Objective To compare the effects of different crown materials on the stress distribution of dental implants by finite ele-ment method.Methods A three-dimensional finite element model of implant-supported crown restoration of the right mandibular first molar was established.Four kinds of crown restoration materials were used,including polymer infiltrated ceramic network Vita Enamic,resin nanoceramic Lava Ultimate,zirconia ceramic Cercon,and polyether-ether-ketone breCAM.A displacement load of 0.01 mm or 200 N was applied to the occlusal surface of the right mandibular first molar,and the analysis of reaction force,the maximum variability of shape,the maximum principal stress,the maximum equivalent stress was performed.Results The results of stress analysis under 0.01 mm displacement load showed that the maximum equivalent stress of Cercon was 937.30 MPa,and that of breCAM was 67.09 MPa.The stress concentration of crowns and implants was obvious.The maximum equivalent stress of the surrounding alveolar bone of Cercon was the highest(26.61 MPa),and that of breCAM was the lowest(3.87 MPa),which was about 1/7 of that of maximum group.The stress concentration range of resin-matrix ceramics and polyether-ether-ketone was relatively small,and the distribution was more uniform.When 200 N was applied,the maximum principal stress and the maximum equivalent stress distribution of the four models were similar,and the maximum variability of shape of breCAM was larger at the loading site.Conclusion Under the same displace-ment load,the stress concentration of polyether-ether-ketone and resin-matrix ceramics crown was lower than that of zirconia.There was no significant difference in stress concentration in the implant and surrounding alveolar bone among polyetheretherketone,resin-ceramic composite,and zirconia crown restorations under the same load.Further research on long-term durability of PEEK and resin-matrix ce-ramicsis needed through practical and clinical trials.

Differences in their pharmacognostic features, microscopic appearance of their powders and UV spectrums, can be used to differentiate Artemisia rupestris from its confusable Achillea millefolium. It is of value to identify them in actual practice.