## 4 Discussion
The Tg of PEKK is higher than that of PMMA (163 and 106 \({}^{\circ}\)C, respectively) [34]. The temperature used in the aging protocol was 55 \({}^{\circ}\)C, closer to the PMMA than the PEKK Tg. Therefore, PEKK may have higher marginal fit stability than PMMA. In our study, thermocycling did not affect the marginal fitness of the titanium framework (\(p\) = 0.029). Titanium presented only a small increase in the mean marginal gap size (0.4 \(\upmu\)m), whereas PEKK exhibited an almost five-fold increase (3.24 \(\upmu\)m). The reason for these findings may be the different water absorption abilities and thermal expansion rates ofthe two materials. Thermal expansion is the tendency of substances to change their volume, shape, and area in response to a change in temperature. The degree of expansion divided by the change in temperature is the coefficient of thermal expansion (CTE), where lower CTE values indicate a lower tendency to change in size. In general, solid materials with higher melting points are more likely to have a lower thermal expansion due to their higher bond energy [35]. Therefore, titanium may exhibit more favorable dimensional stability than PEKK, due to its extremely high melting temperature (Tm) of 1660 \({}^{\circ}\)C and lower CTE of 10.3 \(\times\) 10\({}^{-6}\) C\({}^{-1}\), according to the manufacturer's data. For the PEKK material, a melting temperature of 363 \({}^{\circ}\)C and a CTE of 47 \(\times\) 10\({}^{-6}\) C\({}^{-1}\) have been reported [33]. The absorption of water within a material may cause dimensional changes. The volumetric expansion of resin-based polymers has been reported to range between 0.7 and 1% [36]. A study investigating the hygroscopic expansion change of PEKK (PEKKTON(r)) revealed that PEKK (0.14% \(\pm\) 0.14%) specimens had the lowest percent volume change over the entire study in comparisons with Vita Enamic (0.38% \(\pm\) 0.16%) and Lava Ultimate (1.06% \(\pm\) 0.14%) [8]. A similar study has reported that PEEK showed the lowest solubility and water absorption values among different CAD/CAM polymers, including PMMA [37]. Although the marginal fit of PEKK restorations can be affected by water absorption, the values of discrepancy would be minimal compared with those of other dental polymers or composite materials. In our study, cyclic loading was not found to affect the marginal integrity of the tested restorations. Thus, the assumption of marginal gap discrepancy enlargement as a result of framework bending from plastic deformation or creep under fatigue loading of ductile material is rejected. This may be partially due to the very high impact strength at low temperatures, high mechanical fatigue strength, and very low tendency to creep in both experimental materials [38]. A study evaluating the long-term mechanical properties of PEEK has indicated a low creep strain of less than 0.1% after 2000 h of loading, suggesting that the nonrecovery deformation of PEEK would be negligible in clinical practice [39]. Moreover, the load value used in our study was considerably lower than the elastic limits of both materials. Our study was performed on cemented FDPs, and our intention was to simulate actual clinical situations. However, cementation can increase marginal gaps [40] and may affect marginal adaptation. Various factors could potentially affect the cement gap, including cement viscosity, the pressure of cementation, time of pressure application, and degree of preparation taper [41; 42]. Therefore, the cementation of specimens in this in vitro study was carried out using a loading cementation device with a load of 5 kg in order to ensure full seating [42; 43]. No general guidelines or gold standards exist regarding methods that measure the marginal adaptation of FDPs. Various techniques have been proposed to analyze marginal precision, but no conclusive evidence indicates that a single method is superior to others [20; 44]. The direct-view technique is the most commonly used, followed by the cross-sectional method and the impression replica technique [44].