Cementoblastic Response to High vs. Low Level of Mechanical Force in Vitro

One of the possible complications of orthodontic treatment is apical root resorption. During orthodontic treatment, as the teeth are being moved, the alveolar bone is continually being modeled to accommodate the teeth. This process activates specific cells that are responsible for bone resorption and can have the unwanted effect of resorbing the apex of the tooth root adjacent to the bone. It is unclear exactly what aspects of orthodontic treatment may trigger the resorptive process. A positive correlation, however, between root resorption and mechanical loading applied during orthodontic tooth movement has been established implicating orthodontic treatment in this adverse effect. Since cementum is the mineralized tissue covering the tooth root, it is poised to play a role in this process. Cementoblasts, sharing many characteristics of osteoblasts, have been shown to express various bone regulatory proteins such as osteopontin (OPN), receptor activator of NFκB ligand (RANKL), cyclooxygenase-2 (COX-2) and sclerostin (SOST). How the expression of these proteins varies in response to mechanical loading is unclear. As cementum has been shown to have reparative properties, it is uncertain whether a certain level of mechanical loading may have a resorptive or antiresorptive effect. Can a low level of force provide a protective effect on the tooth root, while a higher force level precipitate resorption to occur? To study the role of cementoblasts in external apical root resorption, we examined changes in ATP release and protein production of molecular bone biomarkers in OCCM-30 cells (murine cementoblastic cell line) following application of mechanical loading by fluid shear stress (FSS) for one hour at two different force levels (12 dynes/cm2, 18.5dynes/cm2). FSS is an in vitro model for applying a mechanical load to cells. We found a significant increase in ATP release following FSS at both levels and a significant decrease of RANKL and OPN protein at 12 dynes/cm2. RANKL promotes the differentiation, activation and survival of osteoclasts, while OPN serves to attach osteoclast cells to bone or the root surface to begin resorption. Our findings suggest that cementoblasts play an active role in the mechanical adaptation of cementum in the process of orthodontic root resorption