Influence of different modeling strategies for the periodontal ligament on finite element simulation results.

INTRODUCTION: The finite element method is a promising tool to investigate the material properties and the structural response of the periodontal ligament (PDL). To obtain realistic and reproducible results during finite element simulations of the PDL, suitable bio-fidelic finite element meshes of the geometry are essential. METHODS: In this study, 4 independent coworkers generated altogether 17 volume meshes (3-dimensional) based on the same high-resolution computed-tomography image data set of a tooth obtained in vivo to compare the influence of the different model generation techniques on the predicted response to loading for low orthodontic forces. RESULTS: It was shown that the thickness of the PDL has a significant effect on initial tooth mobility but only a remarkably moderate effect on the observed stress distribution in the PDL. Both the tooth and the bone can be considered effectively rigid when exploring the response of the PDL under low loads. The effect of geometric nonlinearities could be neglected for the applied force system. CONCLUSIONS: Most importantly, this study highlights the sensitivity of the finite element simulation results for accurate geometric reconstruction of the PDL.

Stress distribution and displacement analysis during an intermaxillary disjunction--a three-dimensional FEM study of a human skull.

The goal of this study was to contribute to an understanding of how much expansion force is needed during a maxillary expansion (ME) and where bony reaction takes place. A finite element (FE) model of a dry human male skull was generated from CT scans. The FE model, which consists of cortical and cancellous bone and teeth, was loaded with the same force magnitudes, directions and working points as in rapid maxillary expansion (RME). A three-dimensional finite element stress analysis (FESA) of the forces and displacement was performed. The highest stress was observed in the maxilla in the region where the forces were applied, and spreads more or less throughout almost the whole frontal skull structures. The displacement distribution which causes stress in the skull is highly dependant on the thickness of the bone and its structure. All areas with high compressive and tensile stress are exactly the regions which determine the maximal amount of force to be used during the maxillary expansion and should be examined in case of any complication during a patient's treatment. Regions with significant compressive and tensile stress are the regions observed to have an increase in cellular activity. Further simulations with a given displacement (0.5mm) showed that displacement simulations need extra caution otherwise they will lead to very high forces which are not realistic in an orthodontic treatment.

Clinical experience with a new superelastic Ni-Ti-stainless steel retraction spring.

This article describes the clinical application of a new superelastic Ni-Ti-stainless steel retraction spring used especially for canine retraction following the segmented archwire technique. The use of the new spring and its 2 configurations are demonstrated in a typodont experiment. Clinical experience with the new spring is presented in 2 patients; it produced canine retraction in these patients who required first premolar extraction. The use of the spring in both cases led to bodily tooth retraction without reactivation or any undesirable side effects.

Alignment of displaced or impacted teeth with the traction chain.

To align those teeth in the dental arch that cannot erupt or are displaced, they must be surgically exposed. After the traction device is attached, the tooth is again covered with the mucoperiosteal flap to protect the tissues involved. For this reason, the loss of this traction device must be absolutely avoided in order to prevent another surgical intervention. Here at the University of Ulm, we have developed and tested an optimized traction chain for that purpose. The chain is attached to the tooth with a laser-roughened pad applying the acid-etching technique. The chain links serve as fixation elements for traction devices such as elastic ligatures or piggyback archwires. At the same time, one can determine the distance covered by the tooth by the number of the erupting chain links. We demonstrate the clinical application of this traction device in two cases.

The derotation of premolars and canines with NiTi elements.

BACKGROUND: Canines or premolars must often be derotated during fixed appliance therapy. If such derotation is carried out on the continuous archwire during the leveling phase, it may lead to unwanted side effects on the neighboring teeth. MATERIAL AND METHODS: At the University of Ulm a .018" x .018" nickel-titanium segmental derotation archwire was developed to start derotation as early as possible. This wire can be inserted during leveling. This system consists of a vertical slot bracket, an elastic ligature, and the derotation archwire. We tested the derotation archwire under clinical application. The occurring forces and torquing moments were measured in vitro via a hexapode and a sensor working in three dimensions. RESULTS: In this investigation, a derotation moment of 12 Nmm occurred in the canine with a 140 degrees archwire rotation and a force in lingual direction. This archwire also made overcorrection possible, since it remains active after complete derotation. A complete derotation of the tooth was effected with no reactivation, due to the wire's superelasticity.

Leveling of the second molar with a superelastic spring.

