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 Table of Contents  
ORIGINAL RESEARCH
Year : 2017  |  Volume : 9  |  Issue : 5  |  Page : 202-206

Polyether ether ketone in protocol bars: Mechanical behavior of three designs


São Leopoldo Mandic Dental Research Center, Campinas, São Paulo, Brazil

Date of Web Publication20-Oct-2017

Correspondence Address:
Geraldo Alberto Pinheiro de Carvalho
Rua Vereadora Lucia Carvalho 235, Parque São José, Varginha, Minhas Gerais
Brazil
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jioh.jioh_163_17

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  Abstract 

Aim: This study evaluated the compressive strength (CS) of protocol bars on polyether ether ketone (PEEK) implants compared to metallic bars (NiCr). Materials and Methods: Thirty PEEK bars and thirty metallic bars (control) were produced using three different cross-sectional designs (n = 10): rectangular cross-section solid bar (R), T-type (T), and inverted T-type (T inv). All bars were 30 mm long. The bars were screwed to Cone Morse implants of 3.75 mm × 11.0 mm and submitted to compression strength test in a universal testing machine (0.5 mm/min), with the load applied to the bar cantilever. After the test, we measured the counter torque on the screws. Results: Compression strength (N) and counter torque (N/cm) data were analyzed with two criteria: ANOVA and Tukey (α = 0.05). PEEK bars showed mean compression strength significantly lower than NiCr bars, for all designs (P < 0.001), with R bars showing the best results and T inv, the worst results. PEEK bars showed smaller counter torque values than NiCr bars for R or T designs (P = 0.002). Conclusion: For PEEK bars, T inv design resulted in the smaller compression strength. PEEK solid bars, with rectangular cross-section, showed larger resistance to compression in comparison to the other designs. However, PEEK bars showed lower compression strength than that verified for metallic bars.

Keywords: Dental implants, polymers, prosthesis design


How to cite this article:
de Carvalho GA, Franco AB, Kreve S, Ramos EV, Dias SC, do Amaral FL. Polyether ether ketone in protocol bars: Mechanical behavior of three designs. J Int Oral Health 2017;9:202-6

How to cite this URL:
de Carvalho GA, Franco AB, Kreve S, Ramos EV, Dias SC, do Amaral FL. Polyether ether ketone in protocol bars: Mechanical behavior of three designs. J Int Oral Health [serial online] 2017 [cited 2022 Jan 18];9:202-6. Available from: https://www.jioh.org/text.asp?2017/9/5/202/216951


  Introduction Top


The biggest challenge of oral rehabilitation is the replacement of lost structures and the restoration of its function and esthetics, focusing on emulating a healthy tooth.[1]

Protocol prosthesis on implants is the most commonly used in complete-arch rehabilitation.[2] Co-Cr and Ni-Cr alloys are commonly used as scaffolds for the production of crowns and prosthetic bars.[3] However, they are poorly capable of absorbing and dissipating the stresses applied to the implants, which increases the risk of losses.[4]

In this sense, efforts have focused on the development of non-metal materials for implant rehabilitation. Polyether ether ketone (PEEK), a linear thermoplastic polymer, has been studied and applied to dental devices.[5] This material combines excellent physical properties, high-temperature stability, and resistance to chemical damage.[4],[5] Furthermore, its elastic modulus is similar to that of the bone,[6] it is biocompatible, bioinert, and radiolucent [7] and is compatible with glass and carbon fibers.[8] As a drawback, PEEK alone is not bioactive and has a limited capacity to accommodate its mechanical properties to the implant form.[9]

Protocol prosthesis presents several positive aspects regarding biomechanics, favoring the distribution of occlusal loads. However, its success is tied to the presence of a passive bar connecting the implants and promoting the stabilization of the prosthesis-implant system.[10] The design of the bar is fundamental for the rehabilitation success since it optimizes the material's physical and mechanical properties, particularly on the cantilever. However, conventional metallic structures with solid design present a high incidence of fractures.[11]

A number of bar types and designs have been suggested.[12] The best biomechanical results were obtained with T-type bars, I-type bars, and more recently, the inverted T-type bars. These are lighter and capable of resisting the loads and dissipating strength.

PEEK, associated to different designs of protocol bars, is capable of optimizing several clinical routine situations. Hence, it is important to analyze the mechanical behavior of the three different designs of PEEK protocol bars and to compare it to the behavior of metallic bars. This study aims to do that with the compression test.


  Materials and Methods Top


This is an experimental, laboratory, and retrospective research that took 6 months to be realized. A previous pilot test was carried out to define the bar design and a sample calculation was performed to determine the number of specimens.

Obtaining the test base

A rectangular metallic matrix was built in stainless steel (48 mm × 38 mm × 15 mm) with two holes with 4.9 mm of depth and 7.8 mm of height. The holes were located on the top part of the matrix, equidistant from the matrix edges, and vertically aligned.

