Conical hybrid connection was used in this study as it was proven that the conical hybrid demonstrated the best stress distribution . This connection has a conical union between the implant fixture and the abutment. In the conical abutment, lateral force is resisted by the taper design of the Morse taper connection. Thus, the stress concentration is resisted by the side-wall contact surface of its taper design. This stress concentration increases at the apical end of the side-wall contact surface where the implant fixture is thicker. This thickness might provide more resistance to the force, especially the off-axis loading force (as in case of angled abutment). Thus, in abutments with conical hybrid connection design, the screw is not the only source of resistance to loading force, as it is in abutments with an internal hexagonal design [21, 22].
In this study, abutment screws were tightened to 30 Ncm according to the manufacturer’s instructions with digital torque gauge. Application of the optimum torque to the implant–abutment complex is critical for long-term successful prosthetic implant restoration. Applied torque develops a force within the screw called preload .
Ten-minute interval was left after the first torque application, and all screws were retightened to the same tightening torque (30 Ncm) with the same digital torque gauge to compensate for the preload loss due to settling effect of the screw thus ensure achieving optimal preload as only 10% of the initial torque is transformed into preload, where the remaining 90% is used to overcome the friction between the surface irregularities .
The results of this study indicated that there is some torque loss after applying two insertion torques with a 10-min interval before any loading as RTVs were less than 30 Ncm. This finding matches previous studies that reported initial torque loss after 2–10 min [4, 9, 10]. Although there was an increase in %RTL before loading in every group, loosening of screws could not be detected clinically. This may indicate that the remaining tightening torque would serve clinically for a longer period.
In the current study, the results showed that there is a significant difference in %RTL before and after application of dynamic cyclic loading for all angulations and collar lengths. These results are in an agreement with previous researches that found a significant difference between %initial and %postload RTL after mechanical cyclic loading [24, 25]. This result was explained by Bickford et al.  as the process of screw loosening occurs in two stages. Initially, external forces cause sliding between the threads, partially relieving the stretching of the screw and reducing preload. At this stage, the higher the preload (within a certain limit), the greater will be the resistance to loosening. The second stage is attained by a gradual reduction of preload below a critical level, in which external forces cause the turning of the screw in an anti-clockwise direction, and it loses its function.
The results of this study showed that, the removal torque loss ratios, with 15° and 25° angulations, were significantly increased. The removal torque loss ratio was increased significantly with increasing angulation as with the 25° angulation the removal torque loss ratio was significantly higher than 15° angulation.
On the other hand, these results disagree with studies that yielded no significant difference between straight and angled abutments for deflection, rotation, and torque required to loosen abutment screws for any parameter at any time [12, 27]. Hsu et al.  showed that the clinical performance of angled abutments is comparable to that of straight abutments with respect to both soft tissue responses and general survival rates. However, in vitro studies of stress/strain analyses of angled abutments can only agree that stress/strain levels increase as abutment angulation increases.
These results are in agreement with previous studies which stated that the difference in abutment screw RTV after load showed better results when less angulation abutment was used and studies have failed to show any contraindication to their use [4, 19, 29]. Ha et al. , in an evaluation of the influence of abutment angulation on screw loosening of implants in the anterior maxilla, found that the angled abutments showed higher RTV (P < .05) than the straight and gold premachined UCLA-type abutments and the difference between them was not significant. This can be attributed to the off-axis force as loading on angled abutments is mostly off-axis, which raises the concern of how angled abutments generally perform with such an unfavorable loading regimen .
The increase in RTL with increasing abutment angulation can be attributed to the off-axis force as loading on angled abutments is mostly off-axis, which raises the concern of how angled abutments generally perform with such an unfavorable loading regimen . Forces applied off-axis may be expected to overload the bone surrounding single-tooth implants, as shown by means of finite element analysis, which affects abutment screw leading to its loosening .
The greater the angulation, the greater the off-axis force that generates more stress and strain in implant components specially the screw  When off-axis loading is applied to an implant, the magnitude of the stress will be increased three times or more . There was a statistically significant increase in stress and strain when abutment angulation increased. This supports the concept of eliminating unnecessary occlusal and off-axial forces on implant-supported restorations . With clinical loading of implants restored using angled abutments, lateral occlusal forces may increase creating torsional force which increases screw loosening [12, 29]. Any direction of load that is not in the long axis of the implant will magnify the crestal stresses to the implant–bone interface and to the abutment screws in the restoration .
On the other hand, concerning the area of contact between screw thread and abutment, the increase of abutment angulation leads to decrease area of friction that leads to retention and thus screw loosening occurs. Comparing micromotion level between a straight abutment, a 15° to 25° abutment angulation, an increase in the micromotion level by 30% was observed. This micromotion may explain the screw failure. However, no screw failure occurred in a study with 2261 angled abutment evaluated for 96 months [29, 32].
According to the results of this study, it was showed that with straight abutments, %postload RTL was lower than %initial RTL. This result could be explained by Squier et al.  who stated that abutments of the conical hybrid connection showed detorque values higher than the initial torque due to the cold solder on the implant–abutment interface, which agrees with the results of this study. This condition arises from the friction between the two surfaces, which differ slightly; the pressure created by the insertion force determines the maintenance of the connection even after stopping the applied force for insertion.
Several studies have been conducted on this type of implant–abutment connection [23, 34]. Sutter et al.  demonstrated reverse torque values of this hybrid implant–abutment connection that were 124% of the initial tightening torque. These authors suggested that cold welding occurred in the conical hybrid implant–abutment connection.
Schmitt et al.  compared conical and nonconical implant–abutment connection systems in terms of their in vitro and in vivo performances. In vitro studies indicate that conical and nonconical abutments exhibited sufficient resistance to the maximal bending forces and fatigue loading. However, conical abutments were superior in terms of sealing, microgap formation, torque maintenance, and abutment stability.
According to the results of this study, %RTL increased significantly with increasing collar length and the percentage of removal torque gaining after load with 0° abutments with collar height 4 mm was less than that of 2 mm. This finding matches what was found by Siadat et al.  who evaluated the effect of collar length on screw loosening and concluded that an increase in the abutment collar length significantly increase the torque loss of abutment–implant screw after cyclic loading. The abutment collar length acts as vertical cantilever, so increasing abutment collar would lead to an increased vertical cantilever which acts as a force magnifier . Cantilever designs increase bending forces on screws due to the lever effect [17, 18].