Page 76 - Chapter 3 - An Introduction to Laser/IPL Hair Removal

Chapter 3 – Fundamentals of Laser/IPL Hair Removal 1st Edition
Each time a treatment is carried out, fewer follicles are destroyed than previously (see Table 18).
Treatment number
% follicles killed
4 weeks
8 weeks
1
33
33
2
20
27
3
17
23
4
13
10
5
10
4
6
4
3
Table 20 - The difference between leaving 4 or 8 weeks makes a slight difference in the % of follicles killed.
We can also see that leaving an 8 week gap between repeat treatments results in a little more clearance of follicles. But the difference is marginal!
Laser/IPL hair removal requires the target follicles to reach a sufficient temperature for a sufficient time to ensure full denaturation of the germ cells. Higher temperatures require lower heating durations, according to the Arrhenius Damage Equation. If the correct parameters are delivered to the follicles, then the overall efficiency of the process depends entirely on the skills and abilities of the laser/IPL operator.
In reality, the effective kill rate will be typically between 20 and 90% in each treatment session, depending on those skills. That EKR has a significant effect on the number of repeat sessions required to generate a satisfactory end-result.
Our model shows clearly the optimum intervals between sessions. Applying the industry ‘standard’ interval of 6 to 8 weeks into the model shows that between 6 and 10 repeat sessions are required – as is typically found in real-world settings. This confirms the accuracy of our model reasonably well.
A quick calculation of the surface area occupied by hair per square centimetre across the body, reveals a range from around 1.5% (legs/axillae) to around 13% (chin/upper lip). This indicates that most of the light energy delivered in hair removal treatments is not actually absorbed by the targets, thereby reducing the overall efficiency, and increasing the likelihood of generating unwanted tissue damage.
Natural reticence to ‘injure’ patients typically results in operators choosing less than optimum parameters, particularly the fluence. The authors’ experience in this field also indicates that many operators do not properly understand the importance of pulsewidth, which essentially determines the supra-threshold ‘cooking time’ of the target cells. In most cases, the denaturation time is not equal to the pulsewidth. As a consequence, achieving a high EKR can be difficult, in many situations, leading to an excessive number of treatments.
Conclusions
________________________________________________________________________ 76 Chapter 3 Laser/IPL Hair Removal
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