Orthodontic Force Systems Research Articles

Comparison of the Initial Orthodontic Force Systems Produced by a New Lingual Bracket System and a Straight-Wire Appliance

Over the last few years, lingual appliances have become an established orthodontic treatment technique. Many studies have concentrated on various esthetic aspects, on laboratory and clinical procedures, and on patient comfort and compliance. The orthodontic force systems of these appliances, however, have not yet been investigated. The aim of this study was thus to determine the forces and moments produced by a new lingual bracket system during the leveling phase of orthodontic treatment and to compare those with the corresponding force system of a labial straight-wire appliance. The intra-oral situation of ten patients undergoing orthodontic treatment was replicated in measurement casts fitted with lingual and labial brackets. Special care was taken to precisely reproduce each patient's interbracket geometry. We measured each tooth's force systems as generated by a leveling arch inserted into the lingual and labial brackets. The resulting force systems of both appliances were found to be quite similar with regard to the magnitude of most force and moment components. Only the first molars were subjected to considerably greater single forces with the lingual appliance. Tipping moments were found to be significantly smaller with the lingual technique, whereas the rotational moments were significantly smaller with the labial appliance. All in all we noted significant differences between the two techniques only in certain areas which upon closer examination were distributed over only a few tooth types. The initial force systems produced by the new lingual bracket system proved to be comparable with those delivered by a conventional straight-wire appliance. The actual levels of forces and moments, however, were found in certain cases to be too heavy with both techniques. We therefore recommend the development of leveling wires producing considerably lighter forces and moments.

Orthodontic fine adjustment after vertical callus distraction of an ankylosed incisor using the floating bone concept

The outcome of vertical callus distraction of a segment of tooth-supporting alveolar process might be functionally and esthetically unsatisfactory because of the unidirectional impact of intraoral distraction devices. In this case report, we describe how, with a shortened consolidation phase and application of the floating bone effect, the tooth-supporting osteotomy segment can be successfully aligned 3 dimensionally. We applied orthodontic force systems that went beyond the unidirectional vector preset by the mechanical properties of the distraction device.

Experimental model of tooth mobility in the human "in vivo"

The aim of the present study was to investigate experimentally the mechanical properties of tooth deflection under external loading. These properties have a significant impact on tooth movement during orthodontic treatment. The stresses and strains caused by tooth movement influence bone remodelling, which is the basis of orthodontic treatment. The movement of a tooth as a direct reaction to the forces acting on it is termed "initial" movement. It is nonlinear and has a clearly time-dependent component. While the initial tooth movement represents the totality of the reaction mechanisms of all the tissues of the tooth unit, it is determined primarily by the mechanical properties of the periodontal ligament (PDL). The PDL is the softest tissue of the tooth unit and is therefore subject to the largest deformations when forces act on the crown of the tooth. The objective of orthodontic treatment is to achieve as precise and rapid tooth movement as possible, without provoking such undesired effects as bone and root resorption. To enable the implementation of an optimal orthodontic force system that meets these requirements, a thorough knowledge of the biomechanics of tooth movement is a must.

Parametric Finite Element Analysis and Closed-formSolutions in Orthodontics

The goal and clinical relevance of this work was the development of closed formulas that are correct and simple enough for a fast decision making by the orthodontist in the daily praxis. This paper performs a parametric three-dimensional finite element linear analysis on a maxillary central incisor with a root of paraboloidal shape, which is subjected to typical orthodontic force-systems. Parameters of most importance, such as the tooth mobility in translation and in pure moment rotation including orthodontic centers, as well as the stresses inside the periodontal ligament are calculated for a large variety of over four hundred different couples of root lengths and root diameters around a nominal value. Regression analysis is afterwards performed and establishes closed-form solutions, which are also explained in terms of analytical strain energy and hydrostatic stress considerations within the periodontal ligament characterised by a small compressibility. The obtained expressions include both the root length as well as the root diameter.

A rational approach to orthodontic anchorage

The outcome of orthodontic treatment is often compromised by the loss of anchorage. Yet surprisingly, forces acting on the anchorage unit that bring about the loss have received very little interest. This paper discusses the biologic background for anchorage control; specifically, the impact of the orthodontic force systems and of occlusal forces on the cellular responses of periodontal tissues supporting the anchorage teeth. Also discussed is the influence of various anchorage devices on the interaction between vertical and sagittal directions of dentofacial development. The importance of taking the vertical forces into consideration is stressed. A rational approach to orthodontic anchorage is suggested. We advocate the use of intra-oral and extra-dental devices when possible, as well as rigid appliances that stimulate the patient's sensation of occlusion. Clinical cases are presented to demonstrate the clinical application of this new approach.

