We all know that the ‘correct’ fluence should be applied to the skin when treating hair and blood vessels. But just how sensitive is the result to that applied fluence?
Well let’s go back to basics. The fluence is merely the concentration of energy at the skin surface. The energy then penetrates into the epidermis and the dermis, resulting in some absorption by a chromophore and a resulting temperature rise.
The temperature rise is directly proportional to the absorbed fluence. So if you double the fluence at the skin surface, you will double the temperature rise in the absorbing target.
But the amount of tissue damaged through the denaturation process is exponentially dependent on the temperature. What this means is that a small rise in temperature can result in a big difference in total denaturation.
The animation below shows how the denaturation, which we call “Omega”, changes as the temperature in the target increases and then falls.
Clearly the amount of denaturation increases quite rapidly, towards the end of the pulse, and then essentially plateaus as the temperature subsequently falls. That’s what we would expect to see in cells within the skin. We are trying to reach a value of omega equal to 1 – at that level the cells are deemed to be irreversibly denatured. In other words, they are ‘kaput’. They will not grow back!
Now let’s compare two different fluences and see what the final value of Omega is in each case (see animation below). In the left-hand panel we can see that an applied fluence of 20 J/cm² results in a final Ω of 0.67. This means that an insufficient volume of cells has been destroyed by the heat. This will allow the surviving cells to regenerate and regrow. For a hair follicle it means that a new finer hair will regrow at some point in the future. Since this hair is finer (i.e. a smaller diameter), it will hold less melanin. That will make it more difficult to kill in the future.
However, the right-hand panel shows an applied fluence of 22 J/cm², which results in a final Omega of 2.17 – those cells are fully denatured. That follicle or blood vessel will not be able to regrow.
The right-hand panel, with its fluence of 22 J/cm², is only 10% more than the left-hand panel. We can see that a relatively small increase in fluence makes a big difference in terms of the final result – 2.17 compared with 0.67.
Unfortunately, it seems that many laser operators are trained to use sub-threshold fluences, similar to the left-hand panel. They often report the regrowth of finer hairs after a certain number of treatments. When I hear this I immediately think that they have used too low a fluence. Had they used the correct fluence from day one, they would not see that result.
In conclusion, we can see that using a fluence that’s just below the threshold value will result in a poor clinical outcome. Now you know why!
Ciao for now,
Mike.
