Safety of the Staff-Reflections and Probability of Exposure

An examination of laser-accident records indicates that the source of accidental ocular exposure of the laser operator and staff is most frequently a reflected beam. Figure 24.4 illustrates the types of mirror-like (specular) laser-beam reflections that can occur from flat or curved specular surfaces, which are characteristic of metallic instruments used in other surgical procedures, but are not frequently encountered in skin resurfacing. Skin injury of the hand holding an instrument is also possible. Normally, a collimated beam is considered the most hazardous type of reflection, but at very close range, a diverging beam may pose a greater likelihood of striking the eye [1,14,15,25,29].

CURVED

SPECULAR

SURFACE

ROUGHENED SURFACE

DIFFUSE REFLECTION

Figure 24.4 Potentially hazardous reflections from specular (mirror-like) surfaces can extend some distance if the beam and reflecting surface are flat. If the beam is convergent, divergent, or the surface curved, the reflected beam will normally diverge rapidly, limiting the hazard zone.

A number of steps can be taken to minimize the potential hazards to the staff and bystand­ers. Preventive measures will depend upon the type of laser. One of the most commonly employed lasers in surgical applications today may still be the CO2 laser. Since the CO2 laser wavelength of 10.6 pm is in the far-infrared spectral region—and invisible—the pres­ence of hazardous secondary beams could go unnoticed. This added hazard resulting from an infrared laser beam’s lack of visibility is common to other infrared lasers, such as the

2.1 pm holmium or the 1064 nm Nd:YAG laser. The 755-795 nm alexandrite lasers are visible, but these wavelengths are weakly visible, with the result that many users have mis­takenly thought a high-power beam is “safe.” Because there have been a number of serious retinal injuries caused by improper attention to safety with Nd: YAG lasers [15], the use of the Nd:YAG laser must be approached with even greater caution than the CO2 laser. By contrast, the dye, argon laser, and the second-harmonic Nd:YAG (sometimes referred to as the “KTP”) laser emit highly visible, blue-green (488, 514.5, 532 and 588 nm) beams, and in some ways pose a lesser potential hazard.

Many dermatological lasers, such as the CO2, Nd:YAG, holmium, or argon, are continu­ous-wave (CW), or nearly so. Even the so-called super-pulse laser is quasi-CW compared to single-pulse or multiply pulsed fractional laser systems. The biological effects and potential hazards from high-peak power-pulsed lasers are quite different from those of CW lasers. This is particularly true of lasers operating in the retinal hazard region of the visible (400-780 nm) and near-infrared spectrum (IR-A, 780 to 1400 nm), as shown in Figs. 24.1 and 24.2. The severity of retinal lesions from a visible or near-infrared (IR-A) CW laser is normally considered to be far less than from a short-pulse laser. Another major factor that influences the potential hazard is the degree of beam collimation. Many dermatological lasers are focused, thereby limiting the hazardous area (referred to as the “nominal hazard zone” in IEC 60825-1 and ANSI Z136.1-2007 [3,9]. An exception is the highly collimated beam from the initial laser prior to the delivery system optics, which may remain hazardous at some distance from the instrument; this can be a problem during laser servicing [1].

Reflections are most serious from flat mirror-like (specular) surfaces—characteristic of many metallic surgical instruments and glass plates. Many surgical instruments now have black anodized or sandblasted and roughened surfaces to reduce (but not eliminate) poten­tially hazardous reflections. The strong curvature and surface roughening spread the reflected energy and greatly reduce the reflection hazard. The surface roughening is gener­ally more effective than the black (ebonized) surface, since the beam is diffused. However, in some cases, combining a special black surface with roughening provides increased pro­tection, and adding a black polymer finish to surgical implements placed in or near the beam has been shown by experiment measurements to offer the greatest protection at the CO2 wavelength—despite initial skepticism by investigators [29]. However, other groups argue against blackening the surface, since the instrument will become hotter than without for visible wavelengths. Therefore, the use of the special blackened surfaces must be approached with caution for each application; fortunately, such instruments are not com­mon in most procedures.

It should be noted that both the surface finish and reflectance seen in the visible spectrum do not indicate these qualities in the invisible, far-infrared spectrum. In fact, a roughened surface that appears to be quite dull and diffuse at a shorter, visible, or IR-A wavelengths, will always be more specular at far-infrared wavelengths (e. g., at 10.6 m). This results from the fact that the relative size of the microscopic structure of the surface relative to the incident wave­length determines whether the beam is reflected as a specular or diffuse reflection [1,14,29].

A specularly reflected beam with only 1% of the initial beam’s power can still be quite hazardous. Hence, the rougher the surface of an instrument likely to intercept the beam, the safer the reflection. For example, even a 1% reflection of a 40 watt (40 W) laser beam is 400 mW! It is somewhat surprising that there have been few cases reported of eye injuries to residents and other persons observing Nd:YAG laser surgery without eye protectors. Hazard­ous specular reflections from a laser beam emerging from an optical fiber are limited in extent because the beam rapidly diverges—just as would a focused beam, as shown in Fig. 24.5.

Most invisible beam surgical lasers have a visible alignment beam. Infrared lasers most often make use of a low-power coaxial He-Ne (632.8 nm) or diode (e. g., 635 nm) red laser. It is desirable where feasible for this alignment beam to be 1 mW (milliwatt) or less, since the maximum CW, visible laser beam power that can safely enter the eye within the aver­sion response (i. e., within the blink reflex of 0.25 seconds) is 1 mW. This type of laser is then classified as Class 2, and poses a very low risk to the user.