Initial tooth displacement in vivo as a predictor of long-term displacement.

In the past, the dry skull has been used as a hypothetical model to test initial orthodontic and orthopaedic force systems. However, the question as to whether this hypothetical model can be used as a predictor of long-term displacements in vivo remains unanswered. In this study, an attempt was made to compare initial tooth displacement with the long-term effect after application of the force system for a longer period of time, in six adult dogs. Tooth displacement was obtained by applying a force by means of a coil spring (push) system. Following application of a force of 50 g in the first series (n = 3) and 80 g in the second series (n = 3), initial displacements were registered by means of speckle interferometry. The long-term displacement was registered by means of standardized cephalometry in the same dog by leaving the force system in place for 5 weeks. The mean values of the displacement vectors of the second premolars in the six dogs were compared. A paired t-test revealed no significant differences between the initial and long-term displacements in any of the dogs. The results show that both groups of measurements belong statistically to the same sample and that initial tooth displacement measured by means of speckle interferometry is a valuable predictor for forecasting long-term displacement in vivo after 5 weeks.

Measuring system for in vivo recording of force systems in orthodontic treatment–concept and analysis of accuracy

The aim of our developments is three-dimensional in vivo recording of those orthodontic force systems inducing tooth movements during treatment with fixed appliances. The concept presented here is the first to permit the forces and torques of these statically multiply undetermined systems to be recorded in vivo. For this purpose the force systems transmitted to the teeth from the archwire are isolated from the respective tooth by means of divisible special-design brackets and introduced into a 3D force–torque sensor via a gripping appliance. The sensor is fixed with a purpose-developed device relative to the patient’s dental arch. The patient’s head is positioned relative to the system by means of a bite fork as well as a forehead and chin support. Electrical measurement of the mechanical quantities is carried out by a six-axis force–torque sensor with semiconductor strain gauge elements, an electronical evaluator and a mobile measuring computer (PC). Extensive calibration of the sensor system has shown that the measuring uncertainty of the electrical measuring is less than 2%. Precise spatial fixing of bracket slot and archwire in the therapeutic position is crucial to the measuring accuracy of the system, as even minimum displacements affect the force system to be measured. Movements of the measuring system up to 0.04 mm result from a therapeutic force of 1.5 N. The results of extensive in vitro studies have already demonstrated that the system developed by us is suitable for the specified in vivo measuring function.

In vitro analysis of the initial tooth mobility in a novel optomechanical set-up

An optomechanical set-up was developed to accurately measure displacement/force curves of the initial tooth mobility in vitro. The system presented is capable of recording all six components of a tooth movement resulting from an applied orthodontic force system, and consists of a laser diode-based optical part and a six degree of freedom mechanical part. Three laser diodes were mounted in orthogonal arrangement on the tooth of a specimen and their light ( λ=670 nm) was focused on the surfaces of three position sensing detectors. The laser beams thus defined a cartesian rigid body coordinate system and the movements of the tooth could directly be derived from the movements of the laser spots on the surfaces of the optical detectors. The force system was applied and simultaneously measured via a three-dimensional force–torque transducer mounted on a six-axis positioning table. Measuring accuracy of the tooth displacements was in the range of 9.0 μm and 0.022 deg for translations and rotations, respectively. Resolution and accuracy of the mechanical system was approx. 0.02 N for the measurement of forces and 0.5 Nmm for torques. Displacement/force diagrams of the specimen of a swine's mandible are presented, showing the relationships between applied force system and tooth displacement of the lower first premolar. The accuracy reached proved to be sufficient for the verification of numerical (Finite Element) models of the initial tooth mobility.

Biomechanical analysis of arch-guided molar distalization with super-elastic nickel-titanium springs

The typical characteristics of orthodontic devices and their effectiveness during tooth movement are of major importance for the success of orthodontic treatment and the stability of the result. To investigate this situation, two typical tooth movements were simulated experimentally using an Orthodontic Measurement and Simulation System (OMSS). These were Distalisation of an upper last molar and Distalisation of a molar including fixation of the guiding arch to a distal supporting last molar. The movements were simulated by employing various different NiTi coil compression springs and two standard steel guiding arches with the dimensions 0.016" x 0.022" and 0.017" x 0.025". The six NiTi springs differed significantly in shape and force characteristics. The eccentric application of the orthodontic force system relative to the centre of resistance brings about a tilting of the tube and arch via molar rotation. Although the forces developed by the springs are relatively constant, frictional losses result in variations in the effective orthodontic forces. With progressive distalisation, the average frictional losses varied between 50% and 80% of the spring-generated force, and frictional variations increased. Lower frictional losses were seen with a 0.016" x 0.022" guiding arch as compared with one measuring 0.017" x 0.025". In comparison with distalisation of a last molar, fixation of the arch to a third molar was associated with higher frictional losses. The interaction of the springs with the tube-guiding arch system could be so powerful that frictional losses were detected already during the initial phase of movement, and in extreme cases led to cessation of distalisation. The rate of distalisation was not affected by the forces generated by the springs. For physiological reasons, therefore, springs developing smaller forces should be used preferentially.

