Threshold levels for damage of the cornea following irradiation by a continuous wave carbon dioxide (10.6 millimicron) laser

Laser-induced corneal injury: validation of a computer model to predict thresholds

The exposure and emission limits of ICNIRP, IEC 60825-1 and ANSI Z136.1 to protect the cornea are based on a limited number of in-vivo studies. To broaden the database, a computer model was developed to predict injury thresholds in the wavelength range from 1050 nm to 10.6 µm and was validated by comparison with all applicable experimental threshold data (ED₅₀) with exposure duration between 1.7 ns and 100 s. The model predictions compare favorably with the in-vivo data with an average ratio of computer prediction to ED₅₀ of 0.94 (standard deviation ± 30%) and a maximum deviation of less than 2. This computer model can be used to improve exposure limits or for a quantitative risk analysis of a given exposure of the cornea.

Ocular damage due to chlorhexidine versus eyeshield thermal injury

BACKGROUND

Ocular damage may occur from a number of mechanisms during laser use.

OBJECTIVE

To review issues relevant to ocular protection during laser resurfacing.

METHODS

The authors were consulted to evaluate the thermal energy transferred from the outer to the inner (ie, corneal contact) surface of stainless steel eyeshields following direct exposure to the carbon dioxide (CO2) resurfacing laser beam. Measurements were obtained using thermocouples (attached to the inner surface of the eyeshields) and analyzed with a computer-based acquisition system.

RESULTS

A maximum eyeshield temperature increase of 13 degrees C above the ambient temperature was noted following one pass with a CO2 resurfacing laser (Sharplan continuous CO2 laser with Clinicon SureScan scanner, 15 W, 950 microsec pulse duration, square spot of 9 mm).

CONCLUSION

The eyeshields analyzed in this study minimized thermal transfer following a single direct hit with a CO2 resurfacing laser. An understanding of the potential mechanisms of ocular injury is essential in preventing its occurrence.

Laser eye injuries

Laser instruments are used in many spheres of human activity, including medicine, industry, laboratory research, entertainment, and, notably, the military. This widespread use of lasers has resulted in many accidental injuries. Injuries are almost always retinal, because of the concentration of visible and near-infrared radiation on the retina. The retina is therefore the body tissue most vulnerable to laser radiation. The nature and severity of this type of retinal injury is determined by multiple laser-related and eye-related factors, the most important being the duration and amount of energy delivered and the retinal location of the lesion. The clinical course of significant retinal laser injuries is characterized by sudden loss of vision, often followed by marked improvement over a few weeks, and occasionally severe late complications. Medical and surgical treatment is limited. Laser devices hazardous to the human eye are currently in widespread use by armed forces. Furthermore, lasers may be employed specifically for visual incapacitation on future battlefields. Adherence to safety practices effectively prevents accidental laser-induced ocular injuries. However, there is no practical way to prevent injuries that are maliciously inflicted, as expected from laser weapons.

Safety parameters for avoiding acute ocular damage from the reflected CO2 (10.6 microns) laser beam

Reflections from instruments in the surgical field involving the CO2 laser beam present a serious ocular hazard. In addition to the use of plastic or glass protective eyewear, this hazard can be minimized by utilizing anodized instruments and recognizing the specific distances at which various reflections are no longer hazardous depending upon certain variables, including laser output wattage, emergent beam lengths, and surface characteristics of the reflecting instruments.

Laser interactions with the cornea

Principles of laser-tissue interactions in the eye are reviewed. Corneal structure and function are summarized, with particular regard for features related to laser treatment. A summary of argon and carbon dioxide laser techniques in the cornea is presented, followed by a review of studies on corneal response to ultraviolet radiation. A detailed description is then given of the characteristics of excimer laser tissue ablation. Potential applications of this process in corneal and keratorefractive surgery are reviewed.

Laser applications in medicine and biology: a bibliography

This bibliography covers the period from 1963 through 1974; 916 references are classified under 23 subject headings. The references are arranged chronologically.