Sessions frequency: understanding the hair cycle

Every hair develops according to a cycle divided into three phases: the young and adult life (anagen phase), the oldness (catagen phase), and the death and fall (telogen phase).

We have seen that it is the accumulation of heat in the hair stem and bulb that induces hair destruction. In the anagen hair, the stem is attached to the bulb, which is full of melanin. Anagen hair are our real targets. Of course, it will be impossible to destroy all anagen hair at 100% in one session (it depends on the power of the shot, and on the hair’s intrinsic repair capacity).

Hair physiology

As all the hair in a same area are not in the anagen phase at the same time (only 20 to 60% of the hair depending on the areas), permanent hair removal of all the hair will only be possible after several sessions, once all the hair are caught in their anagen phase. According to areas, maintenance sessions might be necessary times to times.

Anagen hair are full of melanin in their base. During the laser shot, the melanin beam absorption (the pigment determining hair colour) will generate an intense heating (80°C approximately) of the bulbs. This heat will then spread throughout the bulb, and in the germinal cells notably, which will be damaged by such a significant and sudden elevation of the temperature. After several sessions, the injuries accumulated become irreversible, and a permanent hair removal of the area treated is obtained.

Catagen hair contain much less melanin. The heat delivered by the laser flash at the bulb level will not be important enough to damage significantly the germinal cells. The hair will fall after the session, but injuries on the reproductive system of the hair will not be severe enough to destroy it.

Telogen hair are already falling, and damaging the germinal cells of the bulb is hardly possible.

Deducing the treatment frequency

The physician's duty is to obtain the best results possible with as few sessions as possible. The regrowth will be weakened as soon as after the first session, but the battle is not yet won: hair do not give in so easily! The following sessions are meant to damage the repair mechanisms of the hair, until lesions become irreversible.

An average of four to five sessions suffice to obtain impressive results, but the utmost skill of the physician consists in determining a frequency of sessions as efficient as possible.

Frequency based on the understanding of the hair life cycle

It is only after years of experience that we managed – my team and myself – to find a treatment we were completely satisfied with. One of the keys was to understand that the duration of the hair cycles as they are defined in reference books are “theoretical” values and that they correspond to a hair “on rest”.

When the hair is attacked, like after a laser shot for example, it “drives crazy” and reproduces itself much faster.

This is why the first sessions are quite close in time, approximately two to three weeks between the two first sessions.  Then sessions become less frequent, and depend on a possible hair regrowth.

In men, the hormonal stimulation, as well as a deeper root, can make it more difficult, so that the treatment can last 12 to 18 months approximately.

Which laser to choose?

The choice of the device, as we have seen, is very important to obtain satisfying results. As a future patient, it is useful for you to know about the existing devices in order to decipher the different advertising discourses. It is the laser wavelength that will determine the skin penetration and the targets aimed.

- The Ruby laser at 694 nm : its wavelength is the best absorbed by melanin : one shot can heat up the hair to more than 200°C (R.M. Clement, M.N. Kierman). But its side-effects on dark skins render it less attractive than wavelengths less absorbed by skin melanin. The small dimensioned laser spot is also considered as an inconvenience to professional use.

- The Alexandrite laser at 755nm: its wavelength is well absorbed by melanin. As a matter of fact, it is sufficiently absorbed to treat blond hair on light skins. Its skin cooling systems allow advances in the treatment of tanned or dark skins.

The device is very big (it weights 150 kg, with a 1 m3 volume). The new generation of devices is equipped with a shooting system created to adapt to the different skin types.

From our viewpoint, the Alexandrite remains the “Rolls Royce” of hair removal. Powerful, multifunctional and comfortable, it provides an excellent visibility of the treatment area, spot diameters up to 18 mm, and the most sophisticated improvements and accessories. Its wavelength enables a deep penetration of the light beam without using a high fluence. We believe that all these characteristics guarantee unequalled efficiency, comfort and safety as compared to other laser families. The only disadvantage of this device is its high cost (130 000€ approximately or 200 000 CHF), a very big investment!

- The laser diode at 790-810nm : its wavelength is not absorbed much by melanin but allows a deeper penetration of the light beam. This technology is promising as lasers are manufactured at a low cost, with a tendency to a decrease due to the advances in the miniaturization of semiconductors. One of their advantages consists in being compact and relatively portable. The low absorption by melanin is both an advantage (skin tan and pigmentation are not affected much) and a disadvantage (the treatment is ineffective on lighter and thinner hair).

- The Nd:Yag laser at 1064 nm: its wavelength is even less absorbed by melanin. But it allows a deep penetration of the light beam. The thermal effect on the hair is low and leads to the emission of high fluences.

It provides very interesting results on patients with very dark skins, who could not be treated otherwise. Nowadays, this type of laser can only provide long-lasting hair removal, but after all, patients are glad to undergo only two or three sessions per year and not to bother with their hair the rest of the time.

We have developed new protocols at the CMH, which combine the Yag treatment and the Alexandrite treatment, so as to treat deep hair more easily (men backs or some bikinis). The results are very interesting and many of our colleagues are now testing this new process.

