Histological Evaluation of Dual-depth Poly-l-Lactic Acid and Hyaluronic Acid Injections

This single-case histological study evaluated the safety and tissue effects of dual-depth injections of poly-L-lactic acid (PLLA) and hyaluronic acid (HA)-based skin booster administered into the same anatomical region. A 77-year-old woman received PLLA in the subcutaneous fat layer and HA in the deep dermis/subdermal layer of the right jowl. The untreated left side served as a control. Histological analysis revealed that PLLA induced a localized foreign-body reaction with increased fibroblast activity, dense collagen, and elastic fiber formation in the subcutaneous layer, indicative of extracellular matrix remodeling. HA-treated dermal layers showed mild matrix compaction and low-grade inflammation, consistent with the hydration effects. There was no histological evidence of filler overlap or adverse interaction. Three-dimensional imaging further supported volumetric enhancement and tissue elevation on the treated side. The results demonstrate that when spatially separated by depth, PLLA and HA can exert independent, synergistic effects without compromising safety. This dual-layer approach may offer a novel strategy for targeted skin rejuvenation in clinical practice. In this single-patient case, layer-separated PLLA and HA elicited independent, compartment-specific tissue responses without observed interaction. Findings are descriptive and anecdotal and should not be generalized without larger, controlled studies incorporating quantitative volumetry and histomorphometry.