The effects of a periodontal ligament analogue on occlusal contact forces.

BACKGROUND: Previous bench-top studies examined the details of the mechanical environment of rigidly fixed occluding teeth. It was demonstrated that during each chomp, contacting molar teeth experience in-occlusal-plane forces (Flateral ) that are highly transient in magnitude and direction. OBJECTIVES: The objectives of this study are to identify Flateral behaviours that are attributable to the presence of a visco-elastic periodontal ligament (PDL) analogue, and to assess the necessity of incorporating it into future studies. METHODS: A weighted maxillary molar denture tooth was lowered onto, and raised from, a matching mandibular molar 10 times. The latter was supported by a load cell that continuously measured Flateral . For statistical purposes, the test was repeated with 21 (n = 21) different occlusal relationships obtained with 0.05 mm incremental shifts of the lower assembly. RESULTS: Overall, the results are similar to those of rigid attachment but the details of the Flateral profiles are very different. CONCLUSION: The PDL plays a major role in the mechanical environment of occlusion, suggesting that, in general (not necessarily always), it should be integrated into studies of occlusion.

The combined effects of salivas and occlusal indicators on occlusal contact forces.

BACKGROUND: Some occlusal detection products are designed for use on dry teeth, but this is not always achieved. Others are suited for dry and wet applications. OBJECTIVE: The objective of this study is to assess the combined effects, on occlusal contact forces, of two previously studied affecting variables-occlusal detection products and saliva. METHODS: We used a full-arch dentiform with three occlusal detection products (an articulating film, an articulation paper and T-Scan) in combination with human (HS) and an artificial saliva. The maxillary arch assembly, weighing ~54 N (the maximum bite force), was lowered onto (occlusion) and lifted off (disclusion) of the mandibular arch through 10 cycles by a mechanical testing machine. The forces and moments acting on the mandibular arch were continuously recorded by a load cell that supported it. RESULTS: The maximum values of Flateral (the in-occlusal plane component of the occlusal contact force) were analysed by occlusion/disclusion separately using one-way ANOVA, with factor for group type to identify the significant effect of salivas on products, effect of products, effect of salivas with products, effect of human saliva. A difference in occlusion and/or in disclusion was considered different. Statistical differences (P < 0.0001) in Flateral were found in: dry product vs product + HS, dry product vs product + artificial saliva (with articulating film and T-Scan) and HS vs product + HS (with articulation paper and T-Scan). CONCLUSION: All products were affected by the salivas, except articulation paper by artificial saliva.

The mechanics of dental occlusion and disclusion.

BACKGROUND: The mechanical environment associated with occluding teeth is the foundation for a wide range of research topics, clinical practice and dogma, product development and marketing, and medico-legal issues. The purpose of this study was to experimentally examine the relationships between occlusal factors and their impact on the associated contact forces. METHODS: Matching pairs of 0°, 20°, 33° and 40° cusp first molar denture teeth were placed into Class I, II and III molar relationships. As the teeth were brought together into occlusion and then separated, the loads experienced by the mandibular tooth were continuously measured by a load cell that was supporting it. FINDINGS: Loess smoothing splines were fitted to all data curves to calculate 95% confidence intervals. All lateral force magnitudes and directions were statistically different when compared between classes of occlusion, cusp angles, and occlusion vs. disclusion. Noteworthy counterintuitive observations were that the lateral contact force components were generally higher during disclusion than occlusion, the peak lateral force magnitudes did not always occur when the occlusal force was maximum, and the lateral contact force component magnitude can be larger than the occlusal force. INTERPRETATION: Wedging and friction account for these unexpected results. The data indicate that each occlusion/disclusion cycle is characterized by complex transient loads that may impact wear facet and non-carious cervical lesion formation, implant and restorative failures, various aspects of occlusal trauma, and the concept of axial occlusal loading.

Biology of biomechanics: Finite element analysis of a statically determinate system to rotate the occlusal plane for correction of a skeletal Class III open-bite malocclusion.

