The clinical rationale for selecting a long pulse width (10 to 20 ms) lies in optimizing the safety and efficacy of the treatment through the principle of selective photothermolysis.
By extending the pulse duration, you allow the surrounding skin sufficient time to dissipate heat and cool down, while simultaneously maintaining destructive heat within the targeted hair follicle. This specific range is chosen because it aligns closely with the thermal relaxation properties of the coarse, terminal hairs typically associated with hirsutism.
The core objective is to match the laser's pulse duration to the Thermal Relaxation Time (TRT) of the target. A 10–20 ms pulse is sufficiently long to protect the epidermis but short enough to ensure thermal damage is confined to the coarse hair follicle.
The Mechanism: Selective Photothermolysis
The science behind this setting is rooted in how different tissues absorb and release heat.
Understanding Thermal Relaxation Time (TRT)
Every object has a Thermal Relaxation Time (TRT), which is the time required for the object to cool by 50% after being heated.
Large objects, such as the coarse hair follicles found in hirsutism, have a longer TRT (they cool slowly).
Small or thin structures, such as the epidermis (skin surface), have a much shorter TRT (they cool quickly).
Targeting the Follicle while Sparing the Skin
To destroy the follicle without burning the skin, the laser pulse width must be less than or equal to the TRT of the hair follicle.
However, it must also be longer than the TRT of the epidermis.
A pulse width of 10 to 20 ms allows the skin to dissipate heat during the laser pulse, preventing collateral thermal damage. Meanwhile, the heat accumulates within the follicle, which cannot cool down fast enough, leading to its destruction.
Why This Matters for Hirsutism
Hirsutism presents a unique clinical challenge because it involves the transformation of fine vellus hair into coarse terminal hair due to excessive androgen secretion.
Addressing Coarse Terminal Hair
Terminal hairs are thicker and deeper than normal facial hair.
Because these hairs are larger, they require significant energy accumulation to reach the temperature necessary to destroy the hair matrix.
A shorter pulse width might heat the hair too aggressively or fail to penetrate deeply enough, while a pulse that is too long (beyond the hair's TRT) allows the follicle to cool down before it is destroyed.
Improving Safety Profile
Treating the face requires a high safety margin to avoid scarring or pigmentation changes.
Longer pulse widths significantly reduce the risk of epidermal injury.
This is particularly relevant if the patient has a darker skin tone (higher Fitzpatrick types), as the melanin in the skin competes for laser energy. The longer pulse provides a "cooling window" for the skin that protects it from the high energy needed to treat deep follicles.
Understanding the Trade-offs
While a 10–20 ms pulse width is effective for hirsutism, clinical judgment is required to avoid under-treatment or adverse effects.
The Risk of Pulse Widths That Are Too Long
If the pulse width exceeds the TRT of the hair follicle significantly (e.g., much longer than 20-30 ms for medium hair), the heat dissipates from the hair shaft into the surrounding tissue before lethal damage occurs.
This results in suboptimal treatment, where the hair is heated but not destroyed.
The Risk of Pulse Widths That Are Too Short
Conversely, using a very short pulse width (e.g., <5 ms) on coarse hair delivers energy too rapidly.
This can cause immediate vaporization of the hair shaft or "exploding" of the hair, which may damage the epidermis and cause pain without effectively destroying the germinative cells at the root.
Making the Right Choice for Your Goal
When configuring your laser device for a patient with hirsutism, consider the specific characteristics of the hair and skin.
- If your primary focus is Coarse, Terminal Hair: Select a pulse width toward the 10–20 ms range to match the longer TRT of these thicker follicles, ensuring deep thermal damage.
- If your primary focus is Darker Skin Types: Lean toward the longer end of the pulse width spectrum (20 ms or higher) to maximize epidermal cooling and minimize the risk of hyperpigmentation.
- If your primary focus is Fine or Residual Hair: You may need to shorten the pulse width slightly (<10 ms), as thinner hairs cool faster and require a more rapid delivery of energy to be destroyed.
By precisely tuning the pulse width to the physical dimensions of the follicle, you transform a standard procedure into a targeted, medical-grade treatment that balances aggressive hair reduction with superior skin safety.
Summary Table:
| Feature | Targeted Hair Follicle (Terminal) | Epidermis (Skin Surface) |
|---|---|---|
| Relative Size | Large / Coarse | Small / Thin |
| Thermal Relaxation Time (TRT) | Long (Cools slowly) | Short (Cools quickly) |
| Effect of 10-20ms Pulse | Heat accumulates to destroy matrix | Heat dissipates during pulse |
| Clinical Result | Effective permanent reduction | Minimized risk of burns/scars |
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References
- Elena Zappia, Luigi Bennardo. Alexandrite and Nd:YAG Laser vs. IPL in the Management of Facial Hirsutism: A Retrospective Study. DOI: 10.3390/photonics10050572
This article is also based on technical information from Belislaser Knowledge Base .
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