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Relationship between molar occlusion and masticatory movement in lateral deviation of the mandible
ORIGINAL ARTICLE
Relationship between molar occlusion and masticatory movement in lateral deviation of the mandible Yuji Suzuki,a Katsuhiko Saitoh,a Ryutaroh Imamura,a Kaori Ishii,a Shinichi Negishi,a Ryuichi Imamura,b Masaru Yamaguchi,a and Kazutaka Kasaia Matsudo, Chiba, Japan
Introduction: The relationship between molar occlusion and chewing patterns was examined in subjects with laterally deviated mandibles. Methods: Twenty-three patients with mandibular deviation from the midline (4 mm or more) and skeletal Class I (0 #ANB #4 ) were divided into 2 groups: normal bite and crossbite. The chewing pattern was classified as normal, reversed, or crossover. Results: The normal bite group had a normal chewing pattern on the affected side 100% of the time and a reversed chewing pattern on the affected and unaffected sides 0% and 7.2% of the time, respectively. Additionally, the normal bite group showed no evidence of a crossover chewing pattern and also had significantly less axial inclination of the mandibular teeth on the affected side compared with the crossbite group; lingual inclination was also evident. The crossbite group had a normal chewing pattern on the affected and unaffected sides 0% and 55.6% of the time, respectively, and reversed and crossover chewing patterns on the affected side 55.6% and 44.4% of the time, respectively. Conclusions: A normal chewing pattern tends to result in lingual axial inclination of the mandibular molars on the affected side, as well as a more consistent chewing pattern. (Am J Orthod Dentofacial Orthop 2017;151:1139-47)
L
ateral deviation of the mandible in a person with facial asymmetry can readily lead to esthetic and functional impairment. In a study of 1460 patients with a dentofacial deformity, Severt et al1 found that 34% had facial asymmetry. They reported that 74% of the patients with facial asymmetry had lateral deviation of the mandible. Additionally, lateral deviation of the mandible presents a number of problems, such as differences in the inclination of the occlusal plane, incongruity of the length of the body or rami of the mandible, and telescopic occlusion or crossbite of the molars. Sato et al2 reported that craniofacial development and occlusion are closely related factors, leading to lateral deviation of the mandible and skeletal crossbite.
From the School of Dentistry, Nihon University, Matsudo, Chiba, Japan. a Department of Orthodontics. b Department of Maxillofacial Orthodontics. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: Yuji Suzuki, Department of Orthodontics, Nihon University School of Dentistry, Matsudo, Chiba, Japan 271-8587; e-mail, yuji. [email protected]. Submitted, July 2016; revised and accepted, November 2016. 0889-5406/$36.00 Ó 2017 by the American Association of Orthodontists. All rights reserved. http://dx.doi.org/10.1016/j.ajodo.2016.11.023
Nakaminami et al3 reported that subjects with lateral deviation of the mandible have a shorter trajectory of chewing than do those with normal occlusion, and that those with lateral deviation of the mandible often have a linear path of masticatory movement. These findings indicate that a lateral deviation of the mandible causes masticatory movements uncharacteristic of that seen in normal occlusion, and this chewing motion may affect stomatognathic function.2,3 In some instances, lateral deviation of the mandible may cause a crossbite in the molars on 1 side of the mouth. If a molar crossbite is evident, then chewing proceeds in a reverse sequence (the mandible moves laterally and then downward), as reported in several studies.3-10 Nonetheless, a study has reported that treatment can normalize the chewing pattern (the mandible will move downward and then laterally) in children with a reverse sequence from a crossbite.11 However, no authors have fully examined the relationship between masticatory movement in laterally deviated mandibles and axial inclination of the teeth. In this study, we aimed to examine the relationship between the presence or absence of molar crossbite and molar occlusion and chewing patterns in patients with a lateral deviation of the mandible. 1139
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Fig 1. Reference points and measurement points on a frontal cephalogram: ANS, Anterior nasal spine; deviation in mental spine, vertical distance between VRL and Me; GO, shifted-side gonion; GO0 , nonshifted side gonion; LO, latero-orbitale (right side); LO0 , latero-orbitale (left side); Me, mental spine; Mo, molar; NC, maximum thinning of the crista galli; occlusal plane, angle formed by Mo-Mo’ from x-axis; VRL, a straight line passing through NC and orthogonal to a horizontal reference line.
