## 4 Discussion  
Patients today retain their natural teeth into old age, thanks to improved medical and dental care, as well as a growing awareness of the importance of oral hygiene [11]. However, younger patients often suffer from a pathological loss of dental hard tissue due to dietary habits (such as acidic foods and drinks) or increased attrition, abrasion, or erosion, either as solitary factors or in combination [15]. The loss of hard dental tissues due to caries, fracture, and/or cavity preparation is associated with a decrease in tooth resistance, as well as a loss of marginal tooth walls. The decrease in strength is even more pronounced [16; 17]. The loss of marginal zones can result in about a 46% loss in tooth hardness [18], significantly increasing the risk of tooth fracture. The choice of restorative material is not only an aesthetic one but also one affected by mechanical properties [19; 20]. An indirect composite resin restoration overcomes the problem of polymerisation shrinkage commonly seen in direct restorations made of similar material [20]. Secondary polymerisation of the composite inlay at high temperatures allows a reduction in the initial polymerisation shrinkage and relieves post-curing stress before the placement of the onlay [21]. Therefore, zirconia, silicate ceramic blocks, polymer-infiltrated ceramic blocks, and composite resin were selected as materials for 3D printing in the presented study [22; 23]. When the data from Group A were compared with those of Group B, the values were comparable to those of Group B, with the results being statistically insignificant. The lowest resistance of ceramic could be explained by the high hardness, with low resistance to adhesion and bending, which is the reason for the resulting compression fractures. Ceramics have a low strain limit, approximately \(0.1\approx 0\), and any bending of the material is capable of causing cracks that will propagate rapidly into its interior, leading to complete failure, as presented in the results for Group A [24]. Another important factor is its limited capacity to distribute localised loads and its lower resistance to deformation, increasing its potential for defects due to the possibility of degradation of the silicon oxide matrix [25]. Adhesive fixation achieved with the selective roughening obtained from hydrofluoric acid conditioning associated with silane coupling enhances the durability of the ceramic. Resin-based cement seems to play a major role in this aspect, as it fills the internal roughness and bonds chemically to the ceramic via silane and to the dentin via an adhesive system. This creates a layer that acts as a stress distribution mechanism for the restorative material [21; 22]. The values were higher when comparing Group A with Groups C and D, revealing statistically significant differences. Zirconia materials contain yttrium, which stabilises their polycrystalline structure, transforming them from a tetragonal crystal structure to a more voluminous monoclinic structure [26]. This stops subcritical crack propagation and prevents stress corrosion cracking, which is the main problem associated with other ceramic materials [19]. Additionally, crack development may be facilitated by any existing defects during the manufacturing process and cyclic loading test [26]. This corresponds with our results, which demonstrate high fracture resistance in the group of zirconia onlays in Group D [27]. Other studies have reported that the flexural or tensile strength of zirconia is approximately 2.5 times greater than that of glass-ceramic-based materials. The lower fracture strength for lithium disilicate onlays can be explained by the glass-ceramic characteristics of the material.