Effect of cyclic loading on reverse torque values of angled screw channel systems

Clinical and Laboratory Outcomes of Angled Screw Channel Implant Prostheses: A Systematic Review

“…15 16 21 Fractographic examinations of anterior two-piece zirconia abutments revealed that critical cracks were initiated in the cingulum from the most apical part of the screw access channel in the friction-fitted system. 15 16 19 Consistent with this finding, some clinical studies have reported early catastrophic failures with the same pattern in two-piece friction fitted zirconia abutments in anterior and premolar SSC restorations. 30 31 As the screw head seats on the internal surface of the zirconia restoration, hoop stresses and/or incompatible hardness between the zirconia and titanium components may be responsible for such pattern of failure.…”

“…An additional finding was the wear of the screw head against the zirconia crown in groups with friction fitted two-piece zirconia abutments (Nobel Biocare). Congruent with this study, Mulla et al 19 evaluated a higher magnitude of cyclic forces for a longer duration of time on 25-degree access channel restorations from three different manufacturers. RTVs were measured 24 hours after initial tightening and after simulated 5 years of functional load.…”

“…The included studies were published between 2017 and 2021, with the majority being published in the past 2 years. Out of seven laboratory investigations, two studies evaluated the fracture strength of the zirconia crowns with ASC (►Table 5), 15,16 four studies evaluated the reverse torque value of nonaxially tightened screws (►Table 6), [17][18][19][20] and one examined both variables. 21 Only four studies reported the exact direction of force in their investigations that was 30 degrees for all of them.…”

“…This is further influenced by the screwdriver's sphere and facet design, as well as, the surface treatment of the screw head. [17][18][19][20] Likewise, difficulty in the engagement of the screwdriver, which was reported with certain systems, led to stripping and wear of the screw head that might impact the crown retrievability and the usability of the screws after multiple tightening in long duration of time. 17,18 The use of nongenuine component can influence the RTV and may jeopardize the joint stability of the implant-abutment complex.…”

The Application of Angulated Screw-Channels in Metal-Free, Implant-Supported Restorations: A Retrospective Survival Analysis

“…Some in vitro studies have reported that as the channel is not perpendicular to the implant, which may receive off-axis loading, the rate of mechanical complications could possibly be higher than conventional, screw-retained, FPD. Additionally, as the force that generates the torque in the fixating screw is not perpendicular to the screw itself, but is positioned at an angle, theoretically a lower percentage of that force is transferred, resulting in a lower preload [23][24][25].…”