Introduction: Screw loosening has been reported to be a common reason for clinical failures. Objectives: To measure the loosening torque values for implant screws after repeated tightening of the implant/abutment connections. Methods: Seven types of implant-abutment assemblies were tested: A.Internal Taper-8⁰ (ITI); B.Internal Taper-11⁰ (AstraTech); C. 4mm multi-level connection (3I); D. Spline (Zimmer); E. Internal Hex (Zimmer); F. External Hex (Nobel Biocare); G. Internal connection-Replace (SteriOss). Implants were mounted into acrylic blocks. Five assemblies of each type were tightened using corresponding screws at manufacturer suggested torque levels and then de-torqued after 20 min rest. Loosening torque values were measured with a commercial torque meter (Snap-On). Torquing and loosening was repeated 10 times for each assembly. Changes between initial tightening torque and the 1^st loosening torque and between the 1^st and 10^th loosening torques were recorded and analyzed statistically.

Results: Tightening torques, loosening torques (Ncm) and SD are shown:

Systems Torque (Ncm)	A	B	C	D	E	F	G
Tightening	35	25	20	30	30	35	35
1st loosening (SD)	36.8 (0.7)	24.64 (0.4)	15.2* (0.8)	29.4 (1.9)	28.2* (1.09)	29.6* (0.5)	29.4* (0.5)
10th loosening (SD)	29.4* (2.9)	24.6 (1.1)	15.4 (0.5)	30.4 (0.89)	27.4 (2.0)	28.6 (0.5)	30.2 (0.4)

*= P<0.01.

Conclusions: Most significant differences occurred between initial screw tightening and the 1^st loosening torque. Systems C, E, F and G displayed initial decreases in loosening torque values that were significant. There were no significant differences between the 1^st and 10^th loosening torques except for system A (P=0.001). Systems B and D did not exhibit any loss of torque throughout the experiment.

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