Bonded retainers--clinical reliability.

Bonded retainers have become a very important retention appliance in orthodontic treatment. They are popular because they are considered reliable, independent of patient cooperation, highly efficient, easy to fabricate, and almost invisible. Of these traits, reliability is the subject of this clinical study. A total of 549 patients with retainers were analyzed with regard to wearing time, extension of the retainer, mean time between failures, operator, and age of patient. The average frequency of breakage or loss was 0.55 per retainer per year. This frequency was dependent primarily on the operator who bonded the retainer and on the extent of the retainer. If the upper canines were involved, reliability was lower. The majority of failures occurred during the first 3 to 6 months. The study showed that bonded retainers represent a highly efficient and reliable retention appliance suited to long-term use.

A formula for the displacement of an arch wire when subjected to a second-order couple.

A new formula, expressing the local angular displacement of an orthodontic beam when subjected to a second-order couple applied at midspan, has been developed and analyzed. The computed displacements were compared with the results of ex vivo testing. There was good agreement between the results from the theoretical evaluation and the bench testing. Second-order activation of an orthodontic beam can be described in four sequential phases. The initial displacement is influenced by the second-order clearance between bracket-slot and wire as well as the relationship between the bracket-slot width and interbracket distance. During phase II there is a nonlinear relationship between applied couple and rotational displacement. Within phases three and four displacement is linearly related to the interbracket distance, provided the relationship between the bracket-slot width and interbracket distance remains constant. For a given tooth size, the second-order beam stiffness is exponentially related to bracket width. The experiments also show that even small deflections of thin stainless steel wires can lead to second-order couples of large magnitudes when using a clinically relevant interbracket distance. Consequently, it is important that the orthodontist evaluates his or her choice of bracket width and arch wire stiffness in each clinical case in order to avoid supra-physiologic force levels.

Alveolar and skeletal dimensions associated with overbite.

The aim of this study was to investigate whether in the maxilla and in the mandible the structure of the anterior medial sagittal alveolar and basal bone is related to the overbite. A total of 460 untreated adult subjects were divided into four groups with either deep bite, normal overbite, end-to-end bite, or open bite and were compared. The overbite, lower face height, and anterior alveolar and basal midsagittal cross-sectional areas from the maxilla and the mandible were assessed on lateral cephalometric radiographs. An index was calculated, dividing the sagittal by the vertical dimension of the midsagittal cross-sectional area. A deeper bite coincided with smaller lower face height, larger alveolar and basal areas, and a more widened shape of the symphysis. If the lower face height was introduced as a covariable, the open bite group showed significantly smaller maxillary and mandibular alveolar and basal cross-sectional areas compared with the end-to-end group, the normal overbite group, or the deep bite group. Vertical variation of the overbite probably coincides with a relative hyperdevelopment or hypodevelopment of the symphysis.

Alveolar and skeletal dimensions associated with lower face height.

In this study, the relationships between the lower face height and the structure of the frontal alveolar and basal bone were investigated. The areas and the dimensions of the anterior alveolar and basal midsagittal cross-sectional bone from the maxilla and the mandible were recorded on lateral cephalograms from 460 untreated adults. An index was calculated dividing the sagittal by the vertical dimension of the midsagittal cross-sectional area. The subjects with a normal overbite between 0.5 and 4 mm (N=165) were divided into three groups according to the lower face height. A larger lower face height coincided with a larger maxillary alveolar and basal area and with a smaller mandibular alveolar index. Correlations between the lower face height and the maxillary alveolar index and the mandibular alveolar and basal area were low. It is concluded that long-faced subjects have a large mandibular alveolar height, which is more associated with a narrowed shape than with a large volume of the symphysis.

Superelastic materials displaying different force levels within one archwire.

