In vitro survey of low energy laser beam penetration in compact bone

The laser used in this study (NEURALASE R1) is a light-emitting diode which emits continuous or modulated impulsional signals in the near infra-red (approximately 850 nm). The power emitted varies between 1 and 11 mW. The instrument is placed at a fixed distance from a photovoltaic sensor whose voltage response is a linear function of the received luminous power. The voltage variations are studied as functions of the thickness of the compact bone interposed between the output of the laser and the diode. On semilogarithmic paper, we find a curve of two linear segments having different slopes. The first (between 0 and 1 mm of bone), of very sharp incline, is most probably due to an important reflexion on the bone surface; the second, with a slight incline, corresponds to the absorption which is exponential. Moreover, if the ray enters in the cortico-medullar direction, the second slope is sharper than in the case of a longitudinal penetration following the diaphyseal texture. A power of 10 mW gives a maximal penetration of about 18 mm in the bone axis direction and approximately 6 mm in the cortico-medullar direction. For eventual therapeutic indications, it is necessary to take account of reflexion on the bone surface as well as the direction of the beam in relation to the bone texture, since these two elements interfere in the diffusion of the stimulation.