Excimer Lasers

The excimer laser has brought more accuracy to corneal surgery and vision correction than ever before. One pulse of the excimer laser removes 0.25 microns of tissue. To put this into perspective, a typical human hair is 70 microns in thickness. Surgeons have never had a device as accurate as the excimer laser in eye surgery.

Electron Micrograph of a Human Hair Ablated by the Excimer Laser

Two types of excimer lasers are available for refractive procedures: broad-beam lasers and scanning lasers. Scanning lasers can be subdivided into two groups: slit scanning and spot scanning. Each kind of laser has its pros and cons. Patients should be aware that while type of beam used does affect outcomes, "often the difference is not substantial enough to dispose end users towards one type of laser or another."¹

The following descriptions are taken from an article by Stanley B. Teplick, MD.2

• Broad Beam Lasers
  A broad beam laser uses a relatively large beam diameter (from 6.0 to 8.0 millimeters) that can be manipulated to ablate the cornea. The broad-beam laser results in the shortest procedure time. This speed creates less likelihood of overcorrection and decentration - a complication caused by movement of the pupil. A disadvantage is an increased possibility of center islands - a complication related to ablation. However, ophthalmologists have learned to decrease the incidence of center islands by using several short laser pulses to ablate the cornea instead of one longer one.
  
  
• Slit Scanning Lasers
  A slit scanning laser uses relatively small beams linked to a rotational device with slit holes that can enlarge. During surgery, the laser beam scans across these holes to gradually increase the ablation zone. A uniform beam and potentially smoother ablations characterize slit scanning lasers. Disadvantages include a slightly higher propensity for decentration and slightly greater risk of overcorrection.
  
  
• Spot Scanning Lasers
  These systems have the potential to produce the smoothest ablations and use radar technology to track the eye's movement. They also have the potential to treat irregular astigmatism and link to topography. These lasers must be linked to eye tracking to ensure proper centration.

U.S. Current Regulatory Status of Excimer Lasers

For the latest information on the US FDA approvals, click here. Patients should be aware that each country has its own regulatory statutes, so some of the devices approved in the US may not be available in your country. At the bottom of this page, we have provided links to various manufacturers wherever possble and have provided contact information for those companies currently not online.

Wavefront Diagnostics & Custom Ablation

Wavefront sensing is an emerging diagnostic tool for measuring the refractive error of the eye. Conventional forms of refraction are limited to measuring the best spherical and cylindrical refraction (myopia or hyperopia and regular astigmatism). But wavefront sensing allows for measurements of other conditions within the cornea that affect a patient's refraction (how they see). Ophthalmologists refer to these conditions as higher order aberrations. Traditionally these higher order aberrations have been described as irregular astigmatism, and considered a limitation to best vision with refraction. Now by understanding and characterizing the components of high order aberrations, doctors have diagnostic power over irregular astigmatism, and the ability to quantify levels of naturally occurring or surgically induced aberration.The diagnostic utility of wavefront sensing has so far been seen in the objective verification of spherocylindrical refraction, diagnosis of complex or subtle conditions of the cornea, such as keratoconus, dry eyes and cataracts, and in the magnitude of induced aberrations after laser vision correction procedures. Therapeutically, wavefront sensing has value in attempts to correct naturally occurring and surgically induced aberrations, and results, so far, have been quite promising.

At its very basic, wavefront sensing uses a simple technique. The patient stares ahead at a field and focuses on an object. While doing so, a beam of light is projected into the eye. This light is reflected off the back of the eye and back out. The machine maps where the exiting rays of light land. This map is then used to determine what is going on in the eye. Earlier diagnostic methods only looked at few data points. Some of the machines that are available today look at nearly 2,000 data points, providing a much more detailed map of the patient's eye.

This diagnostic information can be used both to assist in the development of the non-surgical correction of visual problems through customized contact lens and, to link to an excimer laser system for the surgical correction of the problems. These surgical systems are available outside the US and currently in clinical trials here in the US.
There is an article on line that may be of assistance.³ The focus of the article is on the marriage of the diagnostic with the laser. While it discusses only one of the systems under development, it does have some good overall information in the first few paragraphs. Another article that may be of interest appeared in EyeWorld which recounts data presented at a meeting in early 2000.⁴

Limited numbers of the machines are available throughout the US, and most of the companies listed below have systems in place throughout the world. International data presented was presented at the American Academy of Ophthalmology annual meeting in New Orleans in 2001. Some of the presentations were on the podium, others were at the manufacturer's booths, presented by physicians. In December 2001 one system was granted approval by the US FDA for therapeutic use in treating decentered ablations. We will to provide further data regarding this application as it becomes available.The manufacturers of excimer lasers systems have been working for several years to develop customized ablation systems that tie the wavefront diagnostic image of a patient's eye directly to the surgical ablation map for the excimer lasers. These laser systems have demonstrated very promising results and are increasingly available outside the US. However, clinical trials in the US are still on-going for the systems. Again, we will provide more data once it becomes available.