Recently archwire materials have become available which are claimed to be capable of exerting different force levels within one arch. Such orthodontic archwires are of importance because the forces they exert are supposed to be better adapted to different physiological force levels as required by teeth with different root surface areas. Undesired side effects such as root resorptions and pain should thus be minimized. The purpose of this study was to test these types of wires and to evaluate whether and, if so, to what degree the claimed properties are present. Load-deflection curves of 6 superelastic nickel-titanium orthodontic wires by 4 manufacturers were recorded using a simple beam bending test at mouth temperature. Activation-deactivation cycles with varying maximum deflection were achieved using a computer controlled stepper motor. Each archwire was tested at 4 different points on the wire. The resulting forces were evaluated metrically and graphically. Great differences were found between the superelastic properties claimed by the manufacturers and the observations of this study and also between the different products. Only 2 of the tested wires showed superelastic properties on a force level that seems adapted to the needs of the teeth to be moved.

The effect of friction on the bending stiffness of orthodontic beams: a theoretical and in vitro study.

The purpose of this study was to investigate the effect of friction on the bending stiffness of orthodontic beams. A theoretical and experimental model have been established where tensile and compressive forces are applied to an arch wire to simulate the effect of additional friction during activation and deactivation, respectively. The results show that tensile force increases wire stiffness, and that compressive force increases flexibility. Thus more force will be needed during activation and more force will be lost during deactivation. The amount of force lost increases nearly linearly with increasing friction. During activation, the percentage increase in force due to friction for a given deflection is about equal to the loss of force due to friction during deactivation. Friction affects thin flexible wires more than heavy wires. Careful ligation is recommended in the leveling phase to reduce the negative side effects of friction.

Properties of superelastic wires and their relevance to orthodontic treatment.

In this study tools were developed to test and compare levelling archwires that the manufacturers claim to have superelastic properties. As the classic spring model and Hook's law cannot be applied, new parameters had to be found. It could be shown that three parameters are necessary to describe a superelastic archwire adequately: the distinctiveness of the pseudo-elastic plateau, the deflection at the beginning of the plateau, and the force level of the plateau. The results showed that many materials either did not show any pseudoelastic properties at all or that the wire parameters were such that they did not give any advantage over conventional work hardened NiTi materials. In many archwires the beginning of the plateau and thereby the desired characteristics began only when the archwire was displaced 1 mm or more. For many archwires the force level of this plateau also proved to be rather high with values often above 500 g.

[Improvement in the adhesive strength of orthodontic brackets on unit-cast and fired dental alloys by microsandblasting]. / Verbesserung der Haftfestigkeit orthodontischer Brackets auf dentalen Vollguss- und Aufbrennlegierungen durch Mikrosandstrahlen.

This experimental study employed an Instron test machine to measure the bond strength of metal brackets to cast test specimens made of 6 different dental alloys. An easily replicated method for testing the shear bond strength is presented, which corresponds to the international recommendations for testing bond strength of orthodontic brackets. The study found that the different methods of surface preparation, such as micro-sandblasting and the use of chemical sealants, had distinct effects on the magnitude of bond strength gauged. Also tested was whether the storage medium prior to debonding influenced the bond strength. The measured values from a series of tested bovine teeth, which were bonded by means of the conventional acid etch method and with the same adhesive, served as a control group. An extended statistical analysis showed that the use of the methods described can achieve a clinically acceptable bond of brackets to dental alloys for prosthetic restorations.

[The force level of pseudoelastic leveling wires in relation to interbracket distance]. / Kraftniveau pseudoelastischer Nivellierungsdrähte in Abhängigkeit vom Interbracketabstand.

One of the arguments for the introduction of pseudo-elastic wire materials was "the way they work biologically", that is to say, on the one hand small and uniform force over an extended period of time and on the other the avoidance of excessive forces during the leveling phase. Since an evaluation of the operative force of such an arch wire is dependent on temperature, stress-strain curve, and method of activation, and because the extent of the activation is no longer directly proportional to the operative force, the result is that it is often difficult to determine clinically, whether a safe maximum force is not being exceeded. To clarify this issue arch wires were tested in a three bracket test simulating to the greatest degree possible a clinical situation. The inter-bracket distance was selected in accordance with the situation in the upper and lower incisor region independent of the width of the bracket. The results showed rather high forces for some of the wires. Often these forces were higher in the mandibular situation due to non-ideal force deflection curves than in the supramaxilla. One of the tested materials did, however, effectively limit the operative force in the upper and lower jaw. Especially significant is that the differing characteristics of the tested wire materials result in a completely different action in relation to force development.

