Inadvertent side effects of fixed lingual retainers : An in vitro study

PURPOSE

To better understand the side effects of fixed lingual retainers by means of an in vitro study in a two-tooth model determining the three-dimensional (3D) force-moment components acting at adjacent teeth combined with different composite-wire interfaces.

METHODS

Triple-stranded round retainer wires were embedded in cured disks of flowable composite. At one side the composite-wire interface was untreated and checked to be absolutely fix. At the other side the composite-wire interface was configured as either an isolated compound with (1) petroleum jelly coating, or an adhered compound with (2) no manipulation, (3) ethanol degreasing or (4) ethanol degreasing and rectangular bending of the wire ends. The 3D force-moment components were registered, while the intertooth distance was increased in steps of 0.01 mm leading to increasing tension of the wire. Measurements were repeated after artificially aging the specimens.

RESULTS

Retainer wire specimens with adhered compound (2, 3, 4) showed negative vestibulo-oral moments ranging maximally each between -0.3 and -0.9 Nmm in opposite direction to positive moments of 1.9 Nmm for specimens with isolated compound 1. Significant tipping moments occurred in the group with isolated compound at lower forces than in those groups with adhered compound. Similar effects were observed after artificial aging.

CONCLUSION

Side effects emerge under specific circumstances: an altered adhesive compound combined with the presence of oral forces. Compounds with lost adhesion at the composite-wire interface showed rotational moments in the direction of the wire windings even during low tensile forces similar to those that may occur in clinical settings. Opposite rotational moments leading to unwinding of the wire may occur in cases with adhered compounds at higher tensile forces. Utilization of round triple-stranded retainer wires without bent ends are of higher risk to induce inadvertent side effects.

Are aligners capable of inducing palatal bodily translation or palatal root torque of upper central incisors? A biomechanical in vitro study

OBJECTIVES

Previous studies have shown that aligners have limited ability to control root movements. The purpose of this study was to investigate which modification geometry and foil thickness are optimal for generating the force-moment (F/M) systems required for palatal root torque of maxillary central incisors.

MATERIALS AND METHODS

Tooth 11 was separated from a maxillary acrylic model and connected to a movement unit via a 3D F/M sensor. Different modification geometries (crescent, capsular, double-spherical) with different depths were digitally implemented in the labio-cervical region of tooth 11 to induce an increased contact force. We evaluated the F/M systems exerted by aligners with thicknesses of 0.4-1.0 mm. F/M measurements were taken with tooth 11 in the neutral position and during palatal displacement of tooth 11 (simulating its initial clinical movement).

RESULTS

The mechanical requirements of palatal root torque are a palatally directed force (- Fy) and a palatal root torquing moment (- Mx). These requirements were reliably achieved with modification depths > 0.5 mm. The modification depth and foil thickness had a significant influence on - Fy magnitudes (linear mixed-effect models, p < 0.01). With the 0.75-mm aligners combined with 1.5-mm deep modifications, the palatal root torque range (palTR) started after an initial palatal crown displacement of 0.09, 0.12, and 0.12 mm for the capsular, crescent, and double-spherical modification geometries, respectively.

CONCLUSIONS

A relatively early start of the palatal torque range (after a 0.1-mm palatal crown displacement) and appropriate - Fy magnitudes were achieved with 0.75-mm-thick aligners containing 1.5-mm deep capsular or crescent pressure regions. Subsequent clinical trials are required to confirm the clinical effects of these modifications.

CLINICAL RELEVANCE

In vitro testing indicated that modified aligners are capable of generating the F/M components required for palatal root torque of upper central incisors.

Mechanical loads exerted by different configurations of Burstone's 3-piece segmented mechanics during a simulated intrusion of the mandibular incisors

INTRODUCTION

Burstone's segmented intrusion arch technique allows variable incisor intrusion with lingual or labial tipping, depending on the position and direction of the force vectors exerted by the intrusion springs. To date, systematic biomechanical studies are lacking. This in vitro study aimed to determine the 3-dimensional force-moment systems applied to the 4 mandibular incisors and the deactivation behavior of the appliance by different configurations of the 3-piece intrusion mechanics.

METHODS

The experimental setup consisted of a mandibular model segmented into 2 buccal and 1 anterior segment mounted on a 6-axis Hexapod to simulate different incisor segment malpositions. Active elements were bilateral 0.017 × 0.025-in titanium-molybdenum alloy intrusion springs. Nine geometric appliance configurations at different superpositions of the anterior segment between 4 and 0 mm were evaluated.

RESULTS

For 3-mm incisor superposition, mesiodistal variation of the contact of the intrusion spring at the anterior segment wire resulted in labial tipping moments between -0.11 and -1.6 Nmm. Variation of the height of force application at the anterior segment showed no significant influence on the tipping moments. During the simulated intrusion of the anterior segment, a force reduction rate of 21% per mm intrusion was observed.

