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 characteristic

Torque expression of 0.018 and 0.022 inch conventional brackets

The aim of this study was to assess the effect of the moments generated with low- and high-torque brackets. Four different bracket prescription-slot combinations of the same bracket type (Mini Diamond® Twin) were evaluated: high-torque 0.018 and 0.022 inch and low-torque 0.018 and 0.022 inch. These brackets were bonded on identical maxillary acrylic resin models with levelled and aligned teeth and each model was mounted on the orthodontic measurement and simulation system (OMSS). Ten specimens of 0.017×0.025 inch and ten 0.019×0.025 inch stainless steel archwires (ORMCO) were evaluated in the low- and high-torque 0.018 inch and 0.022 inch brackets, respectively. The wires were ligated with elastomerics into the brackets and each measurement was repeated once after religation. Two-way analysis of variance and t-test were conducted to compare the generated moments between wires at low- and high-torque brackets separately. The maximum moment generated by the 0.017×0.025 inch stainless steel archwire in the 0.018 inch brackets at +15 degrees ranged from 14.33 and 12.95 Nmm for the high- and low-torque brackets, respectively. The measured torque in the 0.022 inch brackets with the 0.019×0.025 inch stainless steel archwire was 9.32 and 6.48 Nmm, respectively. The recorded differences of maximum moments between the high- and low-torque series were statistically significant. High-torque brackets produced higher moments compared with low-torque brackets. Additionally, in both high- and low-torque configurations, the thicker 0.019×0.025 inch steel archwire in the 0.022 inch slot system generated lower moments in comparison with the 0.017×0.025 inch steel archwire in the 0.018 inch slot syste

A comparative assessment of torque generated by lingual and conventional brackets

The aim of this study was to assess the effect of bracket type on the labiopalatal moments generated by lingual and conventional brackets. Incognito™ lingual brackets (3M Unitek), STb™ lingual brackets (Light Lingual System; ORMCO), In-Ovation L lingual brackets (DENTSPLY GAC), and conventional 0.018 inch slot brackets (Gemini; 3M Unitek) were bonded on identical maxillary acrylic resin models with levelled and aligned teeth. Each model was mounted on the orthodontic measurement and simulation system and 10 0.0175 × 0.0175 TMA wires were used for each bracket type. The wire was ligated with elastomerics into the Incognito, STb, and conventional brackets and each measurement was repeated once after religation. A 15 degrees buccal root torque (+15 degrees) and then a 15 degrees palatal root torque (−15 degrees) were gradually applied to the right central incisor bracket. After each activation, the bracket returned to its initial position and the moments in the sagittal plane were recorded during these rotations of the bracket. One-way analysis of variance with post hoc multiple comparisons (Tukey test at 0.05 error rate) was conducted to assess the effect on bracket type on the generated moments. The magnitude of maximum moment at +15 degrees ranged 8.8, 8.2, 7.1, and 5.8 Nmm for the Incognito, STb, conventional Gemini, and the In-Ovation L brackets, respectively; similar values were recorded at −15 degrees: 8.6, 8.1, 7.0, and 5.7 Nmm, respectively. The recorded differences of maximum moments were statistically significant, except between the Incognito and STb brackets. Additionally, the torque angles were evaluated at which the crown torque fell well below the minimum levels of 5.0 Nmm, as well as the moment/torque ratio at the last part of the activation/deactivation curve, between 10 and 15 degrees. The lowest torque expression was observed at the self-ligating lingual brackets, followed by the conventional brackets. The Incognito and STb lingual brackets generated the highest moment

An investigation on fatigue, fracture resistance, and color properties of aesthetic CAD/CAM monolithic ceramics

