Novel approach for characterizing clinical load application of superelastic orthodontic wires.

OBJECTIVE: Current standardized in vitro bending experiments for orthodontic archwires cannot capture friction conditions and load sequencing during multi-bracket treatment. This means that clinically relevant forces exerted by superelastic wires cannot be predicted. To address these limitations, this study explored a novel test protocol that estimates clinical load range. METHODS: The correction of a labially displaced maxillary incisor was simulated using an in vitro model with three lingual brackets. Deflection force levels derived from four different protocols were designed to explore the impact of friction and wire load history. These force levels were compared in nickel-titanium (NiTi) archwires with three commonly used diameters. The unloading path varied between protocols, with single or multiple sequences and different load orders and initial conditions. RESULTS: Deflection forces from the new protocol, employing multiple continuous load/unload cycles (CCincr), consistently exceeded those from the conventional protocol using a single continuous unloading path (CUdecr). Mean differences in plateau force ranged from 0.54 N (Ø 0.014" wire) to 1.19 N (Ø 0.016" wire). The CCinr protocol also provided average force range estimates of 0.47 N (Ø 0.012" wire), 0.89 N (Ø 0.014" wire), and 1.15 N (Ø 0.016" wire). SIGNIFICANCE: Clinical orientation towards CUdecr carries a high risk of excessive therapeutic forces because clinical loading situations caused by friction and load history are underestimated. Physiological tooth mobility using NiTi wires contributes decisively to the therapeutic load situation. Therefore, only short unloading sequences starting from the maximum deflection in the load history, as in CCincr, are clinically meaningful.

Biomechanical model registration for monitoring and simulating large orthodontic tooth movements in the maxilla and mandible.

PURPOSE: Superimposition of digital dental-arch models allows quantification of orthodontic tooth movements (OTM). Currently, this procedure requires stable reference surfaces usually only present in the maxilla. This study aimed to investigate the accuracy of a novel superimposition approach based on biomechanical principles of OTM and the equilibrium of forces and moments (EFM)-applicable in both jaws-for monitoring and simulating large OTM. METHODS: The study included 7 patients who had undergone extraction of the first (PM1-Ex) or second (PM2-Ex) premolar in each quadrant. Digital models taken at start and end of the T­Loop treatment phase were superimposed by applying 3 EFM variants differing in the number of teeth used for registration. Maxillary OTM results for EFM were validated against those for a conventional surface registration method (SRM). In an additional case study, OTM were simulated for PM1-Ex, PM2-Ex and non-extraction treatment strategies. RESULTS: The EFM variant that included all teeth of the dental arch achieved the highest accuracy, with median translational and rotational OTM deviations from SRM of only 0.37 mm and 0.56°, respectively. On average, retracted canines and first premolars were distalized by 3.0 mm, accompanied by 6.2° distal crown tipping and 12.2° distorotation. The share of space closure by molar mesialization was 19.4% for PM1-Ex quadrants and 34.5% for PM2-Ex quadrants. CONCLUSION: EFM allows accurate OTM quantification relative to the maxillary and mandibular bases even in challenging situations involving large OTM. Superimposition of malocclusion and setup models enables realistic simulation of final tooth positions. This may greatly enhance the value of digital setups for decision-making in orthodontic treatment planning.

Effect of variable periodontal ligament thickness and its non-linear material properties on the location of a tooth's centre of resistance.

In orthodontics, the 3D translational and rotational movement of a tooth is determined by the force-moment system applied and the location of the tooth's centre of resistance (CR). Because of the practical constraints of in-vivo experiments, the finite element (FE) method is commonly used to determine the CR. The objective of this study was to investigate the geometric model details required for accurate CR determination, and the effect of material non-linearity of the periodontal ligament (PDL). A FE model of a human lower canine derived from a high-resolution µCT scan (voxel size: 50 µm) was investigated by applying four different modelling approaches to the PDL. These comprised linear and non-linear material models, each with uniform and realistic PDL thickness. The CR locations determined for the four model configurations were in the range 37.2-45.3% (alveolar margin: 0%; root apex: 100%). We observed that a non-linear material model introduces load-dependent results that are dominated by the PDL regions under tension. Load variation within the range used in clinical orthodontic practice resulted in CR variations below 0.3%. Furthermore, the individualized realistic PDL geometry shifted the CR towards the alveolar margin by 2.3% and 2.8% on average for the linear and non-linear material models, respectively. We concluded that for conventional clinical therapy and the generation of representative reference data, the least sophisticated modelling approach with linear material behaviour and uniform PDL thickness appears sufficiently accurate. Research applications that require more precise treatment monitoring and planning may, however, benefit from the more accurate results obtained from the non-linear constitutive law and individualized realistic PDL geometry.

Three-dimensional amplitude characteristics of masseter motor units and representativeness of extracted motor unit samples.

