The information contained herein is highly technical and will be extremely useful as a source for the eye care professional. Any specific information requests can be directed to Dr. Murray in the Guest Book.
Comprehensive information specific to the LASIK procedure (including photos of an actual LASIK procedure) can be found by visiting our sister web site www.lasik1.com
Medical Section Contents
A comprehensive list of medical terms, ophthalmalic definitions, and associated supplies and equipment.
The Magic of PRK
The magic of Photo Refractive Keratectomy (PRK) is a surgical precision unprecedented in human history. 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 PRK the focusing power of glasses or contact lenses is sculpted directly unto the cornea or front window of the eye. The new and special laser actually cleaves individual molecular bonds to remove tissue with no damage to surrounding tissue. Computer programs control the surface sculpting to ensure the highest possible accuracy and success of the intended refractive change.
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.
Everyone familiar with optics will understand that the refractive effect of sculpting a concave or convex lens upon the cornea 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.
Current PRK lasers are best classified by laser source (either Argon-Fluoride excimer lasers or solid state) and beam type (broad beam or scanning beam). Laser technology and computer control software has evolved significantly since the first normally sighted eyes were treated in 1987. Initial PRK treatments used 3.5 and 4mm optical zones so as to minimize the depth of ablation. Since many pupils dilate to 5mm it is not surprising that edge glare and light sensitivity were common complications. Ablation diameter increase with edge smoothing has been implemented to solve many edge glare problems. Wide or broad beam machines initially had problems caused by the use of nitrogen flow to disperse vaporized tissue and with the occurrence of unvaporized central islands. Stoppage of nitrogen flow and modification of computer generated treatment regimes has largely eliminated these problems.
The US Food & Drug Administration (FDA) has been cautious, rigid, and slow to approve PRK for widespread use within the USA. There has been speculation that the reason for the current caution is embarrassment over a previous premature approved of the surgical procedure of radial keratotomy (RK). Many observers have feared that the apparent bureaucratic rigidity might impede the implementation of future needed changes to equipment or procedures prior to long and inflexible testing schedules. However, recently the FDA surprised its critics when, with the final approval of the Summit Laser, they insisted upon increasing the size of the optical zone from that tested in the preapproval trials. In the US a number of other laser manufacturers are progressing or almost through FDA trials. In contrast, most other jurisdictions including Europe and Canada, have, without the "benefit" of as vigorous an approval process, had the freedom to amend and improve equipment and treatment regimes as improvements presented themselves. There is now worldwide a large and expanding experience with many varied laser machines and evolving technical improvements.
Aesculapx- Meditec GMBH
MEL 60- This is an Argon Fluoride 193 nm excimer scanning system. The scanning beam is rectangular and measures 1mm by 10 mm. The system uses a limbal suction cup mechanism to fix centration and computer controlled rotating masks which fit into the suction cup mechanism. The mask for simple myopia is an f-stop like mechanism. There are different masks used for myopia with astigmatism, hyperopia, and for pure astigmatism. The masks rotate within the suction cup in order to control any axis of extra ablation as needed for astigmatism correction. Laser calibration is done by visual inspection of a 1um thick metal foil which requires 9 laser passes for removal. There is a layer of red under the silver foil making efficacy of removal easily monitored. Watch this space for a future "quick-time" or "M-peg" clip of calibration, and actual treatment of hyperopia, myopia, and astigmatism.
LADARVision® System and CustomCornea® - The LADARVision® System consists of the LADARVision® 4000 laser and the LADARWave® Aberrometer. The LADARVision excimer laser is a small-spot scanning laser with a laser radar tracking device. FDA approved for wavefront-guided ablations.
Bausch & Lomb Technolas 217 Excimer Laser with PLANOSCAN
Technolas 217 Workstation - this is an Argon Fluoride 193 nm excimer scanning system. The scanning beam is a circular spot which can be size adjusted. Centration is accomplished by an active "pupil" tracking mechanism which locks on to the pupil image and will have the laser follow any movement by the patient's eye. Active Infra-red Eye Tracker and passive monitor interrupts laser beam on movement in excess of 3 mm range (1.5 mm radius). Astigmatism, myopia and hyperopia can be treated by software adjustment of the beam scans.