BACKGROUND: Second permanent molars often erupt during fixed appliance treatment. When they are to be incorporated into the fixed appliance, it is often not possible to bond the buccal tube in the correct position. MATERIAL AND METHODS: We describe herein a method for aligning second permanent molars by means of a superelastic segmental archwire. This involves the application of a rigid steel archwire extending from the right first molar to the left first molar for anchorage. This method was investigated in-vitro and in its clinical application. The in-vitro test was carried out with a hexapode simulating the vertical deviation of the second molar. RESULTS: A .016'' x .022'' stainless steel archwire achieved forces of 13 N; a TMA archwire attained 5 N (both with a deflection of 2 mm), and the superelastic late-leveler achieved a maximum force of 1.3 N with a 5 mm deflection. The maximum uprighting torquing moments were 55 Nmm with the stainless steel archwire, 25 Nmm with the TMA archwire and 2 mm deflection, and 4 Nmm with the late-leveler and 5 mm deflection. CONCLUSIONS: The forces and moments of stainless-steel and TMA archwires are too high for the alignment of second permanent molars. The moderate forces and moments occurring with the superelastic late-leveler are capable of intruding, derotating and uprighting the second molar. The vertical component prevents artificial opening of the bite.

Initial results regarding force exertion during rapid maxillary expansion in children.

AIM AND METHOD: We developed a method to measure the forces occurring during rapid maxillary expansion and carried out the University of Ulm rapid maxillary expansion method, similar to that of Chatellier and Chateau, in ten patients. RESULTS: The forces occurring on the maxilla increased up to 120 N. There was considerable variation in force level from patient to patient. After "rupture" of the midpalatal suture, the forces decreased. The remaining forces seemed to be the result from stress on soft tissue and bone.

Dental care during orthodontic treatment with electric toothbrushes.

OBJECTIVE: In an in-vitro experimental set-up, we tested the efficacy of different electric toothbrushes in cleaning teeth with a multibracket appliance. MATERIAL AND METHODS: The efficacy of three sonic and four rotating toothbrushes were compared; the brushes removed artificial plaque from plastic teeth. The amount of removed plaque substitute was determined via a comparative photo-analysis with one picture of the teeth before and one picture after the brushing cycle. Two views of the tooth segment were chosen: one vestibular view and one apical view of the areas behind the bracket wings. All tests were carried out with water, elmex and Pearls and Dents as brushing media. RESULTS: The data showed greater brushing efficacy when Pearls and Dents toothpaste was used. The Sonicare toothbrush was the most efficient. The Sonicmax and rotating Oral-B ProfessionalCare 7000 were also very efficacious. On average, all the other toothbrushes performed less well. All toothbrushes were capable of brushing the undercut areas behind the bracket wings, with plaque removal being the greatest when the Sonicare toothbrush was used together with Pearls and Dents. CONCLUSIONS: The best electric toothbrushes did not demonstrate good tooth cleaning. Longer brushing times, mastery of the proper brushing technique, and/or special toothbrush heads are absolutely necessary.

Effective disinfection of orthodontic pliers.

OBJECTIVE: Pathogenic microbes may be transmitted directly from the orthodontist to the patient or from the patient to the doctor, and indirectly from patient to patient. The latter may occur via contaminated instruments or surfaces, and is referred to as cross-contamination. The objective of this study was to evaluate the extent of bacterial contamination of orthodontic pliers and the efficacy of the disinfection techniques applied after clinical use. We also sought to examine under standardized conditions the virucidal, bactericidal and fungicidal effects of disinfection techniques used in practice. MATERIALS AND METHODS: The efficacy of various disinfection methods was determined after clinical use in-vivo on 10 test subjects and in-vitro with deliberate contamination. The following disinfection methods were tested: 1. Iso-Septol spray 2. Incidur spray 3. Trough disinfection in combination with 5% Sekusept Plus solution 4. Ultrasound bath in combination with 5% Sekusept Plus solution 5. Thermal disinfection For in-vitro contamination we used the test organisms Staphylococcus aureus, Escherichia coli, Candida albicans, Coxsackie virus B4, HSV 1, and Adenovirus type 5. The tests were carried out six to eight times for each organism. The Weingart pliers and distalend cutters were tested. The criteria for effective disinfection were a reduction in infectiosity of five log steps (for bacteria and fungi) or four log steps (viruses). Statistical analysis was carried out using the Wilcoxon and Whitney U-test. RESULTS: The presence of contamination following clinical use was not adequately eliminated with all disinfection methods. The spray methods exhibited shortcomings in disinfection. For the type of contamination defined, trough disinfection with 5% Sekusept Plus and the Incidur and Iso-Septol spray disinfection methods provided insufficient disinfection. Conversely, the ultrasound bath with 5% Sekusept Plus solution and steam disinfection met the criteria for effective disinfection for all microbes. No statistically significant difference was found between the oiled and unoiled states. In some cases, there were slightly higher rates of contamination with the Weingart pliers as with the distalend cutters. However, these were not statistically significant. CONCLUSIONS: It should be possible to disinfect lipophilic viruses and the usual bacterial infections adequately with all methods, provided that the use of sprays and trough disinfection is preceded by cleaning with brush and water, followed by drying. With hydrophilic viruses, however, the spray and trough disinfection methods are limited in their efficacy and cannot be considered adequate. Exclusively chemical methods are therefore less effective than thermal or physical-chemical methods. Thermal disinfection and the ultrasound bath in combination with 5% Sekusept Plus are clearly superior to spray disinfection and trough disinfection alone. The ultrasound bath and thermal disinfection can therefore be recommended for the disinfection of orthodontic pliers. We recommend that the pliers be cleaned beforehand due to their uneven surfaces.

