Caution Urged on Laser Spot Size for PRK
A group of Italian researchers has uncovered evidence that lasers used during photorefractive keratectomy create stress waves that propagate within the eye itself. These waves, which are acoustic in nature, have bipolar characteristics of positive and negative pressures and may occur in laser-assisted in situ keratomileusis. While researchers do not expect the compression phase to cause damage, the negative pressure peak may cause local tissue stretching and bubble formation that could affect the eye and the outcome of the procedure.
In photoablation of the cornea, if the laser's spot size is large when compared with the cornea, the shape of the stress wavefront mimics the shape of the cornea and focuses to a spot inside the eye creating potentially dangerous negative and positive pressures (above right). For smaller spot sizes, diffraction makes the wavefront shape large and spherical (above left).
The researchers, led by Roberto Pini of the National Research Council of Italy, discovered that the cornea acts to focus the acoustic waves that are generated as the laser ablates tissue on the cornea's surface. When the laser's spot size exceeds 3 mm, the cornea acts like an acoustic lens for the stress wave. During propagation it converges to an acoustic focal point, Pini explained. "Here the pressure peak can become up to three times greater than that measured below the cornea. This acoustic focal spot typically is located in the eye at a distance from the cornea's surface equal to [the laser] spot's radius."
Pini's group found that the stress waves generated positive pressures up to 250 bar and negative pressures of 90 bar. The pressures are very short, about equal to the length of the laser pulse. Pini said that research has not yet demonstrated that the positive pressures cause significant damage to the eyes. The negative pressures the researchers uncovered, however, may be enough to create cavitation bubbles or stretch the tissue because 90 bar is well above the threshold pressure for cavitation in organic liquids.
Typically, these procedures use a spot size of about 6 to 7 mm. Using a spot size smaller than the radius of the cornea's curvature prevents the stress wave from focusing. Pini added that the laser intensity -- the fluence divided by the pulse duration -- also affects the stress waves.
The scientists plan to advance their research along two lines. In affiliation with the San Raffaele Hospital in Milan, they will conduct clinical tests to examine the possibility of photorefractive keratectomy damaging patients' eyes. They also plan to examine other side effects of laser refractive surgery such as the formation of a "central island" on the cornea's surface.
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