CONCLUSIONS

This study contributes to a more detailed and systematic understanding of the 3-piece intrusion mechanics and confirms the simplicity and predictability of the 3-piece intrusion. According to the measured reduction rate, the intrusion springs should be activated once every 2 months or 1-mm intrusion.

Experimental and numerical evaluation of simulated dental arch expansion during orthodontic therapy

In this publication, methods are presented to improve predictions of perioral soft-tissue changes following the expansion of the dental arches during orthodontic therapy. Acrylic veneers with different thicknesses were reversibly attached to the buccal surfaces of the upper and lower incisors to simulate their protrusion. The resulting morphological changes of the perioral soft-tissue surface were determined by 3D face scans. Experimentally-determined 3D soft-tissue changes are compared to numerical predictions using detailed finite-element (FE) models of the face of two individuals differing in the body mass index (BMI). The results suggest that common estimates of material constants used by the detailed and individualized FE models might be sufficient to explain absolute soft-tissue displacements although differences occurred between experimental and modeling results regarding the directions of displacements. The aim of this investigation is to create predictions of post-treatment appearance that are helpful for therapy planning.

Force decay of polyethylene terephthalate glycol aligner materials during simulation of typical clinical loading/unloading scenarios

BACKGROUND

This in vitro study investigated the effect of three distinct daily loading/unloading cycles on force delivery during orthodontic aligner therapy. The cycles were applied for 7 days and were designed to reflect typical clinical aligner application scenarios.

MATERIALS AND METHODS

Flat polyethylene terephthalate glycol (PET-G) specimens (Duran®, Scheu Dental, Iserlohn, Germany) with thicknesses ranging between 0.4 and 0.75 mm were tested in a three-point-bending testing machine. Measurements comprised loading/unloading intervals of 12 h/12 h, 18 h/6 h, and 23 h/1 h, and specimens were exposed to bidistilled water during loading to simulate intraoral conditions.

RESULTS

A very large decay in force for the PET‑G specimens could already be observed after the first loading period, with significantly different residual force values of 24, 20, and 21% recorded for the 12 h/12 h, 18 h/6 h, and 23 h/1 h loading/unloading modes, respectively (Mann-Whitney U test, p < 0.01). In addition, further decays in force from the first to the last loading period at day 7 of 13.5% (12 h/12 h), 9.7% (18 h/6 h), and 8.4% (23 h/1 h) differed significantly among the three distinct loading modes (Mann-Whitney U test, p < 0.01).

CONCLUSION

Although the initial material stiffness of PET‑G is relatively high, the transmission of excessive forces is attenuated by the high material-related force decay already within a few hours after intraoral insertion.

Motor learning might contribute to a therapeutic anterior shift of the habitual mandibular position-An exploratory study

BACKGROUND

Passive mandibular advancement with functional appliances is commonly used to treat juvenile patients with mandibular retrognathism.

OBJECTIVE

The aim of this study was to investigate whether active repetitive training of the mandible into an anterior position would result in a shift of the habitual mandibular position (HMP).

METHODS

Twenty adult healthy subjects were randomly assigned to one of two groups: a training group receiving six supervised functional training sessions of 10 min each and a control group without training. Bonded lateral biteplates disengaged occlusion among both groups throughout the 15-day experiment. Customised registration-training appliances consisted of a maxillary component with an anterior plane and a mandibular component with an attached metal sphere. Training sessions consisted of repeated mouth-opening/closing cycles (frequency: 30/min) to hit an anteriorly positioned hemispherical target notch with this metal sphere. The HMP was registered at defined times during the experiment.

RESULTS

The HMP in the training group showed a statistically significant anterior shift of 1.6 mm (interquartile range [IQR]: 1.2 mm), compared with a significant posterior shift of -0.8 mm (IQR: 2.8 mm) in the control group (p < .05). Although the anterior shift among the training group showed a partial relapse 4 days after the first training block, it then advanced slightly in the 4-day interval after the second training block, which might indicate neuroplasticity of the masticatory motor system.

CONCLUSIONS

Motor learning by repetitive training of the mandible into an anterior position might help to improve the results of functional appliance therapy among patients with mandibular retrognathism.

