B U J O D. Original Research

Transcription

1 Original Research EVALUATATION OF STRESS DISTRIBUTION WITH VARIOUS ORTHODONTIC FORCES IN MAXILLARY CENTRAL INCISOR WITH VARIOUS ROOT MORPHOLOGY - A FINITE ELEMENT STUDY AUTHORS: Parikshit Rao*, Narayan Kulkarni** ABSTRACT: Introduction: Root resorption is unavoidable sequel to orthodontic treatment especially in maxillary incisor. Application of orthodontic forces produces a stress and depending upon the magnitude of these stresses root resorption takes place. No biomechanical study has been done that clarifies the influence of different orthodontic forces on various root morphology. Finite Element Model Analysis is non-invasive and accurate method which permits detailed analyses of tooth movement in 3-Dimensions and allows reasonable approximation of the biological tissues. The purpose of the study was to investigate stress distribution on various types of root morphology on application of different types of orthodontic forces using FEM model. Method: Finite Element Model of Maxillary central incisor with different root morphologies (normal, short, blunt, dilacerated and pipette) were constructed and optimum orthodontic forces in various directions (intrusion, extrusion, tipping, bodily and rotational) were applied to the tooth axis at the bracket level. Result: On application of various forces, significantly increased stress was seen at the apex of root with dilacerated and pipette morphology. Short and blunt root showed increased stress at the cervical 1/3 region when bodily and rotational forces were applied. Conclusion: In the present study, stress distribution pattern suggests increased root resorption in maxillary central incisor with deviated root morphology. Introduction Application of orthodontic forces produces a stress condition at the tooth, periodontal ligament and bone interface further causing tooth movement. Depending upon the magnitude of these stresses favorable or unfavorable movement occurs and may cause irreversible root resorption. Therefore accurate analysis of stresses at this interface appears to be a basic step towas correct understanding of its mechanical behavior and also to optimize tooth movement. ADDRESS FOR CORRESSPONDENCE: Dr Parikshit Rao. C1/. Ramgiri, Ojas Park. Vasna road, Near vasna jakatnaka, Baroda. Gujarat. raoparikshit83@yahoo.in One such analytical approach; to study stresses during tooth movement, which allows reasonable approximation of the biological tissues, is Finite Element Model (FEM). It is noninvasive, accurate and permits detailed analyses of tooth movement. Also it provides with quantitative data that increases the understanding of the physiologic reactions that occur after force application and may yield an improved understanding of the reactions and interactions of individual tissues. Such detailed information on the stresses and strain in the tissues is difficult to obtain and accurately analyzed by any other experimental technique because of the interaction of the surrounding tissues, which may then distort the data [1] obtained for any individual material response. *, ** Sr. Lecturer Dept of Orthodontics, K.M.Shah Dental College, Baroda. BHAVNAGAR UNIVERSITY'S JOURNAL OF DENTISTRY Vol. Issue-3 Sept. 01 4

2 Rao et al Sameshima and Sinclair in a comparative study using radiographs taken before and after the treatment reported that teeth with abnormal root morphology frequently show external root resorption compared to normal root [] morphology. Mirabella and Arthur found that in abnormal root shape, if orthodontic force is concentrated at a particular region of the deviated root shape [3] then root resorption may occur. Lee Y stated that it is possible that patients with dental anomalies have increased risk for apical resorption during orthodontic treatment. The mechanism may be that cementum and dentin are affected during root formation in such patients, thus reducing the ability of the cementum and dentin to resist resorption in [4] situations with excess of pressure. However, no biomechanical study has been done that clarifies the influence of different orthodontic forces (intrusion, extrusion, tipping, bodily and rotational) on various root morphology (normal, short, blunt, dilacerated and pipette) during orthodontic force application. Thus, the present study aims to study the effects of different types of orthodontic forces on various types of root morphology using FEM model. Material and Method: Preparation of FEM : 3-dimensional finite element model of permanent maxillary central incisor, periodontal ligament and alveolar bone (cancellous bone) with normal root morphology and its variation as classified by Lavender and Malmgren (four variations i.e. short root, blunt root, dilacerated root and pipette shaped root) were prepared using Ansys version 10 software.(fig 01 A-E).[] Tooth model with normal root morphology was constructed based on the data given by Wheeler's[ 6] (crown length of 10. mm and a root length of 13 mm) (Fig 1-A). Model with short root was constructed with root length of 8mm. (Fig 1-B). Model with dilacerated root morphology had distally bent root apex. (Fig 1- D). Model with pipette root morphology had constriction at middle 1/3 root apex. (Fig 1-E). Figure Legends: Fig 1 Various Root Morphologies Fig 1 A Normal Root Morphology Fig 1 D Dilacerated Root Morphology The periodontal membrane which surrounded the root surface was 00 µm thick and the alveolar bone was represented by cancellous bone of.0 mm thickness. Material properties: Fig 1 E Pipette Root Morphology Fig 1 C Blunt Root Morphology Each structure of central incisor i.e. enamel, dentine and pulp, periodontal ligament and cancellous bone was meshed using automeshing routine in the finite element analysis program (Ansys 10). (Fig 0 A-E)The material properties for all models were defined as linear and are shown in Table 1. BHAVNAGAR UNIVERSITY'S JOURNAL OF DENTISTRY Vol. Issue-3 Sept. 01

