Low-angle Group Research Articles

Craniofacial morphological characteristics of Chinese adults with normal occlusion and different skeletal divergence

The aim of the present study was to examine the craniofacial morphologic characteristics of different vertical dysplasias in a population of Chinese adults with normal occlusion. Sixty-nine subjects (39 males and 30 females) were selected from 800 healthy students between 18 and 24 years of age. Lateral cephalograms were obtained and 27 hard and 10 soft tissue measurements were analysed. The subjects were then divided into three groups: high angle, low angle, or control according to the value of FH-MP. Intraclass correlation coefficient was determined for the repeated measurements. One-way analysis of variance was used to determine the differences between the groups. The results showed that the low-angle group had a larger cranial basis angle (N-S-Ar) and the high-angle group had a shorter maxilla (Ans-Ptm; P < 0.01). The high-angle group displayed vertical hyperdivergency with increased PP-OP, OP-MP, gonial, and lower gonial angles, whereas the low-angle group showed significant hypodivergence with decreased values for all variables (P < 0.01). The low-angle group displayed a more protrusive chin and the high-angle group a more retrusive chin (P < 0.01). Differences in dentoalveolar measurements in the divergent groups were mainly in the anterior region. Moreover, the low-angle group had a thicker and the high-angle group a thinner lower dentoalveolus (P < 0.01). For face height measurements, the main differences in the divergent groups were at the anterior lower third (P < 0.01). Soft tissue deviations were less obvious in the high-angle group and in general less significant than those of the hard tissues in both divergent groups. Significantly different morphological characteristics exist in Chinese adults with vertical dysplasia but normal occlusion. Major skeletal cephalometric changes were found for the lower facial third. The soft tissues showed a well-adapting mechanism of soft tissue coverage for the skeletal dysplasia.

Relationship between vertical skeletal pattern and success rate of orthodontic mini-implants

The objective of this research was to determine which clinical and skeletal factors are related to the success rate of orthodontic mini-implants in the maxillary and mandibular posterior buccal areas. A total of 778 orthodontic mini-implants (Dual-Top Anchor System, Jeil Medical, Seoul, Korea; 1.6 mm diameter, 8 mm length, cylinder shape, self-drilling type) in 306 patients were retrospectively examined. The success rate was calculated according to clinical variables (sex, age, soft-tissue management, placement position, sagittal skeletal classification, arch-length discrepancy, and side) and skeletal variables (articular angle, mandibular plane to palatal plane angle, Frankfort-mandibular plane angle, mandibular plane angle, gonial angle, upper gonial angle, and lower gonial angle). Analysis of variance (ANOVA), chi-square tests, and multiple logistic regression analysis were used for statistical analysis. The overall success rate was 79.0%. Almost 80% of the failures occurred within the first 4 months. The clinical variables sex, age, soft-tissue management, sagittal skeletal classification, arch-length discrepancy, and side did not show significant differences in the success rate. Although the success rates were significantly different according to placement position (P <0.01), there was no significant difference in the odds ratios among placement positions. In the skeletal variables, average upper gonial angle (84.2%) had a significantly higher success rate than low (75.7%) and high (71.2%) upper gonial angles (P <0.01). High Frankfort-mandibular plane angle (P <0.05) and low upper gonial angle groups (P <0.05) showed significant lower odds ratios than did the other types. Vertical skeletal pattern might be an important factor for the success of orthodontic mini-implants placed in posterior buccal areas.

Effect of coronary bifurcation angle on clinical outcomes in Chinese patients treated with crush stenting: a subgroup analysis from DKCRUSH-1 bifurcation study

