Impact of the occlusal contact pattern and occlusal adjustment on the wear and stability of crowns

Objectives: To investigate the impact of the occlusal contact situation and occlusal adjustment on wear, roughness, and fracture force of molar crowns.Materials and methods: CAD/CAM crowns (lower right first molar, n = 64; 4 groups a` 8, 3Y-TZP zirconia and resin composite) and corresponding antagonists (upper right first molar; 3Y-TZP zirconia) were manufactured. Crowns were constructed according to two principles of occlusion (group T: Peter K. Thomas' point-centric cusp-tofossa tripodization concept, with 15 contact points; group RA Sigurd P. Ramfjord and Major M. Ash, freedom in centric concept with four contacts). On one half of the crowns, occlusal adjustment was performed (groups T adjusted and RA adjusted). All crowns underwent combined thermal cycling (TC) and mechanical loading (ML) (ML: 1.2 x 106 cycles, 50 N, 2 Hz, mouth opening 1 mm; TC: 2 x 3000 cycles, 5/55 degrees C). Wear area and depth of each contact point on the occlusal surfaces of crowns and antagonists were determined using a digital microscope. Surface roughness (Ra, Rz) was measured in and besides (reference) the worn area (3D laserscanning microscope). Fracture force of the crowns was determined (statistics: Levene-test, one-way-ANOVA; Bonferroni-post-hoc-test; between-subjects effects, Pearson correlation, alpha=0.05).Results: The resin composite crowns yielded significantly higher mean values for wear area and depth (p < 0.001) and lower fracture forces (p < 0.001). Resin composite surfaces showed increased roughness after TCML while zirconia exhibited smoothened surfaces. The occlusal design significantly impacted wear depth (p = 0.012) and fracture force (p < 0.001). Resin composite crowns with fewer contact points (group RA) showed more wear and lower fracture force. Adjusted resin composite crowns showed increased wear areas and depths (p = 0.0090.013). For zirconia crowns, the adjustment impacted wear area (p = 0.013), wear depth (p = 0.008), and fracture force (p = 0.006), with adjusted zirconia crowns exhibiting more wear and lower maximum forces until fracture. Zirconia wear depth was also impacted by the occlusal design (p = 0.012). Antagonistic wear was influenced by the restorative material, the occlusal contact pattern, and the adjustment. Conclusions: The investigated materials show strongly varying performances with zirconia being significantly influenced by the adjustment, while for resin composites, contact design and adjustment had a major impact. Clinical relevance: The results show the necessity of adapting occlusal design and adjustment in order to improve roughness, wear, and stability of zirconia and resin composite crowns.

Automated summary

This study investigated the impact of occlusal contact situation and occlusal adjustment on wear, roughness, and fracture force of molar crowns. The results showed that resin composite crowns had higher wear area and depth, as well as lower fracture forces. Adjusted crowns exhibited increased wear areas and depths. For zirconia crowns, the adjustment resulted in greater wear area and depth, as well as lower fracture forces. These findings emphasize the importance of adapting occlusal design and adjustment to improve the performance and stability of zirconia and resin composite crowns.