Computational modeling and histologic analysis of 6.78- and 2-MHz monopolar radiofrequency-induced thermal reactions

Monopolar radiofrequency (RF) devices are widely used for skin tightening, wrinkle reduction, and body contouring. However, frequency-dependent differences in energy absorption and tissue remodeling remain insufficiently characterized. This study aimed to compare the thermal distribution and histological responses induced by 6.78‑ and 2‑MHz monopolar RF, as well as their sequential combination. Finite-element computational modeling was used to simulate electric field propagation and heat diffusion in multilayered skin with varying subcutaneous fat thicknesses and fibrous septa configurations. In vivo experiments were conducted on porcine skin treated with 6.78‑, 2‑, and dual‑frequency RF modes. Histologic changes were evaluated using hematoxylin and eosin, Masson's trichrome, and Verhoeff-van Gieson staining. Apoptotic cell death was assessed using the terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay to evaluate adipocyte viability. Computational modeling demonstrated that 2‑MHz RF produced broader and deeper thermal effects within the adipose tissue, whereas 6.78‑MHz RF generated more localized heating along the fibrous septa. Dual-frequency RF combines these effects, creating pronounced thermal reactions at the dermosubcutaneous junction. Histological analysis revealed significant collagen and elastin remodeling across the dermis and fibrous septa in dual-frequency-treated specimens, with no evidence of adipocyte apoptosis. Moreover, remodeling changes were more extensive and persistent at later time points, suggesting that dual-frequency treatments have a greater tissue remodeling potential compared with single-frequency applications. Dual-frequency monopolar RF effectively promoted extracellular matrix remodeling in the dermis and subcutis while preserving adipocyte viability, suggesting its use as a safe and versatile modality for skin rejuvenation and contouring.