Background Bifurcation angles may have an impact on the clinical outcomes of crush stenting. We sought to compare high (≥60°) with low (&lt;60°) bifurcation angle in patients who underwent either classical or double kissing (DK) crush stenting for bifurcation lesions from the DKCRUSH-1 data base. Methods There were 212 patients with 220 lesions, some with low-angle (n=138) and some with high-angle (n=74). Angiography was indexed at 8-month after procedure. Primary endpoint was the occurrence of major adverse cardiac events (MACEs), defined as cardiac death, myocardial infarction and target lesion revascularization (TLR). Secondary endpoint included late lumen loss, the rate of restenosis, and final kissing balloon inflation (FKBI). Results At 8 months, clinical follow-up was 100%; angiographic follow-up was 75% in the low-angle group and 83.3% in the high-angle group. There were no significant differences in the FKBI between the high-angle group (91.43%) and the low-angle group (82.39%). In the high angle group, there was a significant difference in contrast volume used (P=0.005) but no significant difference in acute gain, minimum lumen diameter (MLD), late loss and diameter stenosis in the pre-bifurcation segment, post-bifurcation segment or side branch. When lesions were assigned into with-(n=133) and without-FKBI (n=42), significant side-branch late loss was seen in the group without-FKBI ((0.65±0.49) mm vs (0.47±0.62) mm, P=0.02), with a resultant greater restenosis rate (37.68% vs 18.32%, P=0.001). No difference was detected in the MACE free survival rate between the high and low angle groups (82.39% vs 82.36%, P=0.84). The rate of stent thrombosis tended to be higher in the lower-angle group although there was no significant difference (P=0.38). The TLR free survival rate was 87.2% in the with-FKBI group vs 73.5% in the without-FKBI group (P=0.001). Cox regression analysis showed that the independent predictors for target vessel revascularization were the side branch stent MLD post stenting (hazard ratios (HR) 1.028, 95% CI 2.357–16.233, P=0.002), lack of FKBI (HR 4.910, 95% CI 4.706–8.459, P=0.001) and unsatisfactory kissing (HR 3.120, 95% CI 2.975–5.431, P=0.001). Conclusions Bifurcation angles do not influence the clinical outcome of crush stenting. Successful final kissing balloon inflation, regardless of bifurcation angles, can predict TLR.

Differences between high- and low-angle subjects in arch form and anterior crowding from 23 to 33 years of age

The aim of this study was to compare changes in arch width and length in high- and low-angle subjects. The material comprised the cephalograms and study models of 17 males and 15 females with a high-angle and 12 males and 16 low-angle females. The age of the subjects at the first examination (T1) was 22 years 10 months and 22 years 8 months in the high-angle groups, 22 years 5 months and 22 years 8 months in the low-angle subjects, respectively, for males and females. The second assessment (T2) was 10 years and 9 years 10 months later in the high-angle and, 10 years 3 months and 9 years 10 months in the low-angle groups, respectively. Statistical analysis of changes for each group during the observation period T1-T2 was undertaken using a paired t-test, and the differences between the groups with two-sample t-tests. The findings showed a general reduction in upper and lower arch length, an increase of intermolar width, and a decrease of intercanine width in both genders. Anterior perimeter and anterior space in both angles decreased in both males and females. The mean changes were small, but with large individual differences. There were no significant differences between the genders or the high- and low-angle groups in the changes, except that the female high-angle group had a statistically significant (P = 0.05) greater decrease in mandibular arch length than the low-angle female group.

Longitudinal Intermaxillary Relationships in Class III Malocclusions with Low and High Mandibular Plane Angles

To analyze the sagittal, vertical, and transverse relationships of the maxilla and mandible in Japanese girls with Class III malocclusions with different inclination of mandibular plane. This longitudinal study utilized serial posteroanterior and lateral cephalograms of 56 untreated subjects from the age of 8 years until the age of 14 years (low mandibular plane angle group: n = 20; average mandibular plane angle group: n = 15; high mandibular plane angle group: n = 21). Sagittal and vertical growth was analyzed on the basis of lateral cephalograms, and transverse growth was analyzed on the basis of posteroanterior cephalograms. There was no significant difference in intermaxillary sagittal relationships among the three groups from age 8 until 14. On the other hand, there were significant changes in the vertical and transverse intermaxillary relationships during this period. When comparing the three groups at the same age, there were significant differences in vertical and transverse intermaxillary relationships in some ages, whereas no significant difference was found in sagittal relationships in any ages. The inclination of mandibular plane might play a role in anticipating changes in vertical and transverse intermaxillary relationships from 8 until 14 years of age.

