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Correction to Volume 64, Issue 4, page 283.

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Extraction has been a controversial subject for as long as the specialty of orthodontics has existed. Some authors believe that the extraction of premolars leads to temporomandibular disorders. This occurs, they say, because the vertical dimension collapses. Concomitantly, over-retraction and retroclination of the incisors cause the facial profile to flatten, bring about premature anterior contacts, and distally displace the mandible and mandibular condyle. Numerous correlation studies in the dental literature do not support this contention. There appears to be no higher incidence of temporomandibular disorders in patients treated with the extraction of premolars than in nontreated patients or those treated without extractions. Analysis of premolar extraction cases reveals that there is no collapse of the vertical dimension; on the contrary, the vertical dimension is either maintained or slightly opened. Similarly, there is no evidence that premolar extraction causes undesirable flattening of the facial profile. The facial profile established during treatment is primarily the result of diagnosis and treatment mechanics. Excessive anterior interferences resulting in possible posterior condyle displacement are the result of treatment mechanics. When arches are leveled properly and space closure and overjet reduction are adequately controlled, there is no reason that such interferences should occur.

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Contemporary segmented-arch space-closure springs require activation techniques that have the side effect of producing a significant amount of geometric nonlinearity. This nonlinearity makes these springs difficult to manipulate because it allows tipping during space closure which may be considered adverse. A new spring mechanism has been designed in response to these difficulties so that the advantages of the segmented-arch technique can be extended to those clinicians presently using simple sliding mechanics as a means of space closure. This spring mechanism has eliminated a significant portion of the geometric nonlinearity by using force-application devices (activators) such as elastics or coil springs as the means of activation. By selecting the right activator it may be possible to close an entire extraction site (approx. 7 mm) with one activation. The proposed mechanism essentially consists of two units: an anchorage unit (M/F = 18 mm), and a translational unit (M/F = 11 mm). These units are combined in order to achieve anterior retraction, posterior protraction, or a combination of the two, which is termed reciprocal attraction. The finite-element method was used in place of bench studies to test the new spring design. The commonly used reciprocal-attraction spring tested with a nearly constant M/F ratio equal to 11 ± 1 mm over an effective force range of 50 gm to 450 gm. The results from the other two tests also showed that precisely controlled couple-to-force (M/F) ratios can be maintained over a wide range of effective forces.

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Initial movement of the maxillary first molars under the application of straight-pull, cervical-pull, and high-pull headgear was measured in human subjects.

Facebow deflection can influence molar movement as the relationship of the force vector to the molar's center of resistance changes with an increase of force. The present study proposes using headgear with a combination of variable-pull headcap and short outer bow. A variable-pull headcap allows a great range in force direction. The direction of the headgear force system can be accurately determined using a short outer bow.

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The absorption and desorption of water by a polymer matrix of composite orthodontic resin could cause debonding of the filler-matrix or hydrolytic degradation of fillers and loss of bond strength. In this study, the bond strength of brackets directly bonded with orthodontic composite to the enamel surface of premolars was measured with an Instron machine; the debonding interface distribution was analyzed by scanning electron microscope and energy dispersive x-ray spectrometry following water immersion for 1, 2, and 3 days, and 1, 2, 4, 8, 16, 24, and 32 weeks, respectively. The results show that, under water immersion, bond strength may gradually weaken over time. The greatest loss occurs initially, followed by a period of relative stabilization, and then a weaker reduction after 24 weeks. The greater the time in water immersion, the less the bond strength and the greater the destruction of the composite resin. The debonding interface occurs between bracket and resin.

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The role of intercuspation of the teeth in transverse maxillary growth and dental arch development was investigated radiographically with the aid of implants in Macaca fascicularis monkeys. Fourteen animals were randomly allocated to a control group (n=7) and an experimental group (n=7) and were followed from 29 to 100 weeks of age.

Intercuspation was eliminated in the experimental group by grinding the canines and molar cusps in both dental arches as soon as possible after emergence. Maxillary occlusal radiographs were taken at regular intervals. Linear and angular analyses of skeletal changes revealed that midpalatal sutural growth seems to be independent of intercuspation. The developing dental arch, however, showed a significantly greater increase in width in certain areas in the experimental group than in the control group. Most findings support the hypothesis that the width of the maxillary dental arch is guided by the width of the mandible through the intercuspation of the posterior teeth.

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The horizontal relationship of the jaws has been defined as the angles or distances between the reference planes of the craniofacial complex and points A and B, which are representative of the anterior limits of the denture bases. The aims of this study were (1) to examine statistically and geometrically the different cephalometric measurements which are used to indicate the A-P jaw relationship, and (2) to provide a more reliable parameter by means of comparative cephalometric analyses with various clinical examples. The APDI and Wits appraisal are parameters for evaluating the anteroposterior relationship of the dentition rather than the jaws. FH to AB plane angle (FABA) may provide not only a reliable cephalometric measurement of the anteroposterior relationship of the jaws but also a clue to the facial profile.

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A three-dimensional method to quantify facial asymmetry is introduced. Stereophotogrammetry was applied to determine three-dimensional (3-D) coordinates for eight pairs of surface landmarks of 106 individuals, including 16 with an operated complete unilateral cleft lip and palate. Facial asymmetry was quantified from four different reference planes that were defined perpendicular to and bisecting lines between pairs of bilateral landmarks related to the eyes, nose and mouth. Significant differences (P< 0.01) between these four planes were determined using multivariate analyses of variance (MANOVA). It is concluded that the best reference plane to select in studies of facial asymmetry is formed by the one which is perpendicular to and bisects the line that connects the landmarks Exocanthion. Reproducibility and validity of the method is demonstrated.