Development of a model for the simulation of orthodontic load on lower first premolars using the finite element method.

AIM: This study was undertaken to calculate the stress in the tooth, surrounding periodontal ligament, and in the alveolar bone when a lower first premolar is subjected to intrusion or torque movement using a constant moment. Root resorptions occur even when very low forces and moments are used in orthodontic therapy. It is therefore of great interest to determine and measure the stress that occurs under particular treatment conditions in the periodontal ligament. MATERIAL AND METHODS: In this study, three finite element calculations were carried out with a realistic 3D model developed by CT data that consisted of a lower premolar, the surrounding periodontal ligament and alveolar bone. In close reference to the in-vivo experiments carried out by Faltin et al. in São Paulo, Brazil, our model was subjected to an intrusive force on the premolar of 0.5 N and a lingual root torque of 3 Nmm. RESULTS: The three main stress directions and hydrostatic stress were quantified in all the surrounding tissues, revealing that the hydrostatic stress profile in the periodontal ligament correlated closely with resorption findings in Faltin et al.'s patients. Resorption occurred in the experimental study in Brazil when the hydrostatic stress exceeded capillary blood pressure in the periodontal ligament. CONCLUSION: We maintain that hydrostatic stress represents a suitable indicator for potential root resorptions caused by higher forces and moments, making it a helpful tool in the development of new orthodontic appliances. We must of course mention that there are many factors other than forces that are responsible for resorptions. But at the moment, only the force can be influenced by the orthodontist.

Dental care with manual toothbrushes during fixed orthodontic treatment--a new testing procedure.

AIM: The aim of this investigation was to employ a new in-vitro testing system for manual toothbrushes in order to distinguish the more effective from those less so for dental care during fixed appliance treatment. MATERIALS AND METHODS: The testing apparatus consisted of a sliding carriage able to execute a horizontal brushing movement, and a row of artificial teeth upon which the various toothbrushes were manipulated. The artificial row of teeth was fixed on a sensor that recorded in all three dimensions the forces and moments caused by the toothbrushes on the toothbrush field. All the tests were executed with a weight of 110 g on a tooth field with a multibracket appliance. Tests were also carried out with five toothbrushes having weights of 200 g, 250 g and 300 g. Here, the decisive target values were 1) the degree of exertion necessary in the brushing direction to move a brush over the artificial teeth, and 2) the maximum force occurring in the brushing direction. High target values indicated high interaction between toothbrush bristles and the surfaces being brushed. RESULTS: From testing five toothbrushes with four different weights, we have established profiles confirming the beneficial and less beneficial properties of certain toothbrushes involving various high contact forces.

The influence of intermittent forces during daytime application of the SIII appliance.

Daytime application of the SIII appliance results in mainly intermittent forces. Permanent force occurs only for a few minutes, if at all. These intermittent forces may be as high as 40 N in individual cases. Depending upon the patient's activity and temperament, 10 to more than 100 intermittent force impulses per minute can occur. The intermittent forces account for the therapeutic effects found in young patients.

Experiments to determine the material properties of the periodontal ligament.

