Effect of tightening torque on the marginal adaptation of cement-retained implant-supported fixed dental prostheses.
BACKGROUND
The final position of the abutment changes with the amount of tightening torque. This could eventually lead to loss of passivity and marginal misfit of prostheses. The aim of this study was to evaluate the effect of three different tightening torques on the marginal adaptation of 3-unit cement-retained implant-supported fixed dental prostheses (FDPs).


METHODS
Two implants (Straumann) were inserted in an acrylic block so that one of the implants was placed vertically and the other at a 15° vertical angle. A straight abutment and a 15° angulated abutment were connected to the vertically and obliquely installed implants, respectively, so that the two abutments were parallel. Then, 10 cement-retained FDPs were waxed and cast. Abutments were tightened with 10, 20, and 35 Ncm torques, respectively. Following each tightening torque, FDPs were luted on respective abutments with temporary cement. The marginal adaptation of the retainers was evaluated using stereomicroscope. FDPs were then removed from the abutments and were sectioned at the connector sites. The retainers were luted again on their respective abutments. Luting procedures and marginal adaptation measurement were repeated. Data were analyzed by ANOVA and least significant difference tests (α = 0.05). After cutting the FDP connectors, the independent samples t-test was used to compare misfit values (α = 0.05).


RESULTS
Following 10, 20, and 35 Ncm tightening torques, the marginal discrepancy of the retainers of FDPs significantly increased (P < 0.05). There was no significant difference between the marginal discrepancies of these two retainers (P > 0.05). The marginal gap values of angulated abutment retainers (ANRs) were significantly higher than those of the straight abutment after cutting the connectors (P = 0.026).


CONCLUSIONS
Within the limitations of this study, the marginal misfit of cement-retained FDPs increased continuously when the tightening torque increased. After cutting the connectors, the marginal misfit of the ANRs was higher than those of the straight abutment retainers.
Department of Prosthodontics, School of Dentistry, Mashhad University of Medical Sciences, Mashhad, Iran.
Jalil
J
Ghanbarzadeh
Department of Prosthodontics, School of Dentistry, Mashhad University of Medical Sciences, Mashhad, Iran.
Hossin
H
Dashti
Department of Prosthodontics, School of Dentistry, Mashhad University of Medical Sciences, Mashhad, Iran.
Reza
R
Karamad
Department of Prosthodontics and Implant, Dental Research Center, School of Dentistry, Tehran University of Medical Sciences, Tehran, Iran.
Marzieh
M
Alikhasi
Department of Prosthodontics, School of Dentistry, Mashhad University of Medical Sciences, Mashhad, Iran.
Mohammadreza
M
Nakhaei
Dental implants; fixed partial denture; implant-supported prosthesis; marginal adaptation; torque