The study sample was divided into four treatment groups: filler, filler after 30 seconds of RF treatment, filler after 60 seconds of RF treatment, and filler after 30 seconds of needle tunneling without RF. All RF treatments were performed by using the INNOfill device (Pacific Pharma) at 23 W (level 9 at 1 MHz) with an insulation-coated 27-gauge needle. The needle was inserted along the back of the rabbits, and RF was administered to all treatment areas while the needle was rotated 360 degrees. This procedure was repeated 10 to 20 times for 30 to 60 seconds. After delivering the RF energy, prefilled 1-ml syringes of filler were connected to the end of the needle electrode. Next, a 0.1 ml mixture of filler (HA 23 mg/ml, Glytone 3; Pierre Fabre, Castres, France) and dye (fast green FCF; Sigma-Aldrich) was intradermally injected into the shaved backs of the animals. At 7 weeks, the rabbits were killed to assess the changes in the tissue structure secondary to the filler and/or RF treatments. Specifically, the tissue was stained with hematoxylin-eosin (H&E), Masson’s trichrome (collagen), and Verhoeff-Van Gieson (elastic fibers) to assess the presence of any remaining filler, the overall collagen response, and the presence of elastic fibers in the dermis.