Two implants Titamax CM ExAcqua 3.75 mm × 11 mm (Neodent, Curitiba, Brazil) were used. To avoid displacements, each implant was fixated using high-viscosity epoxy resin. The implants were placed on the holes, at a distance of 18 mm between the midlines of the columns. A straight CM mini column, with a 1.5 mm band (Neodent, Curitiba, Brazil), was installed over the implants following the manufacturer's instructions.

Obtaining the test specimen

Prosthetic components were scanned using Dental Wings 3Series (Dental Wings, Montreal, Canada) to create a virtual model, from which a digital project for three protocol-type bars was employed to connect the implants on the metallic matrix. The project was created on three dimensional Geomagic (Geomagic, Morrisville, USA).

For the bars, we used a PEEK Victrex disc (Polifluor, São Paulo, Brazil) measuring 98 mm × 16.5 mm, milled on a 4-axis Zenotec Mini mill (Wieland Dental, Stuttgart, Germany).

Thirty PEEK bars were produced [Figure 1]: 10 solid bars with a rectangular section, 10 T-type bars, and 10 inverted T-type bars. All bars were 30 mm × 4 mm × 6 mm. The cantilever was 9 mm long.
Figure 1: Milled polyether ether ketone bars.

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To obtain the 30 Ni-Cr bars, the same process was applied to a calcinable Burnout Vipiblock VBW acrylic disc (Vipi Produtos Odontológicos Ltda, Pirassununga, Brazil), from which ten bars of each design were produced with equal dimensions. These bars were then submitted to induction casting. The alloy used to produce the bars was Liga N NiCr (Vipi Produtos Odontológicos Ltda, Pirassununga, Brazil). The metallic bars were used as control group [Figure 2].
Figure 2: Milled metal bars.

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Compression test

The test specimens were attached to the mini columns using 3.0 mm prosthetic screws (Neodent, Curitiba, Brazil), with 20N of torque, according to the manufacturer's instructions. We used the digital torque meter TQ 8800 (LT Lutron, Taiwan), which was also used to measure the counter torque after the test.

The insertion torque was first applied to the screw adjacent to the cantilever. This procedure was performed for all test specimen, both PEEK and metallic. The counter torque measurements followed the same pattern.

For the mechanical compression test, a universal testing machine EMIC DL2000 (EMIC, São Paulo, Brazil) was used with a load cell of 200N and actuator speed of 0.5 mm/min.

During the test, a straight tip cylindrical shaft was used to apply force to the cantilever center, creating a vertical compressive strength (CS). CS values are registered in MPa. The test ended when reached 1.5 mm of deformation or a strength equivalent to 200N [Figure 3] and [Figure 4].
Figure 3: Specimens and polyether ether ketone mechanical test

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Figure 4: Specimens and alloy mechanical test.

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Statistical analysis

Two-criteria variance analysis was used to verify if counter torque, compression strength, and deformation were influenced by type of material, by design of the bar, or by their interaction. For multiple comparisons, we used Tukey's test.

To check correlation between counter torque, compression strength, and deformation, we used Pearson's test.

The statistical analyses were performed using SPSS 23 (SPSS Inc., Chicago, IL, USA), with 5% of significance.


  Results Top


Regarding counter torque, two-criteria variance analysis showed a significant interaction between type of material and design of the bar (P = 0.002). For solid and T-type bars, counter torque was significantly lower for PEEK bars than for Ni-Cr alloy bars [Table 1]. Inverted T-type bars showed no difference in counter torque between PEEK and Ni-Cr. Comparison of different designs of PEEK bars showed a larger counter torque for the inverted T-type bar [Table 1]. The solid bars and T-type bars showed no difference in this parameter. For metallic bars, the larger counter torque was observed in the T-type bar, followed by inverted T-type and solid bar [Table 1].
Table 1: Mean and standard deviation for torque, counter torque, compression strength, and deformation according to type of material for different types of bars

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Compression strength was affected by the interaction between type of material and design of bar (P < 0.001). Tukey's test showed a lower compression strength of PEEK bars in comparison to Ni-Cr bars [Table 1] and [Figure 5]. However, while alloy bars' resistance to compression was not influenced by the design, inverted T-type PEEK bars showed the lowest compression strength [Table 1] and [Figure 5], 2.6 times lower than that showed by solid bars. T-type bars showed a compression strength 41.2% lower than that showed by solid bars and 34.5% lower than that showed by inverted T-type bars.
Figure 5: Bar chart of compression strength values, according to type of material.

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The two-criteria variance analysis applied to deformation data showed no significant interaction between type of material and design of bar (P = 0.098). PEEK bars showed a significant larger amount of deformation than the alloy bars (P < 0.001), [Table 1]. Furthermore, the design of bars significantly affected deformation (P = 0.028), with solid bars showing the largest values. T-type and inverted T-type showed no statistical difference in terms of deformation [Table 1].

