Dynamic loading leakage test of dental abutment/implant connections based upon a novel implant system with an abutment switch feature: an in vitro study

Introduction

The failure of dental implants is multifactorial and can be attributed to a number of reasons. Previous studies have addressed the question of whether the cause of implant failure is primarily mechanical, resulting from incorrect loading, or whether biological complications, such as microbial colonization, play a greater role [1]. As posited by Kim et al., the notable discrepancies between implants and natural teeth can be attributed to the absence of the periodontal ligament. This is associated with disparate tactile perception and mobility of the implants, which consequently manifests as different behaviors when subject to excessive loading [1]. The tooth mobility provided by the periodontal ligament enables the teeth to adapt to varying load conditions, including those resulting from jaw deformation during mastication. In contrast to natural teeth, dental implants lack the periodontal ligament, which transmits sensory feedback regarding the magnitude of chewing forces. Consequently, it is hypothesized that implants are subjected to considerably higher chewing forces than their natural counterparts [1],2]. It is assumed that occlusal contacts, parafunctions and extra axial loading along with occlusal forces play an important role in their survival rate [3], [4], [5]. Conversely, other studies seek to demonstrate that implant loss is primarily due to the development of peri-implantitis [6].

In relation to this matter, the configuration of the implant-abutment junction is of significance regarding the longevity of dental implants and has already been the subject of investigation in several previous studies [7], [8], [9]. Dynamic loading (e.g. during the chewing process) results in micromotion of the abutment. It has been observed that this micromotion can lead to a bacterial colonization in the interior of the implant and its associated soft tissues around two-part implant systems, despite the absence of this phenomenon in one-part implants [10], [11], [12], [13]. Conventional implant systems feature either a conical or a butt joint, more precisely platform abutment interface with or without anti-rotation protection. The use of conical abutments has become increasingly common over time, as evidenced by the fact that the majority of studies are conducted with these superstructures in mind [14]. However, depending on the initial situation or the practitioner, it is still advisable to use butt joint abutments [15]. The tioLogic^® TWINFIT implant series features a newly developed “abutment switch” and permits a transition between conical and butt joint abutment configuration at any instance. According to the manufacturer Dentaurum^®, this system has been designed to give the dentist maximum flexibility during implant placement.

The purpose of this investigation was to compare the sealing capability of the implant-abutment interface between the tioLogic^® TWINFIT implant series and implants from other manufacturers. A dynamic loading leakage test was performed, according to which it is to be weighed whether the design can withstand intra-oral exposure and to assess the implants’ clinical behavior. The objective was to examine the impact of an increasing dynamic load on the sealing capacities of the implants.

Materials and methods

Tiologic^® TWINFIT

The tioLogic^® TWINFIT implant system features an “abutment switch” that enables the selection of either a “conical” or a “platform” abutment configuration, eliminating the need for the practitioner to decide in advance which abutment type to use. Both abutments have five indexes and an integrated platform switch. It should be noted that the platform abutment offered by Dentaurum^® deviates from the standard platform abutments, as it has a slight cone at the upper part (Figure 1). We utilized the TWINFIT implant (Dentaurum, Ispringen, Germany) in size “M”, which has a 4.2 mm diameter and a length of 15.0 mm.

Figure 1:

Conical and platform abutment in cross-section. A comparison of the conical and platform abutment of the tioLogic^® TWINFIT implant system in cross-section reveals that, although the platform abutment possesses a slight cone in the upper area, it is predominantly positioned with the flat surface on the implant shoulder. (Figure: courtesy of Dentaurum).

Specimen preparation

To conduct the dynamic loading leak test, the implants were altered to include an apical Luer-Lock hose fitting that was linked to the pump (Figure 2). Blue water infused with standard ink (LAMY T 52 ink – blue, LAMY GmbH, Heidelberg, Germany) was then pumped into the specimen. After that, the threaded part of the implants was set in resin (PalaXpress, Haereus Kulzer GmbH, Hanau, Germany) within an 18 mm long copper tube using an embedding guide. The resin level was set to replicate a 3 mm bone loss. The abutments were fitted with a spherical load head on top, with the sphere’s center positioned 8 mm above the implant shoulder, resulting in a total height of 11 mm for the dynamic load applied (Figure 3). Before tightening the abutments with a 30 Ncm torque in accordance with the manufacturer’s instructions, the pump was activated to vent the remaining air from the tubes and the specimen, to prevent the formation of air bubbles within the hose, which could otherwise obstruct its functionality. Each specimen was subjected to an initial leak test at maximum pressure of the pump for 1 h after the screws were tightened, before undergoing the actual dynamic loading leak test.

