I. Mechanism of Action
The Q-switched (QS) laser operates on the principles of selective photothermolysis and photoacoustic (photomechanical) effect. It delivers high-energy pulses at specific wavelengths (typically in nanoseconds), targeting pigmented particles such as melanin or tattoo ink. The absorbed energy causes rapid heating and fragmentation of the pigment into tiny particles, which are then cleared through the skin’s metabolic processes, phagocytosed by macrophages, or eliminated via the lymphatic and circulatory systems.
At the technical level, Q-switched lasers utilize a technique that modulates the quality factor (Q factor) of the laser resonator. Initially, the Q value is suppressed to allow energy to build up in the gain medium. When the Q is suddenly increased, the stored energy is released as a high-intensity pulse—achieving peak power outputs of over 200 MW—delivered in nanoseconds. This enables precise targeting of pigment with minimal damage to surrounding tissues.
Q-switching is essentially an energy gating mechanism that produces extremely short (5–100 ns) and intense laser pulses, making Q-switched lasers the gold standard for treating pigmented lesions. Picosecond lasers (mode-locked lasers) are a newer advancement in this field, showing great promise in pigment treatment.
Key Principles:
- Pulse Duration: 6–20 ns, shorter than the thermal relaxation time (TRT) of melanosomes (~1 μs), ensuring heat does not spread to surrounding tissue.
- Wavelength Selectivity: 532 nm, 755 nm, 1064 nm – selectively absorbed by melanin and tattoo pigments.
- Photoacoustic Effect: Pigments rapidly heat and shatter, creating intracellular cavitation and gas bubbles—resulting in an immediate whitening response (endpoint reaction) and audible “pop” sounds during treatment.
- Resonator Modulation: Q-switching enables sudden energy discharge, achieving ultra-high peak power in a short pulse.
Wavelength Applications:
- 532 nm – Targets superficial epidermal pigment (freckles, sunspots).
- 1064 nm – Penetrates deep dermis (Ota nevus, deep tattoos).
- 694/755 nm – For mixed-type pigment lesions (e.g., melasma).
II. Clinical Indications
Q-switched lasers are used to treat a wide range of conditions, mainly categorized by pigment depth:
1. Pigmented Lesions
- Epidermal: Freckles, solar lentigines, café-au-lait macules, seborrheic keratoses.
- Dermal: Nevus of Ota, Hori’s nevus, blue nevus, Mongolian spots.
- Mixed-type: Melasma (with caution), post-inflammatory hyperpigmentation (PIH), periorbital/perioral pigmentation, Becker’s nevus.
2. Tattoo & Artificial Pigment Removal
- Amateur and professional tattoos (black, blue, green, etc.), traumatic tattoos, cosmetic tattooing (eyebrows, eyeliner).
3. Skin Rejuvenation
- Fine lines, large pores, acne-induced redness, collagen remodeling, dark eye circles.
⚠️ Melasma: Best treated with adjunct barrier repair and anti-inflammatory regimens. Laser alone may cause relapse.
4. Other Uses
- Benign superficial tumors, seborrheic keratosis, acne redness, photoaging.
Diagnostic Aids:
- Wood’s lamp/UV imaging (e.g., VISIA): To assess pigment depth.
- Dermoscopy: Helps differentiate benign vs malignant or precancerous lesions.
- Biopsy: Advised when diagnosis is uncertain.
III. Contraindications
Absolute Contraindications:
- Suspected/confirmed melanoma or malignant lesions.
- Active infections (herpes simplex, pustular acne, cellulitis).
- Photosensitive conditions (porphyria) or recent use of photosensitizing drugs (retinoids, tetracyclines).
Relative Contraindications:
- Pregnancy or breastfeeding (safety not well established).
- Keloid-prone individuals, open wounds, active dermatoses (psoriasis, eczema).
- Within 6 months of deep resurfacing or radiation therapy.
- Poorly controlled systemic diseases (e.g., diabetes, epilepsy, cardiovascular conditions).
- Coagulation disorders or recent anticoagulant use.
- Recent sun exposure (within 3–4 weeks).
Always conduct a full patient evaluation, screen for contraindications, and obtain informed written consent.
IV. Laser Parameters
Laser settings must be individualized based on the indication and skin type. Below are standard reference guidelines:
| Parameter | Recommended Settings |
|---|---|
| Wavelength | 532 nm (epidermal); 1064 nm (dermal); 694/755 nm (mixed/melasma) |
| Energy Density | 2.5–3.5 J/cm² for melasma (low-energy mode); others based on mild whitening endpoint |
| Spot Size | Large (6–8 mm) to improve penetration and reduce epidermal damage |
| Frequency | 1–1.5 Hz for melasma; 1–10 Hz for general use |
| Pulse Width | 6–20 ns (shorter than pigment TRT) |
Treatment Endpoint:
- Slight redness or mild whitening is ideal.
