Dental implantation is one of the primary treatment options for partially or fully edentulous patients and has a consistently high success rate (Pjetursson et al., 2014). Traditionally, a screw-retained connection is used to secure the abutment to the implant body. However, because this design requires the use of a central screw, it is often accompanied by mechanical complications such as damage and fracture that result from screw loosening due to excessive occlusal forces. The 5-year incidence rate of complications is 3.1%–10.8% (Pjetursson et al., 2014). These complications can potentially lead to implant loss and even irreversible implant failure (Kim et al., 2021; Bischof et al., 2024).

To address the above issues, new implant–abutment connection designs have been developed that take advantage of advancements in material technology to improve mechanical stability and reduce screw-related failures. The newest designs are primarily categorized into two types: external connection and internal connection. External connection is achieved by connecting a protrusion in the implant’s upper plane with a concave area in the abutment’s lower plane. Internal connection is achieved between a concave area in the implant and a protrusion in the abutment. Internal connections are associated with a lower incidence of bone resorption and mechanical complications and are more commonly used in clinical practice (Caricasulo et al., 2018; Camps-Font et al., 2023). Conical connections, also referred to as Morse taper connections (Schmitt et al., 2014), provide a tight fit between the implant and abutment, thereby minimizing bacterial penetration and improving long-term stability (Ricomini Filho et al., 2010; do Nascimento et al., 2012; Machado et al., 2013). Popular Morse taper systems include Ankylos (5.7°), ITI (6°–8°), and Astra Tech (11°), all of which use screws for retention. In contrast, the Bicon system (1.5°) uses a screwless Morse taper connection design that relies purely on frictional engagement for retention and thus eliminates the risk of complications that arise from screw loosening or fracture (Tang et al., 2017).

While Morse taper systems show acceptable implant survival rates (Mangano et al., 2011; Urdaneta et al., 2012) and predictability and demonstrate improved convenience in clinical practice, the performance of this system is still under debate. In a previous study on dental implant restoration complications, a screwless Morse taper system was associated with high rates of restoration dislodgement in the maxillary anterior area (Urdaneta et al., 2010). This may be because of two reasons. First, the low bone density and oblique occlusal loading in this area may increase the micromovement of the implant–abutment interface. Second, restorations in the maxillary anterior area mainly experience forces that are non-axial, which can cause rotation or loosening of the abutment when the force becomes excessive. The same study also observed that Morse taper systems may have fractures in abutments with 2.0 mm locking diameters and dislodging in abutments with 3.0 mm locking diameters in the posterior areas (Urdaneta et al., 2010), likely because these areas are subject to greater chewing forces that cannot be sustained by narrow abutments. These findings suggest that the abutment diameter and loading patterns greatly affect the performance of Morse taper systems. However, clear understanding of the mechanical performance of screwless Morse taper connections remains limited because of the lack of data.

To fill this gap, we conducted in vitro tests to evaluate the mechanical strength of a screwless Morse taper implant–abutment connection system, which adopts a narrow-diameter design that was less investigated before. Compared with previous works, we included more types of tests and also greatly increased the extent of mechanical challenge in the tests. Our findings provide useful insights and references for developing more advanced implant restoration strategies.

By simulating the chewing motion, this study evaluated the mechanical performance of a single taper-retained dental implant–abutment system and inspected how the system is impacted by the size of the implant–abutment connection interface. The results demonstrated that the single taper-retained connection demonstrated excellent stability while producing minimal micromotion and wear under various high-stress conditions; this system should therefore help prolong the service life of the implants. These findings not only enhance the understanding of single taper-retained implant–abutment systems, but also provide useful references for clinicians and may help improve the success rate of dental implantation and patient satisfaction.

The single taper-retained connection system tested in this study involved a screwless design with an internal connection taper angle of 1.5°. Retention of this system relies entirely on the friction generated by the tight contact between the implant and the abutment. During implantation, the implant is tapped onto the abutment with external force, which produces a wedging effect at the implant–abutment interface (IAC) to achieve a tight connection—sometimes referred to as “cold welding.” This novel connection mechanism has several advantages over traditional mechanisms. First, it constrains the microgap at the coronal aspect of the IAC, which ensures superior sealing and stability, significantly reducing bacterial penetration and lowering the risk of infection (Baixe et al., 2010; Chu et al., 2012; Liu and Wang, 2017; Gao et al., 2021). Second, the abutment has a solid cylindrical structure, which has high mechanical strength, also ensuring high stability at the peri-implant bone levels and a long service life of the implants (Ribeiro et al., 2024). Finally, owing to the high-precision manufacturing of the implant–abutment interface, consistency can be ensured across implantations.