INTRODUCTION: In the absence of adequate animal or in-vitro models, the biomechanics of human malocclusion must be studied indirectly. Finite element analysis (FEA) is emerging as a clinical technology to assist in diagnosis, treatment planning, and retrospective analysis. The hypothesis tested is that instantaneous FEA can retrospectively simulate long-term mandibular arch retraction and occlusal plane rotation for the correction of a skeletal Class III malocclusion. METHODS: Seventeen published case reports were selected of patients treated with statically determinate mechanics using posterior mandible or infrazygomatic crest bone screw anchorage to retract the mandibular arch. Two-dimensional measurements were made for incisor and molar movements, mandibular arch rotation, and retraction relative to the maxillary arch. A patient with cone-beam computed tomography imaging was selected for a retrospective FEA. RESULTS: The mean age for the sample was 23.3 ± 3.3 years; there were 7 men and 10 women. Mean incisor movements were 3.35 ± 1.55 mm of retraction and 2.18 ± 2.51 mm of extrusion. Corresponding molar movements were retractions of 4.85 ± 1.78 mm and intrusions of 0.85 ± 2.22 mm. Retraction of the mandibular arch relative to the maxillary arch was 4.88 ± 1.41 mm. Mean posterior rotation of the mandibular arch was -5.76° ± 4.77° (counterclockwise). The mean treatment time (n = 16) was 36.2 ± 15.3 months. Bone screws in the posterior mandibular region were more efficient for intruding molars and decreasing the vertical dimension of the occlusion to close an open bite. The full-cusp, skeletal Class III patient selected for FEA was treated to an American Board of Orthodontics Cast-Radiograph Evaluation score of 24 points in about 36 months by en-masse retraction and posterior rotation of the mandibular arch: the bilateral load on the mandibular segment was about 200 cN. The mandibular arch was retracted by about 5 mm, posterior rotation was about 16.5°, and molar intrusion was about 3 mm. There was a 4° decrease in the mandibular plane angle to close the skeletal open bite. Retrospective sequential iterations (FEA animation) simulated the clinical response, as documented with longitudinal cephalometrics. The level of periodontal ligament stress was relatively uniform (<5 kPa) for all teeth in the mandibular arch segment. CONCLUSIONS: En-masse retraction of the mandibular arch is efficient for conservatively treating a skeletal Class III malocclusion. Posterior mandibular anchorage causes intrusion of the molars to close the vertical dimension of the occlusion and the mandibular plane angle. Instantaneous FEA as modeled here could be used to reasonably predict the clinical results of an applied load.

Microdamage generation by tapered and cylindrical mini-screw implants after pilot drilling.

OBJECTIVE: To investigate the relationship between mini-screw implant (MSI) diameter (1.6 vs 2.0 mm) and shape (tapered vs cylindrical) and the amount of microdamage generated during insertion. MATERIALS AND METHODS: Thirty-six cylindrical and 36 tapered MSIs, 6 mm long, were used in this study. Half of each shape was 1.6 mm in diameter, while the other half was 2.0 mm. After pilot drilling, four and five MSIs were inserted, respectively, into fresh cadaveric maxillae and mandibles of dogs. Bone blocks containing the MSIs were sectioned and ground parallel to the MSI axis. Epifluorescent microscopy was used to measure overall cortical thickness, crack length, and crack number adjacent to the MSI. Crack density and total microdamage burden per surface length were calculated. Three-way analysis of variance (ANOVA) was used to test the effects of jaw, and MSI shape and diameter. Pairwise comparisons were made to control the overall significance level at 5%. RESULTS: The larger (2.0 vs 1.6 mm) cylindrical MSIs increased the numbers, lengths, and densities of microcracks, and the total microdamage burden. The same diameter cylindrical and tapered MSIs generated a similar number of cracks and crack lengths. More total microdamage burden was created by the 2.0-mm cylindrical than the 2.0-mm tapered MSIs. Although higher crack densities were produced by the insertion of 1.6-mm tapered MSIs, there was no difference in total microdamage burden induced by 1.6-mm tapered and 1.6-mm cylindrical MSIs. CONCLUSIONS: Pilot drilling is effective in reducing microdamage during insertion of tapered MSIs. To prevent excessive microdamage, large diameter and cylindrical MSIs should be avoided.

