Ideal orthodontic alignment load relationships based on periodontal ligament stress.

OBJECTIVES: To test the hypothesis that periodontal ligament (PDL) stress relationships that yield resistance numbers representing load proportions between different teeth depend on alignment load type. MATERIALS AND METHODS: Finite element models of all teeth, except the third molars, were produced. Four different types of loads were applied, and the third principal stresses of different teeth in standardized areas of most compression were calculated. Based on these results, resistance numbers, representing the load proportions for each tooth derived from PDL stress, were determined. RESULTS: The third principal stress values for typical alignment loads in the areas of most stress were very different for different load types for each tooth. Differences in resistance numbers between teeth also varied with different loads. CONCLUSION: Resistance numbers, that is, load proportion numbers between teeth to achieve similar stress at the compressive PDL zone, depend on the type of applied load.

The influence of force magnitude on intrusion of the maxillary segment.

The purpose of this study was to determine whether the magnitude of intrusive force to the maxillary incisors influences the rate of incisor intrusion or the axial inclination, extrusion, and narrowing of the buccal segments. Twenty patients between the ages of nine and 14 years who needed at least two mm of maxillary incisor intrusion were assigned to one of two equal groups. In group 1 patients, the teeth in the maxillary anterior segment were intruded using 40 g, whereas in group 2 patients, 80 g was used. Records were taken from each patient at the beginning and end of intrusion. There was no statistically significant difference between the 40- and 80-g groups in the rate of incisor intrusion, or the amount of axial inclination change, extrusion, and narrowing of the buccal segments.

The relation between the point of force application and flaring of the anterior segment.

The purpose of this study was to determine whether application of an intrusive force by an intrusion arch at the distal wings of the lateral incisor brackets causes a change in the axial inclination of the anterior segment. Maxillary incisor intrusion was performed, and records were taken from 40 adolescent patients at the beginning and end of intrusion. Intrusion of the maxillary anterior segment caused a statistically significant mean increase in axial inclination of the central incisor of 8.74 degrees. The following correlations were investigated and found not statistically significant. The correlation between the (1) distance from the point of force application to the center of resistance at the start of intrusion and the change in axial inclination of the incisor, (2) distance from the point of force application to the center of resistance at the start of intrusion and the change in distance from the incisal edge to the distal side of the first molar, (3) distance from the point of intrusive force application to the center of resistance at the start of intrusion and at the end of intrusion, (4) distance from the point of intrusive force application to the center of resistance at the start of intrusion and the change in this distance between start and end of intrusion, and (5) amount of intrusion and the change in axial inclination.

The role of a high pull headgear in counteracting side effects from intrusion of the maxillary anterior segment.

Intrusion of incisors is often the preferred treatment of a deep overbite. This study focuses on deep overbite correction by intrusion of maxillary incisors. The purpose of this study is to determine whether high-pull headgear wear can prevent steepening of the buccal segment, extrusion of the buccal segment, maintain arch width, and increase the rate of incisor intrusion. The number of patients needed for this study was calculated to be 20. Patients were between nine and 14 years of age and assigned to one of two groups. In each group, intrusion of maxillary incisors was performed. Patients in one group wore a high-pull headgear at night, and patients in the other group did not. For each patient, a lateral head film, impressions with a wax bite in centric occlusion, and intraoral photographs were taken at the beginning and end of intrusion. This study demonstrated that high-pull headgear had no effect on steepening and extrusion of the buccal segments or on the rate of intrusion but did have an effect on narrowing of the buccal segments. By performing intrusion as described in this study, no statistically significant side effects were observed in the buccal segments, whereas a statistically significant amount of incisor intrusion of 2.24 mm in the no-headgear group and 2.37 mm in the headgear group was observed.

Shear in flexure of fiber composites with different end supports.

