Screw joint

Abstract: The problem of screw stability was approached in this study by calculating the maximum occlusal forces in vivo for patients with single implant restorations (which use only one screw to secure the prosthetic reconstruction to the implant). The measurements of occlusal forces together with geometric parameters for the individual patients were used to determine the necessary holding capabilities of the screw joint. Different screw designs were tried in bench test situations and the results were compared with clinical situations. A gold alloy screw with a flat head and high tightening torque (35 Ncm) produced the best results.

Abstract: A series of 10 incrementally larger, machined ASTM Grade 23 titanium non-segmented (UCLA type) abutments was loaded off axis with 133 N and cycled at 1150 vertical strokes per minute and 28 counterclockwise rotations per minute to determine screw joint stability. Abutment internal hexagonals ranged from 0.1065 to 0.1110 inches. External hexagonal mean flat-to-flat width was 2.684 mm. Rotational misfit between international and external hexagonals ranged from 1.94 degrees for the smallest abutment to 14.87 degrees for the largest. Screw joint failure ranged from 134,000 to 9.3 million cycles. The tightest matrix/patrix hexagonal screw joint failed at a mean of 6.7 million cycles. This study indicated that there was a direct correlation between hexagonal misfit and screw joint loosening. A rotational misfit of under 2 degrees provided the most stable and predictable screw joint.

Abstract: Statement of problem Abutment and prosthetic loosening of single and multiple screw-retained, implant-supported fixed partial dentures is a concern. Purpose The purpose of this study was to investigate stress distribution of preloaded dental implant screws in 3 implant-to-abutment joint systems under simulated occlusal forces. Material and methods Three abutment-to-implant joint systems were simulated by using the 3-dimensional finite element analysis method: (1) Branemark external hexagonal screw-retained abutment, (2) ITI 8-degree Morse tapered cemented abutment, and (3) ITI 8-degree Morse tapered plus internal octagonal screw-retained abutment. A thermal load and contact analysis method were used to simulate the preload resulting from the manufacturers' recommended torques in implant screw joint assemblies. The simulated preloaded implants were then loaded with 3 simulated static occlusal loads (10 N; horizontal, 35 N; vertical, 70 N; oblique) on the crown position onto the implant complex. Results Numeric and graphical results demonstrated that the stresses increased in both the abutment and prosthetic screws in the finite element models after simulated horizontal loading. However, when vertical and oblique static loads were applied, stresses decreased in the external hexagonal and internal octagonal plus 8-degree Morse tapered abutment and prosthetic screws with the exception of the prosthetic screw of ITI abutment after 70-N oblique loading. Stresses increased in the ITI 8-degree Morse tapered cemented abutment after both vertical and oblique loads. Conclusion Although an increase or decrease was demonstrated for the maximum calculated stress values in preloaded screws after occlusal loads, these maximum stress values were well below the yield stress of both abutment and prosthetic screws of 2 implant systems tested. The results imply that the 3 implant-to-abutment joint systems tested may not fail under the simulated occlusal forces.

Abstract: Statement of problem Efforts to reduce the recurrence of abutment screw loosening with single tooth implant-supported restorations have been reported. However, the current knowledge about the role of the implant external hexagon is incomplete. Purpose This in vitro study investigated the effect of lateral cyclic loading with different load positions on abutment screw loosening of an external hexagon implant system. Material and methods Fifteen Branemark implant assemblies were divided equally into 3 groups, A, B, and C. Each assembly consisted of a Mark IV implant (4 × 10 mm) mounted in a brass block, a CeraOne abutment (3 mm), and an experimental cement-retained superstructure. For group A, a cyclic load of 50 N was applied centrally and perpendicular to the long axis of the implant, whereas for group B, the same load was applied eccentrically (at a distance of 4 mm) in a loosening direction. A target of 1.0×10 6 cycles (40 months of simulated function) was defined. Group C (control) was left unloaded for the same loading time period as groups A and B. Reverse torque was recorded before and after loading and the difference was calculated. The data were analyzed with 1-way analysis of variance and compared with the Tukey test (α=.05). Results Group A exhibited a significant difference in the reverse torque difference values ([−5.6 to −3.4] ± 0.86 N·cm) compared with groups B ([−1.9 to 0.5] ± 0.99 N·cm) and C ([−0.7 to 0.0] ± 0.26 N·cm) ( P Conclusion Within the limitations of this study, reverse torque values of the screw joint were preserved under eccentric lateral loading, as compared with centric loading ( P