Loretta K. Rubenstein

Tooth diameters and arch perimeters in a black and a white population

This study was undertaken to test the hypothesis that a sample of black patients will have larger mesiodistal tooth diameters and larger dental arch perimeters than a corresponding sample of white patients. In this study, the black samples mean canine, first and second premolar, and first molar mesiodistal diameters were significantly larger than those of the white sample. The dental arches of the black patients were significantly wider and deeper but did not show significantly more crowding. Gender and race differences did exist, but gender differences were controlled by sampling procedures. The black sample also had a larger mean MP-SN angle but this was not accompanied by the increased crowding and the narrower dental arches that had been reported associated with high-angle white samples.

Increase in arch perimeter due to orthodontic expansion

A mathematical model was developed to compare quantitatively the effects of various types of orthodontic expansion on mandibular arch perimeter. Mandibular arch form was modeled with spline interpolation to fit a smooth curve between assigned molar, canine, and incisor positions. Starting with average arch dimensions, intermolar width, intercanine width, and midline arch length were increased individually and in combination in millimeter increments up to 5 mm, and the consequent changes in arch perimeter were measured. Increasing midline arch length by incisor advancement was nearly four times as effective in increasing arch perimeter as was molar expansion; canine expansion had an intermediate effect. Arch perimeter increments increased slightly with successive amounts of expansion for the molar, canine, and incisor. Combinations of molar-canine and canine-incisor expansion yielded results comparable to the total effects achieved by expansion of those teeth individually. Combined molar-canine expansion created increases in arch perimeter that were only slightly less than those generated by incisor advancement alone.

Lower arch perimeter preservation using the lingual arch

The purpose of this investigation was to determine whether the placement of a mandibular lingual arch maintained arch perimeter in the transition from the mixed to the permanent dentition, and if so, whether it was effective at preventing mesial migration of first permanent molars, or whether this migration still occurred en masse, by increased lower incisor proclination. Thirty patients were randomly assigned to either a treatment group (N = 14, mean age = 11.5 years) or a control group (N = 16, mean age = 11.3 years). Study models, cephalograms, and tomograms of the patients, taken at the beginning and at the end of the study period, were examined. Statistically significant differences between groups were found for positional changes of mandibular first molars and incisors, and changes in arch dimensions. The results indicate that the lingual arch can help reduce arch perimeter loss, but at the expense of slight mandibular incisor proclination.

Arch length considerations due to the curve of Spee: A mathematical model

Arch length analysis should consider discrepancies not only within the sagittal plane but also within the vertical and transverse planes. The vertical deviation of the occlusal plane from a flat plane is known as the curve of Spee. The purpose of this study was to produce a mathematical model of the mandibular arch form in three planes of space and to determine the effect that the curve of Spee has on arch circumference. Two mandibular arch forms, the catenary and the Bonwill-Hawley, were examined. The curve of Spee was modeled as a cylinder perpendicular to the midsagittal plane centered on the arch anteroposteriorly. A mathematical distance formula was used to calculate arch circumferences from the central fossa of the first molars for 10 arches with curves of Spee ranging from 0 to 10 mm. This procedure was repeated for arch circumferences extending from the central fossa of the second molars. Plots for the difference in arch circumferences verses depth of the curve of Spee showed that the relationship between these two variables is not linear and is less than one to one. This model showed that clinical practice of allowing 1 mm of arch circumference for leveling each millimeter of curve of Spee overestimates the amount of arch circumference needed to flatten the curve of Spee.

Moments with the edgewise appliance: incisor torque control.

Traditional edgewise orthodontic mechanics are significantly limited in their ability to provide incisor torque control because of the limitations of bracket-to-bracket mechanics and the poorly defined reciprocal actions inherently produced. Attempts to address this issue clinically have been largely empirical. The science of mechanics dictates that all incisor torque control mechanisms must act through one of two basic principles: the moment of a couple or the moment of a force. The torquing arch is a modification of the traditional edgewise system and employs the moment of a couple to achieve incisor torque control and precise definition of reciprocal effects. The torquing arch force system includes a large moment to rotate incisors in a crown facial/root lingual direction, and concurrent equilibrium forces to extrude incisors and intrude molars. Alternatively, the base arch uses the moment of a force to also rotate incisors in a crown facial/root lingual direction. The base arch, however, includes a large moment to rotate molars in a crown distal/root mesial direction, and concurrent equilibrium forces to intrude incisors and extrude molars. Depending on how they are employed, torquing arches and base arches may also rotate molars in a faciolingual direction, enhance or diminish posterior anchorage, and increase or conserve arch perimeter. Contemporary quality orthodontic care requires an awareness and control of all of the forces created by orthodontic appliances.

Influence of vertical growth pattern on faciolingual inclinations and treatment mechanics.

