Objective: In this study, cadaveric specimens of mandibular premolars from two young adult and two elderly adult donors were tested to determine stress-strain behavior of the periodontal ligament over an orthodontic force range. Methods: The premolars were sliced transversely into sections approximately 0.85 mm thick using a water-irrigated circular saw. Fixtures were fabricated to clamp the bony part of the specimen and allow for extrusive and intrusive loading of specimens immersed in saline solution. Finite element models of a mandibular premolar subjected to low force were constructed and used to compare stresses using linear and nonlinear material properties of the PDL. Results: Stress strain curves for both loading directions had distinct toe and linear regions, demonstrating nonlinear behavior of the periodontal ligament. The average linear shear modulus was higher for intrusion than extrusion. The average linear shear modulus in extrusion was higher for cervical margin and apex regions than midroot regions, suggesting anistropic material properties of the PDL. Preliminary results of the finite element analyses indicate that the nonlinear material properties of the PDL are required for the computer models to accurately predict experimental load/displacement behavior. Conclusion: The data strongly suggest that a nonlinear representation of PDL mechanical properties is appropriate, and a linear, homogeneous assumption for this soft tissue may be overly simplistic. In addition, the mechanical behavior is dependent on load direction and anatomical location along the root. Inclusion of such nonlinear properties in finite element models may improve the accuracy of computer simulations of complex behavior of the human PDL.

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