Microstructure Change Of Alveolar Bone During Orthodontic Tooth Movement In Ovariectomized Rats

Background and PurposeDuring orthodontic treatment,tooth movement was closely related to alveolar bone remodeling.Due to estrogen deficiency and aging,orthodontic treatment,especially for postmenopausal women,could result in the decline of bone mineral density（BMD）and osteoporosis（OP）,which would further affect the balance of bone resorption and formation in periodontal tissues and pose a crucial factor hindering orthodontic treatment to senior patients.In this study,stress distribution of root surface of rat molar was analyzed under orthodontic force loading by means of finite element method（FEM）.And micro-computed tomography（Micro-CT）system was used to develop a new approach to evaluate the dynamic changes of alveolar bone and distance for mesial movement of the molar in ovariectomized rats during orthodontic tooth movement,and explore the change pattern of the trabecular bone microstructure in alveolar bone and orthodontic tooth movement under osteoporosis conditions so as to provide theoretical reference for orthodontic clinic treatment.MethodsA three dimensional finite element model of maxillary first molar and its support tissues of rat was established by means of FEM and Micro-CT system.Different force magnitudes（25 g and 75 g）were exerted simulatly on the tooth cervix of model passing the centers of mesial root and distobuccal root,and Von Mises stress was determined under different force loading.140 female Wistar rats were randomly divided into ovariectomy（OVX）group and sham group of equal number.Three months after surgery,60 rats were selected randomly from each group and orthodontic appliances with force loading were installed to simulate the orthodontic tooth movement.Each group was divided into25-g subgroup and 75-g subgroup of equal number according to diferent force loading.On the 3^rd,7^th and 14^th day after the installation of the orthodontic appliances,three batches of 10 rats from each group being randomly picked and 20 rats which were not installed the appliances as control were anesthetized to death respectively.Their left maxillary bones as samples were separated and scanned using Micro-CT system,microstructural parameters for trabecular bone mesial to the distobuccal root were evaluated.Meanwhile,distance for mesial movement of the molar was measured.ResultsThree dimensional finite element model of maxillary first molar of rat which was established by combining Micro-CT system had satisfactory geometric and mechanical comparability.The characteristics of stress distribution of root surface were similar under 25 g and 75 g force loading.The stress became larger with force loading increased.Among five roots of the maxillary first molar,the stress of distal region to the cervical third of the mesiobuccal root was largest.And to distobuccal root which volume was relative bigger,the stress of mesial region to the cervical third of it was largest.Stress distribution of root surface also included:the distal region of mesiobuccal root,distal region to the cervical and middle third of the distobuccal and mesial roots,distal region to the middle third of the distolingual root.Moreover,the stress of mesiolingual and distolingual roots which lied in lingual side were smaller than that of mesiobuccal and distobuccal roots which lied in buccal side significantly.Microstructural parameters for trabecular bone of four subgroups showed no significant changes from day 0 to day 3（P>0.05）;from day 3 to day 7,BMD,BV/TV and Tb.Th decreased significantly（P<0.05）,whereas Tb.Sp and SMI increased significantly（P<0.05）;the differences lied in that:from day 7 to day 14,in sham 25-g subgroup,BMD,BV/TV and Tb.Th increased significantly（P<0.05）,while Tb.Sp and SMI decreased significantly（P<0.05）.And in sham 75-g subgroup,changes of parameters did carry any statistical significance（P>0.05）.Correspondingly,the various parameters of OVX 25-g subgroup were no obvious change,while the significant increase of BMD,BV/TV and Tb.Th and the significant decrease of Tb.Sp and SMI were found.Meanwhile,in OVX 75-g subgroup,BMD,BV/TV and Tb.Th decreased significantly（P<0.05）,whereas Tb.Sp and SMI increased significantly（P<0.05）.In addition,under the same force magnitudes,the distance for mesial movement in OVX group was larger than that in sham group at different time points（P<0.05）.But in each group,the 75-g subgroup showed larger distance than 25-g subgroup only at day 14（P<0.05）.ConclusionsMesial region to the cervical third of the distobuccal root was most suitable for study region in term of alveolar bone remolding during orthodontic tooth movement.Lack of estrogen could affect alveolar bone remolding and accelerate tooth movement.Apparently,the fast tooth movement at the expense of excessive alveolar bone resorption was not ideal.Even the distance for mesial movement of molar under light force loading was less than that of under heavy force loading,but orthodontic tooth movement was safty.