Enamel - a "metallic-like" deformable biocomposite.

OBJECTIVES: The aim of this study is to compare the mechanical responses of enamel with dental-used metals and to show that enamel has mechanical properties similar to metals rather than ceramics.
METHODS: Four dental-used metals: cast alloy, gold alloy, titanium and amalgam, were compared with enamel. Pure hydroxyapatite (HAP) was used as the ceramic analogue of enamel. A Berkovich and a spherical indenter were selected for nanoindentation tests. The stress-strain (H-a/R) curves and indentation creep behaviour were investigated and analyzed.
RESULTS: Although the primary composition of enamel is hydroxyapatite, the indentation stress-strain curves and creep behaviour of enamel was totally different to HAP. Enamel had similar stress-strain response to that of cast alloy and gold alloy, all of which showed work-hardening effect. Titanium and amalgam had curves that showed lower stress at comparable strain than enamel and showed no work-hardening. Amalgam exhibited the greatest creep behaviour, followed by Titanium, enamel and gold alloy. Similar to HAP, the cast alloy had very limited creep response. Only enamel showed significant backcreep at minimum load. This may be a consequence of the memory behaviour of the minor protein component within enamel.
CONCLUSIONS: The small remnant volume fraction of protein fragments have endowed enamel with metallic-like mechanical properties, which impart it with an ability to sustain repetitive cyclic contact loading over the life of the host. It may be better to choose metallic-like dental restorative materials to sustain the severe cyclic contact behaviour experienced and to protect the opposing teeth from excessive damages.