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Correction to Volume 62, Issue 1, page 59.

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The purposes of the investigation were: i) to compare the condylar displacements from centric slides for 74 sets of casts mounted with an estimated versus a true hinge axis; ii) to determine the error in hinge axes location on the recording of condylar displacements from centric slides; and iii) to define the reproducibility of the Mandibular Position Indicator (MPI), a method of measuring condylar (in vitro) displacements from centric relation position.

The findings revealed that routine use of the true hinge axis to obtain MPI readings for analysis of diagnostic casts is not warranted. The mean standard error of MPI double readings attributed to both hinge axes location was 0.4 mm for each component in the sagittal plane. Of the 0.4 mm of mean standard error, 0.2 mm was contributed by the lack of reproducibility of the MPI. The centric relation bite registration technique outlined in this study always captured a superior articulator position of the condylar mechanism. The condylar distractions and deflections from centric relation to maximum intercuspation were primarily in an inferior direction with a smaller posterior component.

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The purpose of this study was to evaluate the effectiveness of heavy (S2) Alastik separators in relieving bruxist activity as monitored through masseter muscle area EMG activity, muscle palpation, and self-reporting in 21 Caucasian subjects. The subjects, all of whom suffered from both bruxism and myofascial pain-dysfunction, were randomly assigned to one of three groups: experimental (separator group); placebo (separator placed and removed); and control groups (no separator).

The findings from this study indicate that there were no observable differences in either subjective or objective responses to the pretreatment versus posttreatment questionnaire and clinical examination for tooth clenching or grinding, facial pain, and fatigue of the jaws. In addition, no statistical differences were found between pre and posttreatment data. The EMG data did not show any statistical differences between pretreatment and posttreatment evaluations or among the 3 groups.

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The relative stage of maturity of a child may be determined by comparing the child's hand-wrist radiograph to known standards of skeletal development. Hand-wrist radiographs of 70 adolescents were used to categorize the individuals by skeletal maturation into early, average and late maturation groups using the Fishman SMA method of assessment. The rates of mandibular and maxillary growth relative to the last stages of the pubertal growth spurt were measured. Statistical evaluation of the data was performed using an analysis of variance. The magnitude of change in growth increments of the mandible was greater in the late maturers than in the early or average maturers. Incremental differences in growth between the maxilla and mandible during the last stages of puberty were noted, with the mandible growing significantly more than the maxilla.

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The use of samarium-cobalt (Sm-Co) magnets for light force application is a relatively new concept in orthodontic tooth movement. This study reports on the forces generated by these magnets.

Magnets were attached to aluminum rods mounted in a universal testing machine. The magnets were initially separated by 10mm and were moved toward each other at 2.5mm/min in repulsion or attraction, depending upon the magnetic pole orientation. The magnets were also positioned initially in contact and then moved apart at a rate of 2.5mm/min, again producing repulsion or attraction, depending upon the pole orientation.

The Sm-Co magnets exhibit very large forces when in close approximation but forces decrease markedly at separations greater than 2mm. The force, P, generated between magnets is determined by their separation, d, and follows the relationship P=dⁿ. At magnet separations of 0 to 2mm, the exponent n is equal to −0.4; at separations of 2mm to 7mm, exponent n equals −2.1 for both attraction and repulsion. Thus the classic Coulomb law of magnetic force was followed only at magnet separations of greater than 2mm.

Force-separation behavior and the high cost of these magnets may not justify their routine clinical use.