Figure 2:

Schematic representation of the prepared test specimen. The cross-section shows the implant with abutment, embedded in a copper tube, as well as the apical modification using a Luer connection. (Figure: courtesy of Dentaurum).

Figure 3:

The four different tested abutment systems with spherical load head on top. (A) tioLogic^® TWINFIT M conical specimen. (B) tioLogic^® TWINFIT M platform specimen. (C) Astra Tech OsseoSpeed™ EV specimen. (D) CONELOG^® specimen.

Loading criteria

The objective is to analyze the sealing capability of the implant-abutment interface under extra-axial loading. The criteria for dynamic fatigue testing of endosseous dental implants are defined in the DIN EN ISO 14801 standard, which offers a solid foundation for comparing results among various specimen types [17]. A specimen holder was prepared with a central bore for the above-mentioned copper tube to mount the implant in a way that the loading direction of the apparatus is in a specific angle without any lateral restriction. Contrary to the DIN standard, the loading angle was set at an angulation of 20° relative to the implant’s long axis, to allow comparison with the test on which our setup is based [16]. Following the ISO standard, the loading force was applied via the custom designed spherical head of the abutments (see Figure 3) without constraining lateral movement. The applied force varied between 100 and 10 % of maximum loading. To conduct the leakage test, specimens were submerged in distilled water (aqua dest, Ampuwa, Fresenius Medical Care, Bad Homburg v.d. Höhe, Germany) at a temperature of 37 ± 2 °C. The overall setup including a scheme of the loading geometry is shown in Figure 4.

Figure 4:

Schematics and photo of the overall setup. (A) Scheme of the loading geometry [17]. D: angle of the incoming force, which is changed to 20° in our setup. I: The loading is applied at a height of 11 mm above the implant shoulder. (B) Embedding apparatus with copper tube and embedded specimen. The outer frame provides space for the copper tube. An implant with the appropriate diameter fits into the inner recess so that it is held in position during the embedding process. The apical Luer-Lock-hose fitting at the bottom of the specimen is visible. (C) Specimen holder. A wedge ensured the angulation of 20°. (D) Specimen in the specimen holder mounted in the Dyna-Mess device in the water bath.

Results

Table 2 gives a full overview of all specimens and the corresponding test results from this study.

Failure force

Figure 5 presents the average failure force for the tested implant-abutment systems, along with a detailed breakdown of results for each specimen tested. The conical tioLogic^® TWINFIT implant-abutment connections did not leak prior to fracture at an average force of 580 N. The machine reached maximum deflection, resulting in fracture of one abutment and fixation screw, while the other two specimens bent at the abutment level. Each specimen in the Conelog^® Progressive series fractured in the neck region at the resin level before leaking, at a rather high force around 630 N. However, the Astra Tech™ implants exhibited a permanent loading fracture at a relatively small force level of 330 N, and the implants fractured below the resin level. The platform version of the tioLogic^® TWINFIT implant-abutment connection was the only one to leak prior to fracture at a relatively low force of 320 N.

Figure 5:

Failure force in Newton (N). Detailed display for every specimen of each implant type. The columns display the mean force of failure, with each point representing an individual specimen. Some test specimens reached the same maximum force, resulting in overlapping points on the diagram. Which points overlap can also be identified through the displacement of the column between two points.

Failure mode

The failure modes manifest in fracture, permanent deformation, and leakage. According to Figure 6, the conical connection type of the tioLogic^® TWINFIT series exhibits better performance than the other samples. These implants were capable of withstanding relatively higher forces and were permanently deformed. The Conelog^® Progressive and Astra Tech™ implants exhibited evidence of fracture prior to leakage, whereas the tioLogic^® TWINFIT platform connection types demonstrated leakage in all cases.

