Biomechanics of the Periodontal Ligament Clinical, Experimental and Numerical Studies
The periodontal ligament (PDL) is a complex, multiphasic structure consisting of fibres, cells, vessels and a fluid phase. This complex structure results in a nonlinear, time dependant behaviour that is difficult to describe by constitutive laws. This talk reports about experimental, numerical and clinical studies to determine the biomechanical behaviour of the periodontal ligament (PDL) with respect to long-term orthodontic force application and short-term masticatory loading. In-vitro experimental studies have been performed on rat, pig and human specimens and results were compared to numerical analyses using finite element models of the respective specimens. In a clinical in-vivo study, a new self-developed device was used to measure the time-dependant behaviour of the PDL and to monitor the changes of the constitutive behaviour of the PDL due to orthodontic therapy. Biomechanical consequences are described and compared to other changes during orthodontic tooth movement, such as geometrical changes of the alveolus.
The Biomechanical limitations of TADs
The advent of TAD is one of the most important contribution to broadening the scope of contemporary orthodontics. TAD readily removes the reciprocal forces on the reactive unit, however it is the only one source of many other side effects in the orthodontic force system. Therefore, TAD is not an "all-time solution" and still has many limitations in clinical applications. By redirecting the line of action, adding more forces for convenience, and making the force system consistent in the application of TAD, one can reduce the adverse side effects and overcome the biomechanics limitations of TAD.
Avoiding Side effects in Orthodontics
The force system from orthodontic appliance is very complicated that there is no ideal device that only produce the movement of the tooth desired by orthodontist. Many orthodontic appliances are inadvertently applied, leading to many unexpected side effects, such as anchorage loss, cuspid rotation during retraction, bite deepening by sliding mechanics, occlusal plane caning after leveling. No single magical appliance can treat the patient without side effects. If we could predict the tooth movement by analyzing the complex force system, we could avoid the side effects or even utilize it to improve treatment outcome. The appliance designed upon sound biomechanical principle will allows us predictable treatment result with minimal side effects. We will discuss about types of side effects, proper management of side effects, and strategic extraction.
This presentation will deal with biomechanical solutions for maximizing therapeutic efficiency and minimizing side effects during space closure in an extraction case. Although various treatment techniques have been developed, it remains quite difficult to predict or compare actual tooth movements subsequent to the application of different treatment mechanics. Therefore, the prediction and planning of orthodontic tooth movement have been largely dependent on clinical experiences. With such a background, the demand for simulating long-term tooth movement under the various treatment mechanics has been increasing to improve therapeutic efficiency and reduce the treatment time.
We developed a simulation system of long-term orthodontic tooth movement after going through bone remodeling process using the finite element method. Using this system, the optimal loading condition for achieving speedy and controllable tooth movements can be determined.
The optimal loading condition on the application of sliding mechanics in combination with power arms and/or implant anchorage will be described. Also for loop mechanics, we developed a simple loop design which produces an optimal force and an M/F ratio for en-masse retraction, and is applicable in the 0.022-in slot system.
In the end, future perspectives on optimizing treatment mechanics using numerical methods or artificial intelligence (AI) will be discussed.
Rodrigo F. Viecilli
Forces applied to the teeth by orthodontic wires are often statically indeterminate and difficult to predict. Burstone and Koenig developed their six-geometries of a two-bracket model to enable clinicians to estimate the force system acting on brackets and recognized additional complexity in large deformation scenarios, which is especially relevant in new orthodontic wires with non-linear properties. This paper aims to quantify the effect of wire material, dimension, and deflection on the most identifiable feature of the six-geometries: the moment dissociation point (force system with no moment on the lesser angled bracket), which may or may not occur at the classically defined geometry IV.
A six-degree of freedom load cell was used to measure the force systems in different combinations of wire materials, wire dimensions, total angle of bracket, and interbracket distance. Brackets were progressively rotated through Burstone and Koenig’s six geometries and the moment on the right bracket was plotted against the ratio of the angle of the two brackets. Regression analysis was used to determine the angular relationship where the actual moment dissociation point occurred for each variable combination. The moment dissociation points were statistically compared.
There were significant differences in the moment dissociation points in the variables studied. A shift in the moment dissociation point toward what is classically considered a geometry III, with lower interbracket distance ratios (IBDr = the ratio of distanes from a) the higher angle bracket to the bracket slot plane intersection point compared to the b) total interbracket distance) with linear materials and low wire deflections was observed. Higher deflections showed a pattern more consistent with the theoretical geometry IV (IBDr 0.33). Superelastic phase transformation at extremely high deflections led to a shift towards a geometry III (lower IBDrs). The moment dissociation point was not always coincident with a geometry IV as classically defined by Burstone and Koenig. Variables including wire material properties, dimension, and wire deflection affect the location of the moment dissociation point to different extents. The classic geometries as defined by Burstone and Koenig are a simplification of a complex wire deflection problem, especially with phase transforming pseudoelastic wires. In clinical situations, where one is attempting to create or predict the force system on brackets, these data should be taken into consideration, especially to avoid inconsistent force systems.
Comparison of Two methods of Transpalatal Arch and Lingual Arch Symmetric Activations
The TPA and LA can be activated with two methods: the “shape-driven method” where the appliance is made to match the final tooth position and the “force-driven method” where the appliance is made to make sure the force system delivered by it at the beginning is consistent with the planned tooth movement. This study compared the two methods in vitro quantitatively with typodont teeth and multi-axis force torque transducers in all three dimensions for symmetric moment activations in all 3 planes of space.
