An in vitro analysis of cantilever effect on the reverse torque measurement for a screw retained single molar crown

Statement of the problem. Surgical limitations in implant placement may result in the production of a single molar crown that has a cantilever effect relationship to its implant fixture. This cantilever effect relationship may decrease the reverse torque value, the most common implant complication, and decrease the success rate of the implant restoration when compared to non-cantilever effect on single molar crown.;Purpose of the study. The first aim of this study was to compare the difference in the amount of implant abutment screw reverse torque value loss between cantilever and non-cantilever effects on single molar crowns under cyclic loading in wet condition. The second aim was to determine if the presence of the interproximal tooth contact support in the screw-retained single molar crown will increase the amount of implant abutment screw reverse torque value compared to not having the interproximal tooth contact support in the screw-retained single molar crown under cyclic loading in wet condition.;Materials and Methods. The specimens in this study are prepared using an internal connection regular root form implant fixture bone level type. Four specimens are prepared. Specimen "A" (control) is a non-cantilever effect on screw retained single crown without interproximal contact support. Specimen "B" is a non-cantilever effect on screw-retained single crown with interproximal contact support. Specimen "C" is a cantilever effect on screw-retained single crown without interproximal contact support. Specimen "D" is a cantilever effect on screw-retained single crown with interproximal contact support. The pilot study estimated a sample size of 6 per specimen would be adequate to obtain a type I error rate of 0.05 and a power of more than 90% for each factor.;All samples were torqued to 20 N-cm with a digital torquing devise and loaded with a controlled load profile utilizing the Texture Analyzer Machine for 500,000 cycles under wet condition. At the end of a complete loading profile, each specimen was then measured for the amount of screw reverse torque value utilizing a digital un-torquing devise. Statistical analyses were done using the two way analysis of variance test (ANOVA) to compare the amount of screw reverse torque value and to test for any statistical significance, independent sample T-test for between specimens significance.;Results. None of the tested specimens failed before the end of the loading profile. The mean (SD) for the reverse torque after one complete loading profile were as follows: straight crown with no interproximal contact specimens was 14.4333 (0.163) N-cm; straight crown with interproximal contact specimens was 14.7333 (0.137) N-cm; cantilever effect on crown with no interproximal contact specimens was 13.2667 (0.103) N-cm; and cantilever effect on crown with interproximal contact specimens was 13.5667 (0.121) N-cm. The two way analysis of variance demonstrated that there was a statistically significant difference between straight and cantilever effect (P < 0.001) and a statically significant difference between contact and no contact (P < 0.001). An independent t-test showed significant difference between specimens. The straight crown with interproximal contact specimen showed the least values for screw reverse torque after one complete loading profile, followed by straight crown without interproximal contact specimen, cantilever effect on crown with interproximal contact specimen and lastly cantilever effect on crown without interproximal contact specimen (P < 0.05).;Conclusion: Within the parameters of this in vitro study with its limitations, the following conclusions can be drawn: Restoring molars with cantilever effect on single crown design will cause a decreased amount of screw reverse torque value as compared with the use of straight single crown design. Presence of interproximal contact support in implant supported single crown will increase the amount of screw reverse torque value as compared to not having it, regardless of the crown design.