The null hypothesis was partially rejected as the difference in angulation of the screw channels significantly affected fracture resistance. However, it did not significantly affect the percentage differences of reverse torque values after thermomechanical cycling.

In line with the current study, Swamidass et al. [6] evaluated the alterations in abutment screw torque in Nobel Biocare implants with straight and angled screw-access channels. They concluded that the percentage difference between RTVi and RTVf did not significantly differ between the straight and angled screw-access channel groups.

Angled abutments are occasionally recommended for non-ideal anterior implant placements. However, titanium abutments may sometimes impart an unnatural bluish appearance in the gingiva. As an alternative solution, zirconia abutments have been proposed to address this concern. Saker et al. [4] reported that straight zirconia implant abutments supporting three-unit lithium disilicate fixed dental prostheses in anterior region were more fracture resistant than those with a 15° angulation. However, this difference was not statistically significant. They also detected that the mean maximum occlusal force in the anterior region ranged from 250 N to 300 N, with some patients generating up to 605 N of force. Regardless of fatigue loading, fracture resistance in both straight and angulated specimens fell within the physiologic range, consistent with the findings of the present study.

Garcia-Hammaker et al. [7] detected that zirconia abutments with a straight channel (0°) were significantly more fracture resistant than the angulated ones (30°). They noted that for zirconia abutments with a 25° screw channel angulation, failure occurred at the apical portion of the zirconia piece within the two-piece abutment, with minor damage to the screw head. Notably, no visible plastic deformation was observed in the titanium base or implant replica, aligning with the findings of the current study.

In the present study, angulating the screw channel to 15° yielded the highest compressive strength. This result is likely due to the greater thickness of zirconia between the incisal edge and screw hole compared to the 0° group. Conversely, the 25° group exhibited the lowest fracture resistance, possibly because of the reduced circumferential ceramic material and the non-axial force.

Elsayed et al. [22] examined the fracture strength of various abutment types, including titanium, zirconia, and lithium disilicate with titanium inserts, as well as the combination of lithium disilicate abutments and crowns featuring titanium inserts. All abutments were restored with lithium disilicate crowns and attached to standard diameter titanium implants. They observed that the one-piece zirconia abutments exhibited the lowest fracture resistance, with fractures occurring at or above the implant shoulder level. In contrast, the other abutment types with titanium inserts were significantly more fracture resistant, with failure attributed to the bending of the titanium inserts and screws.

Katsavochristou et al. [18] investigated the fracture resistance and performance of zirconia implant abutments with different angulations. Similar to the present study, they discovered that an implant-to-abutment angulation of 15° resulted in a significantly higher fracture resistance compared to 0° and 25°. They also noted that, based on standard deviations, the data for the 0° group were notably more reliable. The standard deviation can be influenced by various factors including the material intrinsic properties, manufacturing factors during abutment milling, and operator’s potential errors when using the universal testing machine. Moreover, the specimens with a 25° angulation were less fracture-resistant than the straight ones (0°); which agrees with the findings of the current study.

Conversely, Albosefi et al. [23] detected that compromised implant positions requiring 15° angulated zirconia custom abutments were less fracture-resistant than the straight ones. This disparity can likely be ascribed to the reduced bulk thickness in zirconia abutments when compared to the custom monolithic zirconia restorations utilized in the current study.

Adolfi et al. [24] evaluated the impact of resin cement on torque loss, vertical misfit, and stress concentration in zirconia restorations cemented to titanium base abutments, as compared to those notched to a titanium base using the hexagon shape of the inner surface of zirconia crowns and the outer surface of the titanium base. Their findings revealed that cement-retained restorations showed a significant decrease in torque loss, stress concentration, and vertical misfit as compared to notched-retained restorations.

Goldberg et al. [25] found no significant differences in the removal torque values and fracture strength of abutment screws among the restorations with dynamic abutment angulations of 0°, 20°, and 28°, nor within the groups. However, they observed different failure patterns ranging from damaged implant platform (although the crowns remained intact) to deformed/loosened screws. This could be attributed to the fact that in that study, the specimens featured casting crowns produced on castable abutments. Apparently, all-ceramic restoration are a more favorable choice compared to metal-ceramic ones, probably due to the fracture mode that does not affect the implant and screws.

Al-Zordk et al. [26] observed that the superstructure material (zirconia, lithium disilicate, or polyetheretherketone) did not significantly affect the torque loss in titanium bases. Internal and marginal fit of titanium base abutments were comparable to other materials. The authors also found that, in cement-retained restorations with titanium base abutments, superior fit and reduced stress were observed compared to one-piece screw-retained restorations.

In the present study, the 15° group exhibited the lowest torque loss, as expected, given the torque loss associated with the increased screwdriver insertion angle. Conversely, the 0° group exhibited the highest torque loss among the three groups, possibly due to the universal screwdriver design, which features a spherical tip that becomes cylindrical as it moves downward. The asymmetrical design of the driver head might facilitate closer engagement between the screwdriver and abutment screw at 15° angulation, potentially transferring less torque to the screw at 0° and 25°. However, the difference in torque loss values in the current study was not statistically significant.

Another study, which did not involve cyclic loading reported no significant difference in RTVs between the 0° and 15° angulation groups. However, when the angulation reached or exceeded 25°, a significant reduction in RTV was observed [27]. An in-vitro study revealed that a 10° angulation yielded higher RTVs compared to 0° and 20° angles. The authors speculated that the inherent asymmetry in the driver head design facilitated closer contact between the screwdriver and the abutment screw at 10°, as opposed to the 0° and 20° angles. Hu et al. [28] reached a conclusion consistent with the present study, suggesting that 20° of angulation yielded the lowest mean RTVs, while the mean RTVs of the 10° group were the highest among the groups.

Mulla et al. [2] investigated the impact of cyclic loading on RTVs in ASC crowns with a 25° angle correction using angled titanium bases. Their findings revealed a significant difference in the RTVi between angulated specimens (25°) and straight control crowns (0°). Although angulated crowns (25°) produced lower torque values than the straight ones (0°), their resulting RTVs after cyclic loading did not exhibit significant differences, consistent with Swamidass’s study [6].

Bonyatpour et al. [5] reported that implant angulation significantly influenced the fracture resistance of one-piece screw-retained hybrid monolithic zirconia restorations. In line with the present study, they observed that 15° of angulation resulted in a significantly higher resistance than 0°. However, in contrast to the present study, they reported that 25° of angulation performed even better than 0°. The difference may stem from the type of specimens used, which were premolars in their study (versus the incisal specimens in the present one). Additionally, it is noteworthy that as the angulation decreases, the hole angle becomes larger, resulting in reduced circumferential material. They observed fractures only in the restorations, not at the screw levels, which aligns with the findings of the current study.

The findings of this study suggest that clinicians can confidently consider one-piece angulated screw channel hybrid monolithic zirconia restorations to ensure both strength and stability in the esthetic zone, thereby enhancing patient outcomes. These restorations exhibit satisfactory fracture strength and minimal torque loss, highlighting their potential clinical utility in implant dentistry.

In this in-vitro study, replicating precise oral conditions and dynamic occlusal movements proved challenging due to the nature of the applied loads (cyclic and static). Therefore, clinical research is required to validate these findings. Furthermore, future studies could explore other ceramic restoration materials and angulated titanium bases to expand upon the findings of this study.