A Novel Mechanism of Ultrasound's Effect on Orthodontic Root Resorption

Objectives: Serious root resorption (RR) occurs in approximately 20% of patients undergoing orthodontic treatment, leading to serious medico-legal issues. Low intensity pulsed ultrasound (LIPUS) has been reported to be effective in enhancing bone growth and fracture healing. Not only in orthopedics LIPUS was also reported to be able to decrease the number of resorption lacunae on root surface during orthodontic tooth movement. However, the mechanism of LIPUS's effects on RR is unknown. We hypothesize that LIPUS prevents RR through inducing an anabolic response of cementoblasts. To testify this, we subjected OCCM.30 cementoblasts (in comparison to MLO-Y4 osteocytes) to LIPUS (30mW/cm2, 1.5 MHz frequency, 1 kHz repetition).

Methods: Cells were seeded on 35mm Petri dishes and grown till 90% confluent. Starved for 24 hours, the cells were subjected to LIPUS for 10 minutes to test signaling pathway or 20 minutes followed by 6 hours post-incubation to examine functional changes, using static cells as control.

Results: In response to LIPUS, ERK1/2 activation, COX-2 production was increased in both cell types. SOST was increased in MLO-Y4 but decreased in OCCM.30 cells. Remarkably, OPG/RANKL ratio was slightly increased in OCCM.30 but dramatically decreased in MLO-Y4 cells.

Conclusions: LIPUS's prevention of RR is through not only promoting cementogenesis but more likely increasing osteoclastic alveolar bone resorption which indirectly alleviates the compression-induced hyalinization in periodontal ligament – a pathological basis for the formation of RR.