## 4 Discussion  
Fiber-reinforced composite materials are used as alternative materials for metal restorations. Modifications of composite resins, through various types of fibers improving the mechanical properties of restorations, allow for their use not only in prosthetic crowns, but also in solid partial dentures (FPD)--e.g., in adhesive bridges [31]. These additions differ from the total permanent dentures by the replacement of the crowns, on which the prosthetic bridge structure is based, with crown inserts, i.e., onlay and inlay. This construction of prosthetic bridges is an alternative for patients who do not agree to a complete permanent replacement due to the need of significant tooth tissue reduction, or when minimal tooth reduction is possible [32; 33]. In the construction of bridges, the connectors between the individual elements of the bridge are most vulnerable to destruction. Solid partial dentures of fiber-reinforced composite resins are made by placing the fiber in the structure and surrounding it with a composite resin. During their fatigue cyclic tests, [PERSON] et al. [34] observed the slow spread of cracks in brittle materials, such as composite resins. The literature lacks studies describing polyaramid fiber due to the fact that this is a new material in dentistry. Research results derived from the study on composites reinforced with polyaramid fibers provided by [PERSON] et al. acknowledge that the addition of polyaramid into composite impact/increases composite strength [35]. Previous studies on dental composites used to make prosthetic bridges were mainly based on standardized samples. Meanwhile, such samples subjected to cyclic loads show lower values of bending strength (30-50%) than those obtained in static studies and are considered more sensitive in assessing the effectiveness of clinical materials [29]. When testing the strength of dental composites, [PERSON] et al. [28] stated that fatigue strength is associated not only with the type of filler, but also with the resin matrix. [PERSON] et al. [27] used standardized samples with dimensions of \(3.0\times 4.0\times 40\) mm in their tests, which underwent cyclic fatigue loads with a force of 100 N. They noted that the strength of fiber-reinforced dental composites increases the bending strength. They noted better strength of composites reinforced with glass fiber under the influence of increasing force compared to the unreinforced control sample. However, these studies did not take into account the physiological interactions between the opposing teeth during chewing. In a study by [PERSON] et al. [26], who analyzed the FEM model of a three-point conventional bridge reinforced with fiber glass, the authors applied a load of 629 N in the rebuilding the lack of the first molar in the mandible. The significant improvement in connector rigidity under vertical load conditions causing twisting and bending movement was stated. As a consequence, the stiffness of connections between individual elements of the sample structure improved, which significantly reduced the deflection of the span because the stresses generated by the vertical load were transferred to the reinforcing fiber. However, no relationship was found between the number of fatigue cycles and bending strength. Similar dependences on the better strength of samples reinforced with polyaramid fiber were recorded in the present research, whereby the actual strength of the manufactured bridges was tested with a strictly established number of loading cycles. Although the experimental groups required significantly lower force in order to break, the destructive force was still in the reported range of habitual and maximal biting forces in molar teeth, which are 300 N and 500 N, respectively [36; 37]. The authors are aware that the tests were carried out in laboratory conditions and do not perfectly reflect the conditions in the oral cavity (the authors did not use any bonding system between sample