Laser Vision Correction     

Lasers are becoming a very important tool in many delicate procedures. Laser vision correction is part of the growing industrial application of micromachining. Physicians are able to use laser correction on the cornea of a patient to correct ocular refractive problems and restore the cornea to a more ideal shape.

 

Types of Lasers

Similar to industrial applications there are two different types of lasers which are used on the eye:

• High pulse energy and a broad beam spot
• Low pulse energy, smaller beam spot, and higher repetition rate.

Types of Refractive Errors:

There are three types of errors which afflict the eye:

• Myopia (nearsightedness) - the curvature of the cornea is too steep and the images are created in front of the retina.
• Hyperopia (farsightedness) - the curvature of the cornea is too flat and the image is created in a plane behind the retina.
• Astigmatism - curvature of the cornea or lens is different in vertical and horizontal axes.

Surgical Techniques

To correct ocular refractive problems, physicians use 193 nm excimer lasers to remove material from the cornea. There are two different ways that the excimer lasers can be used on the patient.

1. Photorefractive Keratectomy:

   In photorefractive keratectomy, the laser removes epithelial cells and material from the top surface of the cornea. The epithelial cells grow back after the surgery, but the underlying corneal cells - including Bowman's membrane - do not. Perforating this membrane can produce effects such as hazy vision, so it takes patients a few days after surgery to see clearly again.

2. Laser-assisted keratomileusis:

   In laser-assisted in situ keratomileusis, the surgeon cuts a flap about 160 micrometers deep across the cornea, then opens this flap like a door to reveal interior corneal tissue. After treatment, the surgeon repositions the flap. Because the technique only slightly disrupts Bowman's membrane, no haze results; patients recover faster and can see shortly after surgery.

    

Full Area Laser Technique

In general laser surgery uses the full area, broad-beam laser which has a uniform beam intensity and high energy levels (100-200 mJ/cm^2) at low repetition rates (10-50 Hz). As shown in Figure 2, To correct myopia, laser pulses with a small diameter are fired into the center of the cornea. The diameter of the laser is increased for each successive pulse until the desired contour results. As a result, the corrective surface profiles is "stair-stepped" and not smooth.

The physician must be very careful while using the large-diameter laser shots. As the diameter of the laser increases in size, there is an increased chance that ablated material will redeposit on the cornea and cause irregularities. Figure 3 on the right demonstrates ablasion. To correct hyperopia, the physician removes material from the cornea's edges with diminishing annulus-shaped apertures. There are two ways to correct astigmatism. A physician can either use elliptical spots to make the corrections or use round spots aligned through the center of the eye.

Flying-Spot Scanning Lasers

Flying-spot scanning laser systems operate by software which directs the X and Y scanners around the cornea. The software then controls the small diameter laser spots to remove the material in a pattern necessary to produce the desired profile. This system is very efficient because it can adjust the number of pulses for a particular position which results in a smoother corrective profile than in the full area approach. Currently, beam diameters of 1-2 mm are used which require several thousand shots at 50-200 Hz.

Article and figures fromHeinz Huber, "Medical Technology Borrows from Industrial Laser Systems." Photonics Design & Solutions, Feb 2000.

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