3 Fig Preparation of FEM model Fig A Enamel and Dentine Fig B Pulp Fig C Cancellous Bone Fig D Model after meshing Table 1: Material Parameters used in the Finite Element Model. M a te ria l Y o u n g s m o d u l u s (N /m m ) E n a m e l D e n tin P o isso n s R a tio P e rio d o n ta l lig a m e n t B o n e Boundary conditions and Solution: Nodes at the mesiodistal and the bottom surface of the alveolar bone were restricted with six degrees of freedom. An area of 3.3 X 4.8 mm was fixed as the loaded portion assuming a size reflected by the size of the bracket. (Fig 03) Experimental orthodontic forces in various directions to the tooth axis were applied to the bracket base area as stated in Table no. Table : Force System Applied Force T ype Intrusion E xtrusi on R otation T ipping Force M agnitude (gm ) 1 gm 0 gm 0 gm 0 gm D irection Intrusion force acting parallel to the lon g axis and tow as the ap ex of tooth fro m centre of cro w n. E x trusion fo rce acting parallel to l ong axis and tow as the incisal edge of tooth f rom centre of th e crow n. H orizo ntal force along the disto-facial and m esiolingual line angles of the incisor in opposi te directions. H orizo ntal force in a distal direction applied perpendicula r to long ax is of the too th. B odily M ovem ent 100 gm H orizo ntal force in a lingual direction at the center of crow n w it h a couple in a buccal crow n d irection at the incisal tip. Stresses and strains for each model on application of each force were calculated with Ansys 10 software using linear structural analysis. BHAVNAGAR UNIVERSITY'S JOURNAL OF DENTISTRY Vol. Issue-3 Sept. 01 6

4 Rao et al Results:The stress distribution on central incisor under various conditions is tabulated in Table no. 3,4 (Fig 04-8) Table 3 : Stress produced in Normal, Short and Blunt root. S tr e s s p r o d u c e d in d if f e r e n t a r e a s o f r o o t. T y p e f o r c e a p p lie d. o f N O R M A L R O O T S H O R T R O O T B L U N T R O O T C M A C M A C M A I n tr u s io n E x tr u s i o n T ip p in g R o ta tio n B o d ily Bold letters in the table denote maximum stress value in respective region when compared with same region in. - All the values are in N/mm. - C denotes cervical area, M denotes middle 1/3 areas, A denotes apical area. Table 4 : Stress produced in Pippette root. Type of force applied. Intrusion Extrusion Tipping Rotation Bodily Stress produced in different areas of root. DILACERATED ROOT P IPPET TE ROOT C M A C M A Dilacerated and Bold letters in the table denote maximum stress value in respective region when compared with same region in. - All the values are in N/mm. - C denotes cervical area, M denotes middle 1/3 areas, A denotes apical area. DISCUSSION: Fig 4 Effect on application of Intrusion force on Fig 4 A Intrusion force on normal root Fig 4 C Intrusion force Fig 4 D Intrusion force Fig 4 B Intrusion force on Short root Fig 4 E Intrusion force All the models showed stress concentration at the cervical area of root surface with maximum stress in the blunt root. The model with pipette shaped root showed the most uncommon variation when compared to other models along the cervico-apical path. The stress BHAVNAGAR UNIVERSITY'S JOURNAL OF DENTISTRY Vol. Issue-3 Sept. 01 7

