Axes of resistance for tooth movement: does the center of resistance exist in 3-dimensional space?

INTRODUCTION: The center of resistance is considered the most important reference point for tooth movement. It is often stated that forces through this point will result in tooth translation. The purpose of this article is to report the results of numeric experiments testing the hypothesis that centers of resistance do not exist in space as 3-dimensional points, primarily because of the geometric asymmetry of the periodontal ligament. As an alternative theory, we propose that, for an arbitrary tooth, translation references can be determined by 2-dimensional projection intersections of 3-dimensional axes of resistance. METHODS: Finite element analyses were conducted on a maxillary first molar model to determine the position of the axes of rotation generated by 3-dimensional couples. Translation tests were performed to compare tooth movement by using different combinations of axes of resistance as references. RESULTS: The couple-generated axes of rotation did not intersect in 3 dimensions; therefore, they do not determine a 3-dimensional center of resistance. Translation was obtained by using projection intersections of the 2 axes of resistance perpendicular to the force direction. CONCLUSIONS: Three-dimensional axes of resistance, or their 2-dimensional projection intersections, should be used to plan movement of an arbitrary tooth. Clinical approximations to a small 3-dimensional "center of resistance volume" might be adequate in nearly symmetric periodontal ligament cases.

Polyphenylene polymers as esthetic orthodontic archwires.

INTRODUCTION: There is continuing interest in an esthetic, effective labial archwire. In this study, we evaluated the potential of new, high-strength polyphenylene polymers to fill this need. METHODS: Polyphenylene (Primospire, Solvay Advanced Polymers, Alpharetta, Ga) polymer was extruded into wires with clinically relevant round and rectangular cross sections. Tensile, flexure, spring-back, stress-relaxation, and formability characteristics were assessed. Arch forms and secondary shapes were formed. RESULTS: Smooth wires with consistent cross-sectional dimensions, high spring-back, and good ductility were produced. Forces delivered were generally similar to typical beta-titanium and nickel-titanium wires of somewhat smaller cross sections. The polyphenylene wire did experience stress relaxation for up to 75 hours. The force magnitudes place polyphenylene wires in the category of an alignment or leveling wire. High formability allowed shape bending similar to that associated with stainless steel wires. CONCLUSIONS: Polyphenylene polymers could serve as esthetic orthodontic archwires; further study is warranted.

Viscoelastic properties of an aesthetic translucent orthodontic wire.

The objective of this study was to evaluate the time-dependent viscoelastic properties of an aesthetic orthodontic archwire. The wire is based on a recently developed translucent polyphenylene thermoplastic, whose rigid molecular structure provides high strength. While the wire has good instantaneous mechanical properties, over time all polymers may relax so it is important to understand the potential impact of the relaxation on orthodontic force systems. Four samples of 0.020 inch round and six samples of 0.021 × 0.025 inch rectangular wire were loaded in tension to a range of initial stresses, and relaxation of the stress was monitored for 7 days. Sixty-three additional samples were maintained in edgewise bracket pairs with vertical displacement for up to 6 weeks. The deformation of these wires was measured immediately after removal from the brackets and for 2 days as the samples recovered. Tensile stress decayed about 10-30 per cent over 24-48 hours depending on the initial stress. The relaxation behaviour was proportional to the initial tensile strain and therefore these data were combined into a single curve using regression. Deformation of the samples placed in the bracket pairs increased with increasing vertical displacement and time, evaluated with analysis of variance, but 19-100 per cent of the deformation was recoverable. The force systems from polyphenylene wires could vary with time and activation, but this behaviour is predictable.

Miniscrews in orthodontic treatment: review and analysis of published clinical trials.

INTRODUCTION: A systematic review of effects related to patient, screw, surgery, and loading on the stability of miniscrews was conducted. METHODS: Reports of clinical trials published before September 2007 with at least 30 miniscrews were reviewed. Parameters examined were patient sex and age, location and method of screw placement, screw length and diameter, time, and amount of loading. RESULTS: Fourteen clinical trials included 452 patients and 1519 screws. The mean overall success rate was 83.8% + or - 7.4%. Patient sex showed no significant differences. In terms of age, 1 of 5 studies with patients over 30 years of age showed a significant difference (P <0.05). Screw diameters of 1 to 1.1 mm yielded significantly lower success rates than those of 1.5 to 2.3 mm. One study reported significantly lower success rates for 6-mm vs 8-mm long miniscrews (72% vs 90%). Screw placement with or without a surgical flap showed contradictory results between studies. Three studies showed significantly higher success rates for maxillary than for mandibular screws. Loading and healing period were not significant in the miniscrews' success rates. CONCLUSIONS: All 14 articles described success rates sufficient for orthodontic treatment. Placement protocols varied markedly. Screws under 8 mm in length and 1.2 mm in diameter should be avoided. Immediate or early loading up to 200 cN was adequate and showed no significant influence on screw stability.

Sagittal and vertical load-deflection and permanent deformation of transpalatal arches connected with palatal implants: an in-vitro study.

