Page 89 - Lasers and IPLs in Medical/Aesthetic Applications v1.2
P. 89

Chapter 1 – Fundamentals of Medical/Aesthetic Lasers and IPLs v1.2
Clinically ‘Useful’ Penetration Depth - Definition
• While this is all very nice and technical, it is pretty useless when considering laser/IPL treatments of targets in the skin! Merely getting the light energy to a particular depth is not sufficient – there must be enough energy at that depth to do the actual job! This is a different definition!!
• We now define the ‘Clinically Useful Penetration Depth’ – this is the depth where a ‘useful’ amount of energy can reach which will carry out the job we want.
Click here for an animation on this topic.
The ‘Clinically Useful Penetration Depth’ not only considers the physics of the interaction (i.e. where the light goes in the skin), but also the effect it has on the target tissue. If the light can penetrate to a depth of 4 mm, but have such low energy at that depth that it has no real effect in the target, then it is quite useless!
We need to know what depth the light can reach AND have the desired effect on the target.
This is known as the ‘Clinically Useful Penetration Depth’ (CUPD) and it differs significantly from the standard ‘Penetration Depth’ definition.
 The CUPD depends on:
 Wavelength Spot diameter Pulse duration and
Fluence (see animation)
The fluence is critical here because it is this factor which determines whether the target tissue responds in the way we want. If the fluence is too low at the skin surface, then it will have little effect at any depth inside the skin
We must understand that the fluence will generally drop with depth – failure to understand this concept is why so many treatments ‘hit the wall’ and never result in complete success!!
● Contrary to popular ‘myth-conception’ smaller spot sizes DO NOT penetrate deeper into the skin – see the ‘useful zone’ in figure 49!
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Chapter 1 LEVEL A Fundamentals of Lasers/IPLs
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