Thermal Damage During Thulium Laser-Assisted Partial Arytenoidectomy: A Comment

This month’s copy of The Annals of Otology, Rhinology & Laryngology contains an article on laser induced tissue damage in calf vocal folds: Minimizing Thermal Damage During Thulium Laser-assisted Partial Arytenoidectomy: Pulsed Versus Continuous Cutting in an ex-vivo Calf Model. ¹ Its main message is the fact that the use of continuous wave (CW) leads to less thermal tissue change than the use of pulsed waves (PW). This is a counterintuitive finding.

The premise of pulse modes is that the energy is delivered in a pulsed beam, varying in frequency, pulse width, and peak power, which enables the tissue to heat up rapidly and cool down between the pulses, reducing thermal damage. ² Tissue damage depends, among other things, on the length of time over which the energy is released. ³

During cuts made with pulsing, four cut-off times were tested in the study: 0 ms (ie, CW), 250 ms, 500 ms, and 750 ms per second (1 Hz repetition rate). The findings presented demonstrate that pulsing the thulium laser at any of the pulse durations studied does not seem to reduce thermal change, but only just adds time to the procedure. The implication is that trying to achieve less tissue damage by introducing PW only seems to have a detrimental effect.

As far back as the 1970s, our predecessors showed that excessive heat can lead to epithelial sloughing, direct damage to the tissue adjacent to the laser cut, and excessive scarring and fibrosis. In those days, it was impossible to predict collateral damage caused by ablation or vaporization. Interactions between laser and tissue are currently better known, the equipment is currently more sophisticated and accurate, and cumulative experience has made it possible to reduce the likelihood of unwanted effects from laser treatments. ⁴

Tissue alterations induced by laser surgery occur in the sequence denaturation, vaporization, carbonization. Thus, the warmer the tissue gets, the more carbonization will occur. Carbonization used to be a frequently occurring finding that should be avoided: there is concern about its effect on granulation tissue and scar formation. Therefore, the experiments are useful.

Out of the Hippocratic principle primum non nocere one should avoid unnecessary damage in patients. Therefore this study is relevant. Partial arytenoidectomy is a glottis widening intervention, with the goal of improving the patient’s airway lumen. Partial arytenoidectomy inherently damages the voice: it induces roughness, and lowers the pitch. From the moment that the thyroarytenoid muscle has been cut from the vocal process, an important decrease in the anteroposterior tension of the muscle occurs. This can be seen during the intervention: the length of the vocal fold diminishes. During post-intervention videolaryngostroboscopy the vocal fold moves like a flag flapping in the wind, showing unstructured oscillations. Theoretically, there is additional detrimental thermal damage to the superficial lamina propria (SLP), further degrading the voice. However, the effect of lack of anteroposterior tension far outweighs the change caused by damage to the SLP.

It has not been made completely clear why a surgeon would use thulium instead of another laser. Of parameters studied in experiments with a fiber-routed CO₂ laser, the greatest influence on collateral thermal expansion was induced by laser pulse duration. ⁵ Pulsed emission increased incision depths. ⁴ Unfortunately, the absolute temperature increases were not reported in those studies. It is known that temperature rise depends on the laser used: 4°C to 6°C with picosecond infrared laser (PIRL) in vocal fold incisions, significantly less than with Er:YAG laser cuts (170°C-200°C). ⁶ The temperature rise in the study described in this month’s journal is in the order of 6.5°C when using CW, as opposed to 16°C to 19°C during PW, depending on the pulse duration. Remarkably, the temperature increase in the reported experiments was not linear with pulse duration.

Temperature rise in laryngeal tissue depending on laser modus has not been published yet. It has, however, been examined in stapedial surgery. ⁷ During stapedotomy, the thulium and CO₂ laser show more extended thermal effects below the stapedial footplate compared to KTP. ⁷

The authors of Minimizing Thermal Damage During Thulium Laser-assisted Partial Arytenoidectomy: Pulsed Versus Continuous Cutting in an ex-vivo Calf Model cannot fully explain their findings, but suggest future research that may yield a better understanding of thulium laser parameter effects on vocal fold surgery. In addition, a prospective clinical series where laser settings are compared might be useful. Such research should be conducted shortly.