Evaluation of orthodontic loads and wire-bracket contact configurations in a three-bracket setup: Comparison of in-vitro experiments with numerical simulations

So far, no practicable procedure exists to quantify the orthodontic loads applied to teeth in vivo. Dentists therefore rely on experience and simplified mechanical in-vitro experiments comprising deflection of orthodontic wires. Predicting the mechanical behaviour of orthodontic wires during clinical therapy requires understanding of the different contact states at multi-bracket-wire interfaces. This study experimentally investigates the effect of different bracket-wire contact configurations in a three-bracket setup and uses two numerical approaches to analyse and complement the experimental data. Commonly used round stainless-steel wires (diameter: 0.012″ and 0.016″) and titanium-molybdenum alloy wires (diameter: 0.016″ and 0.018″) were tested. All six force-moment components were measured separately for each of the three brackets. The results indicate that a specific sequence of distinct bracket-wire contact configurations occurs. Several transitions between configurations caused substantial changes of effective wire stiffness (EWS), which were consistent among experimental and numerical methods. The lowest EWS was observed for the configuration in which the wire touched only one wing of the lateral brackets. Taking this stiffness as 100%, the transition to a configuration in which the wire touched two opposing wings of the lateral brackets resulted in an increase of EWS of 300% ± 10%. This increase was independent of the wire type. Additional contacts resulted in further increases of stiffness beyond 400%. The results of this combined experimental and numerical study are important for providing a fundamental understanding of multi-bracket-wire contact configurations and have important implications for clinical therapy.

Effects of preventing intercuspation on the precision of jaw movements

BACKGROUND

Closing movements are among the jaw's basic physiological motor actions. During functional movements, the jaw changes position continually, which requires appropriate proprioception. However, the significance of the various proprioceptive receptors involved and how they interact is not yet fully clear.

OBJECTIVES

This study's main objective was to test whether preventing intercuspation (IC) for 1 week would affect the precision of jaw-closing movements into IC and the functional space of habitual chewing movements (HCM). A secondary objective was to compare precision of jaw-closing movements into IC with the precision of movements into a target position (TP) far from IC.

METHODS

Fourteen participants' HCM and jaw-closing movements into IC were recorded on two sessions (T1 and T2) 1 week apart. Between sessions, participants wore posterior bite plates to prevent IC. They also received a 10-minute training session at T1 to guide their jaw-closing movements into TP. The precision of the closing movements into IC and TP was analysed. For HCM, the vertical amplitude, lateral width and area of chewing cycles were evaluated.

RESULTS

The precision of jaw movements into IC increased as the jaw gap decreased, but precision did not differ significantly between T1 and T2. For HCM, the vertical amplitude and area of chewing cycles increased significantly between T1 and T2. The precision of the closing trajectory into TP increased significantly during the training session.

CONCLUSION

Our results confirm the excellent adaptability of the craniomandibular system, controlled by stringent motor programmes that are supported by continuous peripheral sensory input.

Mechanical load exerted by PET-G aligners during mesial and distal derotation of a mandibular canine : An in vitro study

INTRODUCTION

The six force-moment (F/M) components exerted by aligners of different thickness during simulated mesiorotation and distorotation of a mandibular canine were studied.

MATERIALS AND METHODS

An acrylic mandibular model with a separated right canine mounted on a hexapod via a 3D F/M sensor was used. Duran+^® aligners (Scheu Dental, Germany) of thickness 0.5, 0.625, and 0.75 mm were fabricated on plaster models with the measurement tooth in its neutral position. The F/M values were recorded during progressive mesiorotation or distorotation of tooth 43 in 1° steps up to ±15°, corresponding to 0.5 mm displacements of the tooth's interdental contacts. Each rotation step included renewed seating of the aligner on the acrylic model. Three aligners were tested three times each for each thickness and direction of rotation.

RESULTS

The median rotational moments for the 0.5 mm aligner and 15° distorotation of tooth 43 was 27.49 Nmm (interquartile range, IQR 1.45 Nmm). The corresponding values for the 0.625 and 0.75 mm aligners were 41.04 Nmm (IQR 5.62 Nmm) and 42.48 Nmm (IQR 2.17 Nmm), respectively. The average rotational moments for distorotation were 15% higher than for mesiorotation (p = 0.01). Relatively high collateral F/M components, specifically an intrusive force and labiolingual and mesiodistal tipping moments, were observed.

CONCLUSION

To avoid overloading of periodontal structures, derotation of lower canines should be limited to 10° per setup step, leading to rotational moments of about 15 Nmm. The mechanical behavior of the 0.625 and 0.75 mm aligners were similar; thus, it may be omitted from the aligner sequence. Further studies are required to investigate specific aligner modifications or attachments for minimizing collateral F/M components or unwanted movements, respectively, during canine derotation.

Forces and moments applied during derotation of a maxillary central incisor with thinner aligners: An in-vitro study

INTRODUCTION

Recent studies have shown that therapeutic loads applied to individual teeth by aligners may substantially exceed recommended values. The primary purpose of this study was to quantify force and moment components during derotation of a maxillary central incisor when 0.3-mm-thick or 0.4-mm-thick polyethylene terephthalate glycol aligners were used instead of conventional polyethylene terephthalate glycol aligners with a minimum thickness of 0.5 mm.