Dental Arch Widths and Mandibular-Maxillary Base Width in Class III Malocclusions with Low, Average and High MP-SN Angles

To analyze the development of the dental arches and skeletal mandibular-maxillary bases in untreated Class III malocclusions with low averages and high mandibular plane angles in subjects aged 10 to 14. The records of 50 untreated Japanese girls with Class III malocclusions at age 10 were selected from the files of patients pending orthodontic surgery. The patients included those with low (< or =27 degrees), average (27 degrees through 37 degrees) and high (>37 degrees) mandibular plane angles. The maxillary skeletal base width, biantegonial width, and maxillary and mandibular intermolar width were determined on posteroanterior cephalograms obtained at annual intervals when subjects were between 10 and 14 years of age. The difference between the maxillary and mandibular intermolar width was also calculated and reported. All skeletal and dental transverse widths in the high-angle group were significantly smaller than those in the low-angle group (P < .05) from ages 10 to 14. On the other hand, the maxillary to mandibular molar difference was the same for the three groups (P > .05) at each age. The deviations in molar differences did increase from age 10 to age 14 in all three groups. Mandibular plane angles might play a stronger role in the transverse skeletal growth of the maxilla and the mandible than the transverse dental growth of the maxilla and the mandible.

Influence of occlusal plane inclination on ANB and Wits assessments of anteroposterior jaw relationships

Although assessments of anteroposterior relationships are vital for orthodontic treatment planning, they cannot be precisely achieved by current cephalometric and noncephalometric resources. ANB angle and the Wits appraisal are the most popular cephalometric measurements applied in clinical orthodontics, although they have drawbacks. Facial vertical features, especially the occlusal plane angle, play an important role in their assessment, contributing to their unreliability. To further understand the influence of occlusal plane inclination, ANB angle and the Wits appraisal were analyzed in cephalo-lateral radiographs of 122 finished orthodontic patients in 2 groups: high occlusal plane angle and low occlusal plane angle. There was a tendency for inconsistency between ANB and Wits assessments in the high occlusal plane angle group and a tendency for consistency in the low occlusal plane angle group. Cranial base influences on ANB and Wits assessments were investigated by subdividing the groups. Any of the considered cranial base factors was decisive for lack or presence of consistency between ANB and Wits assessments. The clinician should beware of possible misinterpretations when using ANB and Wits to assess the anteroposterior relationship of the jaws, especially in high occlusal plane angle patients.

Transverse growth of the maxilla and mandible in untreated girls with low, average, and high MP-SN angles: A longitudinal study

The purpose of this study was to investigate maxillary and mandibular transverse growth in untreated female subjects with low, average, and high mandibular plane angles longitudinally from ages 6 to 18. Eighty-one untreated white girls with low (< or = 27 degrees , n = 16), average (> 27 degrees to < 37 degrees , n = 41), and high (> or = 37 degrees , n = 24) mandibular plane angles at age 6 were selected from the Bolton-Brush and Burlington Growth Studies. For each subject, longitudinal posteroanterior cephalograms at different ages (from ages 6 to 18) were traced, and the widths of maxilla and mandible were measured. All the measurements were converted by using a magnification factor of 8.5% (the subject-to-film distance was set at 13 cm). At age 6, the high-angle group had narrower maxillary and mandibular widths than the low-angle group, and this trend continued until age 18. From ages 6 to 14, maxillary width showed a steady and similar rate of increase for all 3 groups (0.90-0.95 mm per year), yet a plateau was reached at age 14 for all groups. Mandibular width increased at a steady rate (about 1.6 mm/year) for all 3 groups until age 14, and a plateau was reached for the high-angle group. For the low- and average-angle groups, mandibular growth continued from ages 14 to 18 but at a slower rate (0.85 mm and 0.39 mm per year, respectively). Vertical facial patterns (with low or high mandibular plane angles) might play a strong role in the transverse growth of the maxilla and the mandible.

