Performance of PEEK based telescopic crowns, a comparative study
Introduction
Conical or telescopic Double-crowns serve to support and anchor removable dentures [[1], [2], [3], [4], [5]]. The principle is based on a primary crown, which is mounted on a tooth or implant, and a secondary crown covering the primary one, that is integrated into the removable denture.
Becker et al. [6] previously discussed the advantages of this system, such as: (a) physical circular enclosure of the anchor teeth and thus axial and physiological loading of the tooth root, (b) enabling ideal periodontal hygiene, (c) easy handling when inserting and removing of the prosthesis, (d) longevity, and (e) good technical feasibility in the laboratory.
A basic distinction is made between cylindrical and conical double crowns. The cylindrical types are characterized by parallel walls between the primary and secondary crowns, while in the latter the outer walls of the primary crown are at an angle of between 2 and 6 degrees to each other. Accordingly, with cylindrical telescopic crowns, the retention force is already established during assembly due to friction as soon as the parallel walls of the primary and secondary crowns touch, whereas with conical telescopic crowns, the retention force is only established in the final position due to the clamping fit. The retention or friction force is an essential influencing parameter for the success of such a construction [7,8]. Körber and Blum [9] suggested a retention force of 5–10 N, taking into account the possible damage to the abutment teeth. Becker et al. [6] found in measurements on patients that a retention force of 3–3.5 N is sufficient.
In any case, remarkably high demands are placed on the technical implementation. This applies to the selection of materials as well as their machining and processing. So far, metal alloys have been used based on the technical possibilities in the laboratory, especially CoCr alloys, often in conjunction with precious metal alloys due to their higher elasticity and thus better adaptation. However, tribo-corrosion occurs between the metal double crowns, especially when combining differently composed metal alloys. This additionally intensifies corrosion phenomena, which then leads to deposits on the noble metal structures of the outwardly visible superstructure [10].
With the introduction of high-strength ceramics based on ZrO2, another material was available, at least for the primary crowns [11]. However, it was found that the pairing of ZrO2 with CoCr alloy resulted in considerable wear on the secondary crown, which accordingly led to a decrease in the retention force [12]. Due to the high demands on fitting, durability against corrosion and abrasion wear polyetheretherketone (PEEK) appears promising as an alternative metal-free material for the manufacturing of telescopic crowns. PEEK is a high performance polymer with a relatively high elastic modulus of 3−4 GPa, good chemical resistance against almost all solvents with almost zero water uptake, in addition to a lower density of 1.32 g/cm³ compared to alloys and ceramics and exceptional tribological properties. Furthermore, a good bond strength can be achieved with veneering composites [13]. Thus, with the help of CAD/CAM technology, metal-free and lightweight prosthetic frameworks can be produced as an alternative to structures made of conventional materials [14].
Accordingly, there are already studies on the behavior of secondary crowns made of PEEK on primary crowns made of CoCr [15] or ZrO2 [16]. On the one hand, when considering double crowns with a cone angle of 0 degrees, the manufacturing of PEEK secondary crowns by hot pressing appears to have a positive effect on the retention force compared to manufacturing by milling [15,16]. This could possibly be attributed to a certain production-related shrinkage in hot-pressed crowns. When the cone angle is increased by up to 2 degrees, the double crowns produced by machining show higher retention forces, possibly due to the higher stiffness of the PEEK type, so that a higher clamping effect comes into play here [15]. In the combination with primary crowns made of ZrO2, however, this pattern was not confirmed [16].
In general, Schubert et al. [17] discussed retention forces of around 2.8 N from a PEEK secondary crown on a ZrO2 primary crown, which did not change significantly even over a simulated period of 10 years. Attempts to combine PEEK secondary crowns and PEEK primary crowns have also been described [18].
Mostly single crown pairings were tested, with different pull-off speeds under various environmental conditions. In all cases only the maximum forces were analyzed and compared, but the force – distance relationship along the contact area between the primary and secondary crown was not investigated.
Therefore, the aim of present study was to compare the frictional behavior of different material pairings in different numbers of telescopic crowns tested simultaneously at different pull-off speeds, with particular reference to the material PEEK for both primary and secondary crowns. The null hypothesis therefore was that there is no difference in the retention behavior of CAD/CAM manufactured secondary PEEK crowns on primary crowns made of different materials, such as CoCr alloy, ZrO2 and PEEK and the retention force increases linearly with the number of crown pairs.
Additionally, the long-term behavior should be simulated to assess the frictional force over time.
Section snippets
Materials and methods
For the manufacturing of the primary crowns an idealized model of a prepared upper 1. molar (26) was used. As materials a CoCr sinter alloy (Ceramill Sintron, Amann Girrbach AG, Pforzheim, Germany) as an exemplary base metal alloy (BMA), 3Y-TZP ZrO2 ceramic (DD Bio ZX2 color, Dental Direkt GmbH, Spenge, Germany), as well as PEEK (DD PeekMED, Dental Direkt, Spenge, Germany) were used. For each material n = 9 primary crowns with a cone angle of 0 ° were manufactured according to the
Results
Exemplary force (F) - distance (d) curves for each pairing and number of crowns per block at a test speed of 10 mm/min for both mounting and dismounting are shown in Fig. 3. The course of the curves differed partially significantly due to the friction along the contact surfaces of each pairing. The courses of the pairings PEEK + PEEK and ZrO2 + PEEK were similar, in the form of a linear decrease in the retention force from the maximum force at 0 mm distance to non-contact at 6 mm distance. In
Discussion
The aim of this study was to investigate different material pairings PEEK + PEEK, CoCr + PEEK and ZrO2 + PEEK for telescopic double crowns. The focus hereby was on the application of PEEK as primary as well as secondary crown. Telescopic crowns of each material pairing with PEEK as secondary crown were produced in the above shown CAD/CAM workflow (Fig. 1). The final step was a manual post processing of the inner sides of the secondary crowns. For each material pairing crown blocks of one, two
Conclusion
Taking into account the limitations of these investigations, the following conclusions can be drawn.
The combination of primary and secondary PEEK crowns showed adequate performance comparable to conventional material combinations, with this material combination being able to undergo 10,000 pull-off cycles without loss of retention force, corresponding to an average service life of approximately 13 years. Therefore, PEEK seems suitable for the fabrication of primary crowns in combination with
Acknowledgment
The authors would like to thank Christiane Schöpf for conducting experiments.
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