METHODS

The test setup consisted of an acrylic model of a maxilla with a separated right central incisor mounted on a 3-dimensional force and moment sensor. The force and moment components were recorded for aligners with thicknesses ranging from 0.3 to 0.75 mm during ±10° rotation and derotation of the separated incisor.

RESULTS

Moments exerted by the thinnest aligner currently available, 0.5 mm, were 73.57 Nmm for the 10° mesiorotation. In comparison, the corresponding moments with the 0.4-mm and 0.3-mm aligners were 41.08 and 17.84 Nmm, respectively. Moment values for derotation of the maxillary right central incisor into neutral position showed nonlinear return curves indicating viscoelastic material behavior.

CONCLUSIONS

A significant load reduction can be achieved with the new thinner aligners. Because of the form instability of the 0.3-mm aligner during handling, we suggest the novel sequence 0.4, 0.5, and 0.75 mm for aligner systems based on sequentially increased material thickness. This sequence combines sufficiently low initial aligner stiffness and steady load increases in single setup steps. The viscoelastic behavior of polyethylene terephthalate glycol aligners observed during incisor derotation should lead to a reduction of the high initial load exerted directly after intraoral aligner insertion.

Optical 3D scans for orthodontic diagnostics performed on full-arch impressions. Completeness of surface structure representation

OBJECTIVE

The purpose of this work was to evaluate the completeness of surface structure representation offered by full-arch impression scans in different situations of tooth (mal)alignment and whether this completeness could be improved by performing rescans on the same impressions reduced sequentially to different levels of gingival height and by adding extra single scans to the number of single scans recommended by the manufacturer.

METHODS

Three pairs of full-arch resin models were used as reference, characterized either by normal occlusion, by anterior diastematic protrusion (and edentulous spaces in the lower posterior segments), or by anterior crowding. An alginate impression of each arch was taken and digitized with a structured-light scanner, followed by three rescans with the impression cut back to 10, 5, and 1 mm of gingival height. Both the initial scan and the rescans were performed both with 19 basic single scans and with 10 extra single scans. Each impression scan was analyzed for quantitative completeness relative to its homologous direct scan of the original resin model. In addition, the topography of voids in the resultant digital model was assessed by visual inspection.

RESULTS

Compared to the homologous reference scans of the original resin models, completeness of the original impression scans--in the absence of both gingival cutback and extra single scans--was 97.23 ± 0.066% in the maxilla or 95.72 ± 0.070% in the mandible with normal occlusion, 91.11 ± 0.132% or 96.07 ± 0.109% in the arches with anterior diastematic protrusion, and 98.24 ± 0.085% or 93.39 ± 0.146% in those with anterior crowding. Gingival cutback and extra single scans were found to improve these values up to 100.35 ± 0.066% or 99.53 ± 0.070% in the arches with normal occlusion, 91.77 ± 0.132% or 97.95 ± 0.109% in those with anterior diastematic protrusion, and 98.59 ± 0.085% or 98.96 ± 0.146% in those with anterior crowding.

CONCLUSION

In strictly quantitative terms, the impression scans did capture relatively large percentages of the total surface. However, the topographic examinations revealed that regions essential for orthodontic model analysis were missing. The malocclusion models were particularly affected. Thus, impression scans performed with structured-light scanners cannot replace scans of positive casts for diagnostic use in orthodontics.

Smart Bracket for Multi-dimensional Force and Moment Measurement

Atraumatic, well-directed, and efficient tooth movement is interrelated with the therapeutic application of adequately dimensioned forces and moments in all three dimensions. The lack of appropriate monitoring tools inspired the development of an orthodontic bracket with an integrated microelectronic chip equipped with multiple piezoresistive stress sensors. Such a ‘smart bracket’ was constructed (scale of 2.5:1) and calibrated. To evaluate how accurately the integrated sensor system allowed for the quantitative determination of three-dimensional force-moment systems externally applied to the bracket, we exerted 396 different force-moment combinations with dimensions within usual therapeutic ranges (± 1.5 N and ± 15 Nmm). Comparison between the externally applied force-moment components and those reconstructed on the basis of the stress sensor signals revealed very good agreement, with standard deviations in the differences of 0.037 N and 0.985 Nmm, respectively. We conclude that our methodological approach is generally suitable for monitoring the relatively low forces and moments exerted on individual teeth with fixed orthodontic appliances.