Static and dynamic loading tests are commonly used to evaluate the mechanical strength of implant–abutment connections. For instance, Dittmer et al. studied the fracture strength of Ankylos Morse taper implants by applying a 30° oblique load; the failure load was 368 N for the screw-retained connection and 430 N for the Morse taper connection (Dittmer et al., 2012). Imam et al. investigated the static load strength of narrow-diameter implant systems; the average static destructive power was 367.20 ± 98.05 and 568.80 ± 85.24 N for implants with a 3.0 mm or 3.5 mm connection diameter, respectively (Imam et al., 2014). Marchetti et al. conducted fatigue tests on implant systems with an internal hex connection of 3.8-mm diameter; the results showed that the static destructive power was 499.6 ± 50.1 N (Marchetti et al., 2014). Duan et al. conducted fatigue tests on implant systems with a 3.3 mm connection diameter; under a cyclic load of 185 N, the probability of failure after 1 × 10⁶ dynamic loading cycles was 5% (Duan et al., 2018). The study suggested that, compared with narrow-diameter implants, implants with the same connection diameter but using the Morse taper connection mechanism achieved higher static and fatigue load capacity. Notably, in our study, the static destructive powers of the M1 and M2 implants both exceeded those of the previously reported Morse taper and screw-retained connections. Furthermore, Dittmer et al. showed that the internal connection diameter of the implant has a significant influence on its overall mechanical strength (Dittmer et al., 2012), which corroborates our findings in comparing M1 and M2.

Humans can generate great bite forces (Wang, 2012; Zhou et al., 2022). Ferrario et al. reported that in healthy young adults, the occlusal force on a single tooth is between 98.33 and 306.07 N (Ferrario et al., 2004). Shoji et al. found that occlusal forces in healthy adults ranged from 86.40 to 1758.60 N, with an average of 798.33 ± 492.16 N (Shoji et al., 2022). After an implant–abutment system experiences more and more load cycles with such large forces, it is expected to have gradually declined mechanical performance (Ricciardi Coppede et al., 2009). Nonetheless, in this study, both M1 and M2 implant–abutment connections withstood 5 × 10⁶ cycles of dynamic loading at 23.2–232 N, demonstrating excellent mechanical durability.

By performing in vitro fatigue tests, Ugurel et al. found that the mechanical strength of screwless Morse taper implants with a 3° taper angle was lower than that of screw-retained implants. None of the tested screwless Morse taper implants survived 1.2 × 10⁶ cycles of dynamic loading of 120 N applied at a 30° angle; all samples failed within the first 1 × 10⁵ cycles as a result of fracture (Ugurel et al., 2015). In contrast, in the study by Bagegni et al., all tested screwless Morse taper implants survived 1 × 10⁶ cycles of dynamic loading of 100 N (Bagegni et al., 2022). The discrepancy between these two studies may be because the implants tested in the first study had a thinner internal wall. Here, our tested samples also successfully survived 5 × 10⁶ cycles of dynamic loading applied at a 30° angle, supporting the conclusion that single taper-retained connections can achieve high mechanical strength. In contrast with the results from Ugurel et al. (2015), the screwless implant samples in our study showed no surface cracks or abutment fractures after testing, consistent with Bagegni et al. (2022).

Screw loosening or fracture is the most common mechanical complication in dental implant restorations (Schwartz-Arad et al., 1999; Theoharidou et al., 2008). The likelihood of screw loosening and fracture is influenced by the design, material, and diameter of the screw, the applied torque, and the geometry of the implant–abutment connection (Gupta et al., 2015), but was reported to be considerably high across multiple studies (Goodacre et al., 1999; Pintinha et al., 2013; Hsu et al., 2018; Pjetursson et al., 2018). In our study, none of the tested implants showed abutment fracture after five million mastication-simulating load cycles, demonstrating their mechanical reliability and superiority. The main reason for screw loosening after implantation is that the dynamic load cycles during mastication may cause the vertical subsidence of the abutment, which can induce slippage between implant threads and screw threads, resulting in torque loss and screw loosening (Kim et al., 2014; Bagegni et al., 2021). We previously found that during simulated mastication, the total vertical displacement of screwless Morse taper abutments ranged from 0.20 to 0.47 mm (average: 0.32 mm), and the finger-pressed displacement ranged from 0.06 to 0.15 mm (average: 0.10 mm); the degree of subsidence was affected by the loading force, locking depth, locking diameter, and thickness of the implant’s external wall (Ren et al., 2024). Further in vitro studies are needed to determine the optimal design for related key parameters such as locking diameter and depth, so as to prevent or minimize abutment subsidence in patients.

