In vitro analysis of resistance to cyclic load and preload distribution of two implant/abutment screwed connections. 
The aim of the present research is an in vitro evaluation of the preload distribution in screw-retained implant systems under cyclic load.  Two implant systems with internal connection were tested: fifteen 4.5 × 10 mm implants with internal hexagon and fifteen 4.5 × 10 mm implants with internal octagon.  Samples underwent cyclic load that was between 20 N and 200 N for 1 × 10(6) cycles.  After mechanical tests, samples were sectioned along the long axis and analyzed under a scanning electron microscope.  Five 4.5 × 10 mm implants with internal hexagon and five 4.5 × 10 mm implants with internal octagon were collected for photoelastic analysis.  Each fixture was mounted in a wax-made parallelepiped measuring 20 mm × 20 mm × 10 mm.  A mold was made for each wax parallelepiped/fixture assembly using a silicone-based impression material, and an epoxy resin was poured in each mold.  After setting of the resin, 25° angled titanium abutments were screwed onto each replica; afterwards, assemblies underwent photoelastic analysis.  After cyclic load, screw threads and heads were still in contact with internal fixture threads and abutment holes, respectively, suggesting that preload has not been lost during load.  During load, SSO and Xsigñ implants behave in a different way.  SSO samples revealed the presence of fringes radiating from the base of the abutment.  Xsigñ implants showed the presence of fringes radiating from the threads of the retention screw.  From the present in vitro research, it is possible to state that screw-retained abutment based on an internal octagonal connection is less likely to come loose after cyclic load.