Dimensional accuracy of jaw scans performed on alginate impressions or stone models: A practice-oriented study

OBJECTIVES

Digital jaw models offer more extensive possibilities for analysis than casts and make it easier to share and archive relevant information. The aim of this study was to compare the dimensional accuracy of scans performed on alginate impressions and on stone models to reference scans performed on underlying resin models.

METHODS

Precision spheres 5 mm in diameter were occlusally fitted to the sites of the first premolars and first molars on a pair of jaw models fabricated from resin. A structured-light scanner was used for digitization. Once the two reference models had been scanned, alginate impressions were taken and scanned after no later than 1 h. A third series of scans was performed on type III stone models derived from the impressions. All scans were analyzed by performing five repeated measurements to determine the distances between the various sphere centers.

RESULTS

Compared to the reference scans, the stone-model scans were larger by a mean of 73.6 µm (maxilla) or 65.2 µm (mandible). The impression scans were only larger by 7.7 µm (maxilla) or smaller by 0.7 µm (mandible). Median standard deviations over the five repeated measurements of 1.0 µm for the reference scans, 2.35 µm for the impression scans, and 2.0 µm for the stone-model scans indicate that the values measured in this study were adequately reproducible.

CONCLUSION

Alginate impressions can be suitably digitized by structured-light scanning and offer considerably better dimensional accuracy than stone models. Apparently, however, both impression scans and stone-model scans can offer adequate precision for orthodontic purposes. The main issue of impression scans (which is incomplete representation of model surfaces) is being systematically explored in a follow-up study.

Forces and moments delivered by novel, thinner PET-G aligners during labiopalatal bodily movement of a maxillary central incisor: An in vitro study

Objective: To evaluate whether overloading of teeth can be avoided by utilizing aligners with reduced thicknesses of 0.4 mm or 0.3 mm.

Materials and Methods: The experimental setup included an acrylic maxillary jaw model with tooth 11 separated and fixed via a 3-D force-moment transducer to a hexapod for experimental movement. Aligners tested were fabricated on duplicate stone models using commercially available polyethylene terephthalate glycol (PET-G) foils with thicknesses between 0.5 and 0.75 mm, and novel 0.4-mm- and 0.3-mm-thick foils. With the test aligner seated, 11 was bodily displaced in a labiopalatal direction in the range of ±0.25 mm while all six force-and-moment components exerted on this tooth were registered.

Results: With the thinnest commercially available 0.5-mm aligner, median forces of −7.89 N and 8.37 N were measured for the maximum 0.25-mm movement of 11 in a labial and palatal direction, respectively. In comparison, force values were 35% and 71% lower for the novel aligners with a thickness of 0.4 mm and 0.3 mm, respectively.

Conclusions: Novel “leveling” aligners with reduced thickness may reduce overloading of individual teeth during aligner therapy. Due to form instability of 0.3-mm aligners, we suggest a novel sequence of 0.4–0.5–0.75 mm for aligner systems using several foil thicknesses for load graduation within single setup steps. This would combine low stiffness of the initial aligner and relatively constant load increases throughout the treatment.

The clinical utility of botulinum toxin injections targeted at the motor endplate zone in cervical dystonia

BACKGROUND AND PURPOSE

Cervical dystonia (CD) patients usually receive repeated botulinum neurotoxin (BoNT) injections. The aims of this study were to evaluate the feasibility of motor endplate zone (MEZ) detection of relevant cervical muscles in CD patients receiving chronic BoNT treatment and to compare the treatment effect of half-dosed, endplate-targeted injections to standard BoNT injections.

METHODS

In study 1, high-density surface electromyography (HD-sEMG) was recorded from the sternocleidomastoid (SCM) and splenius capitis (SC) muscles in 18 CD patients with ongoing BoNT treatment, by which the location of the MEZ was determined. In study 2, nine additional patients with rotational-type CD participated in a treatment effect study where they received either half of their regular BoNT dose through endplate-targeted injections or their normal BoNT dose through standard injections (crossover design). Dystonia severity was recorded before and 4 weeks after each treatment session (Toronto Western Spasmodic Torticollis Rating Scale severity subscore).

RESULTS

In the SCM muscle the MEZ was located at the lower border of the superior third part of the muscle, and in the SC muscle at half muscle length. Endplate-targeted, half-dosed BoNT injection resulted in a similar treatment effect to injecting the full dose in the standard technique.

CONCLUSIONS

Half-dosed, endplate-targeted BoNT injections lead to a similar treatment effect to the standard BoNT injection protocol. MEZ detection confronts the clinician with some technical challenges, such as the ability of accurate and technically optimal placement of the electrode grid and correct interpretation of the HD-sEMG signal.