OBJECTIVE: This study aimed to characterize amplitude topographies for masseter motor units (MUs) three-dimensionally, and to assess whether high-density surface electromyography (HDsEMG) is able to detect MU samples that represent the masseter's entire MU pool. METHODS: Ten healthy adult volunteers participated in the study, which combined three EMG techniques. A HDsEMG grid covering the entire masseter, and intramuscular fine-wire electrodes were used to obtain two independent MU samples for comparison. The MUs' amplitude profiles in the dimension of muscle depth were determined using scanning EMG. All data were recorded simultaneously during a low, constant contraction level controlled by 3D force feedback. RESULTS: The median medio-lateral diameter of 4.4 mm (range: 1.2-7.9 mm) for MUs detected by HDsEMG did not differ significantly (Mann-Whitney-U test, p = 0.805) from that of 3.9 mm (0.6-8.6 mm) for MUs detected by fine-wire EMG. For individual subjects, the medio-lateral diameters of all HDsEMG-detected MUs spanned 70.5% (19.2-75.1%) of the masseter's thickness. CONCLUSIONS: HDsEMG is able to examine small and large MUs from a great masseter proportion in one single measurement. SIGNIFICANCE: Clinical application of HDsEMG might contribute to a better understanding of neuromuscular adaptations in patients with temporomandibular disorders (TMD) and could allow for monitoring treatment effects.

Novel Method for Superposing 3D Digital Models for Monitoring Orthodontic Tooth Movement.

Quantitative three-dimensional analysis of orthodontic tooth movement (OTM) is possible by superposition of digital jaw models made at different times during treatment. Conventional methods rely on surface alignment at palatal soft-tissue areas, which is applicable to the maxilla only. We introduce two novel numerical methods applicable to both maxilla and mandible. The OTM from the initial phase of multi-bracket appliance treatment of ten pairs of maxillary models were evaluated and compared with four conventional methods. The median range of deviation of OTM for three users was 13-72% smaller for the novel methods than for the conventional methods, indicating greater inter-observer agreement. Total tooth translation and rotation were significantly different (ANOVA, p < 0.01) for OTM determined by use of the two numerical and four conventional methods. Directional decomposition of OTM from the novel methods showed clinically acceptable agreement with reference results except for vertical translations (deviations of medians greater than 0.6 mm). The difference in vertical translational OTM can be explained by maxillary vertical growth during the observation period, which is additionally recorded by conventional methods. The novel approaches are, thus, particularly suitable for evaluation of pure treatment effects, because growth-related changes are ignored.

Comparison of methods to determine the centre of resistance of teeth.

In orthodontic treatment, the locations of the centre of resistance (CR) of individual teeth and the applied load system are the major determinants for the type of tooth movement achieved. Currently, CR locations have only been specified for a relatively small number of tooth specimen for research purposes. Analysing cone beam computed tomography data samples from three upper central incisors, this study explores whether the effort to establish accurate CR estimates can be reduced by (i) morphing a pre-existing simplified finite element (FE) mesh to fit to the segmented 3D tooth-bone model, and (ii) individualizing a mean CR location according to a small parameter set characterising the morphology of the tooth and its embedding. The FE morphing approach and the semi-analytical approach led to CR estimates that differ in average only 0.04 and 0.12 mm respectively from those determined by very time-consuming individual FE modelling (standard method). Both approaches may help to estimate the movement of individual teeth during orthodontic treatment and, thus, increase the therapeutic efficacy.

Electrophysiological method to examine muscle fiber architecture in the upper lip in cleft-lip patients.

OBJECTIVES: The upper lip's musculature comprises several muscle fiber groups with different spatial orientations for various lip movements. In the past, it has not been possible to describe these various groups and their motor units in detail with traditional methods of electromyography (EMG). The purpose of this investigation was to create a viable method for studying the architecture of upper-lip muscle fibers in patients with surgically closed cleft lip by refining the design of a flexible electrode array for multichannel EMG. METHODS: Calibrated perioral photographs of 36 adult subjects were used to determine the basic geometry of an electrode array for use on different facial shapes. A total of 256 chloride silver electrodes, 1 mm in diameter, were distributed over this geometry to form a grid with interelectrode distances of 2.5 mm. Technical implementation of the electrode array was based on a modified technique used for flexible printed circuit boards. A double-sided adhesive tape with perforations was used to apply the array to the skin in the upper-lip area. Surface EMG signals were recorded while the subjects were performing various lip movements and exerting specific lip forces. The latter were recorded with a specially developed transducer. RESULTS: Despite an extremely dense grid of electrodes associated with downsized contacts, the electrode array could be precisely applied to the perforated adhesive tape. The electrode array remained firmly attached to the skin even during extensive lip movements. The recorded EMG signals offered high baseline stability and a strong signal-to-noise ratio, thus, allowing us to extract individual motor unit action potentials (MUAPs). Spatiotemporal tracking of MUAPs succeeded in demonstrating the course of muscle fibers belonging to individual motor units. Pilot measurements taken on subjects with surgically closed unilateral cleft lip revealed an asymmetric distribution of muscle activity surrounding the closed cleft-lip segments. CONCLUSION: Our refined noninvasive electrophysiological method is capable of providing detailed information about muscle fiber architecture and the position, size, and organization of motor units in the lip area. It is thus feasible to conduct further studies to analyze differences in muscle morphology and motor control between surgically treated cleft-lip patients and noncleft individuals.