BACKGROUND:The strength of al-ceramic crowns is affected by many factors. At present there are many studies concerning the neck edge shape, cutting thickness, binder and convergence angle, but little has been reported on the effects of different occlusions on al-ceramic crowns. OBJECTIVE:To explore the maxilary central incisor al-ceramic crowns in different occlusal states by finite element analysis. METHODS: Three-dimensional finite element models of the maxilary central incisor al-ceramic crowns were established. Empress II and In-Ceram Zirconia were selected. The normal overbite position and deep overbite position were applied to force a load of 230 N, with the long axis of the tooth at a 45° angle. Distribution of inner stress and edge stress was analyzed at different occlusal states. RESULTS AND CONCLUSION:(1) The stress distribution of al-ceramic crowns under different occlusion relationship was different. The stress peaks of cementum, alveolar bone and periodontium in the deep overbite were lower than those in the normal overbite. The stress peaks of al-ceramic crowns and binder were higher than in the deep overbite than the normal overbite. The stress peak of the dentin in the Empress II group was higher in the deep overbite than in the normal overbite. The stress peak of the dentin in the In-Ceram Zirconia group was lower in the deep overbite than in the normal overbite. Different al-ceramic crowns had no influence on the stress distribution, but in the deep overbite, the stress was mainly concentrated in the occlusal contact area, and in the normal overbite, the stress was mainly concentrated in the occlusal contact area and at the labial cervical margin. (2) The stress distribution of different al-ceramic crowns under the same occlusal state was different. In the normal overbite, different al-ceramic crowns had no evident influence on the stress peak, and in the deep overbite, there was a certain effect of different al-ceramic crowns on the stress peak, but there was no significant difference. The stress peaks of blinder and dentin were obviously affected by al-ceramic crowns. The stress peaks of blinder and dentin in Empress II group were higher than those in the In-Ceram Zirconia group.

Objective To investigate the imaging manifestation of immature teratoma.Methods Imaging features of 9 cases of immature teratoma.confirmed pathologically were retrospective analysed.Results All 9 cases were male,age ranging from 1 to 38 years,mean age 1 9.5 years,AFP increased varying degrees in six cases .(1)3 cases (mean age 25.5 years)located in the anterior mediastinum which CT characteristics included the following features:lesions were projecting to one side,with irregular shape,obvi-ous necrosis,mild enhancement,wrap or oppression the blood vessels,one case with superior vena cava and head arm vein tumor em-boli and mediastinal lymph node metastasis.(2)6 cases (mean age 9.5 years)located in the pineal region and tricorn region,which MR finding were as following :cystic or solid placeholders,tow cases with small calcification or fat,solid part and the capsule wall enhancement significantly,4 cases with nodular enhancement.Conclusion Immature teratoma tends to occur in the middle struc-ture,and appears in children and young adults frequently,necrosis and cystic changes are seen commonly.CT and MR are of high val-ue in diagnosing immature teratoma.