Exploring polyetheretherketone in dental implants and abutments: A focus on biomechanics and finite element methods

Advancements in dentistry involve the evolution of materials. Biocompatibility, aesthetics, low plaque affinity, and characteristics similar to dental structure are among the essential criteria of modern materials used in dentistry [1,2]. Polymers are materials that have allowed significant technological progress in the second half of the twentieth century, although they are now controversial from an environmental standpoint [3–7]. Polyetheretherketone (PEEK) is among the most innovative polymers, described as “high performance.” It was first used in the 1980s in the aerospace and automotive industries [8,9]. Then, due to its remarkable mechanical properties, biocompatibility, and modulus of elasticity similar to human bone, PEEK began to be studied in the field of orthopedic prostheses in the 1990s [6,10–12]. Today, it is expanding in most clinical disciplines of dentistry. Often shaped conventionally, meaning through extrusion, compression, injection, or machining, manufacturers are now turning to 3D printing of PEEK, or additive manufacturing [13–16].

The advancement of dental implant, abutment, and associated materials necessitates adherence to established in vitro and in vivo tests before they are fit for commercial use. Conducting in vivo tests on a substantial number of samples can be lengthy and may pose challenges to animals or patients involved. Moreover, potential inaccuracies in these assessments could lead to misleading outcomes, impacting both the materials and the results of the experiment. Therefore, the finite element method (FEM) emerges as a valuable tool for predicting potential mechanical complications using theoretical models coupled with in vitro tests [17–20]. The finite element analysis (FEA) is frequently used to assess the biomechanical behavior of implantable biomaterials in vitro, whether in orthopedics, traumatology, or oral implantology, a trend is reflected by the increasing number of publications on the subject (Figure 1). This method allows for the evaluation of stress distribution during the load transfer of an implant to the underlying bone structures and typically follows a consistent procedure.

Figure 1

The number of PubMed indexed publications on finite elements used in dental specialties since 2007. Search query: (peek) AND (dental implants) AND (FEA) OR (FEM).

This article delves into the latest advancements in PEEK-based biomaterials for dental applications, highlighting recent improvements in properties and mechanical testing. We will conduct an integrative systematic review focused on analyzing stress distribution across dental implants and abutments made from carbon fiber-reinforced PEEK composites, utilizing FEM studies as a core analytical tool (Figure 1). The investigation is structured around key research questions, aiming to uncover through a systematic approach:

• 1)

  How are PEEK and its composites applied in the dental field, especially as materials for implant abutments and the core of dental implants?

• 2)

  What do mechanical tests, including those for tensile, compression, bending stress, wear resistance, and particularly fatigue tests, reveal about the suitability of PEEK and its composites for dental implant applications?

• 3)

  How does FEA enhance our understanding of stress distribution in dental implants and abutments made from carbon fiber-reinforced PEEK composites?

A literature search on PubMed (via National Library of Medicine) and ScienceDirect (Elsevier BV) was conducted using the following search terms (Table 1):

• Term 1: (peek) AND (stress analysis, mechanical) AND (dental)

• Term 2: (peek) AND (dental) AND (mechanical phenomena) AND (stress, mechanical) AND (material testing)

• Term 3: (peek) AND (dental) AND (finite element) AND (FEA)

The electronic searches described previously contributed to initial grouping:

• On PubMed: 181 articles

• On ScienceDirect: 275 articles

After eliminating duplicates, the titles, and abstracts of the articles resulting from the searches, only 45 publications were retained.

A complete reading of the text and an analysis of the relevance of the articles finally led to a final result grouping 25 articles (Table 2). The publication dates of the articles range between 2010 and 2023, except for 1 article.

A selection tree of articles (as depicted in Figure 2) was therefore made from the 25 articles, including 6 literature reviews (21–24–27–28–41–43), 12 studies for FEA (22–23–29–30–32–33–34–35–37–39–44–45), and 9 in vitro experimental studies (25–26–31–36–38–40–42).

Figure 2

Flow diagram of the search strategy.

