The success of two-piece dental implants (Figure 1) is threatened by fretting occurring between the ceramic abutment and the titanium-based fixture. A wrong design could generate mismatches between surfaces and a poor choice of materials would result in excessive micromotions between abutment and fixture. Sliding and fretting wear cause a disproportionate ion release in the surrounding tissues, potentially giving the gingiva a greyish appearance. The lost material enlarges the existing microgaps exposing the implant to bacterial colonization and infections. The extreme consequences of these processes result in the removal of the implant and revision surgery.
How can these threats be neutralized by selecting the proper implant materials?
Changing the bulk material of the fixture to zirconia or other ceramic materials would result in a higher risk of brittle failure. Coating the less wear-resistant fixture material allows to improve the tribological properties of the contact while keeping a ductile damage behavior of the system.
Is it possible to evaluate the effectiveness of a coating with a fretting test method?
Figure 1. Schematic of a two-piece dental implant, highlighting the contacts more prone to fretting wear.
Table 1. Physiological parameters for dental fretting in vivo.
Fretting behavior of coatings and dental materials can be studied using Ducom MicroForce Tribometer (Figure 2). Ducom MicroForce Tribometer can apply a normal force that ranges from 10 mN to 10 N and it measures friction coefficient as low as 0.001 at a rate of 2000 data points per second. It uses parallel plate capacitive sensors that prevents crosstalk between normal force and friction force. A reciprocating stage with zero backlash was used to achieve stroke lengths from 20 to 100 µm.
Table 2. Test parameters.
Fretting friction force in terms of stick, partial slip and gross slip was measured for dental implant materials using Ducom MicroForce. The chosen materials were titanium (cpTi), titanium alloy (Ti64) and titanium alloy coated with a 5 µm thick diamond-like carbon coating (DLC), in the form of flat pins. Zirconia balls (3Y-TZP, Ø 3 mm) were used as counterparts.
Figure 2. Ducom MicroForce tribometer.
Figure 3. Evolution of friction coefficient of a zirconia ball against the three materials.
As shown in Figure 3, DLC showed a lower friction response, whereas cpTi and Ti64 differed to some extent only at the stroke length transition 50 µm to 100 µm and they both showed a higher friction coefficient than the DLC coating. No signs of wear on both the metal pins and the alumina balls.
Figure 4. Friction signature of the three tested tribopairs at the beginning of the test and after the two stroke length changes.
The fretting friction signatures are shown in Figure 4. A stick phase was observed for all the materials at a stroke length of 20 µm, so similar that they almost exactly overlapped. When the stroke length was increased to 50 µm each material responded with an increase in friction, shifting to a partial slip condition within few cycles. At a stroke length of 100 µm, gross slip became dominant; cpTi and Ti64 presented a gradual increase in friction. Said increase was reflected in the calculated energy dissipated because of friction (Table 3), showing values of about one third of those found for the two titanium materials.
Table 3. Summary of the friction coefficient values at different moments of the test and the energy dissipated due to friction.
In conclusion, DLC coating the fixture component was proved to be a potential solution as determined using Ducom MicroForce Tribometer, because DLC showed a more stable and lower friction compared to the other titanium materials. However, this is only a preliminary result, that should be integrated by corrosion studies at physiological temperature.