Objectives To evaluate and compare fracture resistance, translucency, and color reproducibility, as well as the effect of aging on the fracture load and color stability of novel monolithic CAD/CAM ceramics. Materials and methods One hundred crowns of uniform thickness were milled from five ceramic blocks (n = 20): partially crystallized lithium disilicate (PLD) and fully crystallized lithium disilicate (FLD), lithium metasilicate (LMS), 4Y-TZP (SMZ), and 5Y-TZP (UMZ) monolithic zirconia. PLD crowns were glazed, LMS was fired, and FLD was polished. SMZ and UMZ crowns were sintered and polished. Crowns were adhesively cemented to epoxy dies. Half of the crowns (n = 10) were subjected to 1.200.000 load cycles with thermal cycling. Color space values L, a, b defined by the Commission Internationale de l´Eclairage (CIELAB) were measured before and after aging, and (∆E) was calculated. Both aged and non-aged specimens were loaded until fracture in a universal testing machine and the fracture load was recorded. X-ray diffraction (XRD) and scanning electron microscope (SEM) fractographic analysis were carried out on fractured fragments of representative samples. For translucency and color reproducibility, 50 rectangular-shaped specimens were fabricated and processed as described previously. Color values were measured over black and white backgrounds, and the translucency parameter (TP) was computed. Using the shade verification mode, (∆E) to shade A3 was calculated. Data were statistically analyzed using one-way and two-way ANOVA, and t-test. Results Aging did not affect fracture resistance significantly (p > 0.05). The highest mean fracture load was obtained for the SMZ and UMZ. A significant color change was observed after aging in all groups. The highest TP was noted for FLD. SMZ and UMZ had the best shade match. Conclusions Zirconia showed higher fracture resistance and color stability than lithium silicate ceramics. Lithium silicate ceramics were more translucent. The experimental FLD demonstrated high translucency. Clinical relevance Tested ceramics showed sufficient stability to withstand masticatory forces. Characterization of final restorations might be mandatory for better color match

A comparative assessment of forces and moments generated by lingual and conventional brackets

The aim of this study was to assess the effect of bracket type on the labiopalatal forces and moments generated in the sagittal plane. Incognito™ lingual brackets (3M Unitek), STb™ lingual brackets (Light Lingual System; ORMCO), and conventional 0.018 inch slot brackets (Gemini; 3M Unitek) were bonded on three identical maxillary acrylic resin models, with a palatally displaced right lateral incisor. The transfer trays for the indirect bonding of the lingual brackets were constructed in certified laboratories. Each model was mounted on the orthodontic measurement and simulation system and ten 0.013 inch CuNiTi wires were used for each bracket type. The wire was ligated with elastomerics and each measurement was repeated once after re-ligation. The labiopalatal forces and the moments in the sagittal plane were recorded on the right lateral incisor. One-way analysis of variance and post hoc Scheffe pairwise comparisons were used to assess the effect on bracket type on the generated forces and moments. The magnitude of forces ranged from 1.62, 1.27, and 1.81 N for the STb, conventional, and Incognito brackets, respectively; the corresponding moments were 2.01, 1.45, and 2.19 N mm, respectively. Bracket type was a significant predictor of the generated forces (P < 0.001) and moments (P < 0.001). The produced forces were different among all three bracket types, whereas the generated moments differed between conventional and lingual brackets but not between lingual bracket

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 mm²), 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

Biomechanical time dependency of the periodontal ligament: a combined experimental and numerical approach

SUMMARY The analysis of the non-linear and time-dependent viscoelasticity of the periodontal ligament (PDL) enables a better understanding of the biomechanical features of the key regulator tissue for tooth movement. This is of great significance in the field of orthodontics as targeted tooth movement remains still one of the main goals to accomplish. The investigation of biomechanical aspects of the PDL function, a difficult area of research, helps towards this direction. After analysing the time-dependent biomechanical properties of pig PDL specimens in an in vitro experimental study, it was possible to confirm that PDL has a viscoelastic anisotropic behaviour. Three-dimensional finite element models of mini-pig mandibular premolars with surrounding tissues were developed, based on micro-computed tomography (μCT) data of the experimental specimens. Tooth mobility was numerically analysed under the same force systems as used in the experiment. A bilinear material parameter set was assumed to simulate tooth displacements. The numerical force/displacement curves were fitted to the experimental curves by repeatedly calculating tooth displacements of 0.2mm varying the loading velocities and the parameters, which describe the nonlinearity. The experimental results showed a good agreement with the numerical calculations. Mean values of Young's moduli E1, E2 and ultimate strain ε12 were derived for the elastic behaviour of the PDL for all loading velocities. E1 and E2 values increased with increasing the velocity, while ε12 remained relatively stable. A bilinear approximation of material properties of the PDL is a suitable description of measured force/displacement diagrams. The numerical results can be used to describe mechanical processes, especially stress-strain distributions in the PDL, accurately. Further development of suitable modelling assumptions for the response of PDL under load would be instrumental to orthodontists and engineers for designing more predictable orthodontic force systems and appliance

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