| Objective:To evaluate the stress distribution within dentine under the influence of different core heights and post materials, concerning post-core crown prosthesis with different crown-root ratios, in the hope of providing a better design of post-core system for clinical practice.Methods:1. A normal maxillary central incisor was chosen as model, according to the statistical data reported by Wang Huiyun; and 47 sectional images were obtained, by spiral computed tomography (CT), scanning perpendicular to the tooth axis, form incisal edge to apex with 0.5mm intervals.2. Sectional images were imported to Mimics software program to generate the 3-D model of this tooth. By processing with Geomagic Studio, analyzable prosthesis models were established with diverse crown-root ratios, core heights and post materials.3. Ansys 11.0 was used to segment these models into meshes, to load them with forces from three different directions respectively, and to analyze the corresponding stress distributions to simulate conditions under normal occlusion, biting and trauma.Results:1. By the CT scanning, Mimics and Geomagic Studio modeling, Ansys software analyzing, models with various crown-root ratios, core heights and post materials were constructed and accessible for 3-D finite element analysis.2. The overall tendency of the stress distribution on post-core crown prosthesis was: the maximum principle stress, minimum principle stress and equilibrium stress all fall on the post-dentine interface, along with the contacting area of the outer surface of root and bone cortex. The stress peak on the outer surface of root was lower than the one on the post-dentine interface. Under the three loading conditions mentioned above, the maximum principle stress and equilibrium stress on post were both larger than those on the root, while post experienced less minimum principle stress comparing to root.3. At the maximum core height, the maximum principle stresses on dentine are 76.951, 44.232 and 3.886MPa respectively, under horizontal, obligated and vertical load, while the equilibrium stresses are 62.492,39.185 and 9.042MPa correspondingly; while for the post, the maximum principle stresses on it are 98.180,48.967 and 9.644MPa under the three loading conditions, while the equilibrium stresses are 95.791,67.671 and 19.600MPa. In contrast, at the minimum core height, the maximum principle stresses on dentine are 77.899,43.743 and 3.469MPa under horizontal, obligated and vertical load, while the equilibrium stresses are 64.549,41.133 and 11.702MPa correspondingly; and for the post, the maximum principle stresses on it are 97.912,48.678 and 10.961 MPa under the three loading conditions, while the equilibrium stresses are 96.809,60.776 and 23.170MPa. According to the data obtained from this experiment, choosing the mean value among the upper and lower limits of post height optimized the stress peak to a relatively lower value. For instance, on dentine, with horizontal, obligated and vertical load, the maximum principle stresses are 76.465,43.030 and 3.212MPa individually, while the equilibrium stresses are 59.477,38.521 and 6.700MPa correspondingly; whereas for the post, the maximum principle stresses on it are 97.632,28.446 and 10.698MPa under the three loading conditions, while the equilibrium stresses are 95.174,60.548 and 16.506MPa. However, the differences made by changing post height usually were not significant, which were less than 5% of the minimum peak data.4. The increasing of crown-root ratios contributed to the evaluation and assembling of stress within prosthesis, and the stress peaks were positively related and kind of proportional to crown-root ratio. For instance, choosing the prosthesis with crown-root ratio of 1 as standard, the peak of equilibrium stress became 1.25 to 1.38 fold of the standard for a prosthesis with a 1.3 crown-root ratio; while this value would drop to 70% to 80% of the standard for a prosthesis with a crown-root ratio of 0.77.5. The peak value of the stress on root cervix decreased along with the elevation of post elasticity modulus; while the peak value on the post-dentine interface raise in responding to the increase of post elasticity modulus.Conclusion:1. The height of core is not a critical determinant on the stress distribution and peak value. However, too high or too low core height will resulted in stress concentration on prosthesis to some extent. Therefore, properly designed core height may relieve the stress on dentine, and it may be suggested as a protective mechanism to lower the core height while keeping it higher than 4mm at the same time.2. Post materials with similar elasticity modulus as dentine are not suitable for restoring teeth with fragile root cervixes; those with high elasticity modulus, such as alloys, may protect the root in some level, by rearranging stress distributions and absorbing stress.3. Under the same force intensity, horizontal load has the most destructive effect on post-core crown prosthesis, followed by obligated load; in contract, vertical load barely plays a harmful role for the prosthesis.4. Regarding the tooth restoration with imbalanced crown-root ratio, gold alloy may be a better choice, which can help the root to achieve a more balanced circumstance for stress distribution. Co-Cr and Ti-alloys, because of their high elasticity modulus, will lead to significant stress concentration on the post-dentine interface. This effect disturbs the protective mechanism of the metal post, which can absorb a certain portion of stress on prosthesis and further balance the stress distributions. While for the glass-fiber post, its low elasticity modulus results in the incapability to absorb stress, change or balance the stress distribution. Therefore it will enhance the stress concentration in dentine around the cervical region for prosthesis which already has imbalanced crown-root ratio. In contrast, the gold ally with the moderate elasticity modulus should be a good candidate for the restoration for these cases. |
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