Optimal placement of bipolar surface EMG electrodes in the face based on single motor unit analysis

Locations of surface electromyography (sEMG) electrodes in the face are usually chosen on a macro-anatomical basis. In this study we describe optimal placement of bipolar electrodes based on a novel method and present results for lower facial muscles. We performed high-density sEMG recordings in 13 healthy participants. Raw sEMG signals were decomposed into motor unit action potentials (MUAPs). We positioned virtual electrode pairs in the interpolated monopolar MUAPs at different positions along muscle fiber direction and calculated the bipolar potentials. Electrode sites were determined where maximal bipolar amplitude was achieved and were validated. Objective guidelines for sEMG electrode placement improve the signal-to-noise ratio and may contribute to reduce cross talk, which is particularly important in the face. The method may be regarded as an important basis for improving the validity and reproducibility of sEMG in complex muscle areas.

Using two-dimensional spatial information in decomposition of surface EMG signals

Recently, high-density surface EMG electrode grids and multi-channel amplifiers became available for non-invasive recording of human motor units (MUs). We present a way to decompose surface EMG signals into MU firing patterns, whereby we concentrate on the importance of two-dimensional spatial differences between the MU action potentials (MUAPs). Our method is exemplified with high-density EMG data from the vastus lateralis muscle of a single subject. Bipolar and Laplacian spatial filtering was applied to the monopolar raw signals. From the single recording in this subject six different simultaneously active MUs could be distinguished using the spatial differences between MUAPs in the direction perpendicular to the muscle fiber direction. After spike-triggered averaging, 125-channel two-dimensional MUAP templates were obtained. Template-matching allowed tracking of all MU firings. The impact of spatial information was measured by using subsets of the MUAP templates, either in parallel or perpendicular to the muscle fiber direction. The use of one-dimensional spatial information perpendicular to the muscle fiber direction was superior to the use of a linear array electrode in the longitudinal direction. However, to detect the firing events of the MUs with a high accuracy, as needed for instance for estimation of firing synchrony, two-dimensional information from the complete grid electrode appears essential.

Smart Brackets for 3D-Force-Moment Measurements in Orthodontic Research and Therapy – Developmental Status and Prospects*

BACKGROUND AND AIM

Quantitative knowledge of the three-dimensional (3D) force-moment systems applied for therapeutic tooth movement is of utmost importance, with regard to the predictability of the course of tooth movement, as well as the reduction of traumatic side effects. The concept of a smart bracket with an integrated sensor system for 3D force and moment measurement has recently been published. The feasibility of this approach has been demonstrated using finite-element (FE) simulations and a 2.5 times enlarged real smart bracket model. The aim of this study was to develop and to mechanically characterize the first wire-mediated, true-scale smart bracket.

MATERIALS AND METHODS

A true-scale smart bracket was built using a stress-sensor chip (having a surface area of 2 x 2.5 mm(2)) and a conventional bracket slot. This bracket was calibrated on a biomechanical system for 3D application and measurement of forces and moments, then its measurement accuracy was evaluated.

RESULTS

With the exception of the bucco-lingually-oriented force component, the embedded sensor system was capable of reconstructing the applied force-moment components with sufficient accuracy. The standard deviations for the differences between applied and inferred values were 0.07 N, 0.07 N and 0.26 N for the components F(x), F(y) and F(z), and 0.76 Nmm, 1.09 Nmm and 0.22 Nmm for the components Mx, My and Mz.

CONCLUSIONS

We were able to construct true-scale, wire-mediated smart brackets. Work on improving the sensor system's buccolingual sensitivity is still in progress. Improved smart brackets with wire-mediated energy transmission could be applied in the near future in orthodontic training as an objective feedback tool, as well as in biomechanical research. Broad clinical application of smart brackets requires integration of telemetric components for data and energy transmission. Such components are now being developed. Future clinical application of smart brackets may contribute in reducing the negative side effects of fixed appliance therapy such as root resorption, while enhancing therapeutic efficiency.

Dentofacial Parameters Explaining Variability in Retroclination of the Maxillary Central Incisors

BACKGROUND AND AIM

The interrelation between retroclination of the maxillary central incisors and dentofacial parameters is a controversial subject in the literature. In contrast to comparisons between malocclusion and control groups, the objective of the present study was to identify skeletal, dentoalveolar or perioral (soft-tissue) factors which primarily determine how severely retroclination is individually manifested.

MATERIALS AND METHODS

For this purpose we evaluated the pretherapeutic lateral cephalograms of 83 patients with an inclination of the maxillary central incisors ranging from physiological values to very severe retroclination (inclination to anterior cranial base between 104 degrees and 64 degrees ). A detailed analysis of the skeletal, dentoalveolar, and soft-tissue morphology was performed using lateral cephalograms taken prior to therapy. The statistical analysis included the calculation of multiple regression models for maxillary central incisor inclination and different parameters describing the lip-to-incisor relationship as dependent variables.