For screw-retained implant–abutment connections, in addition to repeated screw tightening (D’Addazio et al., 2021), the geometry of the connection also affects the pull-out resistance. For instance, smaller angles were found to offer greater resistance (Caballero et al., 2024). A similar result was observed for screwless connections as well (Nogueira Barbosa Marchon et al., 2024), which is likely because of two reasons. First, a smaller taper angle leads to a greater contact area, which increases the friction at the implant–abutment interface and thus strengthens the cold welding effect (Caballero et al., 2024; Nogueira Barbosa Marchon et al., 2024). Second, Hsu et al. showed that smaller taper angles in implant–abutment connections may result in greater axial displacement, thereby increasing friction and thus the pull-out resistance (Hsu et al., 2018). Like in our work, Hsu et al. also knocked the upper component into the implant to achieve the connection, which led to pull-out resistance and mechanical stability comparable to the screw-retained connections (Hsu et al., 2018). The implants in our study use an internal taper of 1.5° with screwless solid abutments that maximize the implant–abutment contact area, ensuring adequate pull-out resistance. The satisfactory pull-out test results of these implants are strongly encouraging and indicate that alternative implant–abutment connection designs should be considered to overcome the issue of screw loosening in traditional designs. In addition, our data suggests that increasing the locking diameter and thus the frictional contact surface can also increase the axial pull force.

A higher torque in screw-based systems leads to greater preload, which reduces the risk of screw loosening and restoration dislodgement (Cibirka et al., 2001; Coelho et al., 2007). However, excessive torque beyond a threshold can create significant friction between the implant and screw threads, which can potentially lead to wrench fracture. Hyun et al. found that adding a positioning hex design to a Morse taper abutment ensures a snug fit with the implant’s corresponding hex slot, and thus can effectively increase torsional resistance (Hyun et al., 2020). However, adding a hex design to a Morse taper abutment also reduces the conical surface area of the abutment, which may compromise the biomechanical performance of the Morse taper connection. Yao et al. demonstrated that in the Cowell implant system with a 7° taper, the single taper-retained connection offered no torsional resistance. The authors found that incorporating an internal octagonal structure provided torsional resistance, but it also lowered the bending strength of the abutment (Yao et al., 2015). Farré-Berga et al. showed that with a 20° angle in the screw channel, connections in the Ball Head System and the Hexagonal Screwdriver System had a mean yield torque of 67.0 ± 12.0 and 45.0 ± 2.0 N cm, respectively (Farre-Berga et al., 2020). In our torsional resistance tests, M1 and M2 samples showed a maximum yield torque of 130.0 ± 12.0 and 156.5 ± 6.6 N cm, respectively, which were substantially higher than those reported by Farre-Berga et al. (2020), even though our abutments use no anti-torsion features. This result suggests that increasing the contact area and friction between the abutment and the implant effectively strengthens the cold welding effect, ensuring reliable torsional stability.

The performance of the implant–abutment connection plays a crucial role in the successful implantation of implant-supported fixed restorations. The abutment is connected to the implant via external or internal connection mechanisms, with the internal connection being biomechanically advantageous. Peri-implantitis and marginal bone loss are two common complications of implant-supported restorations that are induced by the gap space at the implant–abutment interface, with the former often preceding and facilitating the latter. Under repetitive lateral occlusal loading, traditional screw-retained systems, even those with an internal hex design, are prone to screw loosening, which increases the gap space at the implant–abutment interface, leading to bacterial infiltration and subsequent bone resorption, especially in jaw areas of low bone density (Gehrke et al., 2023; Angermair et al., 2024). In contrast, because of its self-locking design, the Morse taper connection offers superior lateral stability, which does not form microgaps under lateral forces (Angermair et al., 2024).

Terrats et al. studied the lateral mechanical strength of single-unit and multi-unit restorations with a narrow diameter implant; the lateral failure force was 118–230 and 488–759 N, respectively, demonstrating clearly greater lateral load resistance in the multi-unit restorations (Terrats et al., 2024). Our study showed that M1 and M2 implants had a lateral resistance of 140.2 ± 14.7 and 238.1 ± 14.3 N, respectively, which are relatively low but within clinically acceptable limits. Furthermore, comparison of the results of M1 and M2 indicates that increasing the abutment diameter enhances the implant’s fracture resistance and lateral load-bearing capacity. In anterior regions, implant restorations may endure non-axial forces 4–5 times greater than normal occlusal forces. Such lateral loading can induce micromotion and microgaps, potentially causing marginal bone resorption. Through continued design optimization (e.g., reducing the taper angle or increasing the contact area), researchers should be able to improve the lateral stability of the implant–abutment connection.

Overall, under standardized conditions, all samples tested in this work withstood fatigue loading, indicating that the single taper-retained implant–abutment connection represents a reliable option for dental restorations. Nonetheless, due to certain limitations of our work such as small sample sizes and lack of high-resolution microscopic imaging (e.g., scanning electron microscopy), as well as the fact that in vitro conditions cannot fully recapitulate the oral environment, further standardized tests are needed to more comprehensively analyze the mechanical behavior of single taper-retained implant systems.

This in vitro study demonstrated that a single taper-retained implant–abutment connection offers strong mechanical performance under both static and long-term dynamic loading conditions. Increasing the locking diameter of the abutment improves the mechanical performance of the connection.