Load Fatigue Performance Evaluation on Two Internal Tapered Abutment–Implant Connection Implants Under Different Screw Tightening Torques

Screw loosening/fracture in implant-supported restorations is one of the most common clinical complications caused by inadequate tightening torque, settling effect, vibrating micro-movement, and excessive bending and fatigue.^1–9  A screw is tightened by applying torque as a clamping force to provide a stable joint between the abutment and implant fixture.^6,9,10,11  This clamping force is also known as the preload, which elongates the screw within the material, increasing the strength with which the abutment and implant are pulled together.^8,12 

Increasing the applied torque (preload) to a screw can provide a more stable joint with greater clamping threshold that overcomes the joint-separating forces that cause screw loosening.¹  Nevertheless, the optimal preload is dependent on the treatment of the interfaces, friction between the components, material properties, and the type of implant-abutment connection.^6,9,10,11  Lang et al¹³  found that the optimal preload was 75% of the screw yield stress. A specified applied torque is usually recommended by implant manufacturers of commercial products. However, applied torque is not the ideal preload for the screw and does not consider the screw's elastic limit.^9,14 

Implant–abutment connection design plays an important role in implant therapy, influencing the force transmission mechanism at the implant–abutment and screw–implant interfaces.^3,15–17  Several studies using external and internal implant-abutment connections showed that the internal conical connection exhibited superior fatigue resistance to that of the external butt joint connection.^3,8,17–21  Two types of internal implant–abutment connections are commonly used for securing the abutment to the implant. The first is a retaining screw (RS), whereas the second is the taper integrated screwed-in (TIS) type. The RS-type connection can secure the implant abutment depending on the screw preload.^16,22–24  The TIS-type connection design combines the press-fit mechanism of the tapered interference fit abutment and the screw at its tapered end.^16,23,25,26  However, the extent of loosening depends on the level of screw preload used in different types of connections.

Quek et al²  evaluated the load fatigue performance of single-tooth implants with different diameters when tightened by applying 3 different torque levels and showed that no significant difference existed among the 3 torque levels but a significant difference was observed between the implant systems.²  However, it remains to be determined whether different applied torques are adequate to offer optimal mechanical stability for different implant-abutment connections under dynamic load. In addition, the dynamic load protocol in this study was rotational fatigue and did not reflect the compressive and shear (lateral) forces recommended by the ISO standard and US Food and Drug Administration (FDA) guidelines for dynamic fatigue testing for dental implants.²⁷ 

The present study evaluates the load fatigue performance of RS- and TIS-type implants tightened using 3 different applied torque levels based on the screw elastic limit. The endurance limit and torque loss in RS and TIS implants under 3 different applied torque levels were also assessed to evaluate the implant stability.

Ninety-six commercially available internal implant–abutment connection implants made by the same manufacturers (IDEOSS, Biotech Co, Taipei, Taiwan) with RS (internal hex screw-in AH7/GH1.5 anatomic abutment) and TIS (AH5.5/GH3 Unicone abutment) connection types were utilized in this study. The implant was made with CP Titanium Grade 5 materials, and the external dimensions were 4 mm diameter and 13 mm length (Figure 1a). The RS connection type was a 2-piece abutment transfixed by a screw and the TIS connection type was a 1-piece solid abutment (Figure 1a). Forty-eight abutments of each type were used.

The tested implant was straight and presented revolution symmetry on its axis. The bone-anchoring part of the specimen was fixed in a rigid clamping device. The embedding material used for sealing the dental implant was epoxy resin (Truetime Industrial Co, Taipei, Taiwan) (4.4 GPa), whose modulus of elasticity should be higher than 3 GPa according to the ISO14801 standard method. The device clamped the specimen at a distance 3.0 mm ± .5 mm apically from the nominal bone level (Figure 1b). A hemispherical loading device (metal cap) was press-fitted onto the abutment to ensure that the load center was located on the central longitudinal axis intersecting the free end of the connecting part and the plane normal to the implant's longitudinal axis. The cap was located 11 mm (i in Figure 1b) from the implant support level.

