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Objective: To test the frictional resistance forces (FRS) generated between several archwires and (1) interactive self-ligating (ISL) brackets and (2) conventionally ligated (CL) brackets. Materials and Methods: Frictional forces produced between three different archwire combinations and self-ligating (SL) brackets (ceramic and metal-slot or all-metal) and CL brackets (metal or ceramic) were evaluated in a dry environment. The three ISL brackets tested were In-Ovation-C, In-Ovation-R, and Damon 3. The three CL brackets were Mystique with Neo Clip, Clarity, and Ovation. Each bracket was tested with 0.020″ SS, 0.019″ × 0.025″ SS and 0.018″ × 0.018″ coated SS. Results: The ISL brackets generally exhibited the lowest frictional forces irrespective of the bracket material and the wire size, and CL brackets exhibited consistently higher frictional forces. Mystique with Neo Clip produced the lowest frictional resistance of all brackets. The In-Ovation-C brackets demonstrated significantly lower frictional resistance than the SL brackets In-Ovation-R and Damon 3 as well as the CL brackets Clarity and Ovation. Conclusions: The ISL ceramic brackets produced the lowest frictional resistance of all the self-ligating brackets. The CL ceramic brackets produced the greatest friction.

Understanding mechanisms of action for orthopedic appliances is critical for orthodontists who hope to treat and retain the achieved corrections in patients with initial Class II mandibular retrognathism. That knowledge can help orthodontists produce clinically significant bone formation and avoid compression at the condyle-glenoid fossa region. It also assists us to understand the differences between short-term and long-term treatment results. It was previously thought that increased activity in the postural masticatory muscles was the key to promoting condyle-glenoid fossa growth. By analyzing results from several studies, we postulate that growth modification is associated with decreased activity, which leads to our nonmuscular hypothesis. This premise has its foundation on 3 key specific findings: significant glenoid fossa bone formation occurs during treatment that includes mandibular displacement; glenoid fossa modification is a result of the stretch forces of the retrodiskal tissues, capsule, and altered flow of viscous synovium; observations that glenoid fossa bone formation takes place a distance from the soft tissue attachment. The latter observation is explained by transduction or referral of forces. Evidence is presented, therefore, that the 3 trigger switches for glenoid fossa growth can similarly initiate short-term condylar growth modifications because the 2 structures are contiguous. These are displacement, several direct viscoelastic connections, and transduction of forces. Histologic evidence further shows that stretched retrodiskal tissues also insert directly into the condylar head's fibrocartilaginous layer. The impact of the viscoelastic tissues may be highly significant and should be considered along with the standard skeletal, dental, neuromuscular, and age factors that influence condyle-glenoid fossa growth with orthopedic advancement. These biodynamic factors are also capable of reversing effects of treatment on mandibular growth direction, size, and morphology. Relapse occurs as a result of release of the condyle and ensuing compression against the newly proliferated retrodiskal tissues together with the reactivation of muscle activity. To describe condyle-glenoid fossa growth modification, an analogy is made to a light bulb on a dimmer switch. The condyle illuminates in treatment, dims down in the retention period, to near base levels over the long-term.

As the frequency of use of the interactive twin (I-twin) edgewise mechanisms and techniques become increasingly prevalent, it is important to consider how they compare with conventional twin (c-twin) edgewise bracket systems. Optimum intrabracket and interbracket forces in I-twins balance with capillary blood pressures. An unbiased, experimental in vitro, scanning electromicroscopy and in vivo clinical investigation of 83 patients was conducted to compare the frictional resistance of three different I-twin bracket systems, type A (Sigma, American Orthodontics), type B (Interactwin, Ormco Corp.), and type C (Damon, A-company) with three c-twins counterparts respectively types D, E, and F. The three interactive twin brackets were each self-seating by an integrated arm component and made significant incremental improvements to the conventional twins in three different ways. First, the I-twin types reduced frictional resistance by using arm engagement with a lower coefficient of friction and a reduced seating force against the arch wire. The reduced seating force friction produced initially small interbracket arch wire deflections for biocompatible tooth movement and, combined with a wide twin bracket, produced accurate rotation corrections. Reduced friction within the I-twin bracket consequently permitted the effective use of light forces for flowing biomechanics that maximized anchorage. In evaluating friction, two distinctly different interaction forces acting to seat the arch wire were also identified. Type A demonstrated active interaction with round arch wires that resulted in a low functional seating force responsible for early and complete tooth control in comparison to the high seating force of c-twins. Types B and C both showed passive interaction with seating force friction approximately equal to zero that required large rectangular dimension arch wires for full bracket expression. Second, a significant reduction in the time taken to change arch wires was found, improving clinical time management. Third, I-twins assisted bracket placement with both center-slot identification markers and coordinated bracket and bonding pad reference planes in three dimensions. In addition, they improved bracket hygiene compared with c-twins by minimizing the use of plaque-retentive conventional ligatures. Despite the reduced elastomer use, experimental and clinical results showed I-twins effectively conserved the traditional four tie-wing design for ease of colored elastomer placement where an increase in friction for anchorage was selectively required and to enhance patient acceptance and motivation. Finally, interactive twins could be used by the practitioner as conventional twins without the interactive arms making them fail-safe.