INTRODUCTION: The purposes of this laboratory investigation were to (1) measure the sagittal and vertical deflection of loaded transpalatal arches (TPAs) connected to a palatal implant, (2) measure the extent of permanent deformation of the connecting TPA in the sagittal and vertical directions, (3) test various wire dimensions in terms of deflection behavior, and (4) evaluate soldering vs laser welding vs adhesive bonding of TPAs in terms of load deflection behavior. METHODS: Stainless steel wires of 6 dimensions were tested: 0.8 x 0.8, 0.9, 1, 1.1, 1.2, and 1.2 x 1.2 mm. For each dimension, 10 specimens were soldered to the palatal implant abutment, 10 were laser welded, and 10 were adhesively bonded to the implant abutment (total, 180 specimens). The measuring device applied increments of force of 50 cN, from 0 to 500 cN. Then the specimens were unloaded. The values were statistically described and analyzed with ANOVA and Wilcoxon rank sum tests. RESULTS AND CONCLUSIONS: Absolute orthodontic anchorage without deformation of TPAs was not observed with the wire dimensions tested. To prevent loss of anchorage greater than 370 mum (sagittal deflection of 1.2 x 1.2 mm adhesively bonded TPA at 500 cN force level), wires thicker than 1.2 x 1.2 mm or cast anchorage elements must be considered for clinical practice. However, larger cross sections might cause more patient discomfort, and laboratory procedures increase costs.

Influence of buccal segment size on prevention of side effects from incisor intrusion.

INTRODUCTION: Deep overbite can be corrected by maxillary incisor intrusion. The purpose of this study was to determine whether the size of the maxillary buccal segment influences the amount of steepening, extrusion, or narrowing of the buccal segments, or the rate of intrusion that occurs with maxillary incisor intrusion. METHODS: Twenty patients, 9 to 14 years of age, seeking treatment at a private practice, were divided into 2 groups. Patients in the long buccal-segment group had maxillary buccal segments that included the canines, both premolars, and the first molars. In the short buccal-segment group, the buccal segments consisted of only the maxillary first molars. Patient records were taken at the beginning and end of maxillary incisor intrusion. RESULTS: Intermolar width increased slightly in the short buccal-segment group and decreased slightly in the long buccal-segment group. More steepening of the buccal segment occurred in the short buccal-segment group, and more proclination of the anterior segment in the long buccal-segment group. The size of the buccal segment had no influence on the rate of incisor intrusion or on the amount of buccal-segment extrusion. In both groups, the mean amount of incisor intrusion exceeded 2 mm. CONCLUSIONS: A buccal segment that extends from canine to first molar will help minimize the side effects of incisor intrusion.

Controlled space closure with a statically determinate retraction system.

We designed a variant of a cantilever spring, the statically determinate retraction system, and studied its mechanical characteristics. This novel system consisted of a single-force cantilever arm made of 0.017 x 0.025-inch titanium molybdenum alloy wire for active retraction and a passive rigid stabilizing unit. Since the active component for space closure is a cantilever, it is simple to measure the force system of the spring with a force gauge (ie, the system is a statically determinate system). A torque tester apparatus was used to examine the property of this retraction spring with a helix at the posterior and a simple bend at the anterior. Both a standard shape and modified shapes of the spring were studied. At full activation, the standard spring delivered 163 g with a load-deflection rate of six g/mm. When the magnitude of the anterior bend of the spring was increased, the horizontal component of the force increased more than the vertical component. In contrast, when the posterior bend of the spring increased, the vertical component of the force increased more than the horizontal component. A clinical case presented here clearly demonstrates the versatility and applicability of the spring.

Initial changes of centres of rotation of the anterior segment in response to horizontal forces.

This study investigated the changes in the initial centres of rotation (Crot) of the upper six anterior teeth in response to a horizontal load. Six upper anterior teeth were extracted, splinted as a unit, and embedded in dental stone after the roots were uniformly coated with silicone. An aluminium fixture was bonded to the anterior segment and three linear variable differential transformers (LVDTs) were attached to measure the microdisplacement of the segment. A pulley and dead weight assembly were used to apply a 200 g occluso-gingivally varying horizontal force to the segment. The changes in the Crot for the anterior segment to the horizontal load were recorded. The results showed that the centre of resistance (Cres) of the upper anterior segment was located 14.5 mm apical and 9.5 mm distal from the incisal edge of the central incisors. A linear functional axis (a trace of the measured Crot) was recorded. The functional axis maintained an angle of 14.5 degrees to the vertical axis of the anterior segment passing through the Cres of the segment. The Crot constant, which determines the tipping sensitivity of the segment, was 23 mm(2). The results demonstrate that the upper anterior segment may be slightly intruded when a horizontal force is applied and is less prone to tipping than a single tooth.

Mecânica do movimento dentário / Mechanics of tooth movement

As forças ortodônticas podem ser expressas matematicamente como vetores. Quando mais de uma força é aplicada a um dente, estas forças podem ser combinadas, determinando uma única resultante. Por outro lado, as forças também podem ser decompostas, possibilitando a análise individual dos seus componentes vertical e horizontal em relaçäo ao plano oclusal, ao plano horizontal de Frankfort ou ao longo eixo do dente. As forças produzem translaçäo (movimento de corpo), rotaçäo, ou uma combinaçäo de translaçäo e rotaçäo, dependendo da relaçäo da linha de açäo da força com o centro de resistência do dente. A tendência de rotaçäo é atribuída ao momento da força, que é calculado multiplicando-se a magnitude da força pela distância perpendicular da linha de açäo da força ao centro de resistência. O único sistema de força capaz de produzir rotaçäo pura (um momento com uma força resultante final igual a zero) é um binário, definido como duas forças de mesma magnitude, aplicadas em direçöes opostas, näo colineares, porém paralelas. O movimento de um dente (ou um grupo de dentes) pode ser melhor definido em relaçäo a um centro de rotaçäo. A proporçäo entre o momento e a força em um dente (M/F), tendo como referência o centro de resistência, determina o centro de rotaçäo. Visto que a maioria das forças é aplicada no bráquete, torna-se necessário considerar sistemas de forças equivalentes no centro de resistência para predizer o movimento do dente. Um gráfico da proporçäo M/F, relacionado com o centro de rotaçäo, ilustra a precisäo necessária para o controle do movimento dentário