Tr. Meling et J. Odegaard, THE EFFECT OF TEMPERATURE ON THE ELASTIC RESPONSES TO LONGITUDINAL TORSION OF RECTANGULAR NICKEL-TITANIUM ARCHWIRES, The Angle orthodontist, 68(4), 1998, pp. 357-368
To investigate responses to longitudinal torsion and the effect of tem
perature on the torsional stiffness of superelastic nickel titanium ar
chwires, eight batches of rectangular wires were tested at 18, 27, 37,
and 40 degrees C ambient temperature. The wires were twisted 25 degre
es and studied in deactivation. The resulting torque-twist diagrams sh
ow that only half the wires had discernible deactivation plateaus at b
ody temperature. The plateaus were generally narrow (1 to 3 degrees) a
nd started at 20 to 23 degrees of torsional twist. The clinical signif
icance of these deactivation plateaus is debatable. Only one wire had
a well-defined plateau that was fairly wide (6 degrees) and started at
a lower level of twist (17 degrees). The wires without plateaus when
deactivated from 25 degrees of twist were retested at body temperature
. All exhibited deactivation plateaus subsequent to activating twists
of 45 and 60 degrees, and the plateaus became more distinct as the deg
ree of prior activation increased. This indicates that the stress impa
rted on the alloys by 25 degrees of activating twist is insufficient t
o induce martensitic transformation at body temperature. As prescripti
ons advocate bracket pretorque of less than 25 degrees for a maxillary
central incisor, the clinical relevance of alloys requiring large act
ivations before they demonstrate deactivation plateaus is questionable
. Half the wires tested were markedly influenced by ambient temperatur
e changes; the other half were relatively insensitive to temperature.
Responsiveness to thermal stimuli seemed closely related to superelast
ic tendency.