Comparative data on facial morphology and muscle thickness using ultrasonography

The purpose of this investigation was to measure the thickness of the masseter, levator labii superioris, and zygomaticus major muscles; to examine the reproducibility of ultrasonographically measured muscle thickness; to evaluate the association between facial and masticatory muscle thickness and vertical facial pattern; and to test whether the variation in muscle thickness is related to the variation in the body mass index (BMI) of different individuals. The thickness of the masseter, levator labii superioris and zygomaticus major muscles was measured bilaterally by ultrasonography in 47 (23 females, 24 males) healthy, fully-dentate young adults who volunteered for the study. The measurements for each individual were performed twice for the masseter muscle: during relaxation and during maximal clenching. Standardized lateral cephalograms of the subjects were traced to determine their facial morphology. The data obtained from the lateral cephalograms were used to divide the subjects into three groups according to their vertical facial pattern: low angle (n = 14), high angle (n = 17) and normal (n = 16). In the low angle group, the mean masseter muscle thickness was 15.20 (+/- 1.90) mm under relaxed conditions and 16.31 (+/- 2.18) mm during maximal clenching. In the high angle group, the respective measurements were 13.29 (+/- 2.52) mm and 14.72 (+/- 2.63) mm. In the vertically normal group, they were 13.56 (+/- 1.95) mm and 14.57 (+/- 1.83) mm. There was no relationship between vertical growth of the face and the thickness of the investigated muscles of facial expression. Masseter muscle thickness was found to be significantly correlated to vertical facial pattern and BMI, showing that individuals with a thick masseter had a vertically shorter facial pattern, whereas the muscles of facial expression showed no relationship with vertical facial pattern.

The effect of cervical headgear on patients with high or low mandibular plane angles and the “myth” of posterior mandibular rotation

The purpose of this retrospective survey was to compare the magnitude of posterior mandibular rotation during orthodontic treatment with edgewise appliances and cervical headgear in patients with high or low Frankfort-mandibular plane angles (FMA). The files of a private orthodontic practice were searched, and the records of growing patients with Class II Division 1 malocclusion were selected. These were divided into 2 groups, according to FMA: the hypodivergent (FMA < 22°) or low-angle group consisted of 29 patients with a median FMA of 19.80°, and the hyperdivergent (FMA > 28°) or high-angle group consisted of 31 patients with a median FMA of 32.70°. All patients had nonextraction treatment with full edgewise appliances (Roth prescription), including second molars, by the same clinician. Cervical headgear and Class II elastics were used, and several patients also had fixed or removable anterior biteplanes for short periods. Pretreatment and posttreatment cephalometric tracings were superimposed on internal basic structures of the mandible. There was no difference in FMA changes between the 2 groups, nor were there statistically significant differences in changes during treatment, with 2 exceptions: the angle SN-GoGn showed a very small (0.86°) mean differential change between groups, generated mostly by the counterclockwise mandibular rotation of the low-angle patients. This was verified with structural superimpositions to evaluate vertical changes. In addition, posterior face height increase was significantly greater in the low-angle group. Structural superimposition of the mandible after treatment showed marked counterclockwise rotations in relation to the anterior base of the skull in both groups, with the high-angle group rotating significantly less. On average, growth and treatment resulted in improvements in the high-angle patients but aggravated the problems in the low-angle patients with deep bite malocclusions.

Analysis of soft tissue profile changes after mandibular advancement surgery

The aim of this study was to analyze changes in soft tissue profile after mandibular advancement surgery, with special emphasis on the effect of skeletal relapse and different Class II facial patterns. The cephalometric radiographs of 30 consecutive patients (24 women and 6 men, mean age 23 years) who underwent sagittal split osteotomy were studied. The radiographs were taken immediately before operation, at one week and 14 months postoperatively. To analyze the possible influence of hyper- and hypodivergent facial patterns, the patients were classified into low- (4 patients), medium- (16 patients) and high-angle (10 patients) groups according to the magnitude of the mandibulonasal plane angle. The main movement occurred in the horizontal plane. Soft tissue pogonion and mentolabial fold were found to follow the underlying skeletal structures in a nearly 1:1 ratio. On final follow-up, skeletal relapse of 1.3 mm was measured at B-point and of 1.5 mm at pogonion. Taking the skeletal relapses into account, the ratios of both corresponding soft tissue references (alternative ratios) dropped to 60%. Soft tissue pogonion is the most reliable reference for the planning of mandibular advancement. The ratio of soft tissue movement to final skeletal position at the chin amounts to 60% for a realistic prediction. However, the low-angle group differed from other groups by showing a markedly low soft-to-hard tissue ratio of only 14% at pogonion and a high ratio of 109% at the mentolabial fold. However, these differences in ratios between the groups were statistically not significant.

