The Solidstate Refractive Lasers symposium held during April 1992 in San Diego, Calif, was a showcase for new approaches to refractive surgery. A dozen leaders in the area of refractive laser development joined to give the audience a comprehensive update on progress in this area. Topics ran across the gamut of solidstate technologies, and included the adjunctive use of topographic analysis to improve clinical results.
One of the most interesting and promising lasers to emerge from these R&D efforts is the erbium: YAG which produces output at the 2.9-micron wavelength. Because light at this wavelength is highly absorbed in water, the erbium laser is fundamentally a very efficient tissue cutting device, with minimal thermal damage. Early tissue studies show that, next to the excimer at 193 nm, the 2.90-micron erbium wavelength has the least thermal effect on adjacent cells during surgery. Other factors, such as cost, size, low maintenance, and the absence of toxic gases, favor the erbium over the excimer as an instrument used in a clinical environment.
Studies of the Q-switched version of the erbium, which produces nanosecond-length pulses, prove that the zone of thermal damage caused by tissue ablation is less than 4 µ. Presumably, with more sophisticated delivery systems and pulsing methods, this level of damage could be further reduced. For instance, techniques such as homogenization of the beam via fiberoptics, and diode laser pumping of the erbium rod are being considered by forward looking design engineers.
Studies show that, like the excimer, the bulk of erbium laser energy is absorbed in the first micron of the cornea. "It's no coincidence that we chose the erbium, which is almost exactly the wavelength that's needed for a refractive alternative to the excimer,'" explained Dr Maloney, who is on the faculty at Jules Stein Eye Institute in Los Angeles, CaIiC and is co-developer of the Premier (Irvine, Calif) erbium system.
All the solidstate cousins of erbium, from holmium to titanium:sapphire lasers, are also under investigation for refractive surgery. Michael Berry, PhD, of Sunrise Technologies (Fremont, Calif) has been working with mid-infrared wavelengths in an effort to perfect a refractive surgery system. His talents have been joined with those of Bruce Sand, PhD, Peter McDonnell, MD, and Jean Marie Pare!, PhD, to commercialize a holmium-based refractive laser designed and manufactured at Sunrise Technologies.
Dr Berry explained the holmium (2.10-micron wavelength) approach to refractive surgery as "laser thermal keratoplasty" where the laser beam is very gently heating the stroma of the cornea to cause a phased transition to stromal collagen. We're doing very gentle thermal processing of stromal collagen within the cornea to change its structure and its mechanical properties in a way that generates a front surface curvature change."
"For a correction of 5.00 diopters, we're typically producing laser heating on a diameter of about 4.00 mm oriented around the central visual axis. The total front surface displacement of the cornea at that position is only 28 µ for a 5.00-diopter correction. I just want to leave you with the clear message that very tiny changes in the front surface curvature of the cornea can lead to large refractive corrections."
Other papers at the symposium covered a variety of novel approaches to vision correction using lasers. One of the most interesting research lasers was discussed by Henry Zenzie, PhD, of Schwartz Electro-Optics (Orlando, Fla). Dr Zenzie has been working with a tunable titanium :sapphire laser under funding from a government grant. Operating parameters of the Titan-P research laser manufactured by Schwartz are as follows: output energy of 100 mJ at 800 nm; tuning range of 680 to 940 nm; a pulse width of 10 nanoseconds at 800 nm; a beam diameter of 2.00 mm; beam divergence of 0.6 mrad; a line width of 0.5 nm; and a repetition rate of 10 Hz.
Using barium borat crystals to convert this laser's wavelength to the 207- to 220-nanometer range, corneal ablation studies were carried out using freshly enucleated calf eyes. The laser beam was directed normal to the corneal surface, and the beam profile at the cornea was near Gaussian with a spot size of about 1.00 mm. A truncated Gaussian beam defined by a 200-micron slit was also used for some samples. Cuts were made at wavelengths ranging from 207 nm to 220 nm. Histological studies revealed that both wavelengths produced sharp cut edges with adjacent damage limited to a layer of collagen less than 1 µ.
Summarizing results with the Ti:sapphire laser, Dr Zenzie pointed out that in spite of "the 20% difference in absorption coefficients between 207 and 220 nm, no significant difference in thermal damage was observed. Further investigation will be needed to delineate the subtle difference between the wavelengths, as well as the differences that may exist between a Gaussian and an aperturized beam's ablation."
by MICHAEL MORETTI