The integrity of fiber-reinforced composite (FRC) prostheses is dependent, in part, on flexural rigidity. The object of this study was to determine if the flexure behavior of uniform FRC beams with restrained or simply supported ends and various length/depth (L/d) aspect ratios could be more accurately modeled by correcting for shear. Experimental results were compared with three analytical models. All models were accurate at high L/d ratios, but the shear-corrected model was accurate to the lowest, more clinically relevant, L/d values. In this range, more than 40% of the beam deflection was due to shear.

Bond strength of fiber-reinforced composite bars for orthodontic attachment.

Long fiber-reinforced composites (FRC) have been shown to have enhanced mechanical properties that allow their use in orthodontic appliances as bars that join teeth to form either anchorage or active units. This study was designed to determine if the bonding of an orthodontic attachment has sufficient strength to withstand loading during clinical use. The experimental model consisted of a hydroxyapatite stone that simulated enamel, FRC bars, and a bonded metal hook. Three specimen types were compared: (1) a metal hook-pad (the control), (2) a woven FRC with a hook-pad, and (3) a unidirectional FRC with a hook-pad. Loads were applied both parallel and at 90 degrees to the tooth surface. Under no condition was the FRC pad combination weaker than the control pad. Under some loading conditions, the loads before failure were as much as 3 times greater than those for the control. The lowest strength was found with loads at 90 degrees to the tooth surface for all 3 types. Failure normally occurred in the FRC and rarely at the bracket or tooth interface. The excellent bonding of the orthodontic attachment to the FRC and the high strengths of the FRC attachment combination demonstrate the ability to form connecting bars between teeth for either anchorage or active segmental movements. These bars offer advantages in simplicity in treatment by reducing the need for some bands, attachments, or wires.

Effect of root and bone morphology on the stress distribution in the periodontal ligament.

To achieve predictable and physiologic orthodontic tooth movement, estimating the axis of rotation of a tooth and the level and location of maximum stress distributed in the periodontal ligament is essential. An extracted upper canine was scanned into a computer 2-dimensionally and divided into 80 nodes along the long axis of the tooth. A mathematical formula was derived, and stress was calculated on each node. The purpose of this study was to reveal the center of resistance, axis of rotation, and an ideal force magnitude associated with various periodontal conditions, such as potential root resorption, alveolar bone loss, and varying anatomic root shape by analyzing the stress distribution in the periodontal ligament. The study demonstrates that the location of center of resistance changes significantly with variation of shape and length of the root embedded in alveolar bone. In contrast, in response to alveolar bone loss, the relative location of the center of resistance to total root length remains constant. Analysis of the stress distribution pattern in our 2-dimensional model reveals that the relationship between location of force and axis of rotation is determined by s(2) (that is) a constant depends on shape and length of a root in alveolar bone. Tapered and short roots that result from alveolar bone loss or apical root resorption are prone to tipping. The optimal orthodontic force may vary depending on the maximum stress in the periodontal ligament.

Force characteristics of nickel-titanium tension coil springs.

Nickel-titanium closed coil springs are commonly used for space closure. The springs possess a high resistance to permanent deformation and the potential for relatively constant force delivery. A study was designed to determine whether relatively constant forces can be delivered and whether the force magnitudes approach the manufacturer's targeted force values. Heavy, medium, and light springs were activated 15 mm at temperatures that ranged from 15 degrees C to 60 degrees C. The forces were measured during deactivation with a specially constructed force transducer temperature chamber. Relatively constant forces can be achieved with an over-activation procedure that allows relaxation to the desired activation. The light springs delivered forces that were near the targeted force; no difference was found between the heavy and medium springs in the constant force range. The force magnitudes varied markedly depending on mouth temperature.

Diagnosis and treatment planning of patients with asymmetries.

The diagnosis, treatment planning, and design of mechanics for the asymmetric patient requires the differentiation between problems of dental and skeletal origin. Although much information can be gleaned from a cephalometric analysis, the clinical examination and study models offer important clues in establishing the diagnosis of skeletal discrepancy. Abnormal and asymmetric axial inclinations can either produce a dental asymmetry or, if compensatory in nature, may mask an underlying skeletal problem. The role of axial inclination in diagnosis is applied to the following situations: subdivision cases, unilateral crossbites, midline discrepancies, arch form deviations, and frontal cants to the occlusal plane. The management of axial inclination asymmetries depends on the treatment plan. Nonextraction patients may require maintenance of asymmetric compensatory axial inclinations. Surgical and extraction patients can be treated to a more ideal symmetry.