- Frosted white indicates over-treatment and risk of PIH.
Patient-Specific Tips:
- Darker skin types: Use longer wavelengths (1064 nm), larger spot sizes, and lower fluence.
- Melasma: Use low-fluence, large-spot protocols; avoid inflammation.
- Spot Test: Always perform patch testing, especially on dark skin.
Device-Specific Considerations:
- Energy density must be adjusted based on the machine; manufacturer guidelines vary.
- PTP Mode (Photoacoustic Twin Pulse): Some models offer gentler sub-pulse emission, better suited for melasma.
V. Treatment Protocols
Session Frequency:
- Epidermal lesions: 1–2 sessions may suffice.
- Dermal lesions: Require 6–10+ sessions.
- Amateur tattoos: <6 sessions; professional, multicolored tattoos may need >20 sessions.
Treatment Interval:
- Typically every 6–8 weeks.
- For Ota nevus or Hori’s nevus: 3–6 months interval recommended for optimal clearance.
Laser Technique:
- Use minimal spot overlap (≤10%).
- Keep the handpiece perpendicular to the skin.
- Treat the entire lesion in one pass.
- Cooling (ice packs or air cooling) before/after helps minimize thermal injury.
VI. Post-Treatment Care & Precautions
Immediate Aftercare:
- Cool compress for 30–60 min post-treatment to reduce erythema.
- For large blisters: seek professional care, avoid puncturing at home.
Wound Care:
- Avoid water exposure for 3–7 days.
- Let scabs fall off naturally—no picking.
Sun Protection:
- Mandatory for 3–6 months post-treatment.
- Use SPF 30+ sunscreen and reapply every 2 hours.
- Combine with physical sun barriers (hat, umbrella).
Skincare Adjustments:
- Avoid alcohol-, acid-, or retinoid-based products for 2+ weeks.
- Use medical-grade repair products (e.g., EGF, tranexamic acid, post-laser masks).
Lifestyle:
- Avoid spicy food, alcohol, intense exercise, saunas, and steam rooms.
- Increase intake of vitamin C, E, and collagen-rich foods.
Monitoring & Risks:
- Recheck in 7–10 days.
- Temporary PIH may occur and typically resolves in 3–6 months.
- Vitamin C supplementation may assist pigment fading.
VII. Summary Table: Indications & Parameters
| Pigment Depth | Condition | Wavelength(s) | Sessions | Notes | Recommendations |
|---|---|---|---|---|---|
| Epidermal | Freckles, lentigines | 532 nm, 1064 nm, IPL | 1–3 | Prevent recurrence with sun protection | QS laser preferred |
| Café-au-lait spots | 532 nm (light skin) | Multiple | ~50% recurrence within 1 year | Partial clearance common | |
| Epidermal nevi | 532 nm | Multiple | Effective for junctional/compound nevi | Spot testing advised | |
| DPN, seborrheic keratosis | QS/ablative/long-pulse | 1–3 | |||
| Dermal | Ota nevus, Hori’s nevus, blue nevus | 1064 nm, 755 nm | Multiple | Ota: low recurrence | QS 1064 nm for dark skin |
| Lichen planus pigmentosus | 1064 nm | Avg 5–6 | Treat stable disease only | 4–6 weeks interval | |
| Mixed | Becker’s nevus | QS/long-pulse/IPL | Multiple | Unpredictable outcome | Caution advised |
| Melasma | QS low energy/LED/IPL | Multiple | First-line: medical therapy | Always spot test | |
| PIH, periorbital pigmentation | 532/1064 nm | Case-dependent |
VIII. Key Takeaways
- Q-switched lasers are nanosecond-pulse lasers used primarily for pigment removal.
- Common wavelengths: 532 nm, 694 nm, 755 nm, 1064 nm.
- Gold standard for pigment disorders and tattoo removal.
- Mechanism: photoacoustic (for pigment blasting) and photothermal (for general tone).
- Careful indication selection and exclusion of contraindications are essential.
- Parameters must match skin type and lesion depth; dark skin requires conservative settings.
- PIH risk can be reduced by avoiding overtreatment (endpoint: mild whitening, not frost white).
- Picosecond lasers may offer better clearance with less PIH—if parameters are well-controlled.
- Q-switched lasers allow wider adjustment of fluence compared to picosecond devices.
- Certain models offer PTP modes ideal for sensitive conditions like melasma.
- Sun protection and post-care are vital for successful outcomes.
