## 4 Discussion  
We investigated and compared the marginal fit and fracture resistance of restorations depending on whether they were manufactured from the latest high-performance polymer, PEKK, vs. conventional metal-ceramic. All examined FPDs survived five years of simulated clinical aging without prosthesis failure. The results support the rejection of the null hypothesis, as significant differences were observed in the comparison of the vertical marginal gap change after aging and the ultimate fracture strengths of the groups. The marginal fit of a restoration is considered one of the most important criteria determining the success of fixed dental restorations [20]. A consensus on the maximum clinically acceptable marginal discrepancy has not been developed; however, the most widely used value in the literature is 120 \(\upmu\)m [20; 21], proposed by [PERSON] and [PERSON] [22]. The authors found that it is difficult to detect marginal gap values of <80 \(\upmu\)m using exploratory or direct tactile stimuli. With a blunt instrument, these values reached 160 \(\upmu\)m; however, gap values > 160 \(\upmu\)m could be seen in the X-ray, which is considered to indicate a poor clinical fit restoration. Thus, a restoration achieving a marginal gap of <120 \(\upmu\)m ensures a proper clinical fit [22]. However, maximum tolerable marginal discrepancies ranging from 50 to 200 \(\upmu\)m have been reported [23; 24]. In the present study, no specimen showed a mean marginal gap value greater than 120 \(\upmu\)m, further indicating that the PEKK framework potentially meets the requirements for successful restoration in clinical practice. Our results are consistent with those of many other studies, which have shown that all marginal gaps of PEKK or PEEK restorations ranged from 20 to 90 \(\upmu\)m, indicating they are clinically acceptable [25; 26; 27]. Dental restorations are subject to changes in moisture, dynamic temperatures, and load occurring within the oral cavity. In vitro experiments have shown that thermomechanical aging is essential for investigating material behavior under clinical-like conditions [28; 29; 30]. Thermomechanical testing may have various effects on the dimensional stability, mechanical properties, and marginal integrity of a restoration. For polymers, the effects may include polymerization stresses, water sorption, plasticizing effects, hygroscopic expansion, and cyclic deformation [8; 29; 30; 31; 32]. The results obtained after a five-year aging simulation suggest that PEKK and titanium, as framework materials, may perform differently in clinical use over time. Titanium frameworks can withstand thermomechanical aging and demonstrate excellent marginal fit stability. On the other hand, a significant increase in the marginal discrepancy was observed in the PEKK group (\(p\) < 0.001). The results reveal a negative impact of thermocycling on marginal fit only in the PEKK group; however, cyclic loading had no influence on the marginal integrity of either experimental material. When dental prostheses undergo various temperature changes, they face dynamic expansion and contraction, especially at the thin margin area, which may cause material distortion and increase the marginal gap [29; 31]. The results of early studies showed that the margin fit of resin-based polymer restorations changes after thermal cycling [30; 31; 32]. [PERSON] et al. (2000) observed a significant increase in the marginal discrepancy after thermal cycling of two conventional polymethyl methacrylate (PMMA) and bis-acryl resin temporary crowns, with mean changes in the marginal gap size of 43.9-548.9 \(\upmu\)m. The mean change in the marginal discrepancy of the PEKK framework (3.24 \(\upmu\)m) in this study was considerably lower than the values reported for other resin-based polymers in earlier studies. According to the temperature performance of thermoplastic polymers, they soften when heated to the glass transition temperature (Tg), changing from a brittle to a more ductile form [33].