A major task in prosthetic dentistry is to maintain or restore a well-balanced occlusion. A dental restoration has to incorporate into the stomatognathic system without interference to fulfil its function and to ensure the patient's satisfaction. The occlusal design has a fundamental influence on the mechanical and tribological properties of the crown [1]. The occlusal contact area as well as the amount and distribution of the contact points are factors impacting the internal stresses, wear, and thus the stability of a dental prosthesis [2].

Individual working steps - from the initial examination of the clinical situation to the insertion of a restoration – may lead to one of the most common complications in dental practice routine: restorations with static and/or dynamic interferences in occlusion [3]. In most cases, correctional intraoral adjustment of the crown has to be carried out [4]. The elimination of occlusal interferences and/or reduction of the vertical height using diamond burs can, however, affect the restoration's mechanical strength, wear performance, and fracture resistance by inducing microcracks [5]. The extent of possible surface damage is dependent on several factors such as the grit size of the diamond bur, speed of rotation, contact pressure, resulting temperature, and grinding time [6]. Materials with a low modulus of elasticity can show less susceptibility to grinding-associated damage compared to rigid materials with low stress absorbing capacity [7]. Subsequent smoothing of the surface by polishing and optional re-glazing are mandatory to improve the mechanical properties and wear performance of manually adjusted restorations [8].

In clinical practice, the general approach for intraoral occlusal corrections is to mark static and dynamic contacts using articulating paper, followed by the elimination of protrusive and laterotrusive interferences and subsequent reduction of centric pre-contacts [9]. These grinding procedures are rarely performed according to a fixed concept, but rather follow the subjective evaluation of the operator relying on the patient's mechanosensitive perception and feedback. Often, the principles of occlusion and the position as well as the distribution of occlusal contacts are not or insufficiently considered due to time constraints [9]. Depending on the contact situation, clinical wear can lead to an equilibration of the chewing surface and deterioration of less firm materials over time. With high-strength restorations, especially monolithic zirconia, particular attention should be paid to the occlusion, as only little or no wear-related adjustment of the occlusal situation can be expected [10]. The purpose of the present in-vitro study was to investigate the impact of the occlusal contact design and occlusal adjustment on wear, roughness, and fracture force of single posterior crowns.

Unfortunately, in-vitro tests with simple test specimens can only reflect the complex clinical situation to a very limited extent. Unlike clinical trials, in-vitro-testing can facilitate timely investigation of clinical procedures by combining reproducible laboratory conditions with the basic requirements of the clinical situation. The prerequisite for comparability include a comparable study design and a clinically relevant contact situation. Thermal cycling and mechanical loading could simulate a clinical-near wear situation in laboratory studies and predict clinical failures [11]. For this purpose, molar crowns fabricated from two different computer aided design/computer aided manufactured (CAD/CAM) materials (resin composite, zirconia) were investigated. The null-hypotheses were that

The type of material,

The occlusal contact pattern, and

The occlusal adjustment