A constitutive model for the periodontal ligament as a compressible transversely isotropic visco-hyperelastic tissue

This study is devoted to the development of a non-linear anisotropic model for the human periodontal ligament (PDL). A thorough knowledge of the behaviour of the PDL is vital in understanding the mechanics of orthodontic tooth mobility, soft tissue response and proposed treatment plans. There is considerable evidence that the deformation of the PDL is the key factor determining the orthodontic tooth movement. The paper focuses on the biomechanical aspect of the behaviour of the PDL. In terms of continuous mechanics, the PDL may be treated as an anisotropic poro-visco-hyperelastic fibre-reinforced compressible material which is subject to large deformations and has an essentially non-linear behaviour. Furthermore, there are issues related to the non-linear tooth and PDL geometry. A new constitutive model for the PDL is proposed. The macroscopic continuum approach is used. The model is based on the non-linear large deformation theory, involving the Lagrangian description. The material is assumed to be compressible, visco-hyperelastic and transversely isotropic. A free-energy function is suggested that incorporates the properties. It also takes into account that the PDL behaves differently in tension and compression. The free-energy function and the associated constitutive equations involve several material parameters, which are to be evaluated from experimental strain-stress data available from the literature and the tooth movement experiments conducted by our team using novel optical motion analysis techniques.