Optimal solder and power density for diode laser tissue soldering (LTS)

Laser tissue soldering (LTS) using albumin and indocyanine green dye (ICG) is an effective technique utilized in various reconstructive surgical procedures. The purpose of this study was to describe in vivo and in vitro temperature profiles of an albumin-based solder while varying ICG concentration and laser power density (PD), and to describe immediate and short-term tensile strength measurements and histology of tissue with variable ICG concentrations and PD.

ICG ranged from 0.31 to 20 mg/mL while PD ranged from 3.2 to 63.7 W/cm(2). Direct solder temperature measurements were obtained at 5-second intervals during laser activation. Differential temperature measurements were determined within the dermis of rat skin and the overlying solder. Eighteen rats were subjected to 2.0-cm incisions (n = 113) created on the dorsal skin followed by closure with LTS at varying PD and ICG concentrations. ICG concentrations included 0.31, 2.5, and 20 mg/mL, while PD ranged from 8.0 to 63.7 W/cm(2). Tensile strength (TS) profiles were measured immediately and 10 days post-operatively. Histological examination was performed at the time of sacrifice.

Temperature profiles of the ICG/albumin solder differed significantly only at the highest concentration of ICG (20 mg/mL), but showed statistically significant variability at different laser PD. Using solder color changes as an endpoint of LTS, average peak solder temperature ranged from 69 degrees C at a PD of 8.0 W/cm(2), 105 degrees -120 degrees C at PD 15.9-31.8 W/cm(2), and > 200 degrees C at PD > or = 47.7 W/cm(2). Peak intradermal temperatures remained below 50 degrees C at all PDs. Varying ICG concentration only had an effect on the immediate TS of wounds at the lowest power densities. Increasing PD resulted in statistically significant increases in immediate TS up to a PD of 23.9 W/cm(2) at an ICG concentrations of 0.31 and up to a PD of 15.9 W/cm(2) at a concentration of 2.5 mg/mL. Statistically insignificant decreases in 10-day would strength resulted from higher PD. Power densities > or = 23.9 W/cm(2) showed significant thermal injury upon histologic examination.

Power density, not ICG concentration, is the primary determinant of solder and tissue temperature during LTS. Effective and reproducible laser tissue soldering may be achieved primarily by power density control when using diode laser and ICG-based albumin solder. Alterations in PD show the most direct and predictable effects on the healing properties of skin closed by LTS. Optimal laser wound closure occurs with an ICG of 2.5 mg/mL and at a PD between 15.9 and 23.9 W/cm(2).