# Clinical Applications of Low-Intensity Pulsed Ultrasound and Its Underlying Mechanisms in Dentistry
## 6 LIPUS Repairs the Dentin-Pulp Complex via Stimulating Tertiary Dentin Formation  
The formation of a dental-pulp complex is always based on severe caries stimulation and the complex plays a reproducible, protective, supportive, and nutritive role [118]. Dental can be divided into primary dentin, physiologic secondary dentin, and tertiary dentin. Especially, tertiary dentin is formed in the corresponding position within the dental pulp cavity, which is a defense response to external injury stimuli [119]. Therefore, the formation of tertiary dentin has great significance in the repair of the dentin-pulp complex after injury. LIPUS was shown to promote tertiary dentin formation and then protect the dental pulp.  
A mouse model of dentin-pulp complex injury was constructed by creating a carious cavity in the maxillary first molar of mice. The use of LIPUS made the tertiary dentin accumulate continuously, and the inflammation of dental pulp tissue decreased [120]. The regulatory mechanism of LIPUS on the dentin-pulp complex was reported. TGF-\(\beta\)1 can stimulate odontoblast differentiation of dental pulp cells, and Smad is involved in its signal transduction [121]. The expression of TGF-\(\beta\)1 and Smad increased after dentin injury in the maxillary first molar of mice. However, through the application of LIPUS, the expressions became higher, which suggests that LIPUS may promote dentin-pulp complex repair by regulating the TGF-\(\beta\)1/Smad signal transduction pathway [122]. In addition, another study found that LIPUS can enhance the expression of calcium ion transport-related proteins (Cav1.2, NCX1, and TRPV1) and stimulate tertiary dentin formation [123]. In the tooth slice organ culture experience, the number of odontoblasts increased after irradiation with LIPUS, also indicating the feasibility of repairing the dentin-pulp complex with LIPUS [124; 125].  
The dentin-pulp complex can be influenced by orthodontic treatment. During orthodontic treatment, the pulp cavity may decrease and the organization structure of pulp may change [126]. The dental pulp was able to react to orthodontic forces. After the orthodontic force invaded, the cell layer of the pulp was damaged, and the blood vessels of the pulp were congested and dilated. More seriously, the pulp was infiltrated by a large number of chronic inflammatory cells, leading to inflammation [127]. On one hand, LIPUS plays a significant role in the repair of dentin-pulp complex injury. On the other hand,LIPUS can shorten orthodontic treatment time and then reduce the invasion of orthodontic force. Therefore, LIPUS may have a protective effect in dental pulp injury induced by orthodontic force and may be a potential application tool to mitigate dental pulp injury.  
In fact, there are limited studies on the application of LIPUS in the dentin-pulp complex. LIPUS may also be harmful to roots because the increase of tertiary dentin may make the root canal narrow. In addition, current studies focus on animals or in vitro, so more studies based on the human body are necessary to determine the true significance of LIPUS to the dentin-pulp complex.