The governing principle behind the 35 to 65 ms pulse width setting is the Theory of Thermokinetic Selectivity. This concept dictates that a longer pulse duration allows heat to dissipate rapidly from smaller structures, such as epidermal melanin, before damage occurs. Conversely, it forces heat to effectively accumulate in larger target structures, specifically the hair follicles, ensuring their destruction without burning the surrounding skin.
Core Insight: This specific pulse width range leverages the physical size difference between skin pigment and hair follicles. It ensures the laser delivers energy slowly enough for the skin to cool down, yet fast enough to irreversibly damage the hair follicle, making it a critical safety standard for treating darker skin types and coarse hair.
The Mechanics of Thermokinetic Selectivity
Differentiating by Structure Size
The Theory of Thermokinetic Selectivity relies on the physical differences between the target (hair follicle) and the non-target (epidermis).
Epidermal structures, specifically melanin granules, are physically small. Because of their small volume, they lose heat very quickly.
Hair follicles, particularly coarse ones, are physically much larger. They retain heat for a significantly longer period.
The Heat Accumulation Dynamic
When the laser is set to a long pulse width (35 to 65 ms), the energy delivery is extended over time.
During this extended delivery, the small epidermal structures have sufficient time to release the heat they absorb into the surrounding tissue. This prevents the epidermis from reaching a temperature that causes burns or hyperpigmentation.
However, the larger hair follicle cannot dissipate heat as fast as the laser delivers it. The thermal energy accumulates within the follicle, eventually reaching the threshold required for permanent destruction.
Matching Thermal Relaxation Time (TRT)
This principle is practically applied by matching the pulse width to the Thermal Relaxation Time (TRT) of the follicle.
The 35-65 ms range roughly corresponds to the TRT of coarse hair follicles.
By matching this time, the laser optimizes the "time-dispersion effect," ensuring the energy remains localized in the follicle germinal center rather than diffusing outward to the skin surface.
Why This Matters for Nd:YAG Lasers
Protecting Darker Skin Tones
Nd:YAG lasers are frequently chosen for patients with darker skin tones (higher epidermal melanin).
The 35-65 ms setting is protective. It ensures that the abundant melanin in the epidermis does not overheat, as the heat dissipates during the long pulse.
Targeting Coarse Hair
This pulse width is specifically effective for thick, coarse hair.
Thicker hair requires more time for thermal energy to conduct from the hair shaft to the entire follicle structure.
A longer pulse width provides the necessary time for this conduction to occur, ensuring the entire reproductive structure of the hair is treated.
Understanding the Trade-offs
The Risk of Pulse Width Mismatch
While the 35-65 ms range is optimal for safety, deviating from it introduces specific risks.
Consequences of Pulse Widths That Are Too Short
If the pulse width is significantly shorter than the follicle's TRT (e.g., 3 ms), the energy is delivered too aggressively.
While this destroys the hair, it does not allow the epidermis time to cool. In darker skin types, this dramatically increases the risk of burns, blistering, and side effects like perifollicular edema.
Consequences of Pulse Widths That Are Too Long
If the pulse width is extended too far beyond the 65 ms range for finer hair, the follicle may lose heat as fast as it receives it.
This results in sub-lethal heating. The hair follicle is warmed but not destroyed, leading to ineffective treatment and hair regrowth.
Making the Right Choice for Your Goal
The correct application of Thermokinetic Selectivity requires balancing patient safety with clinical efficacy.
- If your primary focus is safety on dark skin: Select a pulse width at the higher end of the range (closer to 65 ms) to maximize epidermal cooling and minimize the risk of hyperpigmentation.
- If your primary focus is treating resistant coarse hair: Ensure the pulse width is sufficient (at least 35-40 ms) to allow full thermal conduction throughout the large follicle structure without dissipating too early.
Ultimately, the 35-65 ms window allows you to aggressively treat the hair while passively protecting the skin.
Summary Table:
| Setting Parameter | Pulse Width (35-65 ms) | Short Pulse (<10 ms) | Long Pulse (>80 ms) |
|---|---|---|---|
| Mechanism | Thermokinetic Selectivity | Rapid Energy Delivery | Sub-lethal Heating |
| Skin Safety | High (Epidermal cooling) | Low (Risk of burns/hyperpigmentation) | High |
| Efficacy | Optimal for coarse hair | High (but risky for skin) | Low (Poor follicle destruction) |
| Target Patient | Darker skin tones (Fitzpatrick IV-VI) | Light skin, fine hair | Not recommended for hair removal |
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References
- Semra Akinturk, Ahmet Eroğlu. Effect of piroxicam gel for pain control and inflammation in Nd:YAG 1064‐nm laser hair removal. DOI: 10.1111/j.1468-3083.2006.01979.x
This article is also based on technical information from Belislaser Knowledge Base .
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