Morphometrics for cephalometric diagnosis

This article demonstrates morphometric methods by applying them to an orthodontic sample. A total of 150 pretreatment cephalograms of consecutive patients (84 female, 66 male) were traced and digitized. Fifteen points were used for the analysis. The tracings were superimposed by the Procrustes method, and shape variability was assessed by principal component analysis. Approximately 70% of the total sample variability was incorporated in the first 5 principal components. The most significant principal component, accounting for 29% of shape variability, was the divergence of skeletal pattern; the second principal component, accounting for 20% of shape variability, was the anteroposterior maxillary relationship. It is recommended that Procrustes superimposition and principal component analysis be incorporated into routine cephalometric analysis for more valid and comprehensive shape assessment.

Activator versus cervical headgear: superimpositional cephalometric comparison

Clinical trials comparing activator and headgear treatment have shown comparable effectiveness in the total result achieved, but the mechanism of correction is still uncertain. Most studies have used conventional cephalometric methods to evaluate treatment effects, and this might be a factor for the inconclusive results. The aim of this retrospective investigation was to compare the effects of activator and cervical headgear treatment with a superimpositional cephalometric method that could discern between vertical and horizontal effects as well as skeletal, dental, and rotational treatment results. The sample consisted of 2 groups of Class II Division 1 patients, treated without extraction by the same clinician (22 patients were treated with a modified activator-type functional appliance, and 30 patients were treated with a combination of cervical headgear and fixed edgewise appliances). Lateral cephalometric radiographs taken at the beginning of treatment and after Class II molar correction were evaluated conventionally and with a superimpositional method. Regarding the conventional cephalometric measurements, the only difference in the anteroposterior dimension between the 2 treatment modalities was the significantly reduced SNA angle in the headgear group. Both appliances appeared to produce minimal changes in FMA and GoGn-SN angles, and there were no statistically significant differences between the treatment groups. Regional superimpositions showed differences in the movement of molars: the maxillary molar was found to move more posteriorly and inferiorly in the headgear group. Conversely, the mandibular molar was found to move toward the occlusal plane more in the activator group. Assessment of mandibular skeletal changes showed that the mandible moved anteriorly by approximately 1 mm more in the activator than in the headgear group. The overall effect of the 2 appliances was found to be clinically comparable. However, the individual components of change showed differences characteristic of each appliance.

Estimated natural head position and facial morphology

Estimated natural head position, or natural head orientation (NHO), has been proposed as a preferred reference position for assessing facial morphology. However, because this position is subjectively defined by the orthodontist, it could be influenced by facial form, which would be an undesirable attribute. The aim of this study was to assess whether NHO is influenced by facial morphology. Lateral photographs of 14 patients were used. Each was warped to produce 2 new images, with the chin positioned backwards or forwards relative to the original (+/-2 degrees change of the soft tissue N-Pg line). All 42 images were placed in circular frames and shown to 7 experienced orthodontists, who were asked to orient them to NHO. Image orientation between the 3 chin positions was measured and compared. The results showed that NHO depended on chin position. Images with protrusive chins were positioned with the head rotated more downwards (Frankfort plane 3.10 degrees relative to horizontal) than were images with retrusive chins (Frankfort plane 4.98 degrees ). The difference in head orientation measurements was half of the difference in chin position (1.88 degrees for a chin change of 4 degrees ). These findings call into question the validity of NHO for diagnosis because it depends on the same factor it aims to assess. Use of NHO would result in underestimating the true skeletal relationships.

Acquisition of 3-dimensional shapes from images

Advances in computer vision have started to infiltrate the specialty of orthodontics. During the past few years, a number of new products have appeared that are capable of extracting the 3-dimensional (3-D) structure of an object just by "looking." Examples include laser scanners for creating 3-D models of the face, and hand-held scanners for creating virtual models of the teeth. Such noninvasive methods will surely evolve rapidly and be applied to a multitude of diagnostic and therapeutic modalities, changing the way we think and practice. This article introduces the basic principles behind such technology so that we can better appreciate its advantages, limitations and possibilities. From the large number of methods for acquiring 3-D shapes from images, 4 were selected and are described below. For more comprehensive coverage, see the book by Klette et al (1).

