The hypothesis under examination in this paper is
that the lifetimes of dental restorations are limited
by the accumulation of contact damage during oral
function; and, moreover, that strengths of dental
ceramics are significantly lower after multi-cycle
loading than after single-cycle loading.
Accordingly, indentation damage and associated
strength degradation from multi-cycle contacts
with spherical indenters in water are evaluated in
four dental ceramics: "aesthetic" ceramicsporcelain
and micaceous glass-ceramic (MGC),
and "structural" ceramics--glass-infiltrated
alumina and yttria-stabilized tetragonal zirconia
polycrystal (Y-TZP). At large numbers of contact
cycles, all materials show an abrupt transition in
damage mode, consisting of strongly enhanced
damage inside the contact area and attendant
initiation of radial cracks outside. This transition
in damage mode is not observed in comparative
static loading tests, attesting to a strong
mechanical component in the fatigue mechanism.
Radial cracks, once formed, lead to rapid
degradation in strength properties, signaling the
end of the useful lifetime of the material. Strength
degradation from multi-cycle contacts is examined
in the test materials, after indentation at loads
from 200 to 3000 N up to 106 cycles. Degradation
occurs in the porcelain and MGC after I104
cycles at loads as low as 200 N; comparable
degradation in the alumina and Y-TZP requires
loads higher than 500 N, well above the clinically
significant range.