5 value at the cervical region was approximately similar to that of normal i.e N/mm in pipette shaped root and N/mm in normal root but increased about 9.3 times ( N/mm ) in the middle 1/3 region when compared to normal root model. (Table no.3) This findings maybe due to the constriction at the middle 1/3 region of the pipette shaped root. Dilacerated root showed increased amount of stress at the apex when compared to other models with the stress value of approximately 0.14 times that of normal model. The increased stress in the middle 1/3 region and apex can be due to progressive bent in the root morphology seen from the middle 1/3 region to the apex. Result are comparable with study carried out by Koji Oyama et al 7 and Germany A [8], the result stated all models had tendency to concentrate stress at the cervical area and the bracket base portion of the crown when intrusive force was applied. Bent root showed significant stress concentration at the mesial and distal portion of the apex. Model with pipette shaped root showed stress concentration at the labial and lingual surface at the neck of the root. The findings of present study were not in accoance with the study carried out by Rudolph D et al.[9] Result stated that intrusive forces resulted in the greatest stress at the apex. Based on the results it was concluded that intrusive forces produce more stress at the root apex. The difference in the results in the above study and present study maybe due to difference in construction of Finite element model or due to difference in the properties applied to each tooth structure. Extrusion : Fig Effect on application of Extrusive force on Fig C Extrusion force On comparing cervico-apical stress distribution pattern in all the models maximum stress in the cervical region was seen at the short root which was approximately 3. times (0.04 N/mm ) greater then the normal model (0.01 N/mm ). Dilacerated root showed N/mm stress in middle 1/3 region which was approx 1.8 times that of normal root morphology and pipette shaped showed maximum stress at the middle 1/3 with stress value of N/mm which was approximately.1 times that of normal root model. At the apex, maximum stress was seen at the apex of dilacerated root. The stress value was N/mm i.e. approximately 3 times that of normal model ( N/mm ). Tipping : Fig 6 Effect on application of on Fig 6 A on normal root Fig D Extrusion force Fig E Extrusion force Fig 6 B on Short root Fig 6 C Fig 6 D Fig 6 E Fig A Extrusion force on normal root Fig B Extrusion force on Short root Dilacerated root model showed stress value of 0.09 N/mm in the middle 1/3 region which was maximum at that region. Compared to stress in the normal root for the same region it was approximately 9.60 times. BHAVNAGAR UNIVERSITY'S JOURNAL OF DENTISTRY Vol. Issue-3 Sept. 01 8

6 Rao et al The result for normal root model in the present study was found to be in accoance with the study carried out by Rudolph D et al.[9] Rotation: Fig 7 Effect on application of on Short root model showed maximum stress in the cervical 1/3 region with stress approximately 4 times greater then rest of the models. All the other models showed approximately equal amount of stress i.e. 0.0 N/mm with mild increase in dilacerated model with stress value of N/mm. Maximum stress was seen at the dilacerated root model in both middle 1/3 and at the apex with stress values of N/mm and N/mm. These values state that the stress was 11 times and 19 times greater then the values of normal root model in respective regions. The result for normal root model in the present study was found to be in accoance with the study carried out by Rudolph D et al.[9] Bodily: Fig 7 A on normal root Fig 7 C Fig 7 D Fig 7 B on Short root Fig 8 Effect on application of Bodily force on Fig 8 A Bodily force on normal root Fig 8 B Bodily force on Short root Fig 8 C Bodily force Fig 8 D Bodily force Fig 8 E Bodily force Fig 7 E Maximum stress at the cervical 1/3 region was seen at the blunt root and short root model. Most important finding for bodily force was that at the middle 1/3 region maximum stress was seen in the normal root (0.038 N/mm ). Unlike any other force considered in the study where in root models with variation in their morphology showed greater stress then the normal. Following observation can be explained by understanding the biomechanics of tooth movement. For bodily tooth movement to occur the force level has to pass through the center of resistance which lies approximately 40 % of the way from the alveolar creast to the apex. This point is analogous to the center of the root between the alveolar creast and the apex, the same region wherein increased stress was seen in the normal model. By which we can probably state that bodily movement was achieved in our normal model. At the apex maximum stress was found at the dilacerated root model and was approximately times ( N/mm ) that of normal root model ( N/mm ). By combining and comparing the result of the present study on application of different forces on various root forms, stress distribution pattern is clarified suggesting of increased root resorption in maxillary central incisor with deviated root morphology. It can also be stated that maximum root resorption was seen in dilacerated root form followed by pipette root form although short and blunt root morphology also show increased root resorption when compared to normal regaless of direction of force applied. These results from the study can be substantiated by following clinical studies, Lavender E, Malmgren carried out a radiographic study to investigate the risk of severe apical root resorption after orthodontic treatment with fixed appliance in relation to resorption after initial treatment of 6-9 months and in relation to apical root form. Results stated that the degree of root resorption in teeth with blunt or pipette shaped root was significantly higher than in teeth with a normal root form.[] Mirabella A, Artun J studied risk factors for apical root resorption in adult orthodontic patients. BHAVNAGAR UNIVERSITY'S JOURNAL OF DENTISTRY Vol. Issue-3 Sept. 01 9