Three specimens from each RS and TIS group were fixated on a digital torque machine (SE MODEL 2205NS, SE Test Systems Co, Ltd, Taiwan) with a torque load cell (NTS Technology Co, Ltd, Japan) for torque application (Figure 2a). Vertical force was used to drive the sample with a rotational speed of 3 rpm until the screw fractured. The maximum strength of the insertion torques was recorded to determine the elastic limit of the screws. The ratio of the manufacturer's recommended torque (25 Ncm) to the average maximum strength of the insertion torque was calculated for RS and TIS connection types and recorded as RS_rtr and TIS_rtr, respectively (Table). Three torque levels defined as 10% less than the RS_rtr and TIS_rtr torques, recommended torque (25 Ncm) and 10% more than the RS_rtr and TIS_rtr torques were applied to the corresponding test specimens (Table).

The Instron E3000 with the axial load cell (Instron, Canton, Mass) shown in Figure 2b was used to perform static and dynamic testing. The static and dynamic test setup was performed for 6 groups with RS and TIS connection types under 3 different applied torques according to the ISO 14801 (2007) requirements. A cross section of the test setup is shown in Figure 2b. A preload of 5 N was applied. The compressive load was then applied at a rate of 1 mm/min on the loading device (cap) until implant failure or until the force decreased below 20% of the maximum load (Figure 2b). The maximum force of the static test was determined.

The fatigue tests were then carried out according to the static test's maximum load in each group. The cyclic loads were set as 80% to 25% of the static maximum load depending on each group according to the ISO14801 requirements. The test frequency was 15 Hz. The implant that endured over 5 × 10⁶ cycles of load was determined to have passed. Fracture of any component or material yielding, permanent deformation, or loosening of the implant assembly was determined as failure. The critical failure point and location of failure initiation were identified. Three specimens were tested at each of the four fatigue test loads. The load-cycle diagram was plotted by representing the number of load cycles endured by each specimen and the corresponding peak load. The object endurance limit was determined in each group.

Each static testing specimen was embedded and midsectioned along the longitudinal axis using a diamond saw. The internal configuration was visually inspected and photographed under a reflected-light microscope (SVP-2010, ARCS Co, Ltd, Taichung, Taiwan) at ×17.5 magnification to evaluate the failure mode.

Removal torque loss was calculated for each group after the endurance limit was obtained in the fatigue test. The removal torque loss ratio was calculated using the following formula to determine efficacy of the abutment screw types and the implant connection system (Park et al¹²):

Preload loss (%) = (initial removal torque − postload removal torque) / (initial removal torque) × 100

The average maximum strengths of the insertion torques were 91 Ncm and 41 Ncm for RS- and TIS-type implants, respectively (Table). The RS_rtr and TIS_rtr values were 27% and 60%, respectively. The corresponding 10% less and more than recommended torques were 16 Ncm and 34 Ncm for the RS-type implant and 21 Ncm and 29 Ncm for the TIS type implant, respectively.

The static fracture resistance results for the RS-type implant for the 3 applied screw torques from low to high were 761.7 ± 66.6 N, 788.8 ± 20.7 N, and 747.8 ± 64.7 N (mean value and standard deviation) (Figure 3). Corresponding values for the TIS-type implant were 495.5 ± 34.3 N, 559.5 ± 26.1 N, and 654.1 ± 55.8 N. The significant difference (P < .05) in t test analysis showed that the fracture resistance in the TIS-type implant increased when the abutment screw was inserted tightly (Figure 3). All load-cycle diagrams for the test run until 5 × 10⁶ are shown in Figure 4. The dynamic testing results showed that the endurance limitations were 229 N, 197 N, and 224 N for the RS-type implant and 322 N, 364 N, and 376 N for the TIS-type implant when 3 screw insertion torques were applied from low to high. Fracture on the screw first thread junction and large deformation on the implant neck were the 2 major failure modes after static testing, regardless of the type of implant (Figure 5). However, no visible deformation was found in all test abutments when the test specimen passed the fatigue test endurance limitation.

Removal torque losses for the RS-type implant under 3 different applied torques were 22.1%, 29.2%, and 25.8% and revealed no significant difference. Corresponding values for the TIS-type implant were 13.2%, 5.3%, and 2.6%, respectively. Significantly less torque loss was found when the screw locked tightly.

