Enhancing the Efficacy of Poly‐ l ‐Lactic Acid Injections With Micro‐Focused Ultrasound: An Evaluation of Combined Treatment and Optimal Sequence

INTRODUCTION: Poly-l-lactic acid (PLLA) and micro-focused ultrasound (MFU) are widely used for skin rejuvenation, each promoting collagen remodeling through distinct mechanisms. While combined use is common in practice, the optimal treatment sequence and safety profile remain unclear. This study aimed to evaluate the synergistic effects and ideal sequencing of PLLA and MFU in a preclinical model to guide clinical protocols. METHODS: Three Bama miniature pigs were subjected to six treatment conditions on demarcated abdominal skin zones: (a) PLLA alone, (b) MFU alone, (c) MFU immediately before PLLA, (d) MFU 1 month before PLLA, (e) MFU 1 month after PLLA, and (f) untreated control. Löviselle PLLA was injected subcutaneously, while MFU was administered using the MFUS Pro system. Skin biopsies were collected at day 180. Histological (Masson's trichrome, Sirius Red, Verhoeff-Van Gieson, H&E), SEM, and quantitative image analyses were performed to assess dermal thickness, collagen subtype distribution, elastin regeneration, fat septa organization, and PLLA microsphere stability. RESULTS: PLLA alone increased dermal thickness by 23.7% and raised the type III/I collagen ratio to 0.79 ± 0.06 (p < 0.01). MFU alone mainly enhanced type I collagen (ratio: 0.39 ± 0.04) and improved fiber alignment. The combined treatment with MFU immediately before PLLA achieved the best outcomes: dermal thickness increased by 35.2%, and the type III/I collagen ratio reached 0.92 ± 0.05 (both p < 0.01). This group also showed denser elastin fibers, well-structured fat septa, and the most compact collagen morphology under SEM. MFU did not accelerate PLLA degradation or increase inflammation, with no significant difference in microsphere size or inflammatory infiltration compared to PLLA alone (p > 0.05). CONCLUSION: Sequential application of MFU immediately before PLLA enhances dermal remodeling more effectively than either treatment alone, without compromising PLLA microsphere stability. These findings support the clinical potential and safety of this combinatory approach, offering evidence-based guidance for optimizing skin rejuvenation protocols.