Tightening techniques for the retaining screws of universal abutment

Purpose: This study evaluated the torque maintenance of universal abutment retaining screws using different tightening techniques, and coated or uncoated screws. Material and method: The screws were tightened to implants as following: Control – 32 Ncm torque; H20 – holding 32 Ncm torque for 20 s; R – 32 Ncm torque, repeated after 10 min (retorque); and H20+R – combining the two tightening techniques. Titanium and coated screws were also evaluated. Result: Statistical analysis showed higher maintained torque for titanium screws (p<0.001). The H20+R technique showed the highest maintained torque (p=0.003), but the H20 technique’s maintained torque was similar. Conclusion: Titanium screws associating the two tightening techniques can improve maintained torque. Descriptors: Implant-supported dental prosthesis; dental implant-abutment design; torque.


INTRODUCTION
The clinical success of cemented implant-supported crowns is dependent on the joint stability of the abutment retaining screw, and common clinical complications in implant prostheses include screw loosening 1 .Loosened screws occur through three major mechanisms: lack of a passive fit among components, overload on screw joints and embedment relaxation 2 .This last phenomenon is also known as "settling" and this occurs because the internal threads of the implant and the threads of the retaining screw of the abutment cannot be machined to be perfectly smooth, resulting in high spots on both surfaces.These spots contact only when the initial tightening torque is applied on the screw when developing the preload.Settling then occurs when the rough spots of the screw threads are smoothed under loading, causing loss of part of the initial torque 3 .
The clinical procedure suggested to overcome settling effects is a second torque application (retorque) on the retaining screw of the abutment 10 minutes after the initial tightening torque 4 .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.Moreover, manufacturers of abutment retaining screws have used dry lubricants, such as diamond-like carbon (DLC), to reduce friction during tightening and to allow more screw-turning for a given torque 7,8 .
The aim of the study was to evaluate the effect of the tightening technique and the screw coating on the maintained torque of retaining screws of the universal abutments for cemented implant-supported crowns.The hypotheses tested were as follows: 1) a tightening technique other than that provided by the manufacturer improves the torque maintenance of retaining screws in a universal abutment; and 2) coated screws improve the torque maintenance of retaining screws in a universal abutment.

RESULT
The statistical analysis showed that both the tightening technique and the screw coating significantly affected the maintained torque (p=0.003 and p<0.001, respectively).However, the interaction between the two factors was not significant (p=0.869).The combination of tightening techniques applied in the H20+R group presented the highest overall maintained torque values, though the values for the H20 group were similar, regardless of the screw used.Moreover, the conventional screws in the Ti subgroup presented higher maintained torque values than those in the DLC subgroup, regardless of the tightening technique used (Table 1).

DISCUSSION
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 matching of the techniques used in the H20+R group showed the highest torque maintenance, however the difference between those values and the intermediate values found for the H20 group was not statistically significant.Therefore, the first hypothesis of the study was accepted, as the tightening technique influenced torque maintenance.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.
The second hypothesis tested in this study was rejected, as the uncoated titanium screws presented greater torque maintenance than the DLC-coated screws.This result is in agreement with a previous study 8 .The lower maintained torque found for the coated screws could be due to the reduced friction during tightening, which could also reduce the screw's friction resistance during loosening.Coated screws may decrease torque loss after mechanical cyclic loading due to friction reduction during tightening, helping the screw turn farther for a given torque and increasing the contact between the threads of the screw joint, resulting in less screw vibration and fewer micromovements during mechanical cyclic loading [7][8][9] ; however, this effect was not evaluated in the current study.Tightening techniques can decrease the loosening of the abutment retaining screws, but there is no consensus in the literature concerning the benefits of coated retaining screws for abutment fixation [7][8][9][10][11] , so clinical studies are needed to confirm the results found in the current study and clarify the hypotheses reported.

CONCLUSION
Within the limitations of this laboratory study, it was possible to conclude that conventional titanium screws and tightening techniques holding the tightening torque for 20 s and matching the holding and repeated torque applications (retorque) present higher maintaining torque of the retaining screws of the abutment used for cemented implant-supported crowns.

Table 1 .*
Mean values (standard deviation) of the maintained torque (Ncm) grouped in blocks (tightening technique and screw) Mean followed by different letter are statistical significant (P<0.05).

Figure 1 .
Figure 1.Experimental design: (a) implant submerged in acrylic resin; (b) universal abutments screwed on the implant; and (c) tighten and maintained torque using the digital torque meter.