SOME MORE INFORMATION: the laser must be powerful enough to keep its energy high when the spot gets larger: what is lost in power is gained in skin penetration.

Technical outlines

The last few points of this overview are meant to give you an idea of the technicity of such a treatment and of the importance for it to be carried out by a physician (often called a laserist).

The laser beam is transmitted by an optical fibre extended by a handpiece that allows the operator to see the skin when he shoots. The skin target area of the laser shot is called “spot”. It is a very intense light mark with a diameter of 7 to 18 mm (or sides measuring 9 to 12 mm). The surface of the skin must be entirely covered as homogeneously as possible by a succession of spots before the treatment.

The main parameters to be observed by the practitioner in order to reach optimum results are:

-The spot diameter: a large spot diameter (12 mm or more) provides a good skin penetration and is more efficient on deep hair. You will also be treated faster. Manufacturers are very good at creating devices able to deliver enough energy to obtain large spots. Only very performing devices can shoot 2 shots per second with a 15 mm spot or 3 shots per second with a 12.5 mm spot!

-The fluence (expressed in Joules/cm2): the cornerstone of the treatment. It defines the energy of the shot per unit of area. The practitioner is confronted to two contradictory objectives while choosing the fluence: being efficient by using high fluences, but remaining prudent so as not to burn the skin. An experienced practitioner will be able to use the highest fluence in order to obtain the most efficient results and durable satisfaction. He will thus analyse your skin type (phototype) and will observe the different skin reactions (hair crepitation, retraction, root expulsion, erythematous papular reaction, etc.) in order to be as close as possible to the maximum tolerated dose. Tests progressively increasing the fluence session after session are undergone for this purpose.

- The pulse time (or shot time): it is the laser emission time, which greatly conditions the efficiency of the treatment. The laser pulse time can be compared to a car transmission: it transmits energy according to a reduction ratio adapted to the different hair and skin types. A car with several gears is much more efficient ; similarly it is preferable for a laser to have several pulse times.

- The skin cooling: if the pulse time can be compared to the transmission, the cooling system can be compared to the tires. The choice of the tires gets harder when cars get powerful and when the transmission is sophisticated. Thus, a sophisticated cooling system is necessary for a good laser to deliver its power and to benefit from all the advantages of an adjustable pulse time. A performing skin cooling system allows an increase in both the efficiency and safety of the treatment, and maintains the patient’s comfort.

SOME MORE INFORMATION: the fluence is derived from power, and is also expressed in Watts/second/cm2. A laser must be powerful enough to deliver the fluence necessary to destroy the deepest hair. The necessary fluence depends strongly on the wavelength. It is relatively low with Ruby lasers, as its light is absorbed a lot by melanin (<20 J/cm2). It must be higher with Alexandrite lasers, even higher with diodes, and very high with an Nd : YAG. In practice, a practitioner often needs more power, may it be to increase the efficiency of a shot, to extend its duration, to increase the spot diameter, or to increase the fluence.

Technical evolution

Even more specialized: two new generation Alexandrite lasers have integrated a technology derived from the discovery of a French researcher, Serge Mordon.

Instead of delivering the energy constantly, the laser astutely delivers the energy intermittently in a sequence of shots spaced of a few milliseconds. The skin has all the time to cool down (it evacuates half the accumulated heat in less than a millisecond). But the hair, with a Thermal Relaxation Time of 10 to 40 milliseconds, keeps storing the heat that cannot be evacuated, so it is stored and the temperature keeps increasing until it reaches the thermal damage threshold.

Explanation: the skin cover, a flat surface, cools down much faster than the cylindrical hair. If you are not convinced, try and pour some soup in a bowl and in a plate and wait to see which of the recipients cools down the first! The lasers, applying this technology based on this physics law of energy dissipation, deliver their fluence by intermittent shots. This succession of short impulses enables an accumulation of heat in the hair until its complete destruction, as well as a cooling of the skin. The epidermis has just the time to cool down before the laser delivers a new impulse, which increases the hair heat, and so on… This process is a real advance in selective photothermolysis.

We do not really like the ‘before / after” pictures you can find in many commercial websites. But we could not resist the temptation to show you the results on this bikini area. The second picture has been taken 2 years after the end of the treatment

Patients with slight dark skins and dark hair are the best candidates for laser hair removal

MORE INFORMATION : thermal relaxation time (TRT) is the time necessary for the tissue to diffuse half of the heat received to adjacent tissues. The little melanin grains that colour the skin and the epidermal cells have very short TRT and cool down very fast. By luck, terminal hair have a 10 ms TRT in the stem and a 40 ms TRT in the bulb. With an efficient fluence, the heat stored by large hair during the shot can literally make them explode and expulse them out of the dermis. A short shot duration is used on light skins with thin hair. On tanned, dark or black skins, the shot duration is long.

Personal choice

We use one of the two devices on the market equipped with this system. We are really satisfied by our choice, since the device allows to increase the power transmitted to the skin and to avoid collateral damages. This innovation enables to push the limits of laser hair removal by treating black and tanned skins with a safety margin unknown until then. It enables to envisage the permanent treatment of men backs and to treat blond hair efficiently.

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