Craniofacial growth in untreated skeletal Class I subjects with low, average, and high MP-SN angles: A longitudinal study

Sixty-eight (36 male and 32 female) untreated skeletal Class I subjects with low (≤ 27°), average (>27°-<37°), and high (≥ 37°) mandibular plane (MP-SN) angles were selected from the Bolton-Brush and Burlington Growth Studies. Cephalograms of each subject at ages 9 and 18 were traced, and 28 parameters were measured. The difference in each parameter from ages 9 to 18 was calculated, and comparisons were made between the groups with low, average, and high angles. Results showed that, for boys and girls at age 9, the low-angle groups exhibited significantly larger SNA angle, SNB angle, facial taper, PFH, PFH:AFH, and ramus height, and the high-angle groups showed significantly larger ANS-Me and gonial angle. From ages 9 to 18, all the male and female low-, average-, and high-angle groups showed an increase in SNA and SNB angles, and PFH:AFH, and a decrease in ANB angle, convexity (more flattened face), MP-SN angle, and gonial angle (mandibular forward rotation). The dental measurements showed few changes with growth in all groups. In terms of skeletal measurements from ages 9 to 18, similar growth changes were found between the sexes in most angular measurements, but males had larger values in linear measurements than females.

Craniofacial growth in untreated skeletal Class II subjects: A longitudinal study

Eighty-five (45 male and 40 female) skeletal Class II untreated subjects with low (< 27°), average (27°- 36°), and high (> 36°) mandibular plane angles (MP-SN) were selected from the Bolton-Brush and the Burlington Growth Studies. Cephalograms of each subject at ages 9 and 18 were traced, and 28 parameters were measured. The difference in each parameter from ages 9 to 18 was calculated, and comparisons were made between the groups with low, average, and high angles. Results showed that for children at age 9, the high-angle group showed greater convexity, larger Y-axis and gonial angles, and greater anterior facial height, and the low-angle group had larger SNA and SNB angles, and greater posterior cranial base, mandibular body, ramus height, and posterior facial height. From ages 9 to 18, all the low-, average-, and high-angle groups showed a decrease of convexity (more flattened face) and a mandibular forward rotation (decreased MP-SN). The low-angle group displayed significantly more facial flattening and more mandibular forward rotation than did the high-angle group. Moreover, the mandibular incisors became more retroclined in the low-angle group and more proclined in the high-angle group with age. Comparisons between males and females showed similar skeletal growth patterns in angular measurements. However, a significant sex difference was noted in some linear measurements. (Am J Orthod Dentofacial Orthop 2002;122:619-26)

Cephalometric evaluation of the effects of pendulum appliance on various vertical growth patterns and of the changes during short-term stabilization.

The aim of this study was to evaluate the effects of the pendulum appliance in dental Class II patients with varying vertical growth patterns and to evaluate the changes during the short-term stabilization period of 3 months. The sample (n=30) was divided into two groups based on their FMA degrees. The high-angle group consisted of 14 patients (10 girls and 4 boys) and had a mean age of 157.7+/-8.0 months. The low-angle group consisted of 16 patients (8 girls and 8 boys) and had a mean age of 155.5+/-18.6 months. Pretreatment, posttreatment and poststabilization cephalometric radiographs were obtained to measure the changes. Mann-Whitney U and Wilcoxon tests were used for statistical evaluation. The amount of upper molar distalization was 5.9 mm (p<0.001) in the high-angle group and 1 mm (p<0.001) in the low-angle group, showing no intergroup difference. The amount of anchorage loss at the second premolars was 4.8 mm (p<0.001) in the high-angle group and 6.6 mm (p<0.001) in the low-angle group. Upper incisors moved anteriorly for 2.1 mm (p<0.05) in the high-angle group and 4.1 mm (p<0.001) in the low-angle group. Intergroup difference was statistically significant (p<0.001). During the stabilization period, 1.5 mm of anchorage loss was measured at the upper molar region in the high-angle group and 1.7 mm of anchorage loss was measured at the upper molar region in the low-angle group. During the stabilization period, upper second premolars and incisors tended to move back to their original places. The results of this study showed that pendulum appliance could move the upper molars distally in a short period of time without depending on the patient compliance. Care should be taken to prevent anchorage loss and to stabilize the upper molars for, at least, 3 months.

