Therapeutic Laser Depth of Penetration

Introduction

• Speaker: Dr. Phil Harrington
• Topic: Understanding how deep therapeutic laser light can penetrate tissues

Key Question

• Not just how deep light penetrates, but to what depth can we deliver a therapeutic dosage?

Key Concepts

Grotthuss-Draper Law

• First Law of Photochemistry: For light to produce an effect, it must be absorbed
• Photons must be absorbed by cells (muscles, connective tissues, nerves, blood vessels) to have an effect

Electromagnetic Spectrum

• Gamma Rays, X-Rays, visible light, infrared
• Therapeutic lasers: Use low-energy photons (red and infrared light), non-ionizing, and safe
• Absorption by tissues is crucial for effectiveness

Semiconductor Diodes in Lasers

• Wavelength Variation: ±10 to ±20 nm is normal and acceptable

Scientific Studies & Findings

Monte Carlo Simulations

• Study Findings: Different wavelengths penetrate to different depths
• Visible Light: Blue and green barely penetrate; red light modestly; infrared deepest

Absorption as a Function of Wavelength

• Y-Axis: Absorption coefficient (logarithmic scale)
• Visible vs. Infrared: Infrared wavelengths penetrate deeper

Practical Insight

• Red Light: Only penetrates a few millimeters; poor for deep tissues
• Infrared Light: Penetrates much deeper (examples: 800-810 nm)

Comparison of Wavelength Studies

• Infrared: 808 nm penetrates significantly deeper than 980 nm
• Red vs. Infrared: Infrared penetrates much deeper than red
• Continuous Wave vs. Super-Pulse: 810 nm continuous wave better than 904 nm super-pulsed

Ice Application Before Laser

• Study Insight: Increases penetration if applied before laser treatment
• Recommendation: Always use ice before laser, not after (delays healing)

Important Figures

Absorption Spectrum of the Human Hand

• Finding: Red light has high optical density (poor penetration), infrared has lower optical density (better penetration)

Depth of Penetration Study

• Method: Water phantom, Monte Carlo simulations, and cadaver tissue
• Result: At 10 cm depth, ~30% of incident light remains

Practical Applications

• Human Thigh: Approx. 20 cm across; significant penetration at 10 cm depth

Summary

• Red Laser: Only for superficial use, penetrates a few mm
• Infrared Laser: Best for deeper conditions (790-810 nm range)
• Key Takeaways: 800 nm reaches 10 cm depth with ~30% light remaining