Three Dimensional Analysis Of The Relationship Between The Alveolar Bone Resorption Ratio And The Periodontal Support Area And Volume Of The Mandibular Premolar

As a mechanics unit, periodontal tissues perform the functions of stabling and supporting, as well as bite force distributing. Alveolar bone loss, attributed by various reasons, may lead to compromisingof periodontal support capacity. It is important to make a general assessment of periodontal support ability of the potential abutment for dentures in clinic. Widely used in the evaluation of the suitabilityof a tooth as an abutment for a fixed or removable partialdenture, one dimensionalvariables, such as thealveolar bone absorption ratio and the crown-to-root ratio, cannot exactly reflect the shape characters of a tooth.Multi-dimensional-index based measurement and analysis are the directionsof abutment evaluation, with the help of cone-beam CT.The aim of this study was to explore the three dimensional way to periodontal support ability evaluation, thus to contribute to better abutment evaluating and prognosis predicting.In our study, a three dimensional way was explored to measure the length, the surface area, the volume, and the crown-to-root ratio of the mandibular premolars by CBCT. The samples were scanned by CBCT and their digital tooth surface images were obtained. After the tooth long axis and the cemento-enamel junction were determined, the root portion was divided into ten equal parts simulating alveolar absorption in 10% increments. And then the relationships between the ratio of alveolar absorption and the CRR (Crown-to-root ratio, CRR), including CRR of the length (CRRL), CRR of the surface area (CRRs) and CRR of the volume (CRRV), were explored. At last,based on tooth long axis, a three dimensional finite element stress analysis model was established and used in furtherevaluation of the effects of alveolar bone loss on stress distribution of periodontal ligament of mandibular premolar.PART ⅠA pilot study of three dimensional evaluation of tooth length, surface area, volume and crown-to-root ratio of mandibular premolarsObjectives:The aim of this study was to explore a three dimensional way to measure the length, the surface area, the volume, and the crown-to-root ratio of the mandibular premolars. Results:The average root length, root surface area, root volume, crown-to-root ratio in length, crown-to-root ratio in area, and crown-to-root ratio in volume were 14.28mm,222.40m2,331.58m3,0.66,1.07, and 0.86 respectively for mandibular first premolars, and 13.24mm,229.96m2,354.02m3,0.67,0.98,0.77 for mandibular second premolars. There were statistically significant differences in total tooth length, crown length, root length, root volume, crown-to-root ratio in area, and crown-to-root ratio in volume between the two kinds of mandibular premolars. Conclusions:Using cone beam CT scanning and three dimensional measuring methods, the length, the surface area, the volume, and the crown-to-root ratio of the mandibular premolars can be measured, as well as the crown-to-root ratio of the length, the surface area, and the volume. These provide new indexes to evaluate the ability and the prognosis of a tooth that may serve as prosthetic abutment.PART ⅡThe relationship between alveolar bone absorption ratio and the extent of periodontal support of mandibular premolarsObjectives:The aim of this study was to explore a three dimensional way to measure alveolar absorption ratio and the remaining area of periodontal ligament support, as well as the relationship between the alveolar absorption ratio and the volume of alveolar bone. Results:With the divisions of root goingapically, the remaining root area and volume were decreased. The remaining area of the periodontal ligament (F(X)) and the degree of alveolar bone loss(X) had a first order function F(X)=1.02-1.12*X, R2=0.994; The remaining volume of the periodontal ligament (F’(X)) and the degree of alveolar bone loss(X) hada quadratic function F’(X)= 0.70*X2-1.81*X+1.06, R2=0.993.Conclusions:With the increase in the extent of alveolar bone absorption, theperiodontal tissues supporting ability was reduced. The proportion of the remaining periodontal ligament and alveolar bone had a function of the extent of absorption. The remaining alveolar bone support volume and extent of absorption of alveolar bone has a quadratic functional relationship.PART ⅢThe relationship between the crown-to-root ratio and the ratio of alveolar absorption of mandibular premolarsObjectives:The aim of this study was to determine the length, the surface area, and the volume of the crown portion and the root portion of the mandibular premolar by computerized image analysis, and to investigate the relationship between the ratio of alveolar absorption and the crown-to-root ratio (CRR). Results:For a certain degree of alveolar absorption, the value of the CRRL is the smallest, and the value of the CRRs is the biggest. Cubic functions and equations fit better than other functions and equations in relating the ratio of alveolar absorption to the CRR. The curvilinear patterns of all the three equations for the CRRL, the CRRS, and the CRRV are similar. From 0 to 20% of alveolar absorption, the value of CRR increases fast. From 20% to 40% of alveolar absorption, the value of CRR increases slowly. If the alveolar absorption exceeds 40%, the value of CRR increases rapidly. Conclusions:Using CBCT scan and computerized image analysis, the CRRL, CRRS and CRRV can be determined. Three times function may explain the relationship between the ratio of alveolar absorption and the CRR.20% and 40% of alveolar absorption are critical values for functional evaluation of mandibular premolars.PART ⅣThree dimensional stress analysis of periodontal ligament of mandibular premolar with alveolar bone lossObjective:To establish three dimensional finite element analysis models based on tooth long axis, and to investigate the effects of alveolar bone loss on stress distribution of periodontal ligament of mandibular premolar. Methods:A sound extracted mandibular premolar was selected. Finite element stress analysis modeling procedures included micro-CT scanning, reconstructing digital tooth model, determining the long axis, determining the cemento-enamel junction, ten-equal dividing of the root, establishing stress analysis model. The loading force was 100 N, and the loading directions were parallel, perpendicular, and being 45°of angle to the tooth long axis. The values of maximum Von Mises stress were calculated. Results:Three dimensional finite element analysis models were established based on tooth long axis and the cemento-enamel junction. The values of maximum Von Mises stress increased along with the ratio of alveolar bone loss. In loading force parallel to the long axis of the tooth, the maximum Von Mises stress of periodontal ligament in 50% alveolar bone loss is two times higher than normal teeth. In loading force perpendicular or with an angle of 45°to the long axis of the tooth, the maximum Von Mises stress of periodontal ligament in 30% alveolar bone loss was two times higher than normal teeth. CRRL, CRRS, CRRV, the ratio of clinical crown length/the remaining periodontal ligament area and the ratio of clinical crown length/volume of remaining root were highly correlated with maximum Von Mises stress of periodontal ligament. Conclusion:For mandibular molars when the alveolar bone loss up to 30% will cause damage to the periodontal tissue.CRRL, CRRs, CRRv, the ratio of clinical crown length/the remaining periodontal ligament area and the ratio of clinical crown length/volume of remaining root can accurately estimate the potential of the periodontal ligament stress values.