Comparison of stainless steel and titanium alloy orthodontic miniscrew implants: a mechanical and histologic analysis.

INTRODUCTION: The detailed mechanical and histologic properties of stainless steel miniscrew implants used for temporary orthodontic anchorage have not been assessed. Thus, the purpose of this study was to compare them with identically sized titanium alloy miniscrew implants. METHODS: Forty-eight stainless steel and 48 titanium alloy miniscrew implants were inserted into the tibias of 12 rabbits. Insertion torque and primary stability were recorded. One hundred grams of tensile force was applied between half of the implants in each group, resulting in 4 subgroups of 24 specimens each. Fluorochrome labeling was administered at weeks 4 and 5. When the rabbits were euthanized at 6 weeks, stability and removal torque were measured in half (ie, 12 specimens) of each of the 4 subgroups. Microdamage burden and bone-to-implant contact ratio were quantified in the other 12 specimens in each subgroup. Mixed model analysis of variance was used for statistical analysis. RESULTS: All implants were stable at insertion and after 6 weeks. The only significant difference was the higher (9%) insertion torque for stainless steel. No significant differences were found between stainless steel and titanium alloy miniscrew implants in microdamage burden and bone-to-implant contact regardless of loading status. CONCLUSIONS: Stainless steel and titanium alloy miniscrew implants provide the same mechanical stability and similar histologic responses, suggesting that both are suitable for immediate orthodontic clinical loads.

An analytical approach to 3D orthodontic load systems.

OBJECTIVE: To present and demonstrate a pseudo three-dimensional (3D) analytical approach for the characterization of orthodontic load (force and moment) systems. MATERIALS AND METHODS: Previously measured 3D load systems were evaluated and compared using the traditional two-dimensional (2D) plane approach and the newly proposed vector method. RESULTS: Although both methods demonstrated that the loop designs were not ideal for translatory space closure, they did so for entirely different and conflicting reasons. CONCLUSIONS: The traditional 2D approach to the analysis of 3D load systems is flawed, but the established 2D orthodontic concepts can be substantially preserved and adapted to 3D with the use of a modified coordinate system that is aligned with the desired tooth translation.

Effects of first- and second-order gable bends on the orthodontic load systems produced by T-loop archwires.

OBJECTIVE: To measure the effects of first- and second-order gable bends on the forces and moments produced by a commercially available closing T-loop archwire. MATERIALS AND METHODS: A dentoform-simulated space closure case was mounted on an orthodontic force tester. Sixteen gable bend combinations were placed in the archwires, which were then activated using standard clinical procedures. At each activation, the three force components and three moment components on the maxillary left lateral incisor and canine were simultaneously measured. RESULTS: The first- and second-order gable bends showed low load coupling effects when used independently, but the load systems became unpredictable when bends were combined. Gable bends affect the magnitudes and directions of the forces and moments that are applied to teeth. The resulting moment to force ratios are sensitive to the bends. CONCLUSION: Gable bends alter the orthodontic load systems; however, the three-dimensional interactions produce complex and unpredictable tradeoffs.

The effects of application time of a self-etching primer and debonding methods on bracket bond strength.