Morphology of the Laminar Junction in Relation to the Shape of the Hoof Capsule and Distal Phalanx in Adult Horses (Equus caballus)

The purpose was to investigate whether differences in equine hoof shape, which are inferred to alter foot function, are accompanied by differences in morphology of the laminar junction. Ten fore feet from adult horses were segregated into normal and low-angle groups, depending on the dorsal angle of the hoof wall. Twenty measurements of external hoof shape and four of the enclosed distal phalanx were tested for differences between groups, and for intragroup correlations. Three measurements of laminar morphology (spacing, orientation and degree of bend) were recorded for samples of up to 50 primary epidermal laminae at each of 20 sample sites. Sites were distributed over the foot in 5 circumferential columns and 4 proximodistal rows. Intergroup differences were investigated, as were correlations among sample sites of the laminar variables with the shape measurements. Results show differences in hoof shape between groups (but not bone shape) and laminar morphology. Six shape measurements are significantly different between groups: dorsal angle, medial and lateral angles, lateral sole width, solar circumference, and dorsal length. In the normal group, shape measurements show patterns of correlation among regions of the hoof, and between hoof and bone measurements. In the low-angle group, shape correlations occur largely within one region of the hoof (the heels) and in the bone measurements. Laminar spacing tends to be nonsignificantly greater in the low-angle group, while variances for laminar spacing and orientation are significantly greater in this group. Laminar spacing correlates with bone width and coronary circumference (CC) of the hoof in the normal group, but only with CC in the low-angle group. When taken as a whole, and interpreted in light of a model of foot mechanical function, the results appear to indicate a deterioration in structural coherence of the foot in the low-angle group.

Masticatory muscle activity in children and adults with different facial types

The purpose of this study was to examine the relationship between the duration of masticatory muscle activity during daytime and vertical craniofacial morphology in children and adults. Thirty children (12 boys and 18 girls) and 30 adults (20 men and 10 women) with normal anteroposterior skeletal relations, acceptable occlusions, and without any temporomandibular joint disorders, were selected as subjects. Activities of the masseter, temporal, and digastric muscles were recorded for 3 hours during daytime, excluding the periods for meals, sleep, and hard-exercise, using a portable electromyogram recording system. A lateral cephalogram was taken of each subject at the intercuspal position to divide the subjects into 3 different facial types, ie, low, average, and high angle groups. Masseter, temporal, and digastric muscle activities mainly consisted of low-amplitude bursts during daytime. Children exhibited longer duration of temporal muscle activity, whereas the masseter muscle presented longer duration of activity in adults. The activities of masseter and digastric muscles were significantly related with the vertical facial type in both children and adults, although temporal muscle activity presented no significant relationship with the craniofacial morphology. The duration of masticatory muscle activity during daytime showed a significant difference between children and adults, indicating a close association with vertical craniofacial morphology in children and adults. (Am J Orthod Dentofacial Orthop 2000;118:63-8)

Fracture system related to geothermal reservoir based on core samples of slim holes. Example from the Uenotai geothermal field, northern Honshu, Japan

In order to clarify the role of fracture permeability in a reservoir, the properties and formation mechanism of fractures were investigated on core samples from the Uenotai geothermal field, Japan. The fractures were located and identified from the records of lost circulation while drilling. Productive fractures are classified into a dominant high-angle (average: 77°) group, and a less common low-angle (average: 28°) group. The high-angle fractures were formed as extensional hydrothermal fractures in response to fault-value behavior triggered by fluid pressure activation under lithostatic conditions. The low-angle fractures formed as extensional and/or extensional shear fractures under hydrostatic-lithostatic transitional conditions, with subhorizontal σ 1 stress. Both kinds of fractures were accompanied by hydrothermal mineral deposition.