[Long-term results after genioplasty]. / Langzeitergebnisse nach Genioplastik.

The serial records of 21 patients were analyzed to study the stability of genioplasty performed in the course of orthognatic surgery and to describe its effect on the soft tissue profile. A pre-operative and a post-operative cephalogram as well as a cephalogram one year after surgery of each patient were available. Sagital correction of the chin from 16.5 mm advancement to 1.1 mm reduction and vertical movements between 4.8 mm lengthening and 9.3 mm shortening remained nearly unchanged during the control period. No post-operative movements of the chin fragment were observed except for minor resorptions that can be attributed to an osseous remodelling and rounding of sharp edges. In cases of a long face syndrome a bone apposition from 1 mm to 5.5 mm appeared at the lower edge of the chin. The average of sagittal soft tissue change in relation to the correction of the bony chin was 71%. The individual values ranged from 4% to 145% and the standard error of the estimate was 3 mm. Therefore the planning of the soft tissue profile is rather unreliable. A genioplasty performed with rigid fixation by compression screws or mini plates, preservation of vascular supply and re-fixation of the soft tissue of the chin results in accurately predictable and stable bony contours. On the other hand, the planning of surgery procedures related to the soft tissue profile is--if at all possible--very insecure.

The use of visible light-curing composites in bonding ceramic brackets.

Three hundred and fifty bovine teeth were bonded with a ceramic bracket using nine different materials. Five of the materials were composites used in conservative dentistry, two were light-curing orthodontic adhesive test materials, and two were chemically curing adhesives. Four of the materials were modified by dilution with resin. With the proper consistency, it was found that the use of primer was not necessary. Adequate strength was observed within a 20-second curing time. No statistical difference in bond strength was observed among the materials. It is concluded that light-curing adhesives can offer several advantages in the bonding of ceramic brackets.

[The determination of the optimal individual position of the mandibular anterior teeth]. / Bestimmung der optimalen individuellen Position der Unterkieferfrontzähne.

Determining the individually optimal position of the lower incisors is one of the important factors in diagnosis and treatment planning. The study compares the accuracy of a number of methods postulated by different authors. It clearly shows that regression models display the highest accuracy in predicting the optimal incisor position. The most important variables in the prognosis are ANB angle and the prominence of the chin as measured by the distance of pogonion to NB line. The best regression model is presented as a formula and in a graphical representation.

Floating norms as a means to describe individual skeletal patterns.

An adequate analysis of the skeletal pattern is essential since a considerable amount of dental compensation can mask the severity of a malocclusion. Disagreement prevails over the standards that should be used to judge the individual values. The paper describes a method of analyzing the skeletal pattern of a patient on an individualized basis. Rather than relating the individual cephalometric values to population means the method employs standards derived from the individual facial type (floating norms). The associations between the basic cephalometric variables can be derived from a box-like graphical representation that is based on the correlations between the different variables. It is thereby an improvement of the 'box' that has been used in the Bergen Technique.

Shear bond strength of metal brackets compared with a new ceramic bracket.

One hundred twenty bovine teeth were bonded with two types of metal brackets and a new ceramic bracket for comparison. Two different adhesives were used, a so-called no-mix and a paste/paste adhesive. The shear bond strength of the ceramic bracket was found to be superior for both adhesives. Bond failure with the ceramic bracket occurred predominantly in the enamel/adhesive interface; the failure site for the metal bracket was mainly in the bracket/adhesive interface. It is concluded that the bond strength between the ceramic bracket and the adhesive in shear mode is stronger than that between the adhesive and the enamel.