The PEEK is a thermally stable semi-crystalline thermoplastic material. Introduced in 1977 by ICI (Imperial Chemical Industries), this material entered the market between 1978 and 1979. It gained traction in the 1980s across orthopedics, traumatology, and spinal surgery, particularly in the creation of interbody cages [46–49]. By the late 1990s, it was recognized as a premier non-metallic counterpart to titanium in these specialties [10]. The literature on implantology began highlighting this material around 1995 [50]. In April 1998, the debut PEEK implant product, branded as PEEK-OPTIMA^TM and compliant with both European Community and Food and Drug Administration standards, was launched [10,51]. PEEK boasts superior mechanical, physical, and chemical attributes. Notably, it has a hardness rating between 99 and 126 on the Rockwell scale [42,52] and an elasticity modulus (4 GPa) that is more akin to cortical bone (20 GPa) than titanium (110 GPa).

As we delve deeper into the applications of PEEK in dentistry, we will explore the use of PEEK in various capacities: as implant abutments, a material for removable partial dentures, a foundation for fixed dental prostheses, and even as the core material for dental implants.

4.1

PEEK implant abutments

In the field of implant dentistry, an abutment serves as the bridge between the prosthetic restoration and the implant itself (Figure 3). Abutments are crafted from a variety of materials, including titanium, gold, zirconium, and ceramics [76]. Recently, there has been an increased utilization of PEEK abutments. Notably, in instances of implant screw damage, PEEK screws offer ease of removal. Studies have shown that PEEK abutments can endure intraoral chewing forces akin to those experienced by titanium abutments. The soft tissue response to PEEK is commendable, often leading to optimal gingival tissue healing [77].

Figure 3

Schematic representation of the structure of a dental implant.

The semi-crystalline nature of PEEK contributes to its reduced brittleness, leading to deformation rather than breakage. Research has shown that when using PEEK for abutments, while the prosthesis remains unharmed, the abutment might deform. Nevertheless, the prosthesis’ functionality can be restored by replacing the deformed abutment [78]. In a study by Koutouzis et al., a randomized controlled clinical trial found negligible differences in bone loss and tissue inflammation between PEEK and titanium abutments (Figure 4). Furthermore, the composition of oral bacteria observed was consistent with that seen with titanium, zirconia, or PMMA abutments [79].

Figure 4

Occlusal view of implants with PEEK abutments (a) following installation and (b) at 3 months. And, occlusal view of implants with titanium abutments (c) following installation and (d) at 3 months [79].

The distribution of occlusal forces and the manner in which stress is distributed during the implant healing period significantly impact the enduring outcomes of implant procedures. In a study by Taha et al. [25], they examined the interplay between various crown-abutment combinations and their impact on the force absorption properties of implant-supported restorations. Their findings indicated that pairing resin-based ceramic crowns with PEEK abutments did not amplify force absorption capabilities. This could be attributed to the notably reduced elastic modulus of PEEK custom abutments, leading to diminished stress within the abutment, but increased stress on the associated crown [35].

Ceramic-reinforced PEEK (RPEEK) is another variant used in implant abutments noted for its good biomechanical properties and biocompatibility, with Al-Rabab’ah et al. [80] reporting stable bone and soft tissue around implants over 2 years.

