key, the program interface permits the loading of application programs, such as a variety of correction profiles. Automated Lamellar Keratectomy (ALK) One of the earliest methods of vision correction, ALK is a purely mechanical method of changing the refractive power of the cornea. It involves removing a top layer of cornea with an automated instrument and then making a second incision (the refractive incision) in order to remove tissue for myopia or adding tissue (i.e. donor cornea) for hyperopia. The first incision is meant to remove a circular button of cornea c. 8 mm in diameter, while leaving one edge hinged so that after the refractive portion of the operation is complete the hinged corneal surface flap can be repositioned. The first incision is easier to perform because the cornea which is only 0.55 mm thick can be flattened and thus held without moving so that a diamond knife can make a slice of uniform thickness from one side leaving the opposite side hinged. In the case of myopia a second incision must be made to remove a curved (lens shaped) piece of tissue from the cornea's middle tissue (stroma). This tissue can be removed from the back surface of the slice or the front surface of the remaining cornea. The first slice, which is usually hinged (i.e. not completely removed) is then replaced on the cornea and held in place with or without a contact lens until the flap can reattach itself to the rest of the cornea - i.e. heal with a change in the shape of the corneal surface equivalent to the change in lens needed to satisfy the refractive needs. The more difficult technical problem with the mechanical ALK procedure is the (second) refractive incision which must remove an extremely thin slice of corneal tissue complete with tapered edges. There is a jelly-like consistency to the corneal tissue underneath the surface and this leads to significant limits to the precision of the procedure. Photorefractive Keratectomy (PRK) A more recent development in vision correction is a procedure called Photorefractive Keratectomy or PRK. Although the approach is similar to ALK, in that the cornea is modified to correct vision, the process is vastly different with remarkable improvements in patient risk and correction capabilities. Rather than making cuts in the cornea, the PRK process uses an excimer laser to sculpt an area 5 to 9 millimeters in diameter on the surface of the eye. This process removes only 5-10% of the thickness of the cornea for mild to moderate myopia and up to 30% for extreme myopia - about the thickness of 1 to 3 human hairs. The major benefit of this procedure is that the integrity and the strength of the corneal dome is retained. The excimer laser is set at a wavelength of 193nm, which can remove a microscopic corneal cell layer without damaging any adjoining cells. This allows the practitioner to make extremely accurate and specific modifications to the cornea with little trauma to the eye. This ability to sculpt, rather than cut, opens up the arena for treating additional vision conditions. At this stage, there are excimer laser machines that with a combination of masks and computer controls, can reliably treat myopia, hyperopia and now astigmatism. Laser In Situ Keratomeleusis (LASIK) An improvement to the ALK method of making the refractive incision is to use the excimer laser. This method is called ALK-E OR LASIK rather than ALK (Automated Lammellar Keratectomy - Excimer laser or Laser in Situ Keratomileusis). The use of the laser to sculpt either a - (myopic) or + (hyperopic) lens in the remaining corneal tissue allows the extreme precision of the refractive laser's surgical ability to significantly enhance the technique. Since the first or surface incision with the microkeratome is technically easier than the second or refractive incision it makes good sense to use the extreme optical precision of the refractive laser to achieve the desired correction of the corneal refraction. This technique allows preservation of the corneal basement epithelial layer knows as Bowman's membrane and in the absence of complications from the reattachment and healing of this flap, the refractive results can be rapid and superb.
|
For more information contact: Email: m2@lasik1.com
Worldwide DirectoryWhat is LASIK? What's New
Price/Payment Options/Patient FinancingFAQs
Free InformationMedical SectionGlossary
Dr. Murray McFadden
(BSc, MD, FRCS(C), Diplomate of the
American Board of Ophthalmology)
Telephone: (604) 530-3332
Fax: (604) 535-6258
Langley, BC Canada V2Y 1N4
This page last updated on November 9, 2004.
Web Page Programmer Turnaround Type and Graphics
|
Disclaimer |