The time-dependent biomechanical behaviour of the periodontal ligament--an in vitro experimental study in minipig mandibular two-rooted premolars

The aim of the present work was to evaluate the biomechanical behaviour of the periodontal ligament (PDL) with respect to force development with different controlled loading velocities. For this purpose, an in vitro experimental study was performed on 18 minipig jaw segments. Displacements with variable increasing loading time were applied to one premolar crown of each jaw segment into the linguobuccal direction through a force sensor provided by a specialized biomechanical set-up. The predefined displacement values to be achieved were 0.1 and 0.2 mm. Each of the given displacement increments was applied on the specimens with a linear displacement increase employing the following time spans: 5, 10, 20, 30, 60, 120, 300, 450, and 600 seconds. Force values were measured during load application to register force/displacement diagrams and after the maximum displacement was reached force decay was monitored for a period of 600 seconds. Force/time curves for each tooth were plotted according to the data obtained. Diagrams of the maximum force values obtained from these plots and the force at the end of each measurement were extracted for all teeth. Forces at the point when maximum displacement was reached ranged from 0.5 to 2.5 N for the 0.1 mm activation and showed extreme variation with the specimens. The factor of volume and surface area of the individual roots were evaluated and found not to be responsible for these deviations. A comparable behaviour was recorded for the 0.2 mm deflection, however, on a higher force level. The results show that the force development at different displacement velocities is complex and dominated by the PDL biomechanical characteristics

Changes in the distribution of occlusal forces in the course of the orthodontic retention phase: A prospective cohort study

PURPOSE: Aim of the present study was to assess the relative distribution of occlusal forces after orthodontic treatment and during the first 3 months of the retention phase using a computerized occlusal analysis system (T-Scan, Tekscan Inc., Norwood, MA, USA). MATERIALS AND METHODS: A total of 52 patients were included in this prospective cohort study and underwent analysis of occlusal forces on the level of tooth, jaw-half, and -quadrant during a 3-month period. Furthermore, differences between three retention protocols (group I: removable appliances in both jaws; group II: fixed 3-3 lingual retainers in both jaws; group III: removable appliance in the maxilla and fixed 3-3 lingual retainer in mandible) were assessed with Wilcoxon signed-rank tests at 5%. RESULTS: Directly after debonding, measured forces distribution were similar to published references for untreated samples. In the following, no significant difference was found between retention protocols II and III with regard to the asymmetry of the anterior occlusal forces. Both groups maintained an asymmetric force distribution in the anterior segment during the study period. There was also no difference between groups II and III in the distribution of occlusal forces for the posterior segments. Both retention concepts kept the symmetrical distribution of occlusal forces stable over the observation period. The retention concept of group I demonstrated a symmetrical distribution of occlusal forces in the anterior segment after debonding and this remained stable during the 3‑month period. In the posterior segment, no improvement of the initially asymmetric masticatory force distribution could be observed. CONCLUSIONS: All three studied retention protocols showed stability in retaining their original symmetrical or asymmetrical occlusal force distribution posteriorly/anteriorly during the 3‑month observation period. Therefore, an even distribution of occlusal forces should be the aim of the finishing phase,as no relative benefit of any single retention scheme in terms of post-debond improvement during the retention phase was seen. // ZIEL DER STUDIE: Ziel der vorliegenden Studie war es, die relative Verteilung der okklusalen Kräfte nach der kieferorthopädischen Behandlung und während der ersten 3 Monate der Retentionsphase mithilfe eines computergestützten okklusalen Analysesystems (T-Scan, Tekscan Inc., Norwood, MA, USA) zu untersuchen. MATERIAL UND METHODEN: 52 Patienten wurden in die prospektive Kohortenstudie aufgenommen und es wurde über einen Zeitraum von 3 Monaten eine Analyse der Verteilung der okklusalen Kräfte bezogen auf die einzelnen Zähne, die Kieferhälften und die unterschiedlichen Quadranten vorgenommen. Darüber hinaus wurden die Unterschiede zwischen 3 Retentionsprotokollen (Gruppe I: herausnehmbare Apparaturen in beiden Kiefern; Gruppe II: festsitzender 3‑3-Lingualretainer in beiden Kiefern; Gruppe III: herausnehmbare Apparatur im Oberkiefer und festsitzender 3‑3-Lingualretainer im Unterkiefer) mithilfe des Wilcoxon-signed-rank-Tests bei einem Signifikanzniveau von 5 % bewertet. ERGEBNISSE: Unmittelbar nach dem Debonding waren die gemessenen Kräfteverteilungen ähnlich wie in bereits veröffentlichten Studien für unbehandelte Probanden. Im Folgenden wurde kein signifikanter Unterschied zwischen den Retentionsprotokollen II und III in Bezug auf eine Asymmetrie der anterioren okklusalen Kräfte festgestellt. Beide Gruppen behielten während des Studienzeitraums eine asymmetrische Kraftverteilung im Frontzahnbereich bei. Auch bei der Verteilung der Okklusionskräfte im Seitenzahnbereich gab es keinen Unterschied zwischen den Gruppen II und III. Beide Retentionskonzepte hielten eine symmetrische Verteilung der okklusalen Kräfte über den Beobachtungszeitraum stabil. Das Retentionskonzept der Gruppe I wies im Frontzahnbereich nach dem Debonding eine symmetrische Verteilung der Okklusionskräfte auf, die über den Dreimonatszeitraum stabil blieb. Im Seitenzahnbereich konnte keine Verbesserung der initialen asymmetrischen Kaukraftverteilung beobachtet werden. SCHLUSSFOLGERUNGEN: Alle 3 untersuchten Retentionsprotokolle zeigten eine stabile Beibehaltung ihrer ursprünglichen symmetrischen oder asymmetrischen Verteilung der okklusalen Kräfte im Seitenzahnbereich bzw. im Frontzahnbereich während des 3‑monatigen Beobachtungszeitraums. Daher sollte eine gleichmäßige Verteilung der okklusalen Kräfte das Ziel bereits der Finishing-Phase der aktiven Therapie sein. Bezüglich der einzelnen Retentionsschemata zeigte keines der untersuchten Protokolle einen relativen Vorteil in Bezug auf mögliche Verbesserung der Kräfteverteilung während der Retentionsphase

