Simultaneous recording of stress-relaxation and video-microscopic images of rabbit periodontium

Objective: The mechanism responsible for the viscoelastic response of the periodontal ligament (PDL) has not been well elucidated. The aim of the present study was to examine the structural basis for the stress-relaxation behaviour of the PDL. Methods: Eight 4-month-old rabbits were used. A tooth-PDL-bone segment was cut in a rectangular prism (1.5 mm wide, 0.25 mm thick, and 6 mm long) from the incisor of a dissected mandible. The tooth and bone parts in each specimen were clamped in a testing machine mounted on a video microscope. Each specimen was initially stretched to a deformation level of 30 mm and then the deformation was kept constant for 5 min to obtain a stress-relaxation curve. Thereafter, stress-relaxation tests were repeated sequentially at deformation levels of 50, 70, 90, 110, 130, 150, 170, and 190 mm at 5 min-intervals. Polarized-light video microscopic images of the specimens were simultaneously recorded during the stress-relaxation tests and analysed with the stress-relaxation curves. Results: The applications of initial deformations to the specimens resulted in stretching of the PDL collagen fibres to various degrees. During the stress-relaxation, the stretched collagen fibres did not show discernible changes at the light microscopic level. However, the image analyses revealed that the brightness of the birefringent fibres initially increased rapidly and then did so gradually. The rates of the increases in the brightness seemed to roughly correspond to the rates of the stress-relaxation. Conclusion: These results suggest that during stress-relaxation, the alignment of the collagen molecules and fibrils within the stretched fibres are in the direction of the stretching and/or some concentration of collagens in a stretched (thinner) specimen may occur due to the strain energy imparted to the specimen on stretching.