Control of faciolingual tooth inclinations is obtained by using rectangular wire with third-order bends and standard edgewise brackets or by using a straight wire in a preadjusted appliance system. Ideal faciolingual inclinations have been determined previously by measuring facial surface contours relative to coronal long axes. This study evaluates faciolingual inclinations based on occlusal table inclinations relative to occlusal planes. The samples compared include untreated ideal occlusions and malocclusions in three different vertical skeletal growth patterns. Faciolingual inclinations of first molars and central incisors were measured relative to the occlusal plane and to selected cephalometric angular measurements. Statistical comparisons between groups revealed significant differences in the inclinations of the upper incisor relative to the occlusal plane (U1-OP) and the inclination of the occlusal plane relative to sella nasion (OP-SN). No statistically significant intergroup differences were found in the inclination of the lower incisor relative to the occlusal plane (L1-OP) or in the faciolingual inclinations of the maxillary and mandibular first molars. On the basis of the large intergroup differences in the mean angle between the occlusal plane and sella nasion (OP-SN), the use of straight-wire appliance therapy is discussed in terms of the potential for creating differential moments. Because differential moments may facilitate or hinder treatment goals, the practitioner must know the biomechanical sequelae resulting from occlusal plane-sella nasion variations, which differ from normative values when preadjusted brackets are used.

Activating a 2×4 appliance

Abstract When the long span of wire in a 2×4 orthodontic appliance enters a bracket in a nonparallel manner, it develops a couple and tendency for rotation called a moment. The moment of a couple creates inherent equal and opposite Newtonian equilibrium forces not readily sensed clinically. Moments at successive brackets, producing rotations in opposite directions, create equilibrium forces also in opposite directions which are subtractive from each other. When these equilibrium forces are equal and opposite, they cancel each other out. If the moments produce rotations in the same direction, the equilibrium forces are also in the same direction and are additive. The apparent simplicity of the 2×4 appliance conceals the fact that it is a powerful orthodontic tool that uses engineering mechanics in a way not possible with fully bracketed appliances. Clearly the application of engineering mechanics is a distinguishing characteristic of modern orthodontics.

Ceramic bracket fracture resistance to second order arch wire activations

The fracture resistance of ceramic brackets to orthodontic activations has been incorrectly estimated by previous investigations that have reported second order loads in terms of force magnitudes rather than moments. Because force magnitudes alone do not reflect the influence of distance from the site of force application on total load, it is impossible to apply previous results to actual clinical situations. The purpose of this study was to determine the average moments (in gram-millimeters) necessary to fracture various ceramic brackets subjected to second order tipping activations and compare them with actual clinical loads. Central and lateral incisor ceramic brackets from seven manufacturers were subjected to mesial-distal tipping arch wire activations at two speeds of load application with a testing apparatus designed for that purpose. Significant differences in fracturability among the brackets of various manufacturers and between central and lateral incisor brackets were found. There were no differences related to the speed of load application. Once the influence of bracket width was considered, the differences in fracture resistance between central and lateral incisor brackets were no longer apparent. Second order activations required to fracture the ceramic brackets in this study were all much greater than measured clinical orthodontic loads. It is unlikely that second order arch wire activations are a significant cause of ceramic bracket failure.

Quantitation of rotational movements associated with surgical mandibular advancement.

Surgical mandibular advancement can be used to correct an anteroposterior and/or vertical malocclusion. The procedure of choice is often the bilateral sagittal split osteotomy (BSSO). By varying the amount of presurgical overbite correction, the rotational movement of the distal segment of the osteotomy can be controlled. Consequently, the malocclusion and the resultant vertical and anteroposterior facial form changes are predictably planned to produce both the desired occlusion as well as the optimal esthetic facial result. Opening rotation of the distal segment elongates the lower face height by varying amounts depending on the nature of the rotation. The amount and type of rotation can be determined and quantified by the technique presented in this paper, which is based on a geometric theorem used to determine the kinematic center of rotation of an object. This technique can shorten treatment time and produce more predictable results. The specific applications are: 1) treatment planning for individual patients, 2) uniform analyses of treatments and grouping of treatment types, and 3) development of more accurate computerized treatment planning programs.

Case Report: Four permanent second molar extractions

A patient who had earlier undergone four second molar extractions and who demonstrated Class II buccal segments and major incisor crowding was treated with the relatively unusual extraction pattern of maxillary lateral incisors. The treatment resolved the crowding and created Class II buccal segments with maxillary canines substituted for extracted lateral incisors. Favorable tooth anatomy produced an acceptable esthetic result accomplished with a predictable outcome over a reasonable duration of treatment. The sequential panoramic radiographs show evidence of the third molars spontaneously progressing toward positions that may result in their successful substitution for the earlier extracted second molars. A relatively unorthodox treatment plan made possible resolution of a difficult existing clinical situation using relatively routine treatment procedures.