Otherwise, Al-Zordk et al. discussed the critical aspect of fixation techniques in implant-supported restorations, highlighting the advantages of cement-screws for their superior passive fit, aesthetic appeal, and force distribution [81]. Zarone et al. further asserted that cement-screw systems have a greater load-bearing capability compared to screw-retained systems [82]. Freitas et al. [83] observed that restorations retained by cement and featuring internal connections display enhanced fracture resistance, leading to their preference in Atsu et al.‘s work [84]. Yet, they caution against the potential for peri-implant diseases due to residual cement in the submucosal area, with prevalence rates ranging from 1.9 to 75% in such restorations [81]. The discussion also touches upon physiological bite forces, noting the range of occlusal forces across different teeth types. Atsu et al. reported that titanium abutments exhibit superior fracture toughness (943.67 N) compared to Zr and RPEEK (770.1 N), though the latter two materials show no significant difference in this aspect (p = 0.001). Ortega et al. presented findings on fracture toughness, with titanium abutments at 468.5 N and PEEK at 200.4 N. They noted that PEEK abutments withstand 1.2 million cycles at 140 N without failure, simulating 5 years of function, but fail under increased loads of 160 N after approximately 89,338 cycles, equating to 4–5 months of use [26]. This behavior indicates PEEK’s role as a “sacrificial material,” absorbing deformation to prevent damage to the implant or screw, in contrast to titanium where deformation affects the internal connection, potentially endangering the implant’s longevity [26]. In another study by Mourya et al. [29], the stress distribution around abutments, subjected to both vertical and oblique forces, was assessed for both titanium and carbon-fiber-reinforced PEEK (CF/PEEK) implants. The findings propose that in patients with molar teeth, using straight abutments in conjunction with PEEK crowns could mitigate intraosseous stress levels and potentially avert implant complications.

The available evidence suggests that PEEK abutments may not meet the biomechanical criteria necessary to supplant traditional titanium abutments. Among available options, zirconium is recognized for its superior biocompatibility as an abutment material [85]. Nevertheless, PEEK exhibits particular merits in applications like customized healing abutment [27], provisional restorations [24,28] for patients lacking functional deficiencies, and for shaping emergency contours during surgical interventions, especially relevant for implants in individuals subject to reduced stress loads.

Figure 5 shows the various PEEK abutments. PEEK healing abutment (Figure 5a) is used as an alternative for classic titanium healing abutment [86], and PEEK with titanium base temporary abutment (Figure 5b) is used for long-term interim restoration, especially in areas with aesthetic consideration [87].

Figure 5

PEEK abutments: (a) PEEK healing abutment (GM customizable healing abutment, Neodent, Curitiba, Brazil), (b) PEEK with titanium base temporary abutment (GM PRO PEEK abutment, Neodent, Curitiba, Brazil).

4.2

PEEK as fixed dental prosthesis

For fixed dental prosthetics, PEEK offers a unique solution with its metal-free crowns and bridges, known for their outstanding biocompatibility and robust mechanical attributes (Figure 6). When assessing PEEK against traditional ceramic and metal materials, it becomes evident that PEEK’s dental bridge infrastructure remains resilient, even when subjected to in vitro aging processes. For implant-linked prosthetic applications, PEEK crowns have showcased effective outcomes [88]. In terms of biocompatibility evaluations, PEEK consistently outperforms many metal-based ceramics. Nevertheless, some experts propose the integration of veneers over PEEK to achieve optimal precision [89]. Given its lightweight nature, PEEK emerges as a promising contender to replace traditional chrome-cobalt prostheses [90–92].

Figure 6

PEEK resin-bonded fixed dental prosthesis: (a) occlusal view and (b) after fixing the teeth [94].

PEEK restorations demonstrate satisfactory resilience against fractures, enduring anterior masticatory forces of around 300 N and forces of 600 N in posterior areas. Extended chewing simulations, replicating up to 5 years of intraoral utilization, revealed no structural damage or debonding incidents in these restorations [77].

4.4

Commercial forms for dental use

Two leading commercial PEEK brands are widely recognized in the dental sector. PEEK-OPTIMA® is the preferred choice within the US, supplied by Invibio Biomaterial Solutions since 1999, and is primarily used for dental applications such as provisional prosthetic abutments and healing screws [98]. In contrast, Europe mainly utilizes BioHPP™ (Figure 7), produced by Bredent GmbH, which is specifically designed for dental applications. It is a modified PEEK material that incorporates ceramic fillers to enhance polishability and is suitable for various dental prostheses and frameworks through injection molding and CAD-CAM processing [99].

Figure 7

PEEK abutment made by BioHPP [99].

