Irradiance vs. Dosage

Lasers Med Sci. 2007 Oct 31; [Epub ahead of print]

Relevance of laser irradiance threshold in the induction of alkaline phosphatase in human osteoblast cultures.

Haxsen V, Schikora D, Sommer U, Remppis A, Greten J, Kasperk C.

Faculty of Science, Biophotonics Group, University of Paderborn, Paderborn, Germany,

Induction of matrix synthesis by low-level laser has been demonstrated extensively. However, the question of dose- or power intensity-dependency is under-investigated. To address this issue we chose human osteoblast cell cultures and measured their alkaline phosphatase (ALP) activity after laser irradiation. The cell cultures were irradiated periodically by 690 nm radiation via optical transmission fiber-based laser needles, reaching into the culture dishes. The osteoblasts showed no induction of ALP activity when we used a single laser needle stimulation with a laser irradiance of 51 mW/cm(2), an increase of approximately 43% at 102 mW/cm(2) irradiance (two needles per well) and a ninefold increase at 204 mW/cm(2) irradiance (four needles per well), leaving the temperature of the culture medium unaffected. We concluded that the osteoblastic response in ALP activity to a laser stimulus shows a logarithmic relationship, with a distinct threshold, rather than a linear dose-dependency. Secondly, the laser irradiance, rather than the dose, is relevant for the impact of the laser.

J Clin Laser Med Surg. 2001 Feb;19(1):29-33.

Biostimulatory windows in low-intensity laser activation: lasers, scanners, and NASA’s light-emitting diode array system.

Sommer AP, Pinheiro AL, Mester AR, Franke RP, Whelan HT.

Central Institute of Biomedical Engineering, Department of Biomaterials, University of Ulm, Germany.

OBJECTIVE: The purpose of this study was to assess and to formulate physically an irreducible set of irradiation parameters that could be relevant in the achieving reproducible light-induced effects in biological systems, both in vitro and in vivo.

BACKGROUND DATA: Light-tissue interaction studies focusing on the evaluation of irradiation thresholds are basic for the extensively growing applications for medical lasers and related light-emitting systems. These thresholds are of central interest in the rejuvenation of collagens, photorefractive keratectomy, and wound healing.

METHODS: There is ample evidence that the action of light in biological systems depends at least on two threshold parameters: the energy density and the intensity. Depending on the particular light delivery system coupled to an irradiation source, the mean energy density and the local intensity have to be determined separately using adequate experimental methods.

RESULTS: From the observations of different research groups and our own observations, we conclude that the threshold parameters energy density and intensity are biologically independent from each other.

CONCLUSIONS: This independence is of practical importance, at least for the medical application of photobiological effects achieved at low-energy density levels, accounting for the success and the failure in most of the cold laser uses since Mester’s pioneering work.