Although inadequate screw torque is accepted as the most important factor that influences force transmission at the implant–abutment and screw–implant interfaces, the optimal screw torque value is still controversial for different implant–abutment connections. Results from the present study indicated that the static fracture resistance and dynamic endurance limitation of the TIS-type implant increased when applying torque on the screw more tightly. Additionally, less torque loss was found with increasing screw insertion torque. These findings revealed that a stable structure was found in the TIS-type implant due to small relative micromotion that occurs between the 2 component structures—a 1-piece solid abutment and implant fixture.

However, no significant differences were found in the static fracture resistance and dynamic endurance limitation for the RS-type implant when the applied torque on the screw varied. This result implied that the screw for the RS-type implant cannot provide sufficient clamping force to provide a stable joint between the abutment and implant fixture. Otherwise, the 2-piece abutment transfixed by a screw in RS-type implants may induce a complicated structural mechanism for relative micromotion and friction force transformation.

Different test methods have been used to evaluate the mechanical strength of implant–abutment connections. The rotational fatigue test was employed in a previous study to investigate the load fatigue performance of narrow-, regular-, and wide-diameter single-tooth implant–abutment systems when tightened using 3 torque levels.²  However, the rotational fatigue test did not reflect mechanical responses on the implant compressive and shear (lateral) forces. The ISO 14801 fatigue test method for single postendosseous dental implants is most useful for comparing endosseous dental implants of different designs or sizes. This method has been used to provide guiding principles for the experimental test design.²⁷  For example, load frequency and wave form represent loading at clinically relevant angles and testing machine characteristics.²⁸  Endurance limit of the object, which can be determined in each group from the load-cycle diagram, is another important feature of this testing approach. Therefore, the ISO 14801 test method has also been recognized by manufacturers to pass FDA 510K certification regulations, and many studies have also applied this standard method to evaluate the long-term in vitro biomechanical behavior of endosseous dental implants.

Static fracture resistance of the RS-type implant was higher than that of the TIS-type implant due to the large relative micromotion and gap found between the abutment and implant in the RS-type implant. Conversely, dynamic endurance limitation of the TIS-type implant was better than that of the RS-type implant. No visible deformation was found in any of the test abutments when the test specimen passed 5 × 10⁶ cycles in the fatigue test regardless of different abutment–implant connections, and all of the endurance limitation values were higher than 197 N (20.10 kg). The 5 × 10⁶ cycles in the fatigue test, which represents the number of occlusal contacts that occur in vivo over approximately 20.8 years (according to Pontius et al²⁹), suggested that there are 1.2 × 10⁶ active cycles over 5 years of service. This result implied that RS- and TIS-type implants can withstand a load over 20 kg for more than 20 years.

Although load fatigue performance for RS and TIS implants was evaluated under 3 different applied torque levels based on the screw elastic limit in this study, no screw loosening or damaged screws from either the abutment or loading cap were found in the fatigue tests. Some assumptions may limit applicability of the experimental results. For example, only typical internal abutment–implant connections for RS- and TIS-type implants were considered in our testing. Angled abutments, settling effect, and repeated insertion/removal cycles on the torque loss of abutments were excluded. Other investigations on the influence of these factors in the maintenance of screw-tightening load should be conducted.

Within the limitations of this study, it can be concluded that static fracture resistance and dynamic endurance limitation of TIS-type implants (1-piece solid abutment and implant fixture) increased when applying torque on the screw more tightly. Less torque loss was also found with increasing screw insertion torque. However, no significant differences were found in the static fracture resistance and dynamic endurance limitation for the RS-type implant when applied torque on the screw varied.

Abbreviations

RS

retaining screw

RS_rtr

ratio of the manufacturer's recommended torque (25 Ncm) to the average maximum strength of the insertion torque of RS connection-type implants

TIS

taper integrated screwed-in

TIS_rtr

ratio of the manufacturer's recommended torque (25 Ncm) to the average maximum strength of the insertion torque of TIS connection-type implants

This study is supported by 105DN08 of the Far Eastern Memorial Hospital and National Yang-Ming University cooperation research project, Taipei, Taiwan.