Exploring the Immunological Implications of Insoluble Particulate Impurities in Hyaluronic Acid Fillers: Integrated Physicochemical and Histological Analyses of the Cohesive Polydensified Matrix Range

Background: Emerging research has identified insoluble particulate impurities in hyaluronic acid (HA) fillers as a potential factor in modulating inflammatory responses to HA fillers. This integrated study aims to characterize insoluble particles and assess the tissue integration and inflammatory potential of the cohesive polydensified matrix hyaluronic acid (CPM-HA) range of fillers. Materials and Methods: Samples were prepared by degrading five CPM-HA fillers of varying concentration and rheology using hyaluronidase. An insoluble particle counter was employed to quantify the number of particles of size ≥10 µm and ≥25 µm present. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were used to characterize insoluble particles. In vivo evaluation of CPM-HA fillers containing 20 mg/mL HA (CPM-HA S and CPM-HA R) in two healthy volunteers was performed at 1-week and 4-weeks after intradermal injection, via punch biopsy and histological assessments. Results: The absolute number of insoluble particles in the CPM-HA fillers were low and beneath established pharmacopeial thresholds for injectables. SEM/EDS analysis determined the presence of expected manufacturing-related constituents including sodium (Na), chloride (Cl) and sulfur (S), while confirming the absence of immunologically relevant particulate matter such as silicon (Si), iron (Fe) and aluminum (Al). Hematoxylin and eosin staining of biopsy specimens revealed good tissue integration and no significant inflammatory response following injection of CPM-HA S and CPM-HA R. Conclusion: CPM-HA fillers exhibited high purity with low levels of insoluble particulate matter and no potential immunologic adjuvants, with preliminary in vivo analyses indicating an absence of inflammatory responses.