Different approaches to anchorage: a survey and an evaluation.

Orthodontic treatment outcome is often compromised by the loss of anchorage. The forces acting on the anchorage unit have, however received surprisingly little attention, and the loss of anchorage is most frequently expressed in the sagittal occlusal relationship. The present paper discusses the interaction between vertical and sagittal components of dentofacial development, and the importance of taking vertical forces into consideration is stressed. The biological background for anchorage is reviewed, i.e., the impact on the cellular reaction of the periodontal ligament around the teeth of the anchorage unit from the orthodontic force system and from occlusion. A new rigid appliance consisting of two occlusal splints connected with transpalatal arches is introduced. The advantage of using the patient's sense of occlusion as part of anchorage by means of this appliance is demonstrated in a number of case presentations.

Radiographic evaluation of the effect of orthodontic retraction on the root of the maxillary canine.

To evaluate possible apical root resorption following orthodontic retraction of the maxillary canine quantitatively and qualitatively. Twenty maxillary canines in 12 patients were retracted using a prefabricated orthodontic spring. Twenty non-treated mandibular canines from the same patients served as controls. Standardized radiographs were taken of all upper and lower canines before (T0) and after (T2) retraction. Root length and tooth length were measured on the radiographs at T0 and T2, and the root length/tooth length ratio was calculated. Ten orthodontists assessed blind the root apex of the pairs of radiographs taken at T0 and T2 for resorption. Digital subtraction was performed between the pairs of radiographs taken at T0 and T2 and the mean density and the standard deviation of the grey shades of the root apex calculated. There were no significant differences between root length/tooth length ratio at T0 and T2 for either the maxillary (p = 0.45) or the mandibular canines (p = 0.79). There was considerable variation between observers judgements of root resorption: nine out of ten observers agreed that a resorption had taken place in only one tooth. There were no significant differences between the mean density values in the subtraction images of the upper and lower canines (p = 0.3). Quantitative assessment with conventional and subtraction radiography failed to demonstrate root resorption in the maxillary canines. Root resorption could be shown qualitatively in only one tooth. The standardized orthodontic force system used for canine retraction appeared to cause no resorption, or only a small degree of resorption of no clinical relevance.

Effects of topical administration of a bisphosphonate (risedronate) on orthodontic tooth movements in rats.

In orthodontics, undesirable movement of anchor teeth during tooth movement and relapse of moved teeth after treatment are the main causes of unsuccessful results. If these tooth movements could be prevented with pharmacological agents, a less complex orthodontic force system and less extensive retention would be required. The purpose of this study was to examine the effect of topical administration of a bisphosphonate (risedronate), a potent blocker of bone resorption, on orthodontic tooth movements in rats. In the first experiment, both the right and left upper first molars were moved buccally with a standardized expansion spring under administration of risedronate. Risedronate solution was injected into the subperiosteum area adjacent to the left upper first molar. The right first molar served as a control with an injection of 0.9% NaCl solution. The topical administration of risedronate caused a significant and dose-dependent reduction of tooth movement after the orthodontic force was applied. In the second experiment, the right and left upper molars were first moved buccally for three weeks. The spring was then removed, and administration of risedronate was begun. The topical administration of risedronate inhibited relapse of the tooth in a dose-dependent manner. The administration of risedronate did not affect either overall growth of the animals or longitudinal growth of tibiae. These results suggest that topical application of risedronate may be helpful in anchoring and retaining teeth under orthodontic treatment.

An analysis of orthodontic force systems applied to the dentition with diminished alveolar support.

Many more adults are undergoing orthodontic therapy than in the past. Since adult dentitions often exhibit diminished alveolar support, it is important to recognize the related apical migration of the dental centre of resistance when applying force systems to the teeth. The force system compensations necessary to produce controlled movement in this circumstance are analysed in a model of a maxillary canine undergoing translation.