Both TPA and LA activated using the force-driven method exhibited lesser unintended side effects in first, second and third order forces and moments than the shape-driven method. We were able to quantify all relevant moment to force ratios to predict tooth movement with both force driven and shape driven activations, with a few unexpected results. As the targeted tooth movements can be helped or hindered by the side effects, clinicians can refer to the results to make the correct activations for the most efficient and effective tooth movements.
Correction of Iatrogenic Asymmetries
There are several reasons for which an orthodontic treatment can be considered a failure and detrimental. Iatrogenic asymmetries are one of the most frequent reasons for which patients require a retreatment after a previous non-satisfactory, treatment.
The reasons for which an iatrogenic asymmetry is created can be an inadequate treatment planning and/or poor biomechanics. In both cases, the outcomes can be detrimental both for esthetics and for function.
In this presentation, a few cases of iatrogenic asymmetry will be presented. The reason for the failure of the first treatment will be analyzed, and the retreatment will be illustrated.
“The Biomechanics of Invisalign” reviews the fundamental concepts of biomechanics in orthodontics and presents their application to clear aligner treatment. SmartForce technology which is comprised of virtual modeling and benchtop measurement of force systems produced by Invisalign aligners to control tooth movement is discussed. The features and innovations developed using this technology and recently introduced in the Invisalign system are presented from a biomechanical perspective. The design principles of SmartForce features, SmartTrack material, and SmartStage movements are presented.
Advanced considerations about the clinical use of statically indeterminate linear wire systems
In this presentation, several concepts that can guide the orthodontist in the clinical use of the “Six Geometries” will be discussed:
1) Beyond the basic general concepts about geometries, the dynamic of geometries and the resistance to displacement are essential notions to predict the clinical outcome of a two-bracket system.
2) Learn how to align consistently, without round tripping, with lower forces and larger springback using segment terminal activation in high geometries.
3) Discover a simple way to assess with good approximation the quantity of force and moment delivered by a “V bend” or by an alpha/beta (truncated V) spring.
Straight Wire Technique became revolutionary method in our era. Easy concepts, ready prescriptions, made orthodontic work flows like magic, but happy moments don’t last. Mixing bad apple with the good one sometimes will lead to spoil both of them, and that what happened in my real clinic life. Side effects and mistakes which i faced led me to search for the right way, and it was just in front of me, The principles of the Biomechanics......Going back to the rules. No more “wait and see!” predict the movement is the real play. Let me explain how Biomechanics drove me to the safe road.
Mandibular Repositioning: Treatment planning and Mechanics Design
dr. Giorgio Fiorelli, dr. Paola Merlo | One day course
Mandibular repositioning in adult patients can be an alternative to surgery in some skeletal class II and asymmetric adult patients.
During this course we will discuss the whole procedure, starting with the analysis of TMJ cbct images to assess the individual possibilities for condylar advancement, the clinical use of Triad gel (the material by which a temporary occlusion is reconstructed) and the evaluation of functional adaptation to the new occlusion.
At this point an orthodontic treatment plan, based on occlusogram executed in the new position is done. The planned orthodontic therapy will lead to the stabilization of the new occlusion.
The treatment of several cases, with explanation of mechanics design, will be discussed in details.
Two-vector mechanics in the treatment of a complex case
In management of complex malocclusion, for instance, when a tooth or a segment of teeth has displaced from the treatment goal in three planes of space significantly, a specific single line of force with reference to the center of resistance can be calculated so that a customized segmented arch appliance can be constructed to deliver that specific single line of force in reaching the target.
Nevertheless, segmented arch mechanics practitioners often come across problems in designing the appropriate appliance for two reasons. Firstly, point of force application could be inaccessible as it is beyond the clinical crown and attached gingivae areas. Secondly, the calculated line of force to the treatment goal in either two planes of space may produce unwanted movement in the third one. Therefore, in 2003 Fiorelli and co-workers proposed the two-vector mechanics. It is a mathematical solution to calculate the equivalent force system from a single point to two points of force application which aids appliance design to overcome the mentioned difficulties.
In this presentation, management of a periodontal case with pathologic tooth migration which the two-vector mechanics treatment plan was discussed in the Biomechanics Summer Course 2014 and was executed in the postgraduate orthodontic clinic of The University of Hong Kong will be discussed in a step by step approach.
The effect of rotation upon dental structure components following appliance placement
The evaluation of the rotation effects upon the elements of a tooth is performed through a FEM analysis as follows: the progressive action of a fixed device on three teeth is modeled and simulated. The three teeth are: first molar,second premolar, first premolar, with the components placed on the buccal and palatal surfaces of the central tooth. The values of resulted stress are relevant for the study – Von Mises equivalent, main maximum stress/main minimum stress with the effect of expansion/compression of the structure tissue and the shearing effect stress, relevant for the pulp. The structure studied is a molar, 1, premolar 1 and premolar 2, with all its constituents and associated elements: enamel – pulp – periodontal ligament – alveolar bone . Corresponding to the geometry, dimensions and morphological data in the specialty textbooks of the tooth, a two-dimensional plane model was created, representing a median section on the height of the structure, perpendicular to the mesial-distal sides. The characteristics of the material are: Young’s E modulus and Poisson’s ratio for the materials of the components of the modeled structure