Residual stress analysis of fixed retainer wires after in vitro loading: Can mastication-induced stresses produce an unfavorable effect?

The aim of the present study was to compare four different types of fixed canine-to-canine retainer regarding the maximum and residual force system generated on a canine during the intrusive in vitro loading of the rest of the anterior teeth. Retainers constructed from Ortho-FlexTech gold chain 0.038×0.016-inch (rectangular, 0.96×0.40 mm), Tru-Chrome® 7-strand twisted 0.027-inch (round, 0.68 mm diameter) steel wire, and Wildcat 0.0175-inch (round, 0.44 mm) and 0.0215-inch (round, 0.55 mm) 3-strand Twistflex steel wire bonded on the anterior teeth of an acrylic resin model, installed in the Orthodontic Measurement and Simulation System. The force system on the canine was recorded during the loading of the anterior teeth as well as the residual force system at the same tooth after the unloading. During maximum loading, the gold chain exerted the lowest and the 0.0215-inch archwire the highest force and moment magnitude. Residual forces and moments were exerted on the canine after the unloading in all retainer types, i.e., the evaluated fixed retainers were not passive after in vitro vertical loading. The lowest magnitude was measured in gold chain retainers and the highest in cases of the high formable/low yield strength 0.027-inch archwire. This fact may explain the unexpected movements of teeth bonded on fixed retainers detected long-term in vivo. © 2015 by De Gruyter 2015

Self-Ligation in Orthodontics

Self-Ligation in Orthodontics is a major new text that reviews, analyses and clarifies the currently available peer-reviewed evidence on a number of applications of this technology, espousing diverse perspectives to make this a unique scientific resource on self-ligation. The book embraces all aspects of self-ligating brackets, extending from therapeutic outcome, to biomechanics, materials, and treatment manifestations. Each chapter addresses a specific question pertinent to the properties and clinical performance of self-ligating brackets including: force and moment generation during engagement; temporal variation of force in active self-ligating brackets; periodontal considerations and oral microbiota alterations; root resorption; treatment efficiency and associated dental effects. A work of substantial, thoughtful, and useful scholarship, Self-Ligation in Orthodontics will be a significant resource for clinicians and researchers in orthodontics, and related dental specialists. •First comprehensive resource on self-ligation in orthodontics •Surveys available peer-reviewed evidence •Epitomizes the management of self-ligating treatment suggesting important treatment concepts and tips for the practicing clinician •Includes diverse perspectives to provide a balanced and scholarly approach •Richly illustrated with detailed color clinical image

Post-treatment changes in permanent retention

OBJECTIVES While permanent retention is today the method of choice to stabilize orthodontic treatment outcomes, recent studies have increasingly reported posttreatment changes in tooth position during permanent retention. We conducted this study to analyze changes in the anterior mandible, whether the changes follow an underlying movement pattern, and, aiming for a preventive strategy, whether any risk factors could be identified comparing findings with the pretreatment situations. METHODS We included 30 patients who had worn fixed Twistflex retainers (UK 3-3) extending from canine to canine in the mandible. Casts reflecting the intraoral situations before orthodontic treatment (T0), directly after completion of active therapy (T1), and 6 months later (T2) were scanned and superimposed using Imageware Surfacer software. Posttreatment changes (T2-T1) of tooth position within the retainer block were analyzed on 3D virtual models and were compared to pretreatment (T0) and treatment-related (T1-T0) findings to identify potential risk factors. RESULTS Almost all analyzed patients revealed three-dimensional changes in tooth position within the retainer block. Comparing these movements, we repeatedly found rotated retainer blocks in labio-oral direction, while the center of rotation was located at the first incisors. This pattern was associated with intercanine expansion and excessive overjet correction during orthodontic treatment. The canines underwent the most pronounced (rotational and translational) movements. CONCLUSIONS In general permanent lingual retainers are safe but in special clinical cases retainers can induce undesired tooth movement. Risk factors seem to be intercanine expansion and excessive overjet correction during orthodontic treatment. In specific cases an additional retention device might be needed

NiTi WIRES FOR ORTHODONTIC APPLICATION

In order to characterize NiTi orthodontic wires microstructure, transformation temperatures, and deformation behaviour have been investigated. Various states of dislocation density were established by additional heat treatment of a work hardened material. The as recieved wire and the change in microstructure due to the chosen heat treatment were described by optical light microscopy. The stress induced transformation behaviour is related with the experimental results in transformation characteristic measurements and microstructural observations