Elsevier

Dental Materials

Volume 37, Issue 11, November 2021, Pages 1667-1675
Dental Materials

Performance of PEEK based telescopic crowns, a comparative study

https://doi.org/10.1016/j.dental.2021.08.020Get rights and content

Abstract

Objective

Telescopic crowns are suitable components of partial dentures to efficiently anchor dental supra-structures to teeth or dental implants and achieve high chewing performance and wear comfort.

Usually alloy- or metal-based structures are used for the primary and the secondary crowns. The advantage is the possibility to produce precise structures with a high perfection and sufficient friction force, but the disadvantage is the corrosion instability. The recent introduction of zirconia ceramics has enabled the fabrication of ceramic primary crowns, thus reducing corrodibility.

The novel application of the high-performance polymer polyetheretherketone (PEEK) as another metal-free alternative material offers a new perspective for such applications.

Therefore, the aim of this work was to assess the performance of telescopic crowns of PEEK by comparing telescopic crowns based on the combination of PEEK (prim. crown) + PEEK (sec. crown) with the pairings ZrO2 (prim. crown) + PEEK (sec. crown) and CoCr-alloy (prim. crown) + PEEK (sec. crown).

Methods

All specimens were CAD/CAM planned and manufactured based on a model of a tooth 26. One master dental technician performed the post-treatment. For each group of material pairing, n = 9 telescopic crown pairs were manufactured and tested. Herein not only the maximum retention force was measured but also the retention force vs. pull-off distance were analyzed. As there is no commonly accepted test protocol available, the influence of various pull off speeds were tested as well. All measurements were first made with three blocks of three crowns (3C), subsequently with three blocks of two crowns (2C) and finally with nine single crowns (1C). The long-term behavior was estimated by performing 10.000 cycles, which is related to a life-time of more than 10 years.

Results

The maximum retention force in case of PEEK + PEEK was higher in comparison to the other tested material pairings. In the range between 1 and 10 mm/ min pull off speed there was no significant influence by the pull off speed. More influence on the friction force would be expected by changes of the number of the crowns acting simultaneously. The friction force was decreasing with decreasing number of crowns but not linearly in any tested case. The long-term test has shown that the friction force remained constant.

Significance

The performance of PEEK + PEEK telescopes is comparable with the usually applied material pairings. Over long time no loss in retention force could be observed. The retention force - distance progression in the PEEK + PEEK pairing offers more security against a possible loss of retention during repair or relining. For further tests of the performance of telescopic crowns or to estimate of friction force limits, a setup with at least two, but preferably three, crowns tested in parallel is suggested.

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.

References (26)

  • K.H. Körber et al.

    Experimental investigations on the influence of the bonding surface size on the reproducible bonding force of conical crowns

    Quintessent Zahntechnik

    (2006)
  • I. Coca et al.

    Long-term experience with telescopically retained overdentures (double crown technique)

    Eur J Prosthodont Restor Dent

    (2000)
  • C. Wagner et al.

    Retention load of telescopic crowns with different taper angles between cobalt-chromium and polyetheretherketone made with three different manufacturing processes examined by pull-off test

    J Protsthodontics

    (2018)
  • Cited by (0)

    View full text