PHYSIOLOGY OF THE PERIODONTAL LIGAMENT: The periodontal ligament is a soft biological tissue that controls tooth movement under physiological loads by joining tooth and alveolar bone. Its various components differ in their material properties. Their spatial configuration and interaction are responsible for the reaction of the tissue in a loading situation. Due to the combination of fluid and elastic elements the periodontal ligament shows a viscoelastic behavior typical of soft biological tissues. It is characterized by non-linearity and time dependency, and additionally depends on loading history. BEHAVIOR UNDER EXTERNAL LOADS: In orthodontics, external loads are applied to the tooth crowns using orthodontic appliances. Since stresses and strains in the periodontal tissue, caused by the initial tooth movement, stimulate alveolar bone remodeling and thus orthodontic tooth movement, knowledge of the material properties of the periodontal ligament is fundamental to selection of an optimal force system for targeted tooth movement. OWN EXPERIMENTS: For this reason, typical properties of the viscoelastic material behavior of the periodontal ligament were tested experimentally in the present study, using samples from pig mandibles. This enabled the properties of force relaxation and hysteresis of this tissue, both of which depend on loading history, to be verified. CONCLUSION: The experimental results allow characterization of the tissue and thus contribute to an understanding of the biomechanics of tooth displacement under externally applied loads.

Contactless measurement of canine retraction by digital macrophotogrammetry during hybrid retractor application.

A new contactless method for measuring tooth movements is presented. Digital macrophotogrammetry (DMP) enables the orthodontist to obtain information on the three-dimensional movement of a tooth at each session. Analysis of the DMP images provides information on the translation and toration of a tooth during treatment. It is conceivable that the introduction of DMP will make an important contribution to quality assurance.DMP application during canine retraction with the Hybrid Retractor((R)) has pave the way for the orthodontic appliance to be correctly adjusted at each session. Despite intra- and interindividual differences during canine movement, for which the anisotropy of the bone seems to be basically responsible, a movement velocity of ca 1.2 mm is to be expected over the entire treatment period. The preconditions for the three-dimensional orientation and description of canine movement are:1. The markers on the brackets and attachments must be clearly visible throughout the treatment.2. The teeth to which the scaling frame is fixed should not move during treatment.3. Four measuring points should be visible on each bracket.4. The control points on the frame must be positioned three-dimensionally, and it must be possible to determine their position with sufficient accuracy in advance. In vitro calibration of the photogrammetry presented here yielded a resolution of 1 micrometer for translation and of better than 1/10 degrees for rotation around the three spatial axes under optimal conditions.

Development and biomechanical investigation of a new compound palatal arch.

BACKGROUND AND AIM: There are many advantages in using a transpalatal arch in orthodontic treatment. In addition to the active movement of individual teeth, such an arch can be used to provide additional anchorage. The aim of this study was to further develop and improve both the clinical and the biomechanical properties of the conventional transpalatal arch. MATERIAL AND METHODS: Currently available compound material components form an established part of orthodontic therapy. The Compound palatal arch broadens the scope of this treatment system. The biomechanical effects of the newly developed Compound palatal arch were verified by comparing them with those of commercially available conventional transpalatal arches. The recently developed Compound palatal arch is made of one compound element: nickel-titanium/stainless steel. The specific dimensions and design of the nickel-titanium element are aimed at exploiting its superelasticity, even during active molar movement. The biomechanical investigation was carried out using six-component measuring sensors to register the forces and moments in the three spatial dimensions (x, y and z) at 37 degrees C. The following appliances were investigated: Goshgarian transpalatal arch, quad-helix appliance, Arndt memory expander, and Compound palatal arch. RESULTS: Transpalatal arches could not be inserted without the arch being under tension. This is particularly important when the arch is being used for anchorage purposes. The Goshgarian arch had to be classified as problematic for the movement of single teeth as the load/deflection rate was high. However, this can be utilized for anchorage purposes. The clinical advantage of a quad-helix appliance is the option for asymmetric tooth movement. CONCLUSIONS: This study showed that the quad-helix was effective for both unilateral and bilateral derotation as well as for transversal tooth movement. However, expansions should be confined to < or = 4 mm, and derotations to < 10 degrees, as forces and moments are otherwise not within the physiologic range. The Arndt memory expander displayed no advantages over either the quad-helix appliance or the Goshgarian transpalatal arch. Nor did it display any superelastic properties. The linear relationship, the relatively high forces and moments, and the lack of clinical adjustability cast doubts on the benefits of this appliance. The newly developed Compound palatal arch showed substantial advantages in molar derotation compared with conventional transpalatal arches. Superelastic properties were achieved through the design and positioning of the nickel-titanium element. Expansion with the Compound palatal arch was comparable with that with conventional transpalatal arches. The clinical advantage is in the fact that this appliance can be reactivated and that dental asymmetries can be treated.

