Hyaluronic acid: in vitro and in vivo analysis, biochemical properties and histological and morphological evaluation of injected filler.

Background: No human model has emerged as an accepted standard to evaluate tissue filler longevity. Objectives: To validate a human model adequate to compare soft tissue filler degradation and tissue reaction. Materials and methods: We evaluated in 18 patients the persistence of hyaluronic acid (HA) filler injected into labial tissue analyzing hyaluronidase (HYAL) activity by means of in vitro and in vivo tests, MRI and histological and ultra-structural examination at 3 and 6 months postop. Results: MRI examination revealed the presence of HA filler in a clear hyperintense area. Histology demonstrated fibroblast activation. The amount and the degradation rate of HYAL and HA did not show a linear correlation. Conclusion: MRI demonstrated the presence of HA in lip tissue even after 6 months. Biopsies at 3 months revealed tissue maturation and at 6 months confirmed the ability of HA to reorganize and integrate the extracellular matrix. The absence of linear correlation between HYAL and HA revealed that the result clinically is probably dependent on systemic factors which can determine HYAL activity and therefore HA longevity.