In this part, we will delve into an in-depth examination of the mechanical behavior properties of PEEK and its associated composites. This analysis will provide a comprehensive understanding of the material’s strengths as well as their ability to withstand force stresses without fracturing or undergoing deformation.

The advancement of dental implant, abutment, and associated materials necessitates adherence to established in vitro and in vivo tests before they are fit for commercial use. Conducting in vivo tests on a substantial number of samples can be lengthy and may pose challenges to animals or patients involved. Moreover, potential inaccuracies in these assessments could lead to misleading outcomes, impacting both the materials and the results of the experiment. Therefore, the FEM emerges as a valuable tool for predicting potential mechanical complications using theoretical models coupled with in vitro tests [22,23,35,39,44,123]. The FEA is frequently used to assess the biomechanical behavior of implantable biomaterials in vitro, whether in orthopedics, traumatology, or oral implantology. This method allows for the evaluation of stress distribution during the load transfer of an implant to the underlying bone structures and typically follows a consistent procedure.

The process involves obtaining a radiographic image of the bone section that will be studied. This image is then converted into a 3D model using computer-aided design (CAD) software. This model becomes the foundation for tests conducted on various models created with each implantable biomaterial.

The articles chosen for this section explore the utilization of FEA in examining the mechanical characteristics of abutments and implants. The process typically involves the creation of a 3D bone model through a CT scan, followed by importing into CAD software such as ANSYS, ABAQUS, SolidWorks, Creo Parametric, or Rhinoceros. A FEA package is then used to generate a 3D mesh which is linked to a suitable mathematical model that captures the properties of the corresponding material. While most of the studies employed ANSYS for simulation [22,23,32,33,39,44], some researchers favored ABAQUS, and CATIA [124–127]. Most of the selected studies assume a linear elastic, isotropic, and homogenous material model. In addition to using fewer parameters, in a linear elastic model, the correlation between these parameters and clinical medical imaging has been well established, which greatly enhances its potential for clinical application [127]. Whereas, an anisotropic material model is found to be more suitable for CFR-PEEK laminates [34]. CFR-PEEK laminates consist of carbon fiber-reinforced PEEK, which exhibits varying stiffness and strength properties in different directions relative to the orientation of the carbon fibers within the laminate. An anisotropic material model can accurately capture these directional dependencies, allowing for more precise simulations and prediction of mechanical behavior. This is particularly crucial in the context of dental implants and abutments, where a comprehensive understanding of material response under diverse loading conditions is essential (Figure 8).

Figure 8

Schematic of an implant-supported prosthesis involving implant, abutment, and a single crown.

The mechanical performance of implants, including factors such as von Mises stress, shear stress, contact stress, and stress shielding, is analyzed under varying loading conditions corresponding to clinical scenarios. An integral consideration in FEA methodologies is the assumption of forces to replicate masticatory loads encountered within the oral cavity of patients. However, defining this parameter precisely remains elusive due to its variability among individuals [39]. The magnitude of axial loading in the selected journals ranges widely from 100 to 1,000 N, while oblique loading typically spans from 50 to 500 N [29,33]. Notably, a predominant trend in these studies is the utilization of axial loading within the range of 100 to 150 N. Furthermore, the angle of oblique loading has slight variation, typically falling between 30° and 45°.

The selected journals aimed to investigate the stresses developed in dental implants and bones using various prosthesis materials, including titanium, zirconium, unfilled, and CFR-PEEK, utilized in abutments and implants, and their combinations. Oblique loading typically results in higher stresses in cortical and cancellous bone, implant components, and restorative crowns compared to axial loading across all mentioned studies. The findings of all authors are consistent with each other. A higher stress concentration is generally observed in the cortical bone near the peri-implant region, gradually decreasing towards the cancellous bone under both axial and oblique loading conditions.

Most studies indicate no significant variation in stress distribution at the implant-bone interface for different material models. Mourya et al.’s study of stress distribution in straight and angled abutments supports this conclusion, suggesting similar stress results with CFR-PEEK and titanium implants in both straight and inclined cases [29].