LaserSight Technologies, Inc.
The new LaserScan LSX utilizes LaserSight's patented scanning delivery system integrating new leading edge technology. The LaserScan LSX uses a patented scanning system to deliver a 1-mm low energy "flying spot" in a proprietary alternating, multi-zone, multi-pass strategy. With each pass, about 2 microns of tissue are precisely removed to produce a finely polished corneal surface. Unlike older broad beam technologies, no rings or ridges are produced. Studies now show that smoother ablations may produce less haze, faster healing and more stable clinical results. Integral to each system is flexibility in treatment parameters including gently tapered transition zones.
Manufacture and sales of refractive laser systems, keratome systems, keratome blade products, and aesthetic lasers. LaserSight pioneered refractive laser systems using 193 nm, high resolution, scanning delivery. Both patient fixation and an optional automated tracking system are available. Astigmatism, hyperopia and myopia can be treated with software adjustments of the scanning mechanism. Watch this space for a future " quick-time" or "M-peg" clip of calibration, and actual treatment of myopia, astigmatism and hyperopia.
The LaserHarmonic-1 and LaserHarmonic-2 are solid state lasers still in the development stage. The former is flash lamp pumped and employs the fifth harmonic of a Nd:Yag at 213nm, and the latter is a diode pumped fifth harmonic Nd:YLF laser at 209nm.
EC-5000 This is an Argon Fluoride 193 nm excimer scanning system. The scanning beam is a rectangular slit which both scans, dynamically rotates, and overlaps. The rotation of the scan is designed to eliminate circular f-stop ridges and increase the smoothness of the ablation. Centration is controlled by the surgeon with a "joy" stick mechanism to follow the patient's eye. Astigmatism and myopia can be treated by software adjustment of the beam scans. At the time of writing we do not have any result data for this machine. Watch this space for a future "quick-time" or "M-peg" clip of calibration, and actual treatment of myopia, and astigmatism.
Novatec LASER SYSTEMS INC
Lightblade (TM) This is an solid state c. 208nm non excimer scanning system based upon the fourth harmonic of a titanium sapphire crystal. The scanning beam is a 200-300um variable size spot. Centration is accomplished by an active tracking mechanism which locks to have the laser follow movement by the patient's eye. Astigmatism and hyperopia can also be treated by software adjustment of the spot scans. At the time of writing we do not have any result data for astigmatism of hyperopia treatment. Watch this space for a future "quick-time" or "M-peg" clip of calibration, and actual treatment of hyperopia, myopia, and astigmatism.
Excimed, Omnimed, Apogee, Apex These are Argon Fluoride 193 nm excimer wide beam systems. The 1990 version of the Eximed machine had optical zones of only 4.5 and 5.0 mm. The Omnimed and Eximed versions increased the optical zone to 6.0 and 6.5mm. (The Apex machine has an optical zone of 6.5mm blending out to 9.4mm transition zone. The mask for simple myopia is an f-stop like mechanism located internally in the beam path. Summit has chosen to use custom crafted ablatable masks in the rail or beam path for the astigmatism and hyperopic correction. These masks protect the corneal tissue under them until the tapered mask is removed by laser pulses. The area without a mask will receive the full laser ablation. We have no data at the time of writing concerning the effectiveness of the ablatable masks for the astigmatic element of myopia treatment. Laser calibration is done by an internal 2 minute beam profile test. Watch this space for a future "quick-time" or "M-peg" clip of calibration, and actual treatment of hyperopia, myopia, and astigmatism.