Figure 6:

Different failure modes for each implant. The following tables present the different types of failure and the corresponding number of test specimens for each implant abutment connection.

Discussion

A microgap can occur between the components of a two-part implant system. Butt joint abutments are seated with a platform on the implant shoulder and feature parallel surfaces at the inside of the implant. Due to the loose fit, it is inevitable that a small gap between the longitudinal contact surfaces appears. If fixated with an abutment screw and under static conditions, this gap is sealed. A loading force higher than the introduced force onto the abutment screw can lead to a micromovement of the abutment [18]. Due to the two parallel cone surfaces, the conical connection is able to eliminate the space between the two parts. In the hypothetical event of tilting when loaded, the abutment cone’s interior portion contacts the implant’s cone surface, preventing tilting and gap formation [19].

In several studies the implant-abutment interface in various two-part implant systems has been investigated. Due to differing testing methods, the exact location of the measured gap varies, thus making the results difficult to compare, but in general, the presence of a microgap and bacterial contamination of the internal implant surface and the abutment screw have been demonstrated [20], [21], [22]. Such contamination was not observed for single-piece implant systems [10].

Dynamic loading simulators have previously been utilized and validated to provide evidence of the presence of a microgap [23]. Suttin et al. utilized a specific method to examine the sealing capacity of the implant abutment interface [16]. The method described in our study adapted the dynamic leak test from Suttin et al. to adhere to the DIN EN ISO 14801 standard where applicable. Implants with an angle that deviates by more than 15° from the original tooth axis require pre-angulated abutments, hence, the angulation criteria were modified to a 20° axis instead of the original 30 ± 2°. In our study, we exclusively examined straight abutments.

Suttin et al. analyzed four different types of implants, including two platform abutment types (Thommen SPI^® Element and 3i T3^® with DCD^®) and two conical abutment types (Astra Tech™ Osseo Speed™ and Straumann^® Bone Level). In their study, the platform implant abutment connections showed leakage prior to failure and the conical abutment specimens exhibited different failure modes [16]. The earliest failure in our study was the tioLogic^® TWINFIT sample, which exhibited a leaking failure mode at an average force of 320 N. This is slightly above to the earliest failure from Suttin et al. (Thommen SPI^® Element implants, which all failed due to leakage at a mean force of 230 N). In contrast, their Astra Tech™ Osseo Speed™ and Straumann^® Bone Level implants failed with an average load of 520 and 570 N, respectively. Our conical tioLogic^® TWINFIT sample sustained an average load of 580 N before fracturing, indicating performance similar to that of the Straumann^® Bone Level implants. Nevertheless, the Astra Tech™ Osseo Speed™ sample behaved differently in both investigations. In the aforementioned investigation, Astra Tech™ implants leaked after surviving a mean loading of 520 N. In our study, similar specimens fractured at a meagre loading of 330 N. Suttins 3i T3^® with DCD^® implant specimens, which are equipped with a platform abutment connection, survived the highest loading forces with an average of 740 N. However, there is a distinction to our specimen: the Thommen, Astra Tech™, and Straumann^® abutments were fitted with titanium alloy screws just like our specimen, whereas the 3i T3^® abutments were fitted with a Certain^® Gold-Tite^® screw [16].

In another study, Suttin and Towse [24] compared the 3i Certain^® Connection with both the Gold-Tite^® screw and the Ti alloy screw in a slightly modified test protocol that involved replacing the screws during testing. They conclude that their findings demonstrate the importance of the screw material on the sealing capability of the implant-abutment connection, given that the sample with the Ti alloy screw failed at an average force of only 500 N, whereas when switched to the gold screw an average sealing capacity up to 780 N could be reached [24]. Future investigations with our specimens could explore whether changing the abutment screw would impact the sealing capability of the tioLogic^® TWINFIT platform abutment type. As for the Astra Tech™ Osseo Speed™ sample, the test could be redone with a lower level of simulated bone loss to investigate whether the early fracture is caused by the narrower implant neck.

