Investigation of optimal orthodontic force at the cellular level through three-dimensionally cultured periodontal ligament cells.

Abstract

Optimal orthodontic force (OOF), important as it is, has generally been addressed at the level of tissue response. This study, for the first time, aimed to investigate its underlying mechanisms at the cellular level. Human periodontal ligament tissue cells (PDLCs) were three-dimensionally (3D) cultured in a thin sheet of poly-lactic-co-glycolic acid (PLGA) scaffold. The 3D cultured PDLCs were treated with static compressive force of 0, 5, 15, and 25g/cm(2) for 6, 24, and 72 hours, respectively. After that, methylthiazolyl tetracolium assay was done to evaluate the cell proliferation. The target gene expression in PDLCs was investigated through real-time PCR analysis. The conditioned media was collected for enzyme-linked immunosorbent assay (ELISA) assay, and also used for the coculture of osteoblasts and osteoclast precursors. Tartrate-resistant acid phosphatase (TRAP) staining was employed to examine osteoclasts. Compressive force inhibited proliferation of PDLCs in a magnitude-dependent manner. Heavier force upregulated expression of the osteoclastogenesis inducers, including RANKL, COX-2, PTHrP, and IL-11, more rapidly; however in the long run, no significant difference was found among different force magnitudes, either in the expression of osteoclastogenesis inducers by PDLCs, or in the osteoclast formation detected by TRAP staining. The results regarding specific force magnitude as OOF should be confined to the present specific model, but not be extrapolated, without caution, to different in vitro models, nor even to in vivo studies or clinical application. Compared with heavier force, lighter force has similar pro-osteoclastogenic whilst less anti-proliferative effects on PDLCs, which provides a novel interpretation for OOF.