###### Abstract  
Purpose: This study aims to investigate the ultimate fractural strength and marginal integrity of a three-unit implant-supported fixed partial denture (FPD) framework fabricated of polyetherketoneketone (PEKK) after simulated five-year clinical aging. Materials and Methods: Twelve FPD frameworks were milled (n = 6 per group). All experimental frameworks were cemented on identical stainless-steel abutment models and subjected to five years of clinically simulated thermomechanical aging. The vertical marginal gap values were analyzed using a scanning electron microscope before and after being subjected to each aging condition. A universal testing machine was used to evaluate the ultimate fracture load. Results: A significant increase in marginal gap values of the PEKK group was observed after five years of simulation aging (\(p<0.001\)), while no significant difference was seen in the titanium group (\(p=0.071\)). After thermocycling, the PEKK group showed a higher statistically significant mean marginal gap value (84.99 + 44.28 \(\upmu\)m) than before (81.75 + 44.53 \(\upmu\)m). The titanium group exhibited superior mechanical strength, with a fracture load significantly higher than that of the PEKK group (3050 + 385.30 and 1359.14 + 205.49 N, respectively). Conclusions: Thermocycling affects the marginal gap discrepancy of PEKK restoration. However, the mean vertical marginal gap values in PEKK and titanium groups after a five-year clinical aging simulation were clinically acceptable. The ultimate fracture load values were higher than the maximum bite force reported in the posterior region. Thus, PEKK could serve as a suitable alternative material to metal in the framework of fixed dental prostheses. vertical marginal gap; polyetherketoneketone; thermocycling; cyclic loading; ultimate fracture load +
Footnote †: journal: Applied Sciences  
traditional metal-based prostheses. Polyaryletterketones (PAEKs) are a family of high-performance thermoplastic polymers with excellent properties. These materials are now well accepted and widely used in many different fields, including the automotive, semiconductor, aerospace, medical, and dental industries [5, 6]. Members of this group of polymers contain ketones and ethers as functional groups that are connected to an aromatic backbone molecular chain. Two commercial PAEKs commonly used in dental applications are polyetheretherketone (PEEK) and polyetherketoneketone (PEKK) [7]. PEKK is the latest member of the PAEK family; it has a second ketone group, which improves its mechanical and physical properties [6, 7], thus currently representing the apex of the quality pyramid of PEAK members. PEKK has the properties of high dimensional stability [8], high chemical and mechanical resistance against wear, and high flexural strength. [PERSON] et al. have reported that the biaxial flexural strengths of pressed and milled PEKKs are 239.9 and 257.8 MPa, respectively [9]. Therefore, PEKK has gained popularity as a framework material owing to its lightweight and manufacturing versatility, being suitable for computer-aided design (CAD)/computer-aided manufacture (CAM) milling and heat-pressed manufacturing [5, 9]. Moreover, compared to PEEK, PEKK has better compatibility with different veneering materials, such as resin composites, ceramics, and acrylic resins [1]. Although the manufacturer (PEKKTON(r), Cendres-Meteaux, Biel/Bienne, Switzerland) has indicated that PEKK is suitable for a wide range of prostheses due to its excellent physiological and biological properties [8], there are limited reports in the literature to date that support its clinical application [1, 10]. Several studies have demonstrated that PEKK is a reliable material for long-term provisional prostheses. [PERSON] stated that a monolithic provisional crown fabricated from PEKK has excellent fractural strength of 2037 N, which can withstand loads in the posterior region [8].