In vitro Residual Stress and Osmotic Swelling of Periodontal Ligament

Objectives: The Periodontal ligament (PDL) has been studied in various loading conditions, but the unloaded PDL has always been considered to be completely devoid of stress. The current study hypothesized that the PDL exhibits residual stress in the unloaded state. This study aimed to: (1) measure osmotic swelling and residual stress in the PDL and (2) determine their effects on the PDL’s behavior under shear loading.
Methods: Transections were prepared from porcine central incisors. Swelling tests were completed in four solutions: (1) deionized distilled water (ddH2O), (2) 50%vol phosphate buffered saline (50%PBS), (3) 100%vol PBS, and (4) silicone oil. Residual strains were measured under a microscope from specimens cut from transections. Uniaxial tests were performed to determine PDL stiffness which was input into a computational model constructed with FEBio. Shear tests to failure were performed in three solutions (ddH2O, 100%PBS, 1000%PBS). ANOVA statistical methods were used to compare the test groups, n>24.
Results: Significant PDL residual strain (effective stretch=1.41) and osmotic swelling (Swelling stretches: 1.19, 1.24, 1.26; in 100%PBS, 50%PBS, and ddH2O respectively) was revealed. PDL specimens submerged in ddH2O exhibited higher failure stresses than those submerged in PBS (1.29, 0.9 MPa respectively). The computational model’s first principal stresses were lower in the swollen PDL than in the non-swollen PDL.
Conclusions: The results suggested that the PDL is capable of osmotic swelling and residual stress behaviors. In the model, PDL swelling reduced the first principal stress during shear, which implied that the tension to collagen fibers would be reduced, and thus that the strains required for failure would be higher in the swollen PDL than in the non-swollen PDL. In the shear tests, the failure stresses (and strains) were higher in ddH2O than in either PBS group which agreed with the model.