Imaging 1927 nm Fractional Thulium Laser‐Tissue Interactions: A Spectrum of Nonablative to Ablative Effects

Infrared fractional lasers that target tissue water are traditionally divided into nonablative and ablative devices. The 1927 nm fractional thulium fiber laser (FTL) may, due to an intermediate water absorption coefficient, offer a range of nonablative-to-ablative effects. This study explored dynamic 1927 nm FTL-tissue interactions produced by different pulse energies in in vivo human skin, using non-invasive optical coherence tomography (OCT) and line-field confocal OCT (LC-OCT) imaging. FTL exposure was performed on in vivo antero-lateral forearm skin at 0, 3, 15, or 20 mJ pulse energy using two separate laser tips (spot size: 200 μm (C1); 350 μm (C5)) at 20-watt power. Immediately after, LC-OCT and OCT imaging enabled qualitative description of microthermal treatment zone (MTZ) morphology, as well as semiquantitative measurement of MTZ diameter and ablation depth. Ranging from nonablative subepidermal effects to frank ablation, imaging revealed a variety of MTZ morphologies depending on pulse energy and laser tip. At low 3 mJ pulse energy, effects were generally limited to the epidermis, with MTZs consisting of subepidermal clefts (C5) or disruptions through the viable epidermis (C1) under a residual stratum corneum. Rising 15-20 mJ pulse energies expanded thermal effects in the lateral and vertical plane, leading to wider MTZs (e.g., C1: 3 mJ vs. 20 mJ: 213 vs. 357 μm), more extensive stratum corneum involvement, and increasing ablation depth to the superficial dermis (e.g., C1: 3 mJ vs. 20 mJ: 93 vs. 101 μm). Visualized by combined LC-OCT and OCT imaging, FTL-tissue interactions are highly modifiable and span the nonablative to ablative spectrum depending on pulse energy and laser tip.