7 Results stated tooth length was associated with root resorption and also that atypical root shape was a risk factor for root resorption for central incisor.[3] Sameshima G, Sinclair P carried out a study to determine whether it will be possible to identify pretreatment factors that will allow the clinician to predict the incidence, location and severity of root resorption before the commencement of orthodontic treatment. The results showed that resorption occurs primarily in the maxillary anterior region with worst resorption seen with maxillary lateral incisor and in teeth with abnormal ro o t m o r p h o l o g y ( p i p e t t e, p o i n t e d, o r dilacerated).[10] Nigul K, Jagomagi T determined the factors related to apical root resorption of maxillary incisor in orthodontic patients. Results stated that the worst resorption was seen with teeth with abnormal root morphology. [11] CONCLUSION: The following conclusions were drawn from the present study: vdeviated root morphology of tooth structure is prone for increased root resorption as compared to normal root morphology on application of different types of orthodontic forces in various directions. vdilacerated root morphology was seen to be the most affected root morphology for root resorption followed by pipette shaped root. vwhen orthodontic forces in different directions were compared, force for bodily movement showed maximum amount of root resorption. It is therefore important to identify the root shape at the beginning of the orthodontic treatment. References: 1. Middleton J, Jones M, Wilson A. The role of the periodontal ligament in bone modeling the initial development of a time-dependent finite element model Am J Orthod Dentofacial Orthop ; 109():1-6.. Sameshima G, Sinclair P. Predicting and preventing root resorption: Part I. Diagnostic factors Am J Orthod Dentofacial Orthop. 001; 119(): Mirabella AD, Artun J. Risk factors for apical root resorption of maxillary anterior teeth in adult orthodontic patients Am J Orthod Dentofacial Orthop. 199; 108(7): Lee R et al. Are dental anamolies risk factors for apical root resorption in orthodontic patients? Am J Orthod Dentofacial Orthop. 1999; 116(): Levander E, Malmgren O. Evaluation of the risk of root resorption during orthodontic treatment. A study of upper incisors. Eur J Orthod.1988; 10: Ash M, Stanley, Nelson J. Wheeler's Dental Anatomy, Physiology and Occlusion. Eighth Edition. Saunders publishers; Oyama K, Motoyoshi M, Hirabayashi M. Effect of root morphology on stress distribution at the root apex. Eur J Orthod.007; 9(): Germany A. Apical thi morphology and intrusive force application: 3D finite element analysis. Tehran J of dentistry. 007; 4(3): Rudolph D, Wiles M, Sameshima G. A finite element model of apical force distribuation from orthodontic tooth movement. Angel orthod. 001; 71 (): Sameshima G, Sinclair P. Predicting and preventing root resorption: Part II. Treatment Factors. Am J Orthod Dentofacial Orthop. 001; 119(): Nigul K, Jagomagi T. Factors related to apical root resorption of maxillary incisors in orthodontic patients. Stomatologiji, basic dental and maxillofacial J.006; 8(3): Source of Support : Conflict of Interest : Date of Submission : Review Completed : NIL NOT DECLARED BHAVNAGAR UNIVERSITY'S JOURNAL OF DENTISTRY Vol. Issue-3 Sept