Background: The mechanics of implant screw loosening or fracture are well understood in the field of engineering. Due to the several factors involved, they have not been as widely explored in dentistry. The current knowledge about the role of the implant external hexagon is incomplete. Two of the factors that seem to influence in this complication are the type of connection and the angulation of the implant.

Aim: The aim of this preliminary in vitro study was to investigate the effect of abutment screw loosening of an external hexagon implant system when a cyclic loading was applied.

Material and Methods: Twelve Mozo Grau Osseous ® (External Hexagonal Connection System) (3, 75 x 11, 5 mm) implants were divided into groups A and B. Group A included six 0˚(4 mm) implant abutments and group B included six 15˚(4 mm) implant abutments. Each implant assembly was tightened using an implant motor (IChiroPro Bien Air®) to 30 Newton centimeters (Ncm) and placed on a custom made implant fixing jig. Cyclic loading was applied (200 N of force at a frequency of 2 Hz for 300, 000 cycles) (12 months of simulated function). Reverse torque was recorded before and after loading. After cyclic loading, the reverse torque value (RTV) data were analyzed using Wilcoxon signed-rank test (p< ,05).

Results: Statistically significant difference was obtained when group A was exposed to a cyclic load (p<,046). Likewise, group B showed statistically significant difference (p<,028).  There were also statistically significant differences in the reverse torque difference values in group A as compared to those of group B (p<,003). The mean RTV for group A before cyclic load were (19,1 ± 2,60) and after cyclic load  (15,31 ± 2,11). For group B the mean RTV before cyclic load were (19,55 ± 1,65) and after cyclic load (12,68 ± 3,93).

Conclusion and Clinical Implications: Within the limitations of this preliminary in vitro study, it was concluded that (1) groups A and B showed decreased RTV after cyclic load of 1 year simulated function (2) group A showed more RTV than group B which may led us to deduce that angled implant abutments have less stability than straight implant abutments 
(3) Although statiscally significance was found, differences when comparing group A to group B were of minor clinical significance.

Machined abutment screw loosening on external hexagonal-connection implant system- A preliminary in vitro study.