20/15, 20/20, STAR (TM) These are Argon Fluoride 193 nm excimer wide beam systems, the STAR(TM) version being the most recent evolution of the machine. The STAR machine has a standard 6mm optical zone which is expandible to 8mm for future applications. The mask for simple myopia is an f-stop like mechanism located internally in the beam path. The astigmatic module masks and hyperopic module masks are located internally in the beam path. The hyperopic module has an ablation zone of 9mm. Laser calibration is performed automatically at the start of each day, and between cases. A plastic test card read on a standard lensometer verifies the calibration. Watch this space for a future "quick-time" or "M-peg" clip of calibration, and actual treatment of hyperopia, myopia, and astigmatism.
As a preamble to discussion of ALK (and RK) it is nessary to remark that as of January 1996 ALK is a anomaly largely confined to the USA. The reason is simple- the FDA in its "wisdom" has only approved one laser machine for PRK use, and only to -7.00 diopters correction. Mechanical surgical operations on the other hand are "approved" and thus American Ophthalmologists effectively have one hand tied behind their backs. They are allowed to perform ALK and RK but not PRK. The precision and efficacy of refractive laser technology have rendered ALK and RK essentially extinct in the rest of the world.
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. 8mm in diameter, but to leave 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.
An improved 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 laserÍs 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.
When comparing only the benefits of Lasik over PRK the first impression is that Lasik has the potential to be a superior procedure. There is however, a very significant list of potential complications or risks and these include:
The overall complication rate for the ALK-E or LASIK in Febuary 1995 was still in the order of 10% in the hands of the world leading innovators in the technique.
1) L'Esperance FA: Ophthalmic Lasers, 2nd Ed,:pg 4, 1983 (ISBN 0-8016-2823-7).
2) Ruby laser 1963- 694.3 nm; Argon laser 1968- 457.9 to 524.7 nm; Krypton laser 1972- 647.1 nm (red), 568.2 nm (yellow), 530.8 nm (green).
3) Neodymium-YAG 1980-1064 nm
4) Trokel S: History and Mechanism of Action of Excimer Laser Corneal Surgery, pg 1,Corneal Laser Surgery; editor Salz JJ, et al, 1995 (ISBN 0-8151-7513-2).
5) Corneal Laser Surgery; editor Salz JJ, associate Editors McDonnell PJ & McDonald MB, 1995 (ISBN 0-8151-7513-2)
6) Trokel S: History and Mechanism of Action of Excimer Laser Corneal Surgery, pg 4,Corneal Laser Surgery; editor Salz JJ, et al, 1995 (ISBN 0-8151-7513-2).
7) Krueger RR, Binder PS, McDonnell PJ: The Effects of Excimer Laser Photoablation on the Cornea pg 17, Corneal Laser Surgery; editor Salz JJ, et al, 1995 (ISBN 0-8151-7513-2).
8) Krueger RR, Binder PS, McDonnell PJ: The Effects of Excimer Laser Photoablation on the Cornea pg 22, Corneal Laser Surgery; editor Salz JJ, et al, 1995 (ISBN 0-8151-7513-2).
9) um=micrometer; 1000 micrometer = 1 millimeter. (an older and no longer used term for micrometer is micron)
10) Krueger RR, Binder PS, McDonnell PJ: The Effects of Excimer Laser Photoablation on the Cornea pg 23, Corneal Laser Surgery; editor Salz JJ, et al, 1995 (ISBN 0-8151-7513-2).
11) depth ablation= (diameter of ablation squared x diopters of correction)/ 3; Munnerlyn CR, Koons SJ, Marshall J: Photorefractive Keratectomy: a technique for laser refractive surgery, J Cataract Refractive Surgery 14: 46-52, 1988.
Dr. Murray McFadden Email: M2@prk.com
For detailed information with actual photos of the LASIK procedure, please visit our sister web site www.lasik1.com
(BSc, MD, FRCS(C), Diplomate of the American
Board of Ophthalmology)
© Copyright 1996-2005 Murray McFadden MD, Inc.
Telephone: (604) 530-3332
Fax: (604) 535-6258
Langley, BC Canada V2Y 1N4
Dr. Murray McFadden
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