Determination of stress levels and profiles in the periodontal ligament by means of an improved three-dimensional finite element model for various types of orthodontic and natural force systems

This study was undertaken to investigate the stress-strain levels and distribution within the periodontal ligament for various types of physiological and orthodontic force systems, assuming that the bone resorption process, leading to tooth movements, is partly controlled by those conditions. Two finite element models were developed, simulating a full and partial mandibular morphology, respectively. Both models were based on morphology and physical parameters of human autopsy material. The effect of changing material parameters and structure, type of boundary conditions, calculation method and fineness of the model on the stress levels and profiles in the periodontal ligament was evaluated by a series of tests. A structure optimization technique was used to investigate the load bearing characteristics of the mandible and the influence of the anisotropic material properties of both the mandible and the segment. A ‘multiple modelling’ technique based on both the mandible and the segment was developed to test various types of boundary conditions in the analysis of the segment. Results presented as ‘stress profiles’ showing the correlation between the applied force system and the stress distribution in the periodontal ligament, based on the improved finite element models, were established.

Sequential directional forces treatment: Two class II case reports

Two types of Angle Class II, Division 1 malocclusion that were treated with Tweed-Merrifield directional forces are presented. The cases are entirely different. Each required a different diagnosis, but the results are similar. In both cases the resultant vector of all the orthodontic force systems was upward and forward in direction.

Tooth displacement analysed on human autopsy material by means of a strain gauge technique

Tooth displacement was analysed on human autopsy material for various types of orthodontic force systems using an electronic strain gauge technique. Tooth movements were registered in two dimensions with low force clip gauges on three sections of the mandible, containing the first premolar and the first molar. The sensitivity was 0.1 microns when horizontal forces of 50, 100, and 150 cN were applied at different horizontal levels producing different M/F ratios. The relationship between a force system and the resulting tooth movement was described by the position of the centre of rotation, CR, and the angle of rotation. The centre of resistance was determined for the three autopsy specimens, and depended on tooth geometry, root length, and level of bony margin. Important differences in location were found. The dynamics of initial tooth movement were analysed within the first minute of load application. Force/displacement curves were generated to evaluate the behaviour of the periodontal ligament. There was a linear correlation between the two variables. Freezing the autopsy material some days before testing had no detectable effect. The results from measurements on human autopsy material were compared with previously derived in vivo measurements.

Electronic determination of centres of rotation produced by orthodontic force systems

A strain gauge measurement device with low force clip gauges, for measurement of initial tooth displacement in two dimensions has been developed. An experimental model simulating a maxillary central incisor is loaded with different known and controlled force systems. The resulting tooth displacements are described by the position of the centre of rotation and the generated angle of rotation for the total tooth movement. The effect of a single force, a moment, and force-moment combinations producing different moment to force ratios were studied. From these results the required force system needed to produce tooth movements with different centres of rotation for a central incisor of average root length are calculated. Furthermore, the model allows us to measure the accuracy of the measuring device by comparing results to analytical and laser holographic data, obtained on similar models. Some preliminary measurements and results, using human autopsy material are presented.

A new orthodontic force system of magnetic brackets

Tooth movement in orthodontic treatment is achieved by a combination of mechanical and physiologic forces acting on the teeth and their supporting structures to effect the biologic changes that cause teeth to move through bone. Traditional orthodontic appliances using wires, springs, and elastic forces inevitably cause some degree of discomfort and pain. In the present study, a new magnetized edgewise bracket was designed. The magnet works to move teeth mesiodistally and is not merely a mechanical force. It is well adapted to the biophysical system of tooth and bone interrelationships. Therefore, it works effectively, gives less discomfort, and produces less stress for the patient. The new magnetic appliance provides a force system that approaches the ideal orthodontic requirements.

Orthodontic force systems: Individualized treatment with open-minded “Begg” technique

Conventional Begg technique is efficacious for alignment, overbite reduction, and anchorage conservation, but it has a number of deficiencies. Biomechanical systems should be determined by the characteristics of each individual malocclusion, not by habitual application of the same recipe to every patient. Check elastics can increase the bite-opening potency of the appliance, thus permitting control of root movement at any stage. Ribbon arch wires can provide torque control for either bolstering or expending anchorage. Extraoral traction can provide anchorage, control the orientation of the dentition within the face, and simplify and/or shorten treatment in some cases.

Orthodontic force systems: Technical refinements for increased efficiency

The efficiency of the Begg technique suffers because Class II elastics overwhelm anchor bends, which normally do not yield enough force for vertical control of maxillary incisors. This can be improved by attention to detail and modifications. Bite-opening bends in the arch wires distal to canines and lateral incisors, long (6.5 mm.) mesiogingivally angulated molar tubes, and placement of intermaxillary elastics at the distal ends of the tubes are all helpful. Intermaxillary “check elastics,” having one end over the posterior end of the maxillary arch wire, both strands under the end of the mandibular arch wire, and the other end up to the maxillary anterior teeth, reinforce anchor bends and extrude upper and lower molars while keeping them upright. They augment bite opening in Stage I and counteract the adverse affects of the auxiliaries in Stage III.