Mechanisms of Cytokine Modulation and Inflammatory Cascade in Bio-stimulatory Skin Rejuvenation: A Comprehensive Literature Review of Poly-L-Lactic Acid, Calcium Hydroxyapatite, Polycaprolactone, and Poly-D, L-Lactic Acid

Biostimulatory fillers, such as poly-L-lactic acid (PLLA), poly-D,L-lactic acid (PDLLA), polycaprolactone (PCL), and calcium hydroxyapatite (CaHA), have transformed aesthetics by promoting neocollagenesis through regulated inflammatory and cytokine-driven cascades. Their clinical effectiveness is well-established, but there is a lack of a synthesis of their mechanisms of action. The proposed comprehensive narrative literature review synthesized the contemporary evidence on the cytokine-modulating mechanisms and pathways of inflammatory responses induced by PLLA, PDLLA, PCL, and CaHA. The study design is a narrative literature review, which is performed using a systematic approach based on PRISMA 2020 guidelines. As a comprehensive literature review incorporating heterogeneous and predominantly non-comparative study designs, a formal risk-of-bias assessment using standardized tools was not performed; however, study limitations were considered qualitatively. It matched the identified mechanistic processes with clinical outcomes and safety profiles. The review included 22 clinical studies published from 1st January 2010 to 28th February 2026, comprising randomized controlled trials, cohort studies, and quasi-experimental designs. Extracted data were systematically categorized into six predefined domains: acute inflammatory profile, macrophage polarization, fibroblast extracellular matrix (ECM) response, angiogenesis, clinical outcomes, and adverse events based on thematic analysis of study endpoints. All four biostimulators elicit a controlled inflammatory response characterized by macrophage infiltration and, in some cases, foreign-body giant cell formation. PLLA and PDLLA promote a transition from pro-inflammatory to pro-regenerative M2 macrophage phenotypes, driving sustained upregulation of Type I and III collagen, elastin, and angiogenesis. PCL induces durable neocollagenesis lasting up to 24 months, supported by neovascularization. CaHA stimulates fibroblast activity and ECM production, with a more pronounced early inflammatory gene signature than PLLA. Clinically, all biostimulators achieve significant and sustained improvements in wrinkle severity, skin quality, and volume restoration, with high patient satisfaction rates. Adverse events were predominantly mild and transient, with nodule formation being most notable with PLLA (4.7-28.6%). The study concluded that PLLA, PDLLA, PCL, and CaHA each elicit distinct yet overlapping inflammatory and cytokine-mediated cascades that culminate in robust neocollagenesis and tissue remodeling. Understanding these mechanistic differences enables clinicians to tailor treatment selection and protocols to achieve optimal, durable aesthetic outcomes with favorable safety profiles