Measurement of radiometric surface temperature and integrated backscattered light intensity during feedback-controlled laser-assisted cartilage reshaping

Abstract

Cartilage undergoes characteristic mechanical stress relaxation following laser irradiation below the ablation threshold. Porcine auricular cartilage (1-2 mm thickness) was irradiated with a Nd:YAG laser (X=1.32 ~m) at two power levels (W/cm2). Surface temperature (So(t) (°C)) (monitored using a single element HgCdTe infrared detector, 10-14 ~m spectral range), and integrated back scattered light intensity I(t) were measured during laser irradiation. A HeNe laser beam (X=632.8 nm) was incident on the back surface of the cartilage specimen and fractional integrated backscattered light intensity was measured using an integrating sphere and a silicon photodiode. Laser irradiation (5.83 W/cm 2, 50 Hz pulse repetition rate (PRR)) continued until surface temperature reached approximately 70°C, during which cartilage mechanical stress relaxation was observed. Integrated back scattered light intensity reached a plateau at about 70°C). At higher laser power (39.45 W/cm ~, 50 Hz PRR), a feedback-controlled cryogen spray was used to maintain surface temperature below 50°C. A similar plateau response was noted in integrated backscattered light intensity. This signal may be used to optimise the process of stress relaxation in laser cartilage reshaping. Several clinical applications involving reconstructive surgery are proposed.

Shape retention in porcine‐septal cartilage following Nd:YAG (λ = 1.32 μm) laser‐mediated reshaping

“…Our results con®rm previous studies [5,10,26], which demonstrated the effectiveness of using laser energy to accelerate stress relaxation in cartilage and reshape grafts. However, in contrast to previous studies that focused on mechanical properties, we measured the shape change accompanying laser-mediated cartilage reshaping and observed signi®cant shape retention.…”

“…Laser reshaping of cartilage has been well documented in both experimental studies [10,14,17,18] and controlled clinical trials [19]. While photography and time-dependent measurements of stress relaxation have illustrated the shape changes in tissue which accompany rapid laser heating, studies to date have neither examined the degree of specimen curvature nor quantitatively investigated memory effects.…”

Laser Reshaping of Cartilage

“…Figure 20.5 shows the acoustic signal to a single laser pulse (a) at the beginning of the laser treatment, and (b) after the tissue temperature has exceeded 70°C and stress relaxation has occurred. (7) Light scattering kinetics in cartilage being irradiated demonstrates an increase (a change of the sign of the slope) when stress begins to drop (Sviridov et ai., 1996;Wong et al, 1998a t b). This Inay arise from the nucleus of a new phase (water droplets, micropores) that increases the number of light scattering centres.…”

“…If the bound-ta-free water phase transition is effected without damage to the surrounding protein or carbohydrate molecules, then stable modified cartilage configurations may be achieved. It was shown that the process of laserassisted cartilage reshaping is accompanied by changes in the mechanical~thermal, and the optical properties of the tissue; changes which can be measured (Ornel'chenko et al'1 1999a;Sobol, 1995;Sobol et 01., 1996a;Sviridov et 01., 1996;Wong et aL t 1997Wong et aL t . 1998a).…”

Diode‐pumped fiber lasers: A new clinical tool?

“…The major advantage of this technique is that complex shapes can be formed without the signi®cant loss of tissue, which usually occurs with mechanical manipulation. There have also been a number of other lasers used for this procedure including the Nd:YAG laser operating at either 1.32 mm [44,45] or 1.44 mm [46,47], the Ho:YAG laser operating at 2.1 mm [48,49] and, the Er:YAG laser operating at 2.94 mm [50,51]. It is very clear from this small survey that a range of laser wavelengths and modes of output are suitable for successful cartilage reshaping, with a number if successful clinical trials already being performed [52,53].…”