Effect of thermo-mechanical loading on marginal quality and wear of primary molar crowns

AIM: To evaluate the effect of thermo-mechanical loading (TML) on marginal quality and wear of different crown types for primary molars. METHODS: Eighty extracted human primary molars were used. After preparation, five groups received different crowns (n=16): preformed metal crowns (3M ESPE) and NuSmile crowns (Orthodontic Technologies Inc.) were inserted as preformed metal crowns; as semi-preformed crowns Protemp crowns (3M ESPE) were luted; and as individually manufactured resin composite crowns Filtek Z250 (3M ESPE) and Heliomolar (Ivoclar Vivadent) were used. Specimens were subjected to 2,500 thermal cycles between 5–55°C and chewing simulation for 100,000 cycles at 50N at a frequency of 0.5 Hz. Before and after thermo-mechanical loading, impressions of the teeth were taken and replicas were made. The replicas received marginal quality evaluation under a SEM at ×200 magnification. Occlusal wear was measured as vertical height loss using a 3-D laser scanning microscope. RESULTS: After TML, all crowns were intact. The adhesively bonded crowns showed significantly better marginal quality to dentine/cementum compared with GIC luted crowns (p<0.05). Laboratory made crowns showed a good fit and nearly transition-free margins also after TML. Preformed metal crowns showed the significantly lowest wear rates compared to the resin composites (p<0.05). Among the resin composite groups, Protemp exhibited the most pronounced wear depths (p<0.05). CONCLUSIONS: The different crown types under investigation showed a good performance concerning the evaluated parameters marginal quality and wear.