Overview of Picosecond Laser & Indications, Parameter Selection and Contraindications

Overview of Picosecond Laser

Picosecond laser refers to a laser with an output pulse width at the picosecond level, mainly including 3 working wavelengths of 755nm, 532nm and 1064nm. Due to its extremely short pulse width, picosecond laser can achieve extremely high peak power in an instant, thereby producing photoacoustic effect (or photomechanical effect) on the target chromophore, crushing tattoo dye particles or melanin particles into smaller particles, making them easier to be cleared by pigment-loving cells such as macrophages, and the inflammatory response is lighter. Therefore, picosecond laser is more effective than Q-switched laser in the treatment of tattoos and most pigment-promoting diseases, and has lighter adverse reactions. In addition, through honeycomb focusing lens or holographic diffraction lens, picosecond laser can be focused into uniformly spaced dot beams. Since each micro beam has extremely high peak power, when it exceeds the photodecomposition threshold of the target color base (melanin or hemoglobin), the target color base absorbs energy nonlinearly to produce photodecomposition effect, forming plasma. The plasma continues to absorb laser energy efficiently and expands continuously, and finally produces a burst phenomenon in the epidermis or dermis, leading to the formation of cavitation bubbles, which is laser-induced optical breakdown (LIOB). There is no damage to the tissues around LIOB, and the inflammatory reaction is also very mild. With the occurrence of LIOB, new collagen and elastic fibers may appear in the dermis. The LIOB effect enables the dot-matrix picosecond laser to improve photoaging and acne pitting scars. Compared with ablative dot-matrix laser and near-infrared non-ablative dot-matrix laser, it has fewer adverse reactions and almost no downtime.

Indications, parameter selection and contraindications & Pigmented diseases

Picosecond laser can be used to treat a variety of epidermal and dermal pigmented diseases. It has an ideal effect on diseases such as freckles, solar lentigo, and nevus of Ota; it is effective for diseases such as coffee spots and freckle-like nevus, but there is still a certain recurrence rate; the effect on pigmented hairy epidermal nevus and Reil's melanosis is still uncertain, and it can be used as one of the improvement methods.

Epidermal pigmented diseases such as freckles, solar lentigo, and coffee spots can be treated with 532nm Nd:YAG picosecond laser or 755nm alexandrite picosecond laser. The treatment parameters should be set according to the patient's skin type and skin lesion color, and generally mild white frost reaction is used as the treatment endpoint.

In the treatment of freckles and solar lentigo, patients with Fitzpatrick III-IV skin types, skin lesions with obvious contrast and clear boundaries with normal skin have better effects; while patients with lighter skin lesions and concurrent chloasma have a higher chance of post-inflammatory pigmentation than the former. For the treatment of coffee spots, freckles, etc., some patients can have obvious curative effects, but the recurrence rate is high.

For epidermal pigmentation diseases such as pigmented epidermal nevus and Reil's melanosis, 532nm Nd:YAG picosecond laser, 755nm alexandrite picosecond laser, and 1064nm Nd:YAG picosecond laser can be used as one of the treatment methods, but the efficacy is still uncertain.

For dermal pigmentation diseases such as nevus of Ota and acquired nevus of Ota-like spots, 755nm alexandrite picosecond laser and 1064nm Nd:YAG picosecond laser can be used for treatment. The treatment interval is generally 3 to 6 months, and the treatment effect is ideal. Generally, recovery can be achieved after several treatments. The number of treatments required for the recovery of nevus of Ota in children is generally less than that of adults. In recent years, studies have confirmed that the alexandrite picosecond laser has a higher skin lesion clearance rate than the single treatment of Q-switched alexandrite nanosecond laser for the treatment of nevus of Ota and acquired nevus of Ota-like spots, and the number of treatments required to achieve recovery is less. The incidence of post-inflammatory pigmentation after treatment of acquired Ota nevus-like spots is high, and the treatment interval can be extended accordingly. For patchy blue nevus, some patients are effectively treated, but the number of treatments is significantly increased. Occasionally, studies have reported that blue nevus can become malignant, and during treatment, attention should be paid to whether the skin lesions increase or ulcerate in a short period of time.