LOW LEVEL LASER / PHOTOBIOMODULATION DOSIMETRY. WHAT ARE THE THRESHOLDS AND WHAT ARE THE LIMITS?

James D Carroll FRSM, AMInstP, THOR Photomedicine Ltd, United Kingdom

Photobiomodulation includes biostimulation and bioinhibition. In vitro, in vivo and clinical studies have established that there are radiant intensities and energy thresholds below which no significant cellular stimulation or clinical effects are achieved.  Conversely there are radiant intensities and total fluence thresholds above which cellular stimulation decreases from its peak effect and inhibition is achieved. Many dose-ranging studies have been published using a variety of laboratory and clinical models and most suggest that cellular stimulation is achieved at radiant intensities in the range of 5 – 50mW/cm2 (at the target tissue ) and that exceeding these levels can inhibit cellular function.

So what is the role of the higher intensity laser products that appear to be popular with some clinicians and their patients ? Analgesia.

Laser (of suitable wavelength and intensity) can disrupt fast axonal transport by causing perturbation of microtubule arrays of small diameter neurons in dorsal root ganglia and by reducing ATP synthesis in axonal mitochondria (Chow and Armati, 2004). There is also evidence that laser inhibits A-delta and C fibre transmission. Laser-induced neural blockade leads to long term altered nociception, similar to local anaesthetics. The repeated application of laser can reduce tonic peripheral nociceptive afferent input to the dorsal horn and facilitate reorganisation of synaptic connections in the CNS producing pain modulation.

By treating nerve supply to a painful area you can give relief without inhibiting repair locally at the site of injury.