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To study the effects of force magnitude on osteoprogenitor cell activity during premaxillary expansion, stainless steel helical springs were attached to the maxillary central incisors of 45 3-month-old male rats. The animals were randomly divided into force levels (0,50, 100, 150,200 gm) and were injected intraperitoneally with tritiated thymidine (1.0 uc/g wt.) 1 hour prior to sacrifice which occurred at 27, 40, and 60 hours. In order to examine cell activity within different regions of the suture, each premaxilla was divided into three geographic areas. Quantitative results were obtained by comparing the percent of labeled cells observed at different force levels, geographic areas, and observation times. The greatest number of labeled cells at each force level was found at 27 hours. Increased forces were correlated with increased numbers of labeled cells up to 100 gm, with decreased cell numbers at higher forces. The numbers of labeled cells at 200 gm were not significantly different from the controls. Histological observations of early bone formation at 60 hours supported the quantitative labeling results at 27 hours. The results also demonstrate a significant correlation between the geographic location of the labeled cells and force magnitude, with maximal cell stimulation occurring more superiorly in the suture as forces increased. The results suggest that early bone formation within the expanded suture can be maximized by varying force magnitude and distance from the point of force application.

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Longitudinal growth changes of the human symphysis were evaluated for 75 children (37 males and 38 females) between 6 and 15 years old. Childhood growth was described by mean yearly rates of change between 6 and 10 years for females and between 7 and 11 years for males; pubertal changes pertain to growth between 10 and 14 for females and between 11 and 15 for males. Cephalometric tracings of the mandible were superimposed using stable reference structures. Vertical growth changes, particularly for landmarks located in the upper 20% of the symphysis, were most pronounced. Annual rates of vertical growth ranged between 0.9 mm/yr for the lingual incisor contact point to −0.2 mm/yr for gnathion. Males showed significantly greater rates of vertical growth than females, especially for the upper half of the symphysis. Vertical growth rates were also greater during puberty than during childhood. The horizontal growth changes indicated lingual movement of most symphyseal landmarks. Annual rates of growth were greatest for landmarks located in the upper half of the symphysis. B-point showed the greatest lingual drift. During puberty, the mandibular incisors in females moved lingually as the upper anterior half of the symphysis remodeled; in males, the incisors maintained their horizontal position as the labial sulcus developed.

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Both the amount of force applied and the duration of the application affect tooth movement. To study the effect of duration, adult male Sprague-Dawley rats were fitted with orthodontic appliances delivering a 40-gm initial mesial tipping force to the maxillary molars. The animals were divided into two longitudinal groups (I: 1 hour and II: 24 hours; N=15). Sham-treated control (III) and 14 day (IV) continuous cross-sectional force tooth movement data were also included for comparison (72 rats per group). Extraoral cephalometric radiographs were obtained at appliance placement and at 1, 3, 5, 7, 10, & 14 days. Tooth movement was determined with respect to palatal implants. ANOVA indicated significant differences existed over time in each group (p=0.0001). Continuous force applied for 14 days generated a classic three-part tooth movement curve. Short-term forces were characterized by initial mesial movement, a distal relapse period (d3–d5), and a late mesial movement period (d7–d14). The results suggest short-term forces of 1 and 24 hours initiate remodeling events, which result in tooth movement 7 to 14 days later and that the minimum effective duration of a 40-gm activation is less than 1 hour in this animal model.

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The purpose of this investigation was to compare growth characteristics of the mandible during puberty with growth characteristics of the hyoid bone, cervical vertebrae, hand bones and standing height. Data were compiled from serial lateral cephalometric roentgenograms, hand-wrist roentgenograms and body height records of 33 Japanese girls between 9 and 14 years old. Records were updated annually. The mandibular growth rate differed from the other growth rates. The timing of maximum growth velocity of the mandible varied more widely than the timing of maximum growth velocity of the other parameters measured, and the total amount of mandibular growth did not correlate to any other measurement. The timing and magnitude of circumpubertal growth acceleration of various components of the body vary within a certain range of difference. However, the amount and timing of mandibular growth seems to be more variable than the other areas studied.

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A technique is presented for the lingual bonded retention of anterior teeth. Functional concepts to guide the goals of orthodontic treatment during finishing are reviewed. The importance of proper anterior guidance is emphasized as the protector of posterior teeth in eccentric jaw position. Clinical procedures for attaining optimum function are suggested, including the elimination of removable retainers.

This paper was presented at the January 1990 meeting of the Midwest Component of the Edward H. Angle Society, Indianapolis, Indiana.

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