OBJECTIVE: To test the manufacturer's recommendation for the application rubbing time of a self-etching primer (Transbond Plus, 3M Unitek) and to compare the resulting bond strength of a resin composite (Transbond XT, 3M Unitek) in the traditional laboratory tension on all four wings with a simulation of the clinical single-wing lift-off debonding instrument (LODI; 3M Unitek). MATERIALS AND METHODS: Flattened stainless-steel maxillary incisor orthodontic brackets (Victory Series, 3M Unitek) were bonded to 108 flattened bovine incisors. The enamel was rubbed with the self-etching primer for 0, 5 (the manufacturer's recommendation), and 10 seconds during a 10-second application. Traditional four-wing and LODI simulated debonding forces and the adhesive remnant index (ARI) were recorded. RESULTS: One-way analysis of variance testing among rubbing times and debonding methods indicated a significant difference in strength with 0 and 5 seconds of rubbing and between traditional and LODI simulated tension. The bond strengths were higher in the ARI  =  1 subset compared to the ARI  =  3-5 subsets. CONCLUSIONS: The manufacturer's recommendation for primer rubbing time produced the highest bond strength. Less force is required for debonding when tension is applied to one wing (LODI simulation) vs on all four wings.

A qualitative engineering analysis of occlusion effects on mandibular fracture repair mechanics.

Objectives. The purpose of this analytical study was to examine and critique the engineering foundations of commonly accepted biomechanical principles of mandible fracture repair. Materials and Methods. Basic principles of static equilibrium were applied to intact and plated mandibles, but instead of the traditional lever forces, the mandibles were subjected to more realistic occlusal forces. Results. These loading conditions produced stress distributions within the intact mandible that were very different and more complex than the customary lever-based gradient. The analyses also demonstrated the entirely different mechanical environments within intact and plated mandibles. Conclusions. Because the loading and geometry of the lever-idealized mandible is incomplete, the associated widely accepted bone stress distribution (tension on top and compression on the bottom) should not be assumed. Furthermore, the stress gradients within the bone of an intact mandible should not be extrapolated to the mechanical environment within the plated regions of a fractured mandible.

The effect of pH levels on nonlatex vs latex interarch elastics.

OBJECTIVE: To evaluate the force decay characteristics of nonlatex vs latex interarch elastics within the normal range of salivary pH levels. MATERIALS AND METHODS: Two nonlatex groups and one latex quasi-control group were tested. Elastics were stretched to 15 mm and were held for 10 seconds (baseline), 4 hours, 8 hours, and 12 hours in artificial saliva solutions with pH levels of 5.0, 6.0, and 7.5. Force magnitudes were measured at 25 mm of activation. Each specimen was used once. Measurements were assessed using three-way analysis of variance (ANOVA). RESULTS: The three-way interaction between group, pH, and time was not significant (P = .13); the group-by-pH interaction also was not significant (P = .70). However, pH-by-time (P = .0179) and group-by-time (P = .0001) interactions were significant. American Orthodontics nonlatex generated significantly higher loads than Auradonics nonlatex. American Orthodontics nonlatex produced significantly higher forces than American Orthodontics latex at 4, 8, and 12 hours, but not at 10 seconds. American Orthodontics latex was significantly stronger than Auradonics nonlatex at 10 seconds, but not at 4, 8, and 12 hours. CONCLUSIONS: No clinically significant correlation between pH and force decay was observed.

Does the center of resistance depend on the direction of tooth movement?

INTRODUCTION: The objective of this study was to compare the locations of the centers of resistance (CRes) in the buccolingual (BL) and mesiodistal (MD) directions of the mandibular central incisors of 6 dogs. METHODS: Based on microcomputed tomography images of the teeth and their supporting tissues, solid models were used to build finite element models. RESULTS: The CRes locations for the 6 specimens were determined for displacements in the BL and MD directions with finite element calculations. Measured from the alveolar crest, the BL and MD locations were 43% to 51% and 31% to 43% of root length, respectively. Their average locations, 46.2% and 38.3%, were statistically different at the 95% CI. CONCLUSIONS: The CRes location for BL tooth movement is significantly more apical than its MD counterpart.

An engineering analysis of dental occlusion principles.