A study was undertaken to determine the contribution of glenoid fossa modification in the correction of skeletal Class II malocclusions. Individually corrected lateral tomograms of 35 patients (18 boys and 17 girls) between the ages of 7.96 and 15.06 years were used. The subjects were selected on the basis of the criteria that they were suitable for treatment with a mandibular protrusive appliance (activator). The mean duration of activator treatment was 1.33 years. Tomograms were taken on a pre- and posttreatment basis. The tomograms were scanned and digitized on screen using a common software program. Points were located on each tomogram and specific linear measurements were used to evaluate any changes in glenoid fossa morphology. Paired t-tests were applied for the left and right sides separately and between the before and after treatment measurements. Paired t-tests were also applied between the left and right sides to test for any difference between the sides. The results of this study demonstrated that there are no positive radiographically depictable contributions from glenoid fossa modification for the correction of skeletal Class II malocclusions treated with mandibular protrusive appliances (activators). In addition, no statistically significant differences were found between right and left sides denoting relative symmetry. It is concluded that, contrary to animal research and magnetic resonance findings, it seems possible that glenoid fossa modeling is not induced by mandibular protrusive appliances during treatment of skeletal Class II problems.

Herbst appliances were activated progressively in growing nonhuman primates, and the results were compared with primate and human controls. The methods and materials of this research are explained in Part 1 of this study. The results are discussed here in Part 2. All experimental subjects developed large super Class I malocclusions, the result of many factors including posterior movement of the maxilla and the maxillary teeth, an increased horizontal component of condylar growth, and anterior displacement of the mandible and the mandibular teeth. The growth modification measured in the glenoid fossa was in an inferior and anterior direction. Restriction of the downward and backward growth of the fossa observed in the control subjects might additionally contribute to the overall super Class I malocclusion. Clinically, these combined effects could be significant at the fossa. The restriction of local temporal bone (fossa) growth cannot be observed clinically; thus, these results might also clarify some Class II correction effects that cannot be explained with functional appliances. Differences in the area and maximum thickness of new bone formation in the glenoid fossa and in condylar growth were statistically significant. The bony changes in the condyle and the glenoid fossa were correlated with decreased postural electromyographic activity during the experimental period. Results from permanently implanted electromyographic sensors demonstrated that lateral pterygoid muscle hyperactivity was not associated with condyle-glenoid fossa growth modification with functional appliances, and that other factors, such as reciprocal stretch forces and subsequent transduction along the fibrocartilage between the displaced condyle and fossa, might play a more significant role in new bone formation. These results support the growth relativity concept.

Changes in the condyle, the glenoid fossa, and the muscles of mastication were investigated in subjects undergoing continuous orthopedic advancement of the mandible with a Herbst-block appliance. The total sample consisted of 56 subjects and included 15 nonhuman primates (in the middle mixed, early permanent, and permanent dentitions), 17 human Herbst patients in the early permanent dentition, and 24 human controls from the Burlington Growth Center. The 8 nonhuman primates in the middle mixed dentition were the focus of this study. Mandibular advancement was obtained progressively in 5 animals by adding stops to the telescopic arms of fixed functional Herbst appliances with occlusal coverage; activations of 5.0 mm, 7.0 mm, and 8.0 mm were achieved. Two primates served as controls, and the third was a sham control. Two experimental animals and the 2 controls also wore surgically implanted electromyographic electrodes in the superior and inferior heads of the lateral pterygoid muscles and in the superficial masseter and anterior digastric muscles. Changes in condylar growth direction and amount were assessed with the Bjork method from measurements made on serial cephalometric tracings superimposed on metallic implants. Undecalcified sections, treated with intravenous tetracycline vital staining, were viewed with fluorescence microscopy to examine histologic changes in the condyle and the glenoid fossa. New bone formation in the fossa associated with continuous mandibular protrusion was quantified by using computerized histomorphometric analysis of decalcified histological sections and polarized light. The unique combination of permanently implanted electromyographic electrodes, tetracycline vital staining, and histomorphometry represents a significant technological advancement in methods and materials. Together, they demonstrated different muscle-bone interaction results for functional appliances than those reported in previous studies. In Part 1 of this study, we describe and discuss the techniques used in this research and give a brief overview of the findings; in Part 2 (to be published next month), we offer a more in-depth discussion of the results and the implications of our findings.

The electromyographic (EMG) activity of masticatory muscles was monitored longitudinally with chronically implanted EMG electrodes to determine whether functional appliances produce a change in postural EMG activity of the muscles. Preappliance and postappliance EMG levels in four experimentals that had been fitted with functional appliances were compared against the background of EMG levels in controls without appliances. The insertion of two types of functional appliance to induce mandibular protrusion was associated with a decrease in postural EMG activity of the superior and inferior heads of the lateral pterygoid, superficial masseter, and anterior digastric muscles; the decrease in the first three muscles was statistically significant. This decreased postural EMG activity persisted for approximately 6 weeks, with a gradual return toward preappliance levels during a subsequent 6-week period of observation. Progressive mandibular advancement of 1.5 to 2 mm every 10 to 15 days did not prevent the decrease in postural EMG activity.