Evaluation of the histologic effects of injectable hyaluronic acid as a filler material in rat’s lip

Abstract Background: Injectable hyaluronic acid is a type of temporary dermal filler. Hyaluronic acid is found naturally throughout the body, with the highest concentrations in the joints, eyes, and skin. Injectable hyaluronic acid is used to reduce the appearance of fine lines and wrinkles, facial folds, and to create structure, framework and volume to the face and lips. Objectives: The present work aimed to examine the early and late histopathological changes occurring in the lower lips of rats, as a result of hyaluronic acid injection, and to determine whether it could really bring us closer to the ideal soft tissue filler material. This research highlights the importance of experimental studies on this subject, since adverse reactions have been observed routinely in dental practice. Material and methods: The present study was carried out on 36 adult male albino rats with age ranging from 2 to 3 months and weight ranging from 150 to 200g. Rats were divided into two equal groups, each consisting of 18 rats; group I (control group) and group II (experimental group). Rats in the experimental group were injected with hyaluronic acid (0.07 ml), whereas rats in the control group were injected with 0.9% sodium chloride. Each group was further subdivided into 3 subgroups: A, B and C according to the time of sacrifice at one week, one and a half months and three months respectively. Results: The results showed inflammation triggered by the injection of hyaluronic acid. On examining the specimens histologically using Hematoxylin and Eosin stains, Toluidine Blue stain and immunohistochemistry with anti-CD68 antibody, there was a significantly greater inflammatory response in the groups at early interval with hyaluronic acid compared with the control (P<0.0001). Higher collagen formation was also observed with Masson’s Trichrome staining. Conclusion: Based on the histopathological, histochemical and immunohistochemical results of this study, undesirable effects of hyaluronic acid filler injection can be detected at early intervals following its injection. Fortunately, all the undesirable effects are transient and decrease by time, reaching levels comparable to normal. Overall, HA was well-tolerated by tissues, reflecting how this filler material possesses a biocompatible property.