Fig 2. Reference points and measurement points on a frontal cephalogram: Col-Go, Distance between condylar (Col) and GO; Col’-Go’, distance between Col’ and GO’; Col-Me, distance between Col and Me; Col’-Me, distance between Col’ and Me; GO-Me, distance between GO and Me; GO0 -Me, distance between GO0 and Me; VRL, a straight line passing through NC and orthogonal to a horizontal reference line.
This study was approved by the ethics committee of Nihon University's School of Dentistry at Matsudo, Chiba, Japan (number EC16-14-042-1).
the maxillary alveolar base; this resulted in a cohort of 23 patients. Additionally, the measurement points and the items measured were selected based on studies by Uesugi et al,12 Damstra et al,13 and Janson et al14 (Figs 1 and 2). The exclusion criteria were the following: (1) patients with temporomandibular joint pain or temporomandibular joint dysfunction (eg, trismus); (2) patients who had previously received orthodontic treatment; (3) patients with dental prostheses, dental caries, or missing teeth; (4) patients with a congenital deformity or syndrome, or previous trauma; and (5) ptients with a functional crossbite. If the maxillary and mandibular first molars on the affected side (the side to which the mandible was deviating) were in normal occlusion on the frontal sections, the subject would be deemed as having a normal bite (14 patients with a normal bite [NB group]: 2 men, 12 women; mean age, 19.5 6 4.5 years). If a crossbite was evident on the affected side, the patient would be deemed as having a crossbite (9 patients with a cross-bite [CB group]: 1 man, 8 women; mean age, 25.1 6 5.1 years).
MATERIAL AND METHODS
Patients with a dentofacial deformity who visited the Department of Orthodontics at the Nihon University's hospital within the past 8 years were considered for enrollment. On a frontal cephalogram, a straight line connecting the crista galli and the anterior nasal spine served as the midline of the face; a straight line connecting the anterior nasal spine and the mental spine of the mandible served as the midline of the mental spine.12 Candidates for enrollment were patients with a lateral deviation of the mandible, which was defined as a deviation of 4 mm or more from the midline of the face to the mental spine. Patients with a lateral deviation of the mandible, who were also classified as skeletal Class I (0 #ANB #4 ), were included. A model of the maxillary and mandibular dentitions in occlusion was used to determine whether the mandibular molars were located buccally with respect to
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Fig 3. Masticatory pattern classification, described as follows. CO, Centric occlusion. In the normal pattern, from centric occlusion, the mandible moves downward and then laterally toward the chewing side or the nonchewing side, before returning to centric occlusion along a concave, convex, or linear path. In the reverse pattern, the reverse of normal chewing, the mandible moves laterally first before moving downward and then returning to centric occlusion. In the crossover pattern, the mandible moves slightly laterally, downward, slightly laterally again, and then returns to centric occlusion.
The Gnatho-Hexagraph II (GC Corporation, Tokyo Japan) was used to measure masticatory movement. A mandibular clutch was placed on the subject's mandibular anterior teeth. The patient was then instructed to relax in a seated position; the patient's head was not immobilized. Once the Frankfort plane was horizontal, a head frame and a facebow were attached. The upper margin of the external acoustic meatus on each side constituted a reference plane. The upper margin of each external acoustic meatus and the inferior margin of the left orbit constituted the Frankfort horizontal plane. The mandibular condyles, the mesiobuccal cusp of the mandibular first molars, and the point of contact between the mandibular central incisors served as measurement points.15 The patients were instructed to freely chew the chewing gum. Once the gum softened, the subject was instructed to start at the maximal intercuspal position and chew the gum on each side for 30 seconds. This masticatory movement was recorded. The test food used was a piece (1.5 g) of normal chewing gum (100% xylitol chewing gum; Oral Care, Tokyo, Japan). Masticatory movement was analyzed at the mandibular incisal point, and a total of 10 chewing strokes (strokes 5-14) on the dominant chewing side were analyzed.6,16 The pattern of masticatory movement was analyzed using the software that came with the equipment to measure jaw movement. Gum was chewed on the affected and unaffected sides for 30 seconds each. The chewing pattern was based on a total of 10 chewing strokes (strokes 5-14). The chewing pattern was classified as normal (from centric occlusion, the mandible moves downward and
then laterally toward the chewing side or the nonchewing side, before returning to centric occlusion along a concave, convex, or linear path), reversed (the reverse of normal chewing; the mandible moves laterally first before moving downward and then returning to centric occlusion), or crossover (the mandible moves slightly laterally, downward, slightly laterally again, and then returns to centric occlusion) (Fig 3).17,18 The items in Figures 4 and 5 were measured using the method of Eguchi et al.19 To model the dentition before orthodontic treatment, the width of the maxillary dentition, the width of the mandibular dentition, the palatal width at the maxillary first molars, the axial inclination of the maxillary first molars, and the axial inclination of the mandibular first molars were measured.20-23 A 3-dimensional scanner (Yasunaga Computer Systems, Co., Inc., Fukui, Japan) was used to obtain 3-dimensional data on the dentition, and the data were analyzed using 3-dimensional analysis software (Medic Engineering Corporation; Kyoto, Japan). The width of the maxillary dentition, width of the mandibular dentition, palatal width at the maxillary first molars, and buccolingual inclination of the maxillary and mandibular first molars were measured.22 A plane passing through the incisal ridge and the cusps of the second premolars and first molars served as the maxillary reference plane. A straight line passing through the cusps of the second premolars and the first molars served as the x-axis, a straight line orthogonal to the x-axis served as the y-axis, and a straight line orthogonal to the intersection of the x-axis and y-axis in the reference plane served as
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Fig 4. Maxillary and mandibular dentition measurements: the distance between the mesial cusps of the maxillary first molars (U6-6CW), the maximum distance from the lingual groove of the maxillary first molars to the neck of the tooth (U6GW), and the distance between the mesial cusps of the mandibular first molars (L6-6CW).
