The thermal effects of therapeutic lasers with 810 and 904 nm wavelengths on human skin.

OBJECTIVE: To investigate the effect of therapeutic infrared class 3B laser irradiation on skin temperature in healthy participants of differing skin color, age, and gender. BACKGROUND: Little is known about the potential thermal effects of Low Level Laser Therapy (LLLT) irradiation on human skin. METHODS: Skin temperature was measured in 40 healthy volunteers with a thermographic camera at laser irradiated and control (non-irradiated) areas on the skin. Six irradiation doses (2-12 J) were delivered from a 200 mW, 810 nm laser and a 60 mW, 904 nm laser, respectively. RESULTS: Thermal effects of therapeutic LLLT using doses recommended in the World Association for Laser Therapy (WALT) guidelines were insignificant; below 1.5°C in light, medium, and dark skin. When higher irradiation doses were used, the 60 mW, 904 nm laser produced significantly (p < 0.01) higher temperatures in dark skin (5.7, SD ± 1.8°C at 12 J) than in light skin, although no participants requested termination of LLLT. However, irradiation with a 200 mW, 810 nm laser induced three to six times more heat in dark skin than in the other skin color groups. Eight of 13 participants with dark skin asked for LLLT to be stopped because of uncomfortable heating. The maximal increase in skin temperature was 22.3°C. CONCLUSIONS: The thermal effects of LLLT at doses recommended by WALT-guidelines for musculoskeletal and inflammatory conditions are negligible (<1.5°C) in light, medium, and dark skin. However, higher LLLT doses delivered with a strong 3B laser (200 mW) are capable of increasing skin temperature significantly and these photothermal effects may exceed the thermal pain threshold for humans with dark skin color.

Is mechanical pain threshold after transcutaneous electrical nerve stimulation (TENS) increased locally and unilaterally? A randomized placebo-controlled trial in healthy subjects.

BACKGROUND AND PURPOSE: It is not fully understood how transcutaneous electrical nerve stimulation (TENS) intensity affects mechanical pain threshold. METHOD: Sixty-six healthy volunteers (13 male, 53 female; 132 hands) without prior experience of TENS participated in the study, which comprised a randomized single-blind controlled trial. TENS was administered for 20 minutes through electrodes (25 x 25 mm) placed on the hands and forearms with a fixed frequency of 100 Hz and pulse duration of 150 micros. TENS intensity was randomized and allocated in a concealed manner so that one arm received TENS with stimulation intensity set at participants' subjective sensory threshold and the other received TENS with stimulation intensity continuously adjusted by physiotherapists to a strong but comfortable non-painful stimulation. Observers were blinded to stimulation intensity levels. RESULTS: Mechanical pain threshold increased significantly, by a mean total of 0.79 kg/cm2 (95% confidence interval [95% CI]: 0.54-1.04) (p < 0.001) on the strong but comfortable non-painful stimulation side. The mean change in mechanical pain threshold on the sensory threshold side was 0.19 kg/cm2 and did not reach statistical significance (95% CI-0.15 to 0.51). The mean stimulation intensity level for sensory threshold was 6.7 mA (95% CI: 5.65 to 7.83) which was significantly lower (p < 0.001) than the mean stimulation intensity for the strong stimulation, which was 20.5 mA (95% CI 16.6 to 24.4), respectively. The strong stimulation levels were, on average, 3.05 times higher than sensory threshold, but individual variations were large (range 1.2-6.1). CONCLUSIONS: TENS administered at a strong but comfortable non-painful stimulation intensity increases mechanical pain threshold ipsi-laterally in healthy subjects, whereas TENS administered at sensory threshold intensity does not. TENS may be ineffective if electrodes are placed contralaterally or distant to the pain site and if stimulation intensity levels are not titrated to subjective strong levels. Further clinical trials are needed to clarify if these findings may also be generalized to populations of chronic pain sufferers.

The variation of heating depth with therapeutic ultrasound frequency in physiotherapy.

In patient treatment, different ultrasound (US) frequencies are attributed to differences in penetration and, as an effect of that, to different heating depths in tissues. A set of 13 experiments was carried out with US frequencies of 0.86, 2 and 3 MHz. A dynamic treatment protocol and a stationary treatment protocol were used. The temperature increase patterns were thermally imaged with a 1-min interval during an insonation of 5 min. At every data point, the temperature in the reference image was subtracted from the thermal image after 1, 2, 3, 4 and 5 min. In every difference thermal image, the distance between the US applicator and the deepest point of heat increase was measured. Results show that US frequencies do not affect the depth limit for the different temperature ranges, in either the static treatment protocol or the dynamic treatment protocol.