Position-Resolved Structural and Mechanical Properties of Narwhal Tusk Dental Tissues

Information about the chemical properties of narwhal tusk is limited to reports about dentin and “cementum” composition obtained from powdered specimens without distinction relative to depth or position along the long tusk. Reported mechanical properties have not been positioned resolved or correlated with chemical and structural properties. Objectives: To map the constituents and mineral-to-collagen ratios (MCR) in cross-sections (CS) cut along male narwhal tusk and to correlate MCR to mechanical properties. Methods: CS were sliced at three locations along the tusk, dry polished, and mapped by Fourier-transform infrared reflectance microspectroscopy (FTIR-RM) with 100x100 μm to 200x200 μm spatial resolution. Microhardness (MH) and Young's Modulus (E) were measured by nanoindentation (200x100μm spatial resolution). Results: Results showed a gradual decreases in the properties measured, moving from pulp to the surface [mean±SD(N)]: MCR 6.73±0.28(5) to 4.16±0.21(5), MH 0.73±0.06(3) to 0.54±0.10(3) GPa, and E 20.54±0.19(3) to 12.99±1.26(3) GPa in a CS close to the tip. At ~1/4 of the length from the tip, MCR went from 5.79±0.06(4) to 4.01±0.40(4), MH 0.51±0.06(4) to 0.18±0.13(4) GPa and E 12.41±1.06(3) to 1.09±0.72(3) GPa moving from pulp to surface. Close to the base MCR went from 4.84±0.26(6) to 3.96±0.48(6), MH from 0.53±0.020(3) to 0.17±0.02(3) and E from 12.87±0.27(3) to 0.67±0.01(3). The crystallinity of cementum mineral was slightly higher than dentin in all CS, and growth rings with different MCR were observed. Conclusions: MH and E both appear to be correlated with the MCR in narwhal tooth tissues away from the tusk base, but all three properties and the degree of correlation are location specific.  All three properties tended to decline in moving from the tip to the base, with modulus demonstrating a dramatic decline moving from pulp to the outer surface in the two cross sections away from the tip.  Supported by ADAHF and NIST.