Biomechanical behaviour of the periodontal ligament of the beagle dog during the first 5 hours of orthodontic force application.

The aim of this study was to describe the mechanical behaviour of the periodontal ligament (PDL) in response to loading with different forces for a period of 5 hours. Seven young adult male beagle dogs (age 1.0-1.5 years) were used. After extractions and placement of implants, custom-made appliances on both sides of the mandible were used to measure the displacement of the second premolars. Tooth displacement was measured during 5 hours of force application. Each dog underwent two measurement sessions. One premolar was moved with a force of 100 cN in the first session and with 50 cN in the second. The contralateral premolar was moved with forces of 100 and 300 cN, respectively. Time-displacement curves showed a rapid instantaneous response lasting only a few seconds followed by a slowly decreasing creep displacement. The instantaneous response demonstrated a large individual variability, caused by both a dog and a force effect. Differences in tooth and PDL anatomy and in the orientation of the periodontal fibres are probably important in this respect. The individual variability faded after the first seconds of tooth displacement, when the viscoelastic properties of the periodontal fibres became more pronounced. The force effect was non-linear for the first minute. Higher forces did not lead to proportionally larger displacements. The non-linearity decreased in the second response. The PDL is a complex material that might be considered as a non-linear fibre-reinforced poroviscoelastic material.