Effects of Sustained Release of RANKL on Rate of Orthodontic Tooth Movement

Objectives: Orthodontic tooth movement (OTM) typically requires 18-36 months of treatment. Current methods to accelerate treatment are either ineffective or only have transient effects. OTM is a process dependent on bone modeling, and RANKL is a key factor in osteoclast activation. We hypothesize a sustained and optimal dose of exogenous RANKL through injectable localized delivery will increase osteoclast formation and increase OTM without detrimental effects.
Methods: Polylactic-acid-co-glycolic acid (PLGA) is used to prepare 250-425µm-sized porous microparticles (MP) using double emulsion technique. 40µg/mL RANKL is adsorbed on the MPs at 37°C for 1 hour, then embedded in 10% hydroxyethylcellulose (HEC) gel at a 1:3 ratio. MPs are characterized for morphology and release behavior in PBS (pH = 7.4). RANKL activity is tested on mice osteoclast precursor cells (RAW 264.7) using tartrate-resistant-acid-phosphatase staining. The effects on OTM is determined using 18-week old male Wistar rats for 14 days. In the experimental group, the formulation is injected in an osteoperforation palatal to the left maxillary first molar, after which OTM is accomplished with a NiTi closed coil spring applying 10 g of force. Control groups will involve only the application of NiTi spring, with and without placebo formulation. The distance of tooth movement determined with microCT is the outcome measure.

Results: The RANKL formulation shows a 20% burst release followed by linear release over 30 days. The formulation maintains bioactivity when tested on RAW 264.7 osteoclast precursor cells, increasing the number of TRAP-positive osteoclast cells in vitro. In vivo experiments show the formulation increases OTM in comparison with control, and the effects of increased bone modeling is localized to the region of tooth movement.
Conclusions: An exogenous source of RANKL supplied by an injectable PLGA formulation is effective at increasing the number of osteoclast cells in vitro and increasing OTM in vivo.