RESULTS

A regression model including 1) the lip-line level measured at the dorsal upper-lower lip contact point, 2) the sagittal intermaxillary relationship, and 3) the inclination of the mandibular central incisors explained 81% of the variability in maxillary central incisor inclination (p < 0.0001 for all three parameters). Statistical analysis of the morphologic base of a high dorsal lip-line level (i.e., the predominant characteristic in the retroclination cases) revealed the significance of soft-tissue, dentoalveolar, and skeletal variables (p < 0.001).

CONCLUSIONS

Complementary to results of previous resting lippressure measurements, this cephalometric study suggests that a high lip-line level is the predominant causative factor for a cover- bite or Class II, Division 2 malocclusion. Therefore, we conclude that (1) lip-line measurements should be included in routine cephalometric diagnostics, and (2) that a high lip-line must be eliminated by therapeutic measures in these malocclusions to prevent a post-orthodontic relapse.

Lip-to-incisor Relationship and Postorthodontic Long-term Stability of Cover-bite Treatment

Objective: To investigate the impact of a persisting high lip line and other potential relapse-inducing factors on long-term stability of orthodontic correction of retroinclined maxillary central incisors.

Materials and Methods: Thirty-one cover-bite (“Deckbiss”) patients with retroinclined maxillary central incisors and a deep frontal overbite were evaluated. The maxillary central incisor inclination was determined odontometrically with study models made pretreatment, posttreatment, and at a follow-up examination (mean posttherapeutic interval: 9.0 years). The lip-to-incisor relationship, the interincisal angle, and the anteroposterior maxillary central incisor position were measured on lateral cephalograms taken after active treatment.

Results: The relapse tendency of the orthodontic correction of the retroinclined maxillary central incisors displayed great interindividual variability with a range of posttherapeutic inclination change of −6.75° to +8.00°. Multiple regression analysis revealed an increased tendency for relapse in patients with (1) a high posttherapeutic (dorsal) lip line level combined with the maxillary central incisor and lower lip contact only in the incisal crown area (P &lt; .01) and (2) a marked therapeutically induced inclination change of the maxillary central incisors (P &lt; .05). Interrelations between the relapse of the corrected maxillary central incisors and other evaluated parameters were not statistically significant.

Conclusions: For maximum treatment stability, the elimination of an excessive overlap of the upper incisors by the lower lip should be regarded as one of the most important therapeutic objectives when treating this malocclusion.

Topographical Characteristics of Motor Units of the Lower Facial Musculature Revealed by Means of High-Density Surface EMG

The objective of this study was to systematically characterize motor units (MUs) of the musculature of the lower face. MU endplate positions and principal muscle fiber orientations relative to facial landmarks were identified. This was done by the analysis of motor unit action potentials (MUAPs) in the surface electromyogram. Thirteen specially trained, healthy subjects performed selective contractions of the depressor anguli oris, depressor labii inferioris, mentalis, and orbicularis oris inferior muscles. Signals were recorded using recently developed, 0.3-mm thin and flexible high-density surface electromyography (sEMG) grids (120 channels). For each subject and each muscle and for different low contraction levels, representative MUAPs ("MU fingerprints") were extracted from the raw sEMG data according to their spatiotemporal amplitude characteristics. We then topographically characterized the lower facial MUs' endplate zones and main muscle fiber orientations on the individual faces of the subjects. These topographical MU parameters were spatially warped to correct for the different sizes and shapes of the faces of individual subjects. This electrophysiological study revealed a distribution of the lower facial MU endplates in more or less restricted, distinct clusters on the muscle often with eccentric locations. The results add substantially to the basic neurophysiologic and anatomical knowledge of the complex facial muscle system. They can also be used to establish objective guidelines for placement of conventional (surface or needle) EMG electrodes as well as for clinical investigations on neuromuscular diseases affecting the facial musculature. The localized endplate positions may also indicate optimal locations for botulinum toxin injection in the face.

A retrospective cephalometric study for the quantitative assessment of relapse factors in cover-bite treatment

BACKGROUND AND AIM

Cover-bite ("Deckbiss") is regarded as a highly relapse-prone malocclusion. In this context the great significance of a high lip line (LipL) as an etiologic factor for the retroclination of the upper central incisors was recently proven within the framework of lip pressure measurements. It therefore seemed likely that a persisting high LipL after correction of cover-bite might have an equally negative impact on the stability of the treatment outcome.

MATERIALS AND METHODS

This issue was investigated in the present retrospective study by cephalometric analysis of the findings prior to therapy (T1), immediately after active mechanotherapy (T2), and after a mean follow-up period of 2 years (T3). The study group consisted of 40 former cover-bite patients with initial linguoversion of the upper central incisors (axial angle to anterior cranial base < 98 degrees ) and anterior deep bite (> or = 4 mm) from the records of the Department of Orthodontics, University of Freiburg i. Br., Germany.

