On the experimental determination of some mechanical properties of porcine periodontal ligament

This work analyzes some aspects of the experimental determination of the mechanical properties of the periodontal ligament (PDL). The necessity of extracting small samples, with a geometry as regular as possible, from a complex biological structure, makes it quite difficult both to establish a correct testing protocol and to obtain reliable results, for instance usable by bioengineers to develop constitutive models. Here, by means of more than 250 experiments performed on small samples of porcine PDL, we try both to provide some statistically significant information, and to clarify some issues related to the testing protocols. Some basic mechanical parameters for the PDL (Young’s modulus, shear modulus, failure stress and strain for tension, compression, and shear tests) are measured, and a relevant statistical analysis is provided. The influence of some experimental parameters (sample conservation procedure, testing modalities), is also studied; on the basis of our results, we can conclude that (i) if conservation is needed, a cooling at −80° is sufficient to guarantee statistically significant results, (ii) it is important to perform at least the compression tests keeping the samples immersed in pressurized fluid, and (iii) preconditioning cycles are necessary only for studying the initial (toe) region of the stress–strain curves. It is also observed that, with these types of samples, some special care is required when computing stresses and strains from force and displacement measurements. In order to illustrate this aspect, some non-linear Finite Element analyses are performed, aimed at evaluating the influence of the sample geometry on the stress and strain calculation. Finally, the issue of fiber damage due to the cutting procedures is briefly discussed.