Development and clinical applications of a light-polymerized fiber-reinforced composite.

STATEMENT OF PROBLEM: After 0 years of intermittent reports in the literature, the use of fiber reinforcement is just now experiencing rapid expansion in dentistry. PURPOSE: This article describes the development and use of a continuous, unidirectional fiber reinforced composite as a framework for the fabrication of fixed prostheses. METHODS: By using various matrix materials and fibers, a number of fiber-reinforced composite formulations were evaluated with the goal of creating a system with optimized mechanical properties and handling characteristics. Fiber-reinforced composite based on a light polymerized BIS-GMA matrix has been used clinically to make 2-phase prostheses comprised of an internal glass fiber-reinforced composite substructure covered by a particulate composite. The clinical and laboratory procedures required for the fabrication and use of reinforced composite fixed prostheses are described for laboratory-fabricated complete or partial coverage fixed prosthesis and chairside prosthesis. RESULTS: Although additional clinical experience is needed, fiber-reinforced composite materials can be used to make metal-free prostheses with excellent esthetic qualities.

T-loop position and anchorage control.

The effect of off-center positioning on the force system produced by segmented 0.017 x 0.025-inch TMA T-loops was measured. A T-loop was designed to produce equal and opposite moments in the centered position. The spring was tested in seven positions, centered, 1, 2, and 3 mm toward the anterior attachment, and 1, 2, and 3 mm toward the posterior attachments. The horizontal force, vertical force, and alpha and beta moments were measured over 6 mm of spring activation. The results showed that the alpha/beta moment ratio was dependent only on the spring position, and independent of spring activation. Eccentric positioning of T-loop springs effectively produces a consistent moment differential through the range of spring activation.

Class II subdivision treatment with tip-back moments.

Unilateral tip-back mechanics are challenging because of a number of undesirable side-effects associated with their use during orthodontic treatment. The purpose of this paper is to review the differential diagnosis and treatment planning of Class II subdivision malocclusions and present a treatment strategy based on a careful biomechanical analysis of the clinical situation. Emphasis is placed on the correction of molar axial inclination using unilateral tip-back moments in the treatment of dental asymmetries. Appliance design and treatment sequencing are also discussed.

Simultaneous intrusion and retraction using a three-piece base arch.

Flared incisors and deep overbite are challenging to treat orthodontically. This paper describes the use of a three-piece base arch and Class I elastics to correct deep overbite while simultaneously closing spaces. An analysis of the biomechanics and a discussion of the appliance design are presented to help understand how the incisor axial inclination can be corrected and controlled during orthodontic therapy. A clinical example illustrates the treatment sequence.

Segmented approach to simultaneous intrusion and space closure: biomechanics of the three-piece base arch appliance.

Deep overbite correction and space closure in patients with flared incisors are mechanically difficult to achieve with conventional orthodontic treatment. The purpose of this article is to present an appliance design that allows simultaneous intrusion and retraction of anterior teeth as well as correction of their axial inclinations. A three-piece base arch was used to achieve simultaneous intrusion and space closure. Various clinical situations are discussed and analyzed from a biomechanical standpoint. Sequences of treatment, appliance design, and management of side effects are described in detail. The segmented approach to simultaneous intrusion and space closure is useful for achieving precise control of tooth movements in the anteroposterior and vertical dimensions.

Screening of matrices and fibers for reinforced thermoplastics intended for dental applications.

