Experimental study on the root fractures of immature rat molars

Methods: Upper first molars of 4-week-old rats were traumatically dislocated toward the palate with a Hoe's pliers at 900gw, and then repositioned immediately. 1 or 2 weeks after injury, the maxillae were resected out from the animals and scanned with micro CT. And then, bucco-lingual serial sections of the maxilla were prepared for the routine histological examination. We examined the root fractures identified in both 3D-CT images and photographs of the histological sections.

Results: 1-week after injury, newly differentiating odontoblasts as well as reparative dentin containing degenerated odontoblasts appeared at the pulpal side of the fracture line in the most cases. At the periodontal side of the fracture line, granulation tissue formation as well as external root resorption were induced in the cases with wide crack. But, cracks remained empty to form dead space in the narrow cases. The periodontal fibers tore at the vicinity of the cracks. 2-weeks after injury, reparative dentin and dentin-like tissue bridged the pulpal side of the fracture line. And, external resorption lacunae as well as cracks were partially covered with cementum at the periodontal side of the fracture line.

Conclusions: Histological data indicated that pulpal side of the fracture line might be covered with immature odontoblasts and reparative dentin in 1 week after injury, and then bridged with differentiated odontoblasts and mature reparative dentin in 2 weeks after injury. The external root resorption lacunae and cracks could be covered with cementum after 2 weeks.