Stress–relaxation and microscopic dynamics of rabbit periodontal ligament

The aim of the present study was to examine the structural basis for the stress–relaxation behaviour of the periodontal ligament (PDL). Seventeen 4-month-old rabbits were used. A tooth–PDL–bone segment was cut in a rectangular prism from the incisor of a dissected mandible. The specimen was mounted in a testing machine built on a video stereomicroscope. Following preconditioning, each specimen was stretched to a deformation of 35 μm and then the deformation was kept constant for 300 s to obtain a stress–relaxation curve. Thereafter, stress–relaxation tests were repeated sequentially at deformations of 55, 75, and 95 μm. Polarised-light video–stereomicroscopic images of the specimens were simultaneously recorded and analysed with the stress–relaxation curves. The image analysis revealed that during stress–relaxation, the brightness of the birefringent fibres tended to initially increase rapidly and then do so gradually. There were negative correlations between the brightness and relaxation modulus at the four deformations. The decreases of normalised relaxation modulus for 300 s were less at greater deformation levels. The stress–relaxation process was well described by a function with three exponential decay terms and a constant. These findings suggest that during stress–relaxation of the PDL, the alignment of the collagen molecules and fibrils within the stretched fibres may occur, which could be driven by the strain energy imparted to the specimen on initial stretching.