Charles J. Burstone

Lip posture and its significance in treatment planning

SINCE malocclusion, tooth stability, and facial esthetics are influenced in part by the total mass, position in space, and general activity of the soft-tissue structures, the orthodontist is vitally concerned with soft-tissue morphology and the posture of the lips. The present article will consider the role and significance of lip posture in orthodontics, particularly as applied to trea,tment planning. Normally, two postural positions of the lips can be observed. In the relaxedlip position, the lips are relaxed, &part, and hanging loosely with no effort made at lip contraction. In the closed-lip position, the lips are lightly touching in order to produce an anterior seal of the oral cavity. The closed-lip position is characterized by minimal contraction in the effort to effect this anterior closure. In the Class II, Division I case in which t,here is a significant overjet, the closedlip position is interpreted a.s that position in which light contact exists between the lower lip and the maxillary incisor. As will be shown, a great deal of confusion can arise if one does not differentiate between the relaxed-lip and closed-lip positions in the evaluation of dental and facial a.bnormalities. For that reason, a detailed description of the relaxed-lip and closed-lip positions will be presented. Certainly, before an attempt is made to describe the more complicated types of lip a&i&y seen in swallowing, mastication, or speech, it, would seem advantageous to consider lhe role of lip posture in subjects with normal occlusion as well as those with malocclusion. The research on lip post,ure has been remaskably sparse and, with few exceptions, has been ignored by America.n investigators, who ha,ve been primarily interested in dentoskeletal variation. To be sure, Brodie’ and others hav*e alluded to the wrap-around muscle sheath as a restraint against forward migration of the dental arches. Furthermore, differences in lip posture in which the lower lip may lie either anterior or posterior to the maxillary incisors have been discussed as etiologic factors in the development of the Class II malocclusion. Schlossberg,2 employing an electromyographic technique, has gone one step further and haa attempted to analyze the muscle areas and their sequence of contraction as the

The integumental profile

Abstract Modern orthodontics implies not only occlusal excellence, but also the positioning of teeth to produce optimal facial harmony for the individual patient. The soft-tissue veneer covering the teeth and bone varies so greatly that study of the dentoskeletal pattern may be inadequate in evaluating facial disharmony. A method of direct integumental analysis is presented, employing angular readings that describe profile components to the skull as a whole (inclination angles) and to each other (contour angles). These readings are made from oriented lateral headplates exposed to show both hard- and soft-tissue detail. The average morphology and variation of acceptable profiles are described, based on the Herron sample (a group of good faces picked by a panel of artists). The hypothesis is explored that average inclination, contour, and proportion is related to profile excellence. Graphic comparison to the Herron sample by use of the integumental profile grid expedites the analysis of malocclusion deformity and the study of soft-tissue changes occurring during growth and treatment.

Three-dimensional finite element analysis for stress in the periodontal tissue by orthodontic forces

This study was designed to investigate the stress levels induced in the periodontal tissue by orthodontic forces using the three-dimensional finite element method. The three-dimensional finite element model of the lower first premolar was constructed on the basis of average anatomic morphology and consisted of 240 isoparametric elements. Principal stresses were determined at the root, alveolar bone, and periodontal ligament (PDL). In all loading cases for the buccolingually directed forces, three principal stresses in the PDL were very similar. At the surface of the root and the alveolar bone, large bending stresses acting almost in parallel to the root were generally observed. During tipping movement, stresses nonuniformly varied with a large difference from the cervix to the apex of the root. On the other hand, in case of movement approaching translation, the stresses induced were either tensile or compressive at all occlusogingival levels with some difference of the stress from the cervix to the apex. The pattern and magnitude of stresses in the periodontium from a given magnitude of force were markedly different, depending on the center of rotation of the tooth.

Mechanics of tooth movement

Orthodontic forces can be treated mathematically as vectors. When more than one force is applied to a tooth, the forces can be combined to determine a single overall resultant. Forces can also be divided into components in order to determine effects parallel and perpendicular to the occlusal plane, Frankfort horizontal, or the long axis of the tooth. Forces produce either translation (bodily movement), rotation, or a combination of translation and rotation, depending upon the relationship of the line of action of the force to the center of resistance of the tooth. The tendency to rotate is due to the moment of the force, which is equal to force magnitude multiplied by the perpendicular distance of the line of action to the center of resistance. The only force system that can produce pure rotation (a moment with no net force) is a couple, which is two equal and opposite, noncolinear but parallel forces. The movement of a tooth (or a set of teeth) can be described through the use of a center of rotation. The ratio between the net moment and net force on a tooth (M/F ratio) with reference to the center of resistance determines the center of rotation. Since most forces are applied at the bracket, it is necessary to compute equivalent force systems at the center of resistance in order to predict tooth movement. A graph of the M/F ratio plotted against the center of rotation illustrates the precision required for controlled tooth movement.