Another study on the performance of titanium/PEEK dental implants simulating different levels of bone loss also indicates that stresses were independent of the implant material [30]. Similarly, Sarot et al.’s work on the stress distribution of CFR-PEEK dental implants concludes that CFR-PEEK implants offer no advantages over titanium implants concerning stress distribution to the peri-implant bone [44]. Shash et al.’s study on fixed prosthesis following the “all on four” framework indicates that 60% CFR-PEEK distributes cortical bone stresses similar to the titanium framework [23] (Figure 9).

Figure 9

The distribution of von Mises stresses (MPa) on the mucosa, cortical and spongy bones, under unilateral force, using Ti and PEEK [27].

Studies by Wazeh et al. [33] and Kaleli et al. [35] agree that crown material does not affect biomechanical behavior concerning stresses in implants and peripheral bone.

However, Mourya et al.‘s findings on stress distribution under parafunctional loading show that the stresses generated in bone with a PEEK crown layered with composite are lower compared to porcelain fused to metal (PFM) crown configurations [29] (Figure 10).

Figure 10

Stress in: (a) CFR-PEEK 15° abutment under vertical loading with PFM crown, (b) CFR-PEEK straight abutment under oblique loading with (PFM) crown, (c) titanium straight abutment under vertical loading with (PFM) crown, and (d) titanium straight abutment under oblique loading with (PFM) crown [29].

Implants with micro threads showed a reduction of maximum von Mises stress by 50–70% in peri-implant bone compared to conventional threading [33], as demonstrated by Wazeh et al. Specifically, micro-threaded titanium implants, when used with a zirconia crown, exhibited the lowest cortical bone stress values under vertical loading, approximately 10 MPa. In contrast, implants made of CFR-PEEK with 30% carbon fibers generated notably higher stresses, around 100 MPa, on the cortical bone.

Titanium implants may induce a stress-shielding effect, leading to implant and bone loss due to their high elastic modulus (110 GPa) compared to bone (14 GPa). The stiffness mismatch between bone and implant can result in stress-shielding phenomena, potentially leading to bone resorption during the bone remodeling stage. In vivo, PEEK material possesses biomechanical properties close to human bone, reducing the risk of bone resorption and osteolysis caused by the stress-shielding effect of implants. A study by Ouldyerou et al. found that Ti-PEEK implants outperform conventional Titanium implants in reducing stress shielding and bone resorption [30]. The application of PEEK veneering on dental implants or abutments was also found to diminish the effects of stress shielding, as indicated by studies conducted by Lee et al. and Sampaio et al. [37,42]. Nevertheless, the thickness of PEEK veneering emerged as a critical factor influencing both stress distribution and the strength of hybrid abutments or implants, as highlighted in the research by Sampaio et al. [37].

Implants or abutments made of PEEK containing 30% carbon fiber exhibited increased stiffness compared to unfilled PEEK. The incorporation of such PEEK composites was found to enhance stress distribution and reduce the load concentration throughout the abutment and implant region, as suggested by Sarot et al. [44].

In weaving together, the detailed insights provided by the various studies, it becomes evident that FEA offers a robust framework for simulating and understanding the biomechanical behavior of dental implants and abutments under various conditions. Through meticulous modeling and simulation, FEA enables researchers and practitioners to predict and mitigate potential complications associated with implant materials and designs, thereby facilitating the development of more effective and safer dental restoration solutions. The consensus across studies on the efficacy of materials like CFR-PEEK and the nuances of stress distribution underlines the complexity of dental biomechanics and the indispensable role of FEA in advancing dental implantology. As the field evolves, the ongoing refinement of FEA methodologies and the exploration of new materials will undoubtedly contribute to enhancing the quality and longevity of dental implants, ultimately benefiting patients with more reliable and durable dental restorations.

The manuscript thoroughly examines the application and mechanical performance of PEEK and its composites in dental implantology, particularly regarding implant abutments and cores. Through FEA, it highlights PEEK’s pivotal role in advancing dental implant technology.