Determination of the mechanical properties of the periodontal ligament in a uniaxial tensional experiment.

BACKGROUND: The periodontal ligament is a soft connective tissue which joins the tooth root to the alveolus and thus provides for anchorage of the tooth in the alveolar bone. Due to its composition of elastic and viscous components, this tissue displays viscoelastic material properties. In a previous study [4], in vitro experiments revealed typical viscoelastic material properties of the periodontal ligament in samples from pig mandibles. These properties included force relaxation, hysteresis, and dependence on loading history. MATERIAL AND METHODS: Based on those experiments, a dependence of tooth displacement on loading velocity was registered in the present study and the stress-strain behavior of the periodontal ligament was examined until the tissue ruptured. For this purpose, segments of the periodontal ligament taken from anterior teeth from the pig mandible were tested in a purpose-developed clamping fixture in a uniaxial tensional experiment. RESULT: It was found that the initial phase of the stress-strain curve in particular was dependent on loading velocity and that the shape of the hysteresis curve was subject to a variation in loading velocity. The stress-strain behavior of the periodontal ligament was characterized, divided into several phases, and the elastic modulus of the initial and the linear phase of the curve was determined at different loading velocities. CONCLUSION: Knowledge of the material properties of the periodontal ligament is fundamental to an understanding of orthodontic tooth movement and thus to selection of an optimal force system for orthodontic treatment.

Dynamic functional force measurements on an anterior bite plane during the night.

BACKGROUND: Anterior bite planes are used in removable and fixed appliance treatment. In removable appliance treatment the question arising is whether the delivered forces can achieve active intrusion in terms of their amplitude and duration. In fixed appliance treatment, the force effect on the incisors and associated pathologic side effects, in particular under the application of intrusion mechanics, have to be considered. SUBJECTS AND METHOD: The aim of the present study was to investigate the effects of an anterior bite plane during the night. For this purpose ten subjects underwent nocturnal sleep investigations by means of a telemetric system. A silicon force sensor was integrated into an anterior bite plane for continuous measurement of bite forces and of the frequency of occlusal contact with the plate. RESULTS: The occlusal forces exerted on the anterior bite planes ranged between 3 and 80 N. The average forces were 5.5-24 N. The number of occlusal contacts varied between 39 and 558, with forces of between 7 and 9 N being registered in most cases. Major interindividual differences were detected in the magnitude of the force as well as in bite frequency. The intraindividual pattern of arising occlusal forces showed an intermittent force effect. No significant differences were found with regard to gender or growth pattern. CONCLUSIONS: In subjects with removable appliances, no active intrusion of teeth is possible during the night owing to the small number of occlusal contacts. Due to the partially very high forces in fixed appliance therapy, the integration of an anterior bite plane has to be assessed as critical in patients with unfavorable root geometry or bruxism.

Corrosion of orthodontic pliers using different sterilization procedures.

AIM: The aim of this study was to investigate the corrosion resistance of orthodontic pliers from several manufacturers following different sterilization procedures, namely heat sterilization and the use of a disinfectant agent (Sekusept Extra N in an ultrasonic bath. MATERIAL AND METHODS: Employed in this study were ten distal-end cutters and ten Weingart pliers manufactured by Aesculap, ETM and Hu-Friedy, examined under electron microscopy for evidence of corrosion following 500 sterilization cycles. The corrosion data were recorded photographically and graphically. Metallographic microsections were prepared from each pair of pliers and were then analyzed by means of EDX measurements to assess the chemical alloy composition. RESULTS: The light and electron microscopic evaluation showed that both heat sterilization and high level disinfection caused corrosive changes on the pliers. However, the type of corrosion differed between the two sterilization methods. The main type of corrosion with Sekusept Extra N was pitting, while with heat sterilization it was surface corrosion. Heat sterilization was the type of corrosion that caused the most corrosive changes, regardless of which type of corrosion was considered. However, these results must be viewed critically, because pitting corrosion is obviously a more severe problem than surface corrosion. In comparing the pliers of various manufacturers, differences were noted. However, it was difficult to rank them when the correct maintenance regime was adhered to. The soldered joint gap was revealed to be a specific weak point, and that area proved to be inadequate due to qualities related to their production, particularly in the Hu-Friedy pliers. CONCLUSIONS: This study showed that heat sterilization leads to less corrosion than cold disinfection. Corroded pliers can be restored to a useable condition by re-polishing, though it is important that the instructions for their care be strictly adhered to.