Evaluation of loosening torque in multi-unit screw retained implant-supported restorations

Objective: The aim was to evaluate the effect of the screw seat finishing procedure on screw loosening torque in implant-supported frameworks with different levels of passivity.

Methods: Twenty commercially pure titanium 3-unit frameworks were produced using a metallic index containing three 3.75mm diameter tapered abutment analogs (10mm center to center). The frameworks were assigned to two groups (n=10) according to its passivity level: (G1) frameworks set over the metallic index; yielding poorer fit; and (G2) frameworks set over custom made stone indexes, yielding better fit. The misfit between the abutments analogs and framework was quantified using a traveling microscope (X120) through the one-screw test protocol. Each framework was set over the respective index (10Ncm) and after 10min delay, the loosening torque was recorded using a digital precision torque-meter before and after the screw seat surface finishing procedure, yielding 4 subgroups: (G1-B) G1 before screw seat finishing procedure; (G1-A) G1 after screw seat finishing; (G2-B) G2 before finishing; and (G2-A) G2 after finishing. The screw seat finishing procedure was carried out using a manual reamer and was assumed to provide a broader contact between the retaining screw head and the framework. Two-way ANOVA and Student t test compared the effect of screw seat finishing procedure on the screw loosening torque for each group in both passivity conditions (P<.05). Results: The framework misfit was: G1 188±61µm; and G2 66±18.5µm. The loosening torque for subgroup G1-B (5.78±0.98Ncm) was significantly lower than G1-A (7.06±0.62Ncm), G2-B (7.42±0.93Ncm), and G2-A (8.08±0.62Ncm) (P<.05); with no significant difference among these 3 subgroups (P>.05).

Conclusions: The screw seat finishing procedure yielded a better screw stability, especially for frameworks with poorer passivity condition.