Orthodontic Anchorage Unit Research Study

Orthodontic Anchorage Unit Research Study Chapter 3 MATERIALS AND METHODS A three-dimensional bone block model (Figure 2) integrated with a miniscrew was constructed with a computer-aided design program (SolidWorks; Dassault Systemes SolidWorks, Concord, Mass) to simulate a miniscrew implanted in bone as an orthodontic anchorage unit. Figure 2. Three dimensional bone block model with screw embedded. The bone block, consisting of cortical and cancellous bone, was constructed with dimensions of 20 mm. in length and width, and 15 mm. in height for evaluation. [1] The cortical bone thickness, elastic moduli, Poisson’s ratio, shear moduli and density of the edentulous maxillary and mandibular alveolar bone as shown on Table 2 was based on the studies of Dechow et al.[2] and Schwartz-Dabney and Dechow[3], respectively; and the dentate specimen based on the study of Peterson et al. [4] These were taken from specific locations of the edentulous maxillary and mandibular alveolar bone as shown in Figure 3. Since cancellous bone density was found to have little or no consequence to results obtained by previous studies[5], a fixed value will be set with elastic moduli of 1.3 GPa, Poisson’s ratio of 0.3, shear moduli of 2.0 and density 0.5g/cm3. Figure 3. Location from which cortical bone specimens were removed from the edentulous crania by Dechow et al.[6] and the edentulous mandibles by Schwartz-Dabney and Dechow[7] The miniscrew geometry was based on the MONDEAL system (MONDEAL Medical Systems, Muhlheim, Germany): ie, the screw thread profile was an isosceles triangle 0.4 mm in height and 0.16 mm along the base.[8] The thread pitch was 1.0 mm. These thread dimensions were fixed in all screw designs in this study. The miniscrew will be assumed to be homogeneous, isotropic and linearly elastic titanium alloy (Ti-6Al-4V) with elastic modulus of 114 GPa, Poisson’s ratio or 0.34 and Yield strength of 880MPa.[9] Table 2. Material properties of maxillary[10] and mandibular[11] alveloar cortical bone. The model was meshed automatically with 10-node tetrahedral solid elements. The interface between the cortex and the cancellous bone was assumed to be fully bonded; i.e., the elements were continuous, sharing the same nodes along the interface. A node-to-node contact condition was given on the interface between the miniscrew and the bone block to imitate a stage without osseointegration.[13] The static load along the x-axis was applied to the head of the miniscrew and perpendicular to its long axis to simulate the orthodontic force. For the nodes located on the 5 exterior surfaces of the bone block, all but the superior surface where the miniscrew entered was constrained in all degrees of freedom to simulate the boundary condition. The nodal solution of the von Mises stress in the bone and the displacement of the miniscrew were calculated for each model with the finite element analysis program.[14] To determine the loading effect, the maxillary and mandibular bone blocks will be subjected to four force magnitudes (1, 2, 4 and 6 N) and three force directions (60o , 90o and 120o) to mimic various clinical conditions. A force direction of 90o was the force perpendicular to the long axis of the miniscrew and parallel to the bone surface.[15] 2 N is the reported clinically safe limit for immediate loading.[16] The loads will be applied at the level of the eye of the screw meant for tying ligatures in a clinical situation.[17] To determine the screw size effect, three screw (outer) diameters (1.5, 2.0 and 2.3 mm.) and five screw length (7, 9, 11, 13 and 15 mm.). The screw length will be measured including the screw head, which had a 2 mm. height for all screw models. To simulate soft tissue thickness, various screw depths (screw length in the bone block) will be modelled.[18] For each screw length, the exposed screw length of 3, 4, 5 mm. will be measured. The von Mises stress and the displacement was determined for the different screw length, diameter and exposed screw length was subjected to the different force magnitude and direction. For every bone specimen there was 540 different scenarios. The stress distribution was then observed for the screws that exhibited the highest stress for each bone specimen and for each force direction. Chapter 5 SUMMARY OF FINDINGS, CONCLUSION AND RECOMMENDATIONS Summary of Findings There is no direct relationship between stress and displacement established. The pattern of the von Mises stress of miniscrews embedded on different bones are similiar with only minor differences. Whereas the maxillary edentulous specimens showing more displacement compared to its mandibular counterpart. Though the highest stress and highest displacement can be observed in 6 N of force at 90o direction at 5 mm. exposed length and the lowest stress and lowest displacement can be observed in 1 N of force at 1200 direction at 3 mm. exposed length. The stress is concentrated on the cortical bone irrespective of the screw length and the exposed screw length. It is more intensified at 5 mm exposed screw length because the moment of arm becomes longer, and lighter at 3 mm exposed length because the moment arm becomes shorter. The cortical bone acts as the fulcrum of the force. All miniscrews embedded in mandibular edentulous specimens were able to establish primary stability except for 2 scenarios in MDE4. 12-13% of miniscrews embedded on maxillary edentulous specimen were not able to establish primary stability. Majority of these occurred with screw diameter of 1.5 mm. (78 85%), exposed screw length of 5 mm. (61 72%), force magnitude of 6 N. (62 – 68 %), and force angle of 900 (40 45%). Conclusion Based on the data gathered, it is found that placement of orthodontic miniscrew on edentulous alveolar bone on both the maxilla and mandible is possible. The use of miniscrew with diameter of 2mm or wider embedding it on attached gingiva of 2mm or thinner, loading it with force 2 N or lower at an angle 60 or 1200 could increase chances of establishing primary stability. Recommendations The proponent of this research would like to recommend the implementation of a Randomized Controlled Trial based on the results of this study. This would ensure the evidence-based practice of the miniscrew anchorage. References [1] T. C. Liu and others, Finite Element Analysis of Miniscrew Implants Used for Orthodontic Anchorage, Am J Orthod Dentofacial Orthop, 141 (2012). [2] P. C. Dechow, Q. Wang, and J. Peterson, Edentulation Alters Material Properties of Cortical Bone in the Human Craniofacial Skeleton: Functional Implications for Craniofacial Structure in Primate Evolution, The Anatomical Records: Advances in Integrative Anatomy and Evolutionary Biology, 293 (2010). [3] C. L. Schwartz-Dabney and P. C. Dechow, Edentulation Alters Material Properties of Cortical Bone in the Human Mandible, J Dent Res, 81 (2002). [4] J. Peterson, Q. Wang, and P. C. Dechow, Material Properties of the Dentate Maxilla, Anat Rec A Discov Mol Cell Evol Biol, 288 (2006). [5] A. Gracco and others, Numerical/Experimental Analysis of the Stress Field around Miniscrews for Orthodontic Anchorage, Eur J Orthod, 31 (2009). [6] Dechow, Wang, and Peterson. [7] Schwartz-Dabney and Dechow. [8] Liu and others [9] Gerhard Welsch, R. Boyer, and E.W. Collings, Materials Properties Handbook: Titanium Alloys, (Asm International, 1994). [10] Dechow, Wang, and Peterson. [11] Schwartz-Dabney and Dechow. [12] Peterson, Wang, and Dechow. [13] Liu and others [14] Ibid. [15] Ibid. [16] A. G. Crismani and others, Miniscrews in Orthodontic Treatment: Review and Analysis of Published Clinical Trials, Am J Orthod Dentofacial Orthop, 137 (2010). [17] S. Singh and others, Three-Dimensional Finite Element Analysis of Strength, Stability, and Stress Distribution in Orthodontic Anchorage: A Conical, Self-Drilling Miniscrew Implant System, Am J Orthod Dentofacial Orthop, 141 (2012). [18] Liu and others