Significance of the soft tissue profile on facial esthetics

The soft tissue profile has been studied extensively in orthodontics, primarily from lateral cephalometric radiographs, under the assumption that the form of the soft tissue outline largely determines the esthetics of the whole face. The purpose of this study was to assess the relative contribution of the shape of the soft tissue profile outline on the attractiveness of the face, as seen from the profile view. Pretreatment color profile facial photographs of 20 female patients were used. The photographs were scanned, and the soft tissue outlines were digitized. The average outline of the 20 original photographs was then calculated and used as a template for modifying the photographs with computer warping methods. This resulted in 20 warped photographs, all with the same soft tissue outline. Three additional photographs were constructed with 1 face-the composite average of the 20 original photographs-and 3 hairstyles from 3 of the original pictures. The photographs were printed and presented to 10 laypersons and 10 orthodontists for scoring. Scoring was performed on 2 occasions separated by at least 1 week. On the first occasion, the original photographs of 10 of the patients and the warped photographs of the other 10 patients were shown. At the next session, the remaining 10 original and 10 warped photographs were shown. The 3 composite photographs were interspersed with the 20 pictures shown to the judges in each scoring session. Judges were asked to score facial attractiveness on a scale of 0 to 10. The judges were unaware of both the computer modification of the photographs and the purpose of the study. Good agreement was noted between the judges, although the orthodontists tended to be more influenced by the profile outline than did the laypersons. The 3 averaged composite photographs were consistently given the highest scores. The modified photographs were given higher scores than their original counterparts, showing that facial attractiveness is influenced by soft tissue outline form. However, the score improvement was not sufficient to reach the level of the composite images, especially for faces initially judged as being unattractive. This shows that factors other than profile outline shape may be more influential in facial esthetics.

Intermaxillary forces during activator treatment

The mode of action of the activator appliance is still unclear. Apart from a possible mandibular growth enhancing effect, some investigators believe that orthopedic forces may be applied to the maxilla, contributing to Class II correction by inhibition of maxillary growth. In addition, orthodontic forces may arise that produce dentoalveolar changes. The purpose of this study was to measure the magnitude of anteroposterior intermaxillary forces during wear of the activator appliance. Ten consecutive patients with Class II dental and skeletal relationships were treated with a modified activator appliance. The appliance had maxillary and mandibular segments that could be detached from each other during the measuring session. A force transducer was placed at the anterior part of the maxillary segment, and the anteroposterior force exerted by the mandibular segment was measured. Measurements were taken in the upright and reclined position at every patient visit for a period of 6 months. Results indicated that intermaxillary forces were generally in the orthodontic range (median values of 100 gf at the upright position and 123 gf at the reclined position). A wide variation in force levels was noted, both between patients and for the same patient during the experimental period. No statistically significant change in force levels was observed during the 6 month period and no difference was noted between upright and reclined posture.

Ideal arch force systems: a center-of-resistance perspective

The analysis of force systems from an ideal arch has shown that the ratio of the moments produced by a straight wire connecting two malaligned brackets depends on the ratio of the angulations of the brackets to the interbracket axis. Although this result permits assessment of the relative forces and moments, prediction of future tooth movement requires knowledge of the position of center of resistance as well. In this study, the forces and moments produced by a straight portion of an arch wire were transferred from the brackets to the center of resistance. The purpose was to compare the force system at the brackets to the force system at the center of resistance and to assess whether bracket geometry can be applied to predict initial tooth movement. A computer model was used to simulate two teeth connected by a straight portion of wire. Forces and moments were calculated with the use of equations derived from elementary beam theory. The results show that the force system at the center of resistance may be of an entirely different "geometry" type than that at the bracket. Factors that influence the force system include the interbracket distance, the angulation of the teeth, the length of the tooth root, and the width of the bracket.

