12 August 2009

Laser eye surgery

The invention: The first significant clinical ophthalmic application of any laser system was the treatment of retinal tears with a pulsed ruby laser. The people behind the invention: Charles J. Campbell (1926- ), an ophthalmologist H. Christian Zweng (1925- ), an ophthalmologist Milton M. Zaret (1927- ), an ophthalmologist Theodore Harold Maiman (1927- ), the physicist who developed the first laser Monkeys and Rabbits The term “laser” is an acronym for light amplification by the stimulated emission of radiation. The development of the laser for ophthalmic (eye surgery) surgery arose from the initial concentration of conventional light by magnifying lenses. Within a laser, atoms are highly energized. When one of these atoms loses its energy in the form of light, it stimulates other atoms to emit light of the same frequency and in the same direction. A cascade of these identical light waves is soon produced, which then oscillate back and forth between the mirrors in the laser cavity. One mirror is only partially reflective, allowing some of the laser light to pass through. This light can be concentrated further into a small burst of high intensity. On July 7, 1960, Theodore Harold Maiman made public his discovery of the first laser—a ruby laser. Shortly thereafter, ophthalmologists began using ruby lasers for medical purposes. The first significant medical uses of the ruby laser occurred in 1961, with experiments on animals conducted by Charles J. Campbell in New York, H. Christian Zweng, and Milton M. Zaret. Zaret and his colleagues produced photocoagulation (a thickening or drawing together of substances by use of light) of the eyes of rabbits by flashes froma ruby laser. Sufficient energy was delivered to cause immediate thermal injury to the retina and iris of the rabbit. The beam also was directed to the interior of the rabbit eye, resulting in retinal coagulations. The team examined the retinal lesions and pointed out both the possible advantages of laser as a tool for therapeutic photocoagulation and the potential applications in medical research. In 1962, Zweng, along with several of his associates, began experimenting with laser photocoagulation on the eyes of monkeys and rabbits in order to establish parameters for the use of lasers on the human eye. Reflected by Blood The vitreous humor, a transparent jelly that usually fills the vitreous cavity of the eyes of younger individuals, commonly shrinks with age, with myopia, or with certain pathologic conditions. As these conditions occur, the vitreous humor begins to separate from the adjacent retina. In some patients, the separating vitreous humor produces a traction (pulling), causing a retinal tear to form. Through this opening in the retina, liquefied vitreous humor can pass to a site underneath the retina, producing retinal detachment and loss of vision. Alaser can be used to cause photocoagulation of a retinal tear. As a result, an adhesive scar forms between the retina surrounding the tear and the underlying layers so that, despite traction, the retina does not detach. If more than a small area of retina has detached, the laser often is ineffective and major retinal detachment surgery must be performed. Thus, in the experiments of Campbell and Zweng, the ruby laser was used to prevent, rather than treat, retinal detachment. In subsequent experiments with humans, all patients were treated with the experimental laser photocoagulator without anesthesia. Although usually no attempt was made to seal holes or tears, the diseased portions of the retina were walled off satisfactorily so that no detachments occurred. One problem that arose involved microaneurysms. A“microaneurysm” is a tiny aneurysm, or blood-filled bubble extending from the wall of a blood vessel. When attempts to obliterate microaneurysms were unsuccessful, the researchers postulated that the color of the ruby pulse so resembled the red of blood that the light was reflected rather than absorbed. They believed that another lasing material emitting light in another part of the spectrum might have performed more successfully.Previously, xenon-arc lamp photocoagulators had been used to treat retinal tears. The long exposure time required of these systems, combined with their broad spectral range emission (versus the single wavelength output of a laser), however, made the retinal spot on which the xenon-arc could be focused too large for many applications. Focused laser spots on the retina could be as small as 50 microns. Consequences The first laser in ophthalmic use by Campbell, Zweng, and Zaret, among others, was a solid laser—Maiman’s ruby laser. While the results they achieved with this laser were more impressive than with the previously used xenon-arc, in the decades following these experiments, argon gas replaced ruby as the most frequently used material in treating retinal tears. Argon laser energy is delivered to the area around the retinal tear through a slit lamp or by using an intraocular probe introduced directly into the eye. The argon wavelength is transmitted through the clear structures of the eye, such as the cornea, lens, and vitreous. This beam is composed of blue-green light that can be effectively aimed at the desired portion of the eye. Nevertheless, the beam can be absorbed by cataracts and by vitreous or retinal blood, decreasing its effectiveness. Moreover, while the ruby laser was found to be highly effective in producing an adhesive scar, it was not useful in the treatment of vascular diseases of the eye. Aseries of laser sources, each with different characteristics, was considered, investigated, and used clinically for various durations during the period that followed Campbell and Zweng’s experiments. Other laser types that are being adapted for use in ophthalmology are carbon dioxide lasers for scleral surgery (surgery on the tough, white, fibrous membrane covering the entire eyeball except the area covered by the cornea) and eye wall resection, dye lasers to kill or slow the growth of tumors, eximer lasers for their ability to break down corneal tissue without heating, and pulsed erbium lasers used to cut intraocular membranes.

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