INTRODUCTION: The purpose of this study was to develop an analytical model of contacting teeth, based on principles of basic engineering statics. The model would be used to demonstrate the interactions between occlusal contacts and tooth loading (forces and moments) and to critique occlusion-related dogma. METHODS: Free-body diagrams were drawn to depict 2 teeth in occlusal contact. In combination with the concept of the center of resistance, the governing equilibrium equations were derived and used to solve for the forces and moments on the teeth and to investigate the influences of tripod and cusp-fossa occlusal schemes. RESULTS: With a specific load on a tooth, it was demonstrated that the load on the opposing tooth and the concomitant occlusal scheme dictated crown-crown contact forces that can be computed. CONCLUSIONS: This engineering analysis suggests flaws in widely held notions about the mechanics of occlusion. Loading that is generally considered clinically desirable is certain to produce undesirable loading on the opposing tooth. The complex relationships between the loads on teeth and crown-crown occlusal contacts make it virtually impossible to control tooth loading with occlusal equilibration. For computational and conceptual reasons, it is essential to consider the center of resistance.

Orthodontic mechanotransduction and the role of the P2X7 receptor.

INTRODUCTION: The P2X7 receptor plays a crucial role in bone biology and inflammation. Its main function is to promote necrotic tissue metabolism by ensuring a normal acute-phase inflammatory response. We used a mouse model to describe and compare orthodontic mechanotransduction in wild-type and P2X7 knock-out mice. METHODS: By using finite element analysis, mouse orthodontic mechanics were scaled to produce typical human stress levels. External root resorption, bone modeling, and bone remodeling were analyzed with fluorescent bone labels, Masson trichrome stain, and microcomputed tomography. Relationships between the biologic responses and the calculated stresses were statistically tested and compared between mouse types. RESULTS: There were direct relationships between certain stress magnitudes and root resorption and bone formation. Hyalinization and root and bone resorption were different in the 2 types of mice. CONCLUSIONS: Orthodontic responses are related to the principal stress patterns in the periodontal ligament, and the P2X7 receptor plays a significant role in their mechanotransduction.

Comparison of the Bolton Standards to longitudinal cephalograms superimposed on the occipital condyle (I-point).

OBJECTIVE: To compare traditional superimposition on sella turcica and the anterior cranial base (SACB) to superimposition referenced at the occipital condyle (I-point) for demonstrating craniofacial growth and development. MATERIALS AND METHODS: Tracings for ages 8, 10, 12, 15 and 18 were chosen from the Bolton Standards of Dentofacial Developmental Growth to compare superimposition with the traditional longitudinal reference at SACB (anterior curvature of sella turcica and anterior cranial base) to reference at I-point on the antero-inferior contour of the occipital condyles in norma lateralis. The serial tracings were superimposed using both the sagittal and postero-anterior (PA) tracings. Incremental growth of landmarks was measured in relation to Cartesian coordinates and compared between the superimposition methods. RESULTS: Sagittal and PA tracing superimpositions displayed an average 7 mm greater cephalad movement of landmarks, an average 2.4 mm greater ventral movement, and comparable transverse dimension with superimposition referenced at I-point as compared to the SACB reference. CONCLUSION: I-point superimposition demonstrates physiologic growth patterns concealed by traditional registration at sella turcica. The evolution of superimposition on SACB was based on convenience and reproducibility. Fundamental principles of bone development are consistent with the occipital condyles as a more biologic reference for relative craniofacial growth. Actual vertical growth is believed to be greater than displayed in this study, due to the cross-sectional and blended nature of the sample.

Force system generated by an adjustable molar root movement mechanism.

INTRODUCTION: Tooth movement simulation is important for planning the optimal force system and appliance design to correct a specific malocclusion. Experimental verification of a 3-dimensional force system is described for a unique molar root movement strategy that can be adapted to many clinical scenarios. METHODS: The force system was measured for molar root movement springs that had adjustable alpha (anterior) and beta (posterior) moments. A 3-dimensional transducer assessed moments and forces in 3 planes during deactivation and simulated molar rotation. Two experimental situations were compared by using 10 springs in each group: spring reactivation was performed to compensate for changes in the force system caused by molar movement, or there was no reactivation. RESULTS: Without reactivation, the force system becomes unfavorable after approximately 5 degrees of molar movement (rotation). With reactivations, a favorable force system through 20 degrees of molar movement is maintained. CONCLUSIONS: Present root-movement appliances require periodic adjustment to achieve optimal tooth movement. Additional studies are needed to design orthodontic appliances for delivering optimal force systems for the entire range of tooth movement.