Plastics reinforced with continuous fibers (FRC) are being developed for dental applications, such as prosthodontic frameworks and orthodontic retainers. Flexure properties, stress relaxation and hydrolytic stability of FRC based on six thermoplastic matrices, three types of fibers, and three fiber volume fractions were evaluated. Samples with clinically relevant dimensions were tested. Polycarbonate was the preferred matrix material. Polycarbonate reinforced with 42 volume percent glass fibers exhibited the highest combination of flexure modulus (17.9 +/- 2.6 GPa), flexure strength (426 +/- 40 MPa), reinforcing efficiency (0.79), and resistance to stress relaxation. No statistically significant difference was observed between E and S2 glass reinforced composites under the experimental conditions used. Kevlar reinforced materials exhibited a low flexure modulus and strength. The apparent flexure moduli of all composites decreased with span length in the range of clinical interest. Generally, the prevalent mode of failure for all FRC investigated was brittle failure under flexure loading. Relatively large sample-to-sample variation in both composition and properties indicated that improved fabrication methods will be needed in future studies. The combination of good flexure properties, formability, and translucency suggests that novel appliance designs for dentistry are feasible with FRC, but further studies of its properties and particularly the effects of fiber/matrix interfacial quality are needed.

Longitudinal clinical evaluation of fiber-reinforced composite fixed partial dentures: a pilot study.

This report describes a clinical pilot study that monitored a group of 12 patients who have received 14 single tooth replacement experimental restorations made with prefabricated continuous fiber-reinforced composite (FRC) frameworks. Because these restorations represent a purely adhesive restorative system, tooth preparation was not performed. The Kaplan-Meier survival probability at 12 months was approximately 50%. The restoration with the longest service life was a mandibular molar replacement that has remained in service 24 months. With improved survival times, bonded FRC definitive restorations should be plausible.

The use of continuous fiber reinforcement in dentistry.

Fiber-reinforced composite (FRC) formulations were developed to serve as structural components for various dental appliances such as prosthodontic frameworks, retainers and splints. Poly(ethylene terephthalate glycol) and poly(1,4-cyclohexylene dimethylene terephthalate glycol) reinforced with continuous S-2 glass fibers were pultruded into continuous lengths with small rectangular cross sections. The microstructure was evaluated with SEM and optical microscopy. Fiber content and flexure properties were measured and compared to previous results by other authors. The present FRC contained 43-45 volume % fiber, which compared favorably with the 5-15 volume % fiber reported by all earlier investigators of dental FRC. The present materials achieved 65% of the theoretically expected modulus, in contrast to the typical value of 40% calculated in the earlier reports. The flexural strength and modulus of the experimental FRC were approximately 565 MPa and 20 GPa, respectively. The present FRC can be formed into individualized devices, and free fibers need not be manipulated by the operator. The improved properties and handling justify further study of these FRC as structural dental materials.

Patterns of initial tooth displacements associated with various root lengths and alveolar bone heights.

The present study was designed to investigate the nature of initial tooth displacements associated with varying root lengths and alveolar bone heights. A three-dimensional model of the upper central incisor was developed for the finite element analysis. Tooth displacements were determined at various levels of the tooth and the apicogingival levels of the center of resistance and centers of rotation were calculated. The results showed that moment-to-force values at the bracket level for translation of a tooth decreased with shorter root length and increased with lower alveolar bone height. In addition, apicogingival levels of the center of resistance shifted more gingivally to the cervix, or the alveolar crest with a shorter root. Alveolar bone loss also shifted the center of resistance toward the alveolar crest, whereas its position was more apical relative to the alveolar bone heights exhibited very slight changes in both cases. The centers of rotation from a single force varied substantially with a short root and alveolar bone loss. However, the relative distances of the centers of rotation from the alveolar crest in comparison with the alveolar bone heights were constant at 0.4 mm, with variations in the root length and alveolar bone height. Because this study showed that root length and alveolar bone height affect the patterns of initial tooth displacements both in the center of resistance and the centers of rotation and also in the amount of displacement, forces applied during orthodontic treatment should take into consideration the anatomic variations in the root length and alveolar bone height so as to produce optimal and desired tooth movement.