Cephalometric analysis of changes in occlusal relationship

One of the main problems in assessing the mode of action of various treatment modalities is the method of measuring the treatment change. The purpose of the present study was to develop a cephalometric method that would permit a detailed evaluation of the individual growth processes (skeletal and dental) that contribute to the overall change in occlusal relationship. The change in molar relationship was resolved in five components, i.e. translation of the maxilla, of the upper molar, of the mandible, of the lower molar, and rotation of the mandible. These movements were recorded using regional superimposition of various structures, with the aid of a computer program. Derotation of the mandible was performed to remove any confounding effects of total mandibular rotation on the interpretation of the measurements. The results show that assessment of treatment effects can be carried out by comparison of the five resulting vectors.

Computer experiments using a two-dimensional model of tooth support

The purpose of this investigation was to study the factors that may affect the position of the center of resistance and center of rotation. A two-dimensional computer model of the periodontal ligament was developed. The model permitted the simulation of an isotropic (responding in the same manner regardless of the direction of the applied force) and nonisotropic periodontal ligament and allowed changes in root shape and in position and direction of force application. The center of resistance was found to depend on the distribution of root surface area. For a model of the upper central incisor, it was located at 42% of the root length measured from the alveolar crest. The presence of anisotropy in the periodontal ligament significantly affected the position of the center of resistance, which was in this case also affected by the direction of the applied force. Forces passing through the center of resistance produced translation of the modeled tooth in a direction not necessarily the same as the direction of the applied force. Tipping forces produced much larger stresses than forces causing translation. Simulation of periodontal involvement resulting in loss of attachment increased the stresses exerted on the periodontal ligament. The model permitted easy assessment of various factors that may influence the position of the center of resistance of teeth and revealed a potentially large variability in the position of the center of resistance and center of rotation, caused by variation of the properties of the periodontal ligament.

Changes in cheek pressure following rapid maxillary expansion

The purpose of the present investigation was to study the effects of rapid maxillary expansion on the pressures exerted by the cheeks on the maxillary arch. The sample consisted of 15 patients (five males, ten females) who received either a Hyrax or Haas type expansion appliance for treatment of a bilateral maxillary constriction of more than 5 mm. The median age of the sample was 12 years. Buccal pressures were measured at the upper first molar on the left and right side, before and after active expansion, and also after an average of 3-4 months of retention with the appliance in place. Buccal pressures on the maxillary first molar averaged approximately 3 g/cm2 before expansion and increased significantly to a value of approximately 9 g/cm2 after expansion. Pressure change was approximately 0.6 g/cm2 for each millimetre of expansion. During the 3-4-month period of stabilization of the appliance, the pressures remained at the post-expansion levels and no adaptation of the soft tissues was observed. These results lead to the conclusion that cheek pressures on the maxillary arch may be implicated in the relapse occurring after rapid expansion, even after the usual 3-month period of stabilization.

Quantitative description of the shape of the mandible

The purpose of this study was to provide quantitative data on the shape of the mandible at the period around the pubertal growth spurt and to test the hypothesis that early mandibular shape may influence the amount and direction of subsequent mandibular growth. Longitudinal data from lateral cephalograms of 55 white female and 39 white male subjects were used. The mandibular outline from articulare to gnathion was analyzed into cosine curves, according to the Fourier equation. The resulting Fourier coefficients, representing mandibular outline shape, were analyzed statistically in relation to age, sex, craniofacial pattern, and mandibular growth rotation. Statistically significant growth changes of the Fourier coefficients were observed, especially during the postpubertal period, indicating a decrease in the gonial angle with age. Sex-related differences in shape were observed at all ages, male subjects showing a more rounded shape of the mandible than female subjects. Mandibular shape, as represented by the Fourier coefficients, was correlated to cephalometric variables, indicating mandibular inclination, but only poorly to cephalometric variables, indicating anteroposterior jaw relation. Total rotation of the mandible during growth could not be predicted by mandibular shape.