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Abstract

In a review of consecutive charts at 5-year intervals from the orthodontic clinic at the University of North Carolina, the number of patients with extraction of all four first premolars increased from 10% in 1953 to 50% in 1963, remained at 35% to 45% until the early 1980s, then declined sharply to the 1950s level by 1993. Extraction for camouflage of Class II malocclusion (maxillary first premolars alone or maxillary first-mandibular second premolars) reached 16% in 1968, then declined, but not as dramatically, and presently is as frequent as the extraction of four first premolars. The rate of extraction of other teeth, done for a variety of individual reasons, has remained almost constant at about 15% for the past 40 years. Thus the total extraction percentage was 30% in 1953, peaked at 76% in 1968, and declined again to 28% in 1993, with almost all the change in the percentage of four first premolar extractions. The increase in first premolar extractions occurred primarily in a search for greater long-term stability; the recent decline seems due to a number of factors. Greater concern about the impact of extraction on facial esthetics, data to suggest that extraction does not guarantee stability, concern about temporomandibular dysfunction, and changes in technique all seem to have played a role. With appropriate orthodontic mechanics, many patients with Class I crowding can be treated satisfactorily with or without premolar extraction.

Abstract

The late mixed dentition stage of development, after the eruption of the first premolars, is a favorable time to start treatment to resolve crowding. This protocol offers the clinician choices. If nonextraction treatment is preferable, arch length preservation can provide the space for alignment in approximately 75% of all patients with crowding. If extraction treatment is indicated, the first premolars are available.

Abstract

Open mouth posture and maxillary arch width were assessed annually for 4 years in a group of children. While younger children exhibited high levels of open mouth posture, this behavior decreased significantly over time. Racial and sex differences, as well as a race-by-time interaction were also evident. The children displayed a significant increase in maxillary arch width across time with sex and racial differences in this growth pattern. Subjects were classified as exhibiting primarily open mouth or closed mouth posture. Although both groups showed increased maxillary arch widths over time, the closed mouth subjects showed significantly greater maxillary arch growth.

Abstract

The purpose of this study was to describe and compare the changes occurring during and after orthodontic treatment in three facial types: short, average and long.

Sixty-six subjects with Class II, Division 1 malocclusion were evaluated. All cases were treated nonextraction, using a fixed edgewise appliance and extraoral forces. The lateral cephalogram and dental casts for each patient were measured at three different stages: pretreatment, immediately after appliance removal and at least two years posttreatment.

There was a wide range of individual variation in posttreatment change for the various skeletal and dental parameters measured. With few exceptions, the three facial types did not show significant differences in posttreatment change. The relative protrusion of maxillary incisor tip (U1:A-Pog) tended to increase after treatment in the long face type while it tended to decrease in the short face type. Long face females, when compared with all other groups, showed greater posttreatment incremental increase in anterior face height as well as the greatest posttreatment decrease in maxillary arch length. Males expressed greater posttreatment incremental increases in the various linear measurements of face height than females.

Differences in posttreatment change for the different facial types do not require special retention consideration.

Abstract

The craniofacial morphology of children with Class II-1 malocclusion with and without deepbite was studied and compared with that of a control group of children with normal occlusion. Common for the Class II-1 children was a short mandibular corpus, which was probably the main reason these children had a distal basal jaw relationship.

The Class II children with deepbite differed from the control group in ways. They had: 1) a distal relationship between points A and B; 2) a distal relationship between points A and pogonion; and 3) a discrepancy in length between the corpora of the two jaws. These deviations discriminated nearly 95% correct between the deepbite group and the controls.

The most characteristic deviations in the Class II children without deepbite, in relation to the controls, were: 1) a distal relationship between points A and B; 2) a small angle between the nasal plane and the anterior cranial base; and 3) a long mental process. These deviations discriminated about 95% correct between the experimental group and the controls.

Many of the craniofacial differences between the Class II groups could, theoretically, be explained by the MP-SN angle being, on average, 9 degrees larger in the children without deepbite. Most typical was that this group has 1) a larger lower anterior face height, 2) larger maxillary and mandibular incisal heights, and 3) a more obvious distal relationship between points A and pogonion. In combination, these differences discriminated correctly between the Class II groups in 90% to 97% of the cases.

Abstract

This study was designed to quantify the intraexaminer and interexaminer reliability of 52 commonly used posterior anterior cephalometric landmarks. The horizontal and vertical identification errors were determined for a sample of 33 skulls and 25 patients. The results show that there is a considerable range in the magnitude of error with different horizontal and vertical values. lnterexaminer landmark identification error was significantly larger than intraexaminer error for many landmarks. The identification error was different for the skull sample compared to the patient sample for a number of landmarks. The relevance of knowing the identification error for each landmark being considered in a particular application was discussed.

Abstract

In this study the effects of prestressing, environmental acidity, oxygen content, and temperature on mechanical properties were measured for three polyurethane-based orthodontic elastomeric chains. Specimens of each chain were treated for 10 and 100 days, and their mechanical properties were compared with those of the untreated specimens via stress-relaxation tests. Conditioning treatments were determined to affect the magnitude of residual load after relaxation, with the largest effect due to prestressing. Among the other variables studied, an increase in temperature of the environment appeared to significantly influence the degradation mechanism responsible for the deterioration of the mechanical properties of polyurethane elastomers (p<0.001); acidity and oxygen content had no significant effects. Force decay profiles from the specimens were derived using a Maxwell-Weichert model consisting of springs and dashpots. This model allows the orthodontist to predict load magnitude supplied by the chain at activation and at any time during treatment.

Abstract

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