the physics of photodisruptive vitreolysis
The first pulsed YAG lasers were developed in 1976 and the ophthalmic YAG laser has been used clinically since 1983. Dr. Franz Fankhauser of Switzerland was responsible for developing the the Q-switched YAG laser in treating vitreous pathology. YAG lasers used to treat eye floaters are FDA-approved ophthalmic lasers. They are approved and used most commonly to treat two conditions: A) POSTERIOR CAPSULE OPACIFICATION, and B) NARROW ANGLE GLAUCOMA
POSTERIOR CAPSULE OPACIFICATION is a common condition that occurs after cataract (lens) removal and replacement with an artificial implant lens. The transparent capsule which surrounds the new, clear implant lens begins to cloud over and starts to resemble wax paper. The laser is very precisely focused onto this membrane and a sequentially enlarged opening is created to once again restore clear vision. In this illustration, the cone-shaped beam is focused on the membrane behind the artificial lens.
NARROW ANGLE GLAUCOMA | PERIPHERAL IRIDOTOMY: There is a
rarer version of glaucoma where the iris and the cornea are very close to each affecting fluid drainage. The decreased outflow of fluid from the eye causes elevated eye pressures risking damage to the optic nerve. Using the YAG laser, a small hole is created in the peripheral iris which allows fro better communication of fluids between the front and rear chambers of the eye. A peripheral iridotomy is the name of the procedure and this photo shows the small opening.
YAG lasers are solid state lasers using Yttrium/Aluminum/Garnet as the excitable doping material. They are stable, reliable, and accurate lasers that require little maintenance or worry. Unlike lasers most people are accustomed to, the optics of our laser does not emit a single narrow beam that many associate with lasers. As an example, this laser pointer light beam will continue in a straight line until it falls on an object or is dissipated by the atmosphere.
The laser energy profile of the Ellex YAG laser is quite different. It has two cone-shaped beams on top of each other. The YAG laser frequency is invisible to the human eye, but there are visible red focusing lasers running longitudinally down the center of the cone. The two focusing lasers, the cone-shaped YAG laser, and the surgeon's narrow depth of field focus all coincide on one small spot where the energy is delivered. This allows the laser to pass though the cornea and lens without delivering any energy there. The pulsed energy is only delivered where the apices of the cone and the focusing beams coincide. This also answers a question we are often asked: "What happens if you miss the target"? The answer is "Nothing". The energy dissipates on the far side of the focus point and so no energy is delivered to the sensitive and important retinal tissue. Unlike other ophthalmology lasers, the YAG laser can deliver energy to non-pigmented tissues. It is interesting that if the laser is focused adjacent to the floater into the clear vitreous or watery fluid, there does not appear to be any delivery of energy.
LASER ENERGY AND EYE FLOATER VAPORIZATION: A common misperception among laser treatment naysayers and critics is the concern that the laser only breaks the floater into smaller pieces. They assume that having many small floaters would be worse than one large one. We have two answers to that concern: A.) small floaters, if they are more than a couple millimeters from the retina may not cast any noticeable shadows and may be imperceptible and B.) The YAG laser, at appropriate energy levels not only breaks the floater into smaller pieces, but also vaporizes the collagen and hyaluronin molecules directly to a gas so that at the end of a treatment, there should be less mass, less material present.
With tight focus of the laser on the surface of the floater and appropriately delivered energy levels, there is a physical process called optical breakdown and plasma formation. There is a high frequency electrical field confined to an area of about 4-8 microns (4-8/1000's of a millimeter). For a duration of approximately 20-30 nanoseconds ( 0.0000002 seconds) the floater becomes opaque and highly reflective acting like a shield preventing the energy rom continuing towards the retina. There is a combination of photochemical, thermal, thermoacoustic, and electromagnetic optical field effects which ionizes the molecules and forms plasmas gases. With a good shot of the laser, both the surgeon and the patient will see a spray of gas bubbles. The surgeon sees them rise to the top of the eye but the patient sees small black dots sinking to the bottom of the visual field.
Next we will describe what the treatment experience is like as well as expectations for improvement HERE.