The cemento-dentine-junction (CDJ) is a durable natural interface connecting root dentine with the cementum that covers it. Though the CDJ connects two nanocomposites of mineralised-collagen-fibres (reinforced by carbonated apatite - cAp nanocrystals), it contains much less collagen [1], and more proteoglycans (Fig. 1a) than either tissue. The CDJ hardly ever fails in vivo, resisting millions of chewing cycles, and this is achieved without cells and with no known healing mechanisms. Such outstanding resilience makes it an interesting role model for the design of cyclically loaded interfaces [2,3].

In our project within FOR 2804 “InterDent”, we investigate the structure and fatigue behaviour of the CDJ, on mm-sized cementum-CDJ-dentine cuboids extracted from the roots of freshly slaughtered pigs. We perform load-controlled compressive micro-fatigue tests (frequency f = 5 Hz, Ringer’s solution, stress amplitude = 25 MPa or 50 MPa, maximum stress = -3 MPa) until the force-displacement graphs lose 50 % of their initial height, or to a maximum of N = 5x10E5 cycles, whichever occurs first. All specimens exhibit initial softening followed by hardening, which is at least partially explained by time-dependent deformation mechanisms (Fig. 1b). Differing somewhat between different root regions and the load sequence, a second, more extended softening peak is observed at N = 10E5. Imaging of the fatigued specimens by phase-contrast enhanced micro-computed tomography (Anatomix @ synchrotron Soleil, Paris) and scanning electron microscopy (SEM) reveal that fatigue cracks form and progress in dentine, presumably leading to pronounced softening. Cracks are always diverted by the CDJ but hardly ever traverse it (Fig. 1c). Further, nano-dynamic mechanical analysis (nanoDMA) shows a lower storage modulus in dentine near the CDJ as compared to regions further away from it (Fig. 1d). This property modification may protect the CDJ during cyclic loading, despite a higher degree of mineralisation and concomitant higher stiffness.

References
[1] KR Craig; JW Harrison, J Endod. 1993;19(7):339-347.
[2] S.P. Ho; M. Balooch; S.J. Marshall; G.W. Marshall, J Biomed Mater Res A. 2004;70(3):480-489. 
[3] S.P. Ho; R.M. Sulyanto; S.J. Marshall; G.W. Marschall, J Struct Biol. 2005;151(1):69-78.