Holographic determination of centers of rotation produced by orthodontic forces

A new tool for measuring tooth movement--laser holography--offers an accurate, noninvasive approach for determining movement in three dimensions. This in vitro study is designed to establish the required force system applied on the crown of a maxillary incisor that would produce different centers of rotation, as in lingual tipping, translation, and root movement. The relationship between moment-to-force ratios and centers of rotation is shown. The experimental data are compared to theoretic approaches. With respect to the location of the center of resistance and centers of rotation, force systems needed to produce different centers of rotation are given for a central incisor of average root length.

Beta titanium: A new orthodontic alloy

Historically, few alloys have been used in the fabrication of orthodontic appliances. This article reviews the gold-based, stainless steel, chrome-cobalt-nickel, and nitinol alloys, as well as beta titanium, a new material for orthodontics. Mechanical properties and manipulative characteristics are summarized to develop a basis for the selection of the proper alloy for a given clinical situation. The beta titanium wire has a unique balance of low stiffness, high springback, formability, and weldability which indicates its use in a wide range of clinical applications. A number of such applications are described.

Chinese NiTi wire—A new orthodontic alloy

Chinese NiTi wire was studied by means of a bending test to determine wire stiffness, springback, and maximum bending moments. Chinese NiTi wire has an unusual deactivation curve (unlike steel and nitinol wires) in which relatively constant forces are produced over a long range of action. The characteristic flexural stiffness of NiTi wire is determined by the amount of activation. At large activations NiTi wires has a stiffness of only 7% that of a comparable stainless steel wire, and at small activations 28% of steel wire. For the same activation at large deflections, the forces produced are 36% that of a comparable nitinol wire. Chinese NiTi wire demonstrates phenomenal springback. It can be deflected 1.6 times as far as nitinol wire or 4.4 times as far as stainless steel wire without appreciable permanent deformation. NiTi wire is highly useful in clinical situations that require a low-stiffness wire with an extremely large springback.

The segmented arch approach to space closure

The clinical application of frictionless attraction springs using the segmented arch technique is described. Differential space closure is achieved by varying the force system between the anterior and posterior segments. A specially designed force transducer allowed accurate force and moment determination for each spring design. By duplicating predetermined spring geometries, the orthodontist can reproduce the required force system within narrow ranges. The most important considerations in the clinical use of attraction springs are the amount of distal activation, the angulation differential between the anterior and posterior teeth, and the centricity or eccentricity of the loop. Improvements in design have lead to a more efficient, hygienic, and comfortable mechanism for space closure.

Force systems from an ideal arch

T he force systems delivered from commonly used orthodontic appliances are relatively unknown. It is little wonder that unpredictable and many times undesirable tooth movement is produced during treatment. In the more sophisticated orthodontic appliances, the force system is produced totally or in part by placing a wire with a given configuration into a series of attachments (brackets, tubes, etc.) on the teeth. In an attempt to determine the force system, orthodontists in the past have used force gauges to measure the amount of force required to seat an arch wire in a bracket. IJnfortunately, this bit of information is inadequate to describe the force system completely in most clinical applications, since the situation is statically indeterminate; in other words, there are too many unknowns to calculate the forces from an appliance using the laws of statics. Clinically, such measurements represent little more than pseudoscience, since they incompletely describe the physical realities and, hence, will not predict the biologic response and the nature of the tooth movement to be expected. The purpose of this article is threefold: (1) to describe the force system which is produced when a straight wire is placed in a nonaligned bracket produced by a malocclusion ; (2) to develop the terminology and the approach to solve and describe force systems from all appliances; and (3) to offer a scientific basis for developing the orthodontic appliances of the future. To reach these objectives, the simplest clinical situation will be considered-the placing of a straight wire in two attachments on two teeth.

Variable-modulus orthodontics☆

Traditionally, orthodontists have varied the size of the wire in order to produce a range of light to heavy forces. A new approach to force control in presented which allows wire size to remain relatively constant and the material of the wire is selected on the basis of clinical requirements. When the material instead of the cross section is varied, superior orientation should be achieved with fewer wires during tooth alignment, and bracket-wire play becomes independent of the forces needed. Since wire stiffness is determined by wire corss section and material, a simplified numbering system is described which aids clinicians in evaluating any orthodontic wire.