Time-dependent mechanical behaviour of the periodontal ligament

The process of tooth displacement in response to orthodontic forces is thou ght to be induced by the stresses and strains in the periodontium. The mech anical force on the tooth is transmitted to the alveolar bone through a lay er of soft connective tissue, the periodontal ligament. Stress and/or strai n distribution in this layer must be derived from mathematical models, such as the finite element method, because it cannot be measured directly in a non-destructive way. The material behaviour of the constituent tissues is r equired as an input for such a model. The purpose of this study was to dete rmine the time-dependent mechanical behaviour of the periodontal ligament d ue to orthodontic loading of a tooth. Therefore, in vivo experiments were p erformed on beagle dogs. The experimental configuration was simulated in a finite element model to estimate the poroelastic material properties for th e periodontal ligament. The experiments showed a two-step response: an inst antaneous displacement of 14.10 +/- 3.21 mum within 4 s and a more gradual (creep) displacement reaching a maximum of 60.00 +/- 9.92 mum after 5 h. Th is response fitted excellently in the finite element model when 21 per cent of the ligament volume was assigned a permeability of 1.0 x 10(-14) m(4)/N s, the remaining 97 per cent was assigned a permeability of 2.5 x 10(-17) m(4)/N s. A tissue elastic modulus of 0.015 +/- 0.001 MPa was estimated. Ou r results indicate that fluid compartments within the periodontal ligament play an important role in the transmission and damping of forces acting on teeth.