Most importantly, screw loosening or fractures are key contributors to implant-abutment system failure [25]. Over time, it becomes more likely for implants to fail under increased loads and unfavorable extra axial forces. In 2015, Gupta et al. published a review of causes of implant failure, such as screw loosening or fracture, and how to prevent or manage these complications [4]. Some studies have suggested a relationship between microleakage and varying closing torques. Specifically, microleakage decreases as the closing torque increases [2]. According to Zipprich et al. [18], forces from different directions affect dental implant crowns during chewing. These include axial, mesiodistal, and orovestibular forces, which result in extra axial forces. Additionally, the authors noted the presence of a torque moment, in which a 100 N loading force at a 30° angle to the implant axis induces a 40 Ncm torque, resulting in abutment screw loosening and micromovements. Therefore, it is crucial to adhere to the correct fixation torques for the abutment screw [12], [26]. Additionally, it is recommended to retighten the screw after a few minutes [4]. Although we followed this procedure in our approach, it did not prevent the platform implant-abutment connection from leaking.

The objective of the fatigue load tests is to determine the forces that result in failure of the implant-abutment connection. In clinical conditions, the applied forces vary over a large range, depending on the muscles involved as well as on the characteristics of the food being processed. These factors, in turn, influence the number of chewing cycles required for mastication [27]. Food consistency changes during the process, making it challenging to replicate the outcome and causing difficulties in measuring the exact chewing forces [28]. In several studies the maximum chewing forces of patients were investigated. In some cases, different regions of the jaw were examined using different methods, making it difficult to compare the results. The average force achieved by healthy young participants in one study was 597 N for female participants, while the male participants achieved an average force of 847 N [29]. Other studies have shown average bite forces of 372 ± 133 N in patients with normal occlusion [30]. The effect of masticatory forces on natural teeth and implants differs, as the natural teeth are anchored to the bone via the periodontium, while implants are in contrast firmly fused to the bone [28]. Other studies investigated older, previously edentulous patients who are then rehabilitated with a prosthesis supported by two implants. Although higher bite forces were achieved after treatment compared to the edentulous situation, these forces were still in the low force range well below 100 N [31], [32]. Van der Built et al. conducted a study on edentulous patients who had undergone the insertion of two implants in the mandible. The maximum bite force of the patients was measured prior to treatment, after 5 and 8–14 months, and after 10 years. The initial decrease in maximum bite force from an average of 183 N before treatment to 162 N after 5 months was followed by an increase to 337 N and 341 N at the subsequent measurements, which is almost equivalent to a doubling of the force [33].

In summary, the majority of studies demonstrated that rehabilitated patients with implant-supported prostheses exhibited lower bite forces in comparison to dentate patients. The load limits of several test specimens fell within this range, while others exceeded it. However, it should be noted that implants lack the periodontal ligament and, consequently, the sensory feedback present in natural teeth [1],2]. This suggests that implants may, in fact, be exposed to very high loads, which may be further exacerbated by incorrect loading.

As this study constitutes a worst-case scenario, with the primary focus being on the identification of leakage, the maximum bite force achieved is of secondary importance. Nevertheless it is important to emphasize that, in real-life situations, appropriate occlusal contacts, elimination of bad habits or parafunctions such as bruxism, and reduction of extra axial loading can help prevent material fatigue [4], [5].

Conclusions

This research investigated the ability of the tioLogic^® TWINFIT implant system to withstand intraoral exposure. It conducted a comparative analysis between the two abutment types and those of other well-established implant systems under conditions of a worst-case scenario. The results of our study confirm those of previous investigations indicating that cylindrical implant-abutment connections are more susceptible to microgap formation, thereby facilitating the penetration of fluids and bacteria into the interior of the implant. It is important to mention that the tioLogic^® TWINFIT platform abutment is distinct from a conventional platform connection. Consequently, the outcomes of this study may not be directly comparable to those of previous research in this field. Furthermore, additional research approaches could facilitate further investigations into the possibility of prolonging the lifespan of the tioLogic^® TWINFIT platform abutments. As the reasons for implant failure are multifactorial, the use of the tioLogic^® TWINFIT implant system remains a viable recommendation. In the final analysis, the choice of abutment connection ultimately falls to the practitioner. However, in terms of the range of materials and equipment required for different implant systems, the tioLogic^® TWINFIT system provides great flexibility while maintaining a manageable number of tools.