Assessment of preload in carbon coated prosthetic screws

Resumo

The mechanical aspects of tightening screws over
implants are important to ensure a successful prosthetic rehabilitation.
Screw loosening is a common problem that can be avoided with
passive adaptation of the components and an increased tensile force
developed in the screw, a preload. Objective: This in vitro study
evaluated the effect on preload of a carbon lubricant deposited on
the surface of titanium alloy prosthetic screws: conventional Ti6Al4V
and surface enhanced. Material and methods: Conventional titanium
alloy prosthetic (n = 7) and carbon coating surface enhanced screws
(n = 7) were compared. Each prosthetic screw supporting a metallic UCLA
over an implant was tightened with the manufacturer’s recommended
torque of 32 N.cm. The removal torque values, recorded for ten
consecutive cycles of tightening and removal, were used to estimate the
preload. Implant blocks were then sectioned and the interfaces were
observed by light microscopy. Results: The lowest removal torque, and
consequently the highest preload values, was achieved for the lubricated
group in most cycles. The contacts between threads were located at the
coronal aspect of all observed screw mating threads. Conclusion: Data
indicate that the lower coefficient of friction of a carbon lubricant can
generate higher preload. The machining precision observed produced
the adaptation and regular contact interfaces.