Effect of retorque on loosening torque of prosthetic screws under two levels of fit of implant-supported dentures

Abstract

This study evaluated the influence of retorque on loosening torque (Lt) of prosthetic screws in implant-supported dentures with different fit levels. Ten mandibular implant-supported dentures were fabricated and then 20 cast models were then prepared using prosthetic structures to create 2 fit levels: passive fit (Pf) and misfit (Mf). Two tightening techniques were also evaluated: initial torque only (T1); and initial torque and retorque after 10 min (T2). Gold or titanium screws were used, resulting in 4 groups to each one: Pf/T1, Pf/T2, Mf/T1, Mf/T2. The Lt was measured 24 h after the tightening torque using digital torque meter. Data were analyzed statistically by two-way ANOVA and Tukey's test (α=0.05), separately for each screw material. For titanium screws, no significant difference (p>0.05) was found between Pf/T1 and Pf/T2, or between Pf/T2 and Mf/T2. However, statistically significant difference (p<0.05) was found between Pf/T1 and Mf/T1, and between Mf/T1 and Mf/T2. Mf reduced the Lt using T1, while and T2 increased the Lt for Mf. Retorque and fit were shown to have no significant influence on the Lt of the gold screws. Retorque application made insignificant the misfit effect on the Lt of the titanium screws, suggesting that this procedure should be performed routinely during the screw tightening in multi-unit dentures.

Tightening techniques for the retaining screws of universal abutment

“…The current study showed the maintained torque was lower than the tightening torque for the screws in all groups, which is in agreement with previous studies 3,5,[6][7][8] . This likely occurs due to the phenomenon known as "settling" or "embedment relaxation" 3 .…”

“…The higher torque maintenance found for the two holding techniques may be because a longer torque application decreases the irregularities in the threads of the implant and retaining screw, increasing the contact between them during the preload and consequently, decreasing the settling effect. The retorque application after the settling effect may act to regain preload and increase the contact area between the threads; for this reason, several researchers have advocated that retorque application 10 min after the initial application should be performed routinely during abutment-implant connections 3,5,6 . The current study showed that this technique was most effective when associated with holding the torque for 20 s.…”

“…Investigators have recommended this procedure as a routine clinical technique 3,5,6 . In addition, holding the torque meter for a longer time period during the tightening torque application on the retaining screw could also decrease the settling effect.…”

Stability of the prosthetic screws of three types of craniofacial prostheses retention systems

“…However, it has been observed that holding the torque meter for a period of time during the tightening of the screws could provide an elongation of the screw and increased pre-load1819.…”

Technique of Framework Cemented on Prepared Abutments to Obtain Passive Fit at Fixed Complete Denture: A 2-Year Follow-Up Report

“…A misfit in the denture frameworks can cause many complications, including biological effects such as inflammation of the surrounding tissue, pain, bone deformation and remodeling, continual resorption or even the loss of osseointegration (5). Technical complications include abutment fractures, screw loosening or fractures, and fracture of the framework (6,7).…”