RESULTS AND CONCLUSIONS

The average relapse was ca. 20% of the total correction of the anterior linguoversion and deep bite, with the relapse tendency, however, displaying substantial interindividual variations. Multiple regression analysis revealed an increased relapse tendency in specific cases: patients with maxillary extractions, cases with a pronounced therapeutically induced change of upper central incisor inclination, and patients with a high post-therapeutic LipL or with poor compliance in the retention phase. In view of the relatively good opportunity to influence the level of the LipL therapeutically, one of the most important therapeutic objectives for cover-bite patients should be to reduce the amount by which the lower lip overlaps the upper incisors ( to a maximum value of 3 mm). This can be achieved by active mechanical intrusion of the upper incisors. If the orthodontist fails to take account of this aspect when planning or performing the treatment, he has to accept an increased risk of relapse.

A thin, flexible multielectrode grid for high-density surface EMG

Although the value of high-density surface electromyography (sEMG) has already been proven in fundamental research and for specific diagnostic questions, there is as yet no broad clinical application. This is partly due to limitations of construction principles and application techniques of conventional electrode array systems. We developed a thin, highly flexible, two-dimensional multielectrode sEMG grid, which is manufactured by using flexprint techniques. The material used as electrode carrier (Polyimid, 50 microm thick) allows grids to be cut out in any required shape or size. One universal grid version can therefore be used for many applications, thereby reducing costs. The reusable electrode grid is attached to the skin by using specially prepared double-sided adhesive tape, which allows the selective application of conductive cream only directly below the detection surfaces. To explore the practical possibilities, this technique was applied in single motor unit analysis of the facial musculature. The high mechanical flexibility allowed the electrode grid to follow the skin surface even in areas with very uneven contours, resulting in good electrical connections in the whole recording area. The silverchloride surfaces of the electrodes and their low electrode-to-skin impedances guaranteed high baseline stability and a low signal noise level. The electrode-to-skin attachment proved to withstand saliva and great tensile forces due to mimic contractions. The inexpensive, universally adaptable and minimally obstructive sensor allows the principal advantages of high-density sEMG to be extended to all skeletal muscles accessible from the skin surface and may lay the foundation for more broad clinical application of this noninvasive, two-dimensional sEMG technique.

A surface EMG electrode for the simultaneous observation of multiple facial muscles

With previous surface electromyography (sEMG) electrodes it has been difficult to combine small outer dimensions and secure skin attachment. We resolved this problem by developing a new skin attachment technique that yields firm electrode fixation without requiring an acrylic housing. Consequently, we could reduce the outer electrode dimensions to 4-mm diameter and only 1.5-mm thickness. In a bipolar montage, this electrode allows an inter-electrode distance of 8 mm. This improves measurement selectivity and, because of the small dimensions, makes possible the non-invasive observation of multiple facial muscles with a minimum of obstruction. Our new technique was tested on a group of 11 professional trumpeters. They were instructed to perform a series of muscle-specific facial poses and to play exercises on their instruments while EMG signals were recorded simultaneously from seven different perioral muscles. Although the skin attachment was subjected to high stress during trumpet playing, more than 98% of electrode placements yielded a secure mechanical and electrical connection. Muscle selectivity of the signals recorded during the facial poses was similar to that obtained in a previous investigation using intra-muscular fine-wire electrodes. Crosstalk in the perioral area was estimated to be lower than 25%. The availability of an unobstructive sEMG electrode for simultaneously observing multiple facial muscles opens up a wide range of applications (e.g. in speech research, psychophysiology and orthodontics).

The importance of the level of the lip line and resting lip pressure in Class II, Division 2 malocclusion

Many clinicians hypothesize that retroclination of the maxillary central incisors in Class II, Division 2 malocclusion is caused by increased resting lip pressure against these teeth. The purpose of this study was (1) to verify this assumption by means of simultaneous lip-pressure measurements at two different levels on the maxillary central incisor crowns, and (2) to examine factors that could possibly contribute to the increased resting lip pressure. This is the first study to prove that individuals with Class II, Division 2 malocclusion (n = 21) have the upper central incisors exposed to significantly higher lip pressure than those with Class I malocclusion (n = 21). Our statistical evaluation revealed that this is primarily attributed to a high lip line and not to a hypertonic peri-oral musculature. We concluded that orthodontic treatment of Class II, Division 2 cases should include intrusion of the maxillary incisors, to eliminate the non-physiologically high pressure exerted by the lower lip on these teeth and, consequently, to reduce the high risk of a post-orthodontic relapse.