Energy density jcm2

Co2 Laser Gain Curve

Fig. 4.64. (a) Ablation curves of fresh and dried bone obtained with a CO2 laser (pulse duration: 250 |s, wavelength: 10.6 ^m). Due to its higher water content, fresh bone is ablated more efficiently. Data according to Forrer et al. (1993). (b) Ablation curve of bone obtained with an Er:YAG laser (pulse duration: 180 |s, wavelength: 2.94 |m). Data according to Scholz and Grothves-Spork (1992)

Fig. 4.64. (a) Ablation curves of fresh and dried bone obtained with a CO2 laser (pulse duration: 250 |s, wavelength: 10.6 ^m). Due to its higher water content, fresh bone is ablated more efficiently. Data according to Forrer et al. (1993). (b) Ablation curve of bone obtained with an Er:YAG laser (pulse duration: 180 |s, wavelength: 2.94 |m). Data according to Scholz and Grothves-Spork (1992)

In the 1980s, research focused on laser radiation at a wavelength of approximately 3 | m which is strongly absorbed by water. For instance, Wolbarsht (1984) compared the effects induced by CO2 lasers at 10.6 |m and HF* lasers10 at 2.9 |m with each other. From his observations, he concluded that the latter wavelength is better suited for orthopedic applications. Similar results were published by Izatt et al. (1990). Unfortunately, though, HF* lasers are very unwieldy machines. Walsh and Deutsch (1989), Nelson et al. (1989a), and Gonzales et al. (1990) reported on the application of compact Er:YAG lasers at a wavelength of 2.94 |m. They stated that this radiation efficiently ablates both bone and cartilage. The ablation curve of bone obtained with the Er:YAG laser is illustrated in Fig. 4.64b.

Another promising laser in orthopedics is the Ho:YAG laser which emits at a wavelength of 2.12 |m. Nuss et al. (1988), Charlton et al. (1990), and Stein et al. (1990) have investigated acute as well as chronic effects of bone ablation with this laser. Its major advantage is that its radiation can be efficiently transmitted through flexible fibers. However, thermal effects are significantly enhanced compared to those induced by Er:YAG lasers at a wavelength of 2.94 |m as observed by Romano et al. (1994). They found that thermal damage is extremely pronounced when applying 250 | s pulses from a Ho:YAG laser. At an incident energy density of 120 J/cm2, a thermal damage zone of roughly 300 | m is determined. On the other hand, pulses from an Er:YAG laser are associated with very little thermal damage. At an energy density of 35 J/cm2, a damage zone of only 12 |m is estimated. The corresponding histologic sections are shown in Fig. 4.65a-b. In the case of the Er:YAG laser, a lower energy density was chosen to obtain a similar ablation depth as with the Ho:YAG laser. One potential application field of erbium lasers is microsurgery of the stapes footplate in the inner ear. This treatment belongs to the discipline of otorhinolaryngology, and it will therefore be addressed in Sect. 4.10.

Due to their high precision in removing tissues, excimer lasers have also been proposed for the ablation of bone material, e.g. by Yow et al. (1989). However, it was soon observed that their efficiency is much too low to justify their clinical application. Moreover, osteotomies performed with XeCl lasers at 308 nm are associated with severe thermal damage as reported by Nelson et al. (1989b). As in the case of CO2 laser radiation, these thermal effects are believed to be responsible for the manifest delay in healing of the laser-induced bone excisions.

An interesting approach to determine laser effects on bone has recently been reported by Barton et al. (1995) and Christ et al. (1995). By using a confocal laser scanning microscope, they were able to analyze ablation rate and morphology as a function of incident pulse energy from a Ho:YAG laser. They concluded that scattering is a dominant factor in the interaction of Ho:YAG laser radiation and bone.

10 Hydrogen fluoride lasers.

Yag Laser And Bone Healing

Fig. 4.65. (a) Histologic section of bone after exposure to a Ho:YAG laser (pulse duration: 250 |s, energy density: 120J/cm2, bar: 100 ^m). (b) Histologic section of bone after exposure to an Er:YAG laser (pulse duration: 250 |s, energy density: 35J/cm2, bar: 200 |m). Photographs kindly provided by Dr. Romano (Bern)

Another discipline for laser applications within orthopedics is arthroscopy. Preliminary results regarding laser meniscectomy, i.e. the treatment of the meniscus, have already been reported by Glick (1981) and Whipple (1981) when using Nd:YAG and CO2 lasers, respectively. At that time, though, suitable delivery systems were not available. Moreover, the CW mode of these lasers led to unacceptable thermal damage. Katsuyuki et al. (1983) and Bradrick et al. (1989) applied Nd:YAG lasers in arthroscopic treatment of the jaw joint. Significant improvements were not achieved until O'Brien and Miller (1990) made use of specially designed contact probes consisting of ceramics. Limbird (1990) pointed out the necessity of blood perfusion measurements after surgery. Major limitations for all infrared lasers in arthro-scopic surgery arise from the optical delivery system. Transmission through flexible fibers can be regarded as a mandatory requirement for an efficient surgical procedure. Therefore, CO2 lasers will never gain clinical relevance in arthroscopic treatments.

A new era of laser arthroscopy began with the application of holmium and erbium lasers. Trauner et a. (1990) reported promising results when using the Ho:YAG laser for the ablation of cartilage. Recently, Ith et al. (1994) have investigated the application of a fiber-delivered Er:YSGG laser emitting at a wavelength of 2.79 |im. The transmittance of novel zirconium fluoride (ZrF4) fibers at this specific wavelength is satisfactory. Ith et al. (1994) have used fresh human meniscus from the knee joint which was obtained during surgery. They have observed a thermally damaged zone of 60 |im when exposing the tissue in air to five laser pulses at a pulse energy of 53 mJ and a pulse duration of 250 |is. On the other hand, when exposing the tissue through water at a slightly higher energy of 65 mJ, thermal damage extended to only 40 | m close to surface and was even negligible elsewhere. In either case -whether exposed in air or through water - a crater depth of roughly 1 mm was achieved. The surprising result of this study is that laser radiation at 2.79 |im can be effectively used for tissue ablation, although it should be strongly absorbed by surrounding water. Thus, Ith et al. (1994) concluded that light - after exiting the fiber - is guided through a water-vapor channel created by the leading part of the laser pulse. The period during which this channel is open was found to be dependent on the duration of the laser pulse. For pulse durations of 250-350 | s, most of the laser energy is transmitted through the water-vapor channel to the target.

The experimental results mentioned above encourage the application of holmium and erbium lasers in arthroscopic surgery. Nevertheless, further investigations need to be performed regarding both thermal and mechanical side effects associated with laser exposure. From today's perspective, though, it is already obvious that arthroscopy belongs to those medical disciplines where minimally invasive techniques based on laser radiation will turn into unrenouncable surgical tools.

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