## 4 Discussion
Before specific preparation designing protocols with a 100% cusp reduction are routinely prescribed in patients, further research is advised. The maximal axial biting forces in males and females who are biting voluntarily have been reported to be much less than the mean fracture strength values of this experiment (1950-2900) N (480-788 N). The voluntary maximum axial biting force is more than the typical range of masticatory forces, which is 17 N to 450 N. People who have bruxism frequently produce involuntary forces between 400 and 1100 N [33]. However, while chewing involves a combination of axial and lateral pressures and movements, in vitro results are based on axial loading. This is in line with the investigation's conclusions, which state that a fracture resistance of greater than 1100 N is necessary to maintain adequate clinical performance [34]. However, the chewing simulator was useful in the current study to simulate oral conditions. Furthermore, the formation and propagation of cracks might cause restorations to shatter. This is particularly crucial to remember while working with restorations made of ceramic materials [17]. With larger preparations, the remaining tooth structure becomes weaker and there is greater cusp bending than ever before due to occlusal stresses. Some researchers feel that onlays with cuspal covering are the best restoration for teeth that have undergone extensive Class II MOD preparations because they may assist in avoiding cuspal displacement under load [35]. The newest generation of zirconia restorative material IPS e.max ZurCAD Prime used in the current study is the future of all-ceramic restorations. This revolutionary material ensures exceptional aesthetics and provides a complete solution for both dental technicians and clinicians. The unique \"gradient technology\" allows two zirconium oxide raw materials, 3Y-TZP and 5Y-TZP, to be combined, offering the strength of 3Y-TZP and the aesthetics of 5Y-TZP [36]. Regarding tooth complex repair, zirconia-based onlays perform better under high compressive pressures than lithium-disilicate-based onlays, according to [PERSON] et al. [17; 37]. Because resin cement, which is used in adhesive restorations, is elastic, it tends to flex under stress, strengthening the material's resistance to fracture. Thus, the key to successful ceramic inlay and onlay placement is creating an adhesive tooth-ceramic contact that is uncompromised [38]. Numerous surface roughening techniques have been developed to achieve micromechanical interlocking between zirconia and resin cement, including sandblasting, laser etching, selective infiltration etching, and nanostructured alumina coating. For zirconia ceramics, sandblasting is the most recommended surface roughening technique [39]. Through the formation of microcracks on the zirconia surface, this technique can degrade the ceramic while increasing surface energy and wettability [39]. However, resin luting agents heal the minor surface flaws created by sandblasting and strengthen the ceramic. Mechanical adhesion achieved by surface roughening is not enough to provide a durable bond between zirconia and resin cement. Thus, adhesive strategies combining surface roughening procedures and chemical bonding, which involve using various primers and resin cement, were utilized in the current study [40]. All studied groups' failure patterns revealed that type III or IV catastrophic fractures, including those involving the underlying tooth structure, predominated. Premolars treated with zirconia onlays of standard preparation (2 mm width of isthmus and reduction in palatal and buccal cusps by 2 mm) typically displayed catastrophic fractures involving tooth structure, according to a recent publication by [PERSON] et al. [41]. This may be attributed to the high modulus of the elasticity of zirconia and its minimal capacity to deform, making it susceptible to fatigue-induced fracture. However, the supporting dentin in this study exposed during cavity preparation has a low modulus of elasticity.