Pearson's tests showed no correlation between torque and counter torque, compression strength, and deformation [Table 2]. On the other hand, counter torque correlated positively, although weakly with compression strength and negatively with deformation [Table 2]. Compression strength was also strongly and negatively correlated to deformation [Table 2].
Table 2: Correlation results

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  Discussion Top


Protocol prostheses are the most commonly used in complete-arch rehabilitations.[11] They are usually made of a metallic structure, mainly of Co-Cr and Ni-Cr alloys, with an acrylic or ceramic coating.[13] However, alternative metal-free materials have been proposed with this end.[14] Among them is the PEEK, a thermoplastic resin used for industrial and medical purposes.[5] PEEK is a colored synthetic organic polymer,[6],[15],[16],[17],[18] chemically resistant and with favorable biomechanical properties,[5],[19] and with elasticity modulus similar to that of the human bone in normal conditions.[5],[7],[20] It is highly resistant to abrasion [16] and is radiolucent, which avoids artifacts in radiographic and tomographic examinations.[7] It does not significantly influence the perimplantar gingival shadow [21] due to the color very similar to that of the tooth, resulting in a more natural appearance.[22] Despite its advantages, PEEK has a limited ability to adapt its mechanical properties to the implant design or to combine with perimplantar tissues.[9]

A prosthesis structural fracture is the most serious and expensive complication and has been correlated to the cantilever length [23] – the longer the cantilever, the higher the stress on the mesial end,[24] and the higher the possibility of deformation.[25] Besides, the design of the bar minimizes flaws.[26] Different designs, a cantilever shorter than 20 mm and larger cross-sectional area, prevent fractures.[27] In this study, the cantilever was 9 mm long, according to the literature above mentioned. In addition to the deformation, tensions on the prosthetic columns are affected by the design of bars, mandibular load conditions, and in a lesser extent, by the coating material properties.[28]

Here, PEEK bars showed a significant larger amount of deformation than the metallic bars. The design significantly affected this property for PEEK bars, with the largest amount of deformation being observed in solid bars and with no statistical difference between T-type and inverted T-type. This result is due to the material's high resilience and in a clinical scenario, it results in a more effective dissipation of mastication loads than observed in metallic bars.[16] This, in turn, could result in a smaller incidence of prosthetic fractures. On the other hand, the smaller deformity of metallic bars is due to its high-elasticity modulus.[25]

This study provides a reliable method for compression test; however, it should be noted limitations considering that this is an in vitro study. Issues such as biomechanical behavior of the implant prosthesis complex were not reproduced as well as their insertion angle. However, the methodology applied aimed to study the proposed material behavior by comparing it with an already established material in clinical use.

The use of resilient materials on prosthetic structures increases the stress on the retention screws.[29] This study found counter torque results that corroborate this observation since there was a significant interaction between type of material and design of bar, with lower values for PEEK bars. Hence, it is worth stressing that resilient materials transfer less mastication load to the implants and improve stress dissipation.[30]

The use of PEEK to make protocol beams is yet being studied, and the available evidence is recent. This study differs from the others using a highly promising material with favorable properties, among them, the ability to absorb loads, resistance to masticatory forces, fast manufacturing, esthetics, and favorable cost.

When different designs of PEEK bars are compared, rectangular solid bars show better compression strength results, followed by T-type and inverted T-type. The poor performance of inverted T-type bars might be due to the smaller contact area, creating a point where the stress is concentrated. It is worth mentioning that solid bars transfer more load to the implants. This difference was not observed in metallic bars.

The present methodology was based on a previous study of metal tests [3] because there are still few papers related to PEEK in the current study proposal.

Furthermore, compression strength of PEEK bars was lower than that observed in Ni-Cr bars. The alloy's resistance figures were higher than PEEK in all compression strength mechanical tests and deformation of prosthetic bars. Nonetheless, the use of PEEK in protocol bars should be considered for solid designs since it combines a high degree of load absorption and compression strength appropriate for a good biomechanical performance of the prosthetic structures.

The load cell used in the tests was based on a cantilever force developed during mastication loads.[31]

This is a pioneer study that will benefit from further tests such as fatigue resistance and finite elements, leading to a better understanding of the material potentialities.

Other studies such as deformation tests, abrasion resistance, tensile strength as well as in vivo analyses will also be relevant to the development of the material on its performance in implant-bone response when subjected to occlusal forces acting.


  Conclusion Top


Based on this study's results, we can conclude:

  • Metallic bars show larger compression strength than PEEK bars
  • The design of PEEK bars influences compression strength. Solid bars are the most resistant and inverted T-type bars, the least resistant
  • Solid PEEK bars shows larger amount of deformation than the other designs
  • Counter torque is smaller in PEEK bars than in metallic bars.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

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