Mandibular rest position and electrical activity of the masticatory muscles

Statement of problem. The determination of a correct vertical dimension of occlusion is a critical procedure in clinical dentistry. Purpose. The objectives of this study were to analyze the relation between mandibular rest position and electrical activity of masticatory muscles and to compare clinical and electromyographic rest position in subjects with different vertical facial morphologic features. Material and methods. Clinical rest position and electromyographic rest position were investigated in 40 subjects. Electromyographic rest position ranged from 0.4 to 12.7 mm (average 7.7 ± 2.7 mm). Clinical rest position ranged from 0.1 to 4.4 mm (average 1.4 ± 1.1 mm). The average difference between electromyographic rest position and clinical rest position was 6.3 ± 2.5 mm (range 0.3 to 10.3 mm). Sixteen subjects were selected according to the Frankfort mandibular plane angle and separated in two groups with the first group (eight subjects) having a mandibular plane angle ≤20 degrees and the second group having a mandibular plane angle ≥28 degrees. Results. Rest position was significantly greater ( p < 0.05) in the low-angle group (2 ± 1.3 mm) than in the high-angle group (0.8 ± 0.8 mm). Electromyographic rest position did not differ between subjects with different facial morphologic features (8.1 ± 1.7 mm low-angle group; 7.6 ± 4.1 mm high-angle group). By varying the vertical dimension millimeter by millimeter, masseter and anterior temporal electromyographic activity demonstrated a considerable decrease over an interocclusal distance of 3 to 4 mm. Further mandibular opening up to 18 mm corresponded to small changes in postural activity. Conclusion. This study suggests that a jaw posture with a few millimeters of interocclusal distance involves a great reduction of masticatory muscle activity. (J Prosthet Dent 1997;78:48-53.)

The relationship between mandibular morphology and apical root curvature in man

Two samples of individuals, a low-angle group and a high-angle group, were selected on the basis of the inclination of the lower border of their mandible, defined by the mandibular plane angle (ML/SNL) in profile radiographs. Apical root curvature of mandibular first and second premolars and molars on panoramic radiographs was compared between the two groups and related to mandibular morphology. In the low-angle group most teeth had more distally curved roots than in the high-angle group. Significant differences were found for the first premolars on both sides ( p < 0.001, p < 0.05), the right second premolars ( p < 0.05) and the distal root of left first molars ( p < 0.05). Correlations were calculated between root curvature and two variables reflecting mandibular morphology, the mandibular plane angle and the gonial angle. The strongest associations were found between the first premolar roots and the gonial angle ( r s = −0.75, p < 0.001, for right and left sides respectively). The association between mandibular morphology and root curvature is probably indirect, caused by tooth migration during root formation.

Mandibular rest position in subjects with high and low mandibular plane angles

Studies have explored the relationship between different facial morphologies and mandibular posture; however, few conclusions can be drawn. The purpose of this study was to explore the effects of contrasting facial morphologies on both clinical and electromyographic rest positions of the mandible. Electromyographic and clinical rest positions were recorded in 10 subjects whose Frankfort horizontal-to-mandibular plane angles were less than 20 degrees (low-angle group) and in 10 whose angles were greater than 30 degrees (high-angle group). The vertical dimension of clinical rest position was recorded from soft tissue markings on the nose and chin after subjects wet their lips, swallowed, and pronounced Mississippi. Electromyographic rest was defined as the interincisal vertical dimension of minimal muscle activity as determined with an Enting Model 220 EMG integrator and the Myotronics K-5 kinesiograph. Head posture was held constant to the subject's visual axis. The vertical dimension of clinical rest position was significantly greater in the low-angle group (X = 4.6 mm., S.D. = 1.4) than in the high angle group (X = 3.1 mm., S.D. = 1.1); however, no differences were observed in the EMG rest positions, which were 9.9 and 9.7 mm., respectively, for the low- and high-angle groups. These results do not support a simple mechanical explanation of jaw posture; however, these measurements need to be repeated with subjects selecting their preferred head inclination.