Tooth positioner effects on occlusal contacts and treatment outcomes.

OBJECTIVE: To determine if an increase in tooth contacts is the principal effect of tooth positioner wear. MATERIALS AND METHODS: Patient charts from a consecutive series were reviewed until a sample of 100 cases that used a tooth positioner was obtained. One hundred control cases were randomly selected from patients treated at the same period. Malocclusion severity and finished occlusion were assessed with the American Board of Orthodontics (ABO) Discrepancy Index (DI) and Objective Grading System (OGS) score, respectively. Finish casts for each patient were mounted on a Galleti articulator. Occlusal registrations were obtained with silicone-based impression material from casts fabricated from impressions taken at the time of fixed appliance removal (control) or at the end of the tooth positioner treatment (experimental). The number of the perforations and transparent areas on the occlusal registrations were quantified. RESULTS: There was no significant difference (P = .20) in the number of total occlusal contacts between the two groups. However, the OGS score of the tooth positioner group (16.7) was significantly (P = .0009) better than for the control group (19.9). CONCLUSIONS: Tooth positioners were effective in improving the occlusal finish, but the effects were independent of an increase in occlusal contacts. Positioners primarily improved first order alignment by tipping teeth into an improved intercuspation.

Three-dimensional mechanical environment of orthodontic tooth movement and root resorption.

INTRODUCTION: The tension-compression theory of bone mechanotransduction is ubiquitous in orthodontics. However, partly due to deficiencies in the characterization of the mechanical environment, there is no consensus on the mechanisms that link stimuli to root resorption and bone response. In this study, we analyzed the predominant directions of tension and compression in the alveolar structures. METHODS: An idealized tooth model was constructed with computer-aided design for finite element stress analysis. The principal stress magnitudes and directions were calculated in tipping and translation. RESULTS: The highest principal stress magnitudes in the root, periodontal ligament (PDL), and alveolar surface occurred predominantly in the longitudinal, radial, and hoop directions, respectively. On the compression side, the only structure consistently in compression in all directions was the PDL; however, magnitudes were different in different directions. CONCLUSIONS: In the same region of root, PDL, and bone, there can be compression in 1 structure and tension in another. At a given point in a structure, compression and tension can coexist in different directions. Magnitudes of compression and tension are typically different in different directions. Because of direction swaps between principal stresses, previously published data of only stress magnitude plots can be confusing and perhaps impossible to understand or correlate with biological responses. To prevent ambiguities, a reference to a principal stress should include not only the structure, but also its predominant direction. Combined stress magnitude and direction results suggest that the PDL is the initiator of mechanotransduction.

Complete orthodontic load systems on teeth in a continuous full archwire: the role of triangular loop position.

INTRODUCTION: A novel approach to characterizing orthodontic spring-generated force and moment systems has been developed. This method allows simultaneous measurement of all 6 force and moment components acting on a tooth. METHODS: A continuous full archwire space-closure technique was simulated, and the complete force and moment systems acting on the teeth adjacent to the extraction space were measured. RESULTS AND CONCLUSIONS: The data showed that, in addition to the intended forces and moments, there are nontrivial activation-dependent interactions with the other load components, and these complex relationships are affected by the position of the triangular loop.

Flaws in root resorption assessment algorithms: role of tooth shape.

INTRODUCTION: The purposes of this study were to analyze and critique the mathematical algorithms used in the clinical assessment of root resorption. METHODS: Geometric constructions and derived mathematical expressions were used to investigate the influence of tooth shape on the relationships between tooth, root, and crown lengths and their respective sizes as measured on before and after radiographs. results: The ambiguities of root-loss assessment protocols are demonstrated. CONCLUSIONS: Root resorption assessment algorithms are unsuited for their intended applications.