Fig 5. Axial inclinations of the maxillary and mandibular first molars: a straight line passed through the transition point from the groove on the buccal aspect of the first molar to the buccal groove on the occlusal surface, and through to the transition point from the groove on the lingual aspect to the lingual groove on the occlusal surface. The angle formed by this straight line and the z-axis served as the axial inclination of the teeth, yielding an axial inclination on the affected side (AI-AS) and an axial inclination on the unaffected side (AI-US).
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Table I. Results of the measurement points on the
lateral and frontal cephalograms
Table II. Results of the measurement points on the lateral and frontal cephalograms
NB group (n 5 14) CB group (n 5 9) ANB ( ) Occlusal plane ( ) Deviation in mental spine (mm)
Mean 1.9 2.5 7.3
SD 1.0 2.1 2.2
Mean 2.0 2.9 8.9
SD 1.3 2.4 4.2
t test NS NS NS
NS, Not significant.
the z-axis. A plane passing through the incisal ridge of the mandibular central incisors and the cusps of the mandibular deciduous second molars (second premolars), and the cusps on the crown of the first molar served as the mandibular reference plane. A straight line was passed through the transition point from the groove on the buccal aspect of the first molar to the buccal groove on the occlusal surface, and through the transition point from the groove on the lingual aspect to the lingual groove on the occlusal surface. The angle formed by this straight line and the z-axis served as the axial inclination of the teeth, yielding an axial inclination on the affected side, and an axial inclination on the unaffected side. Statistical analysis
The percentages of chewing patterns on the affected and unaffected sides were calculated. The chewing patterns were normally distributed in each group. The Student t test was used to test for significant differences between the groups for the widths of the maxillary and mandibular dentitions, palatal widths, and axial inclinations of the maxillary and mandibular first molars on the affected and unaffected sides. RESULTS
Measurements from lateral cephalograms are shown in Tables I and II. Authors of previous studies have measured the distances between condylar and mental spine, condylar and gonion, and gonion and mental spine on the affected and unaffected sides according to lateral cephalograms to identify facial asymmetry.12-14,24 We analyzed lateral cephalograms in accordance with the analysis of Ricketts.25 The deviations in mental spine were 7.3 6 2.2 mm (mean) for patients in the NB group and 8.9 6 4.2 mm (mean) for patients in the CB group. Significant differences in the deviation in mental spine were not noted. Similarly, significant differences in the occlusal plane angle were not noted. The values for condylarmental spine, condylar-gonion, and gonion-mental spine were greater on the unaffected side compared with the
NB group (n 5 14) Col-Me (mm) Col-Go (mm) Go-Me (mm) CB group (n 5 9) Col-Me (mm) Col-Go (mm) Go-Me (mm)
Affected side
Unaffected side
Mean
SD
Mean
SD
t test
112.1 60.9 60.8
9.8 5.3 6.7
117.6 64.1 65.0
9.5 4.0 8.5
NS NS NS
110.1 59.5 58.4
6.0 5.2 4.4
118.6 65.4 63.1
5.6 4.0 6.5
y
* NS
NS, Not significant; Col-Me, distance between Col and Me; Col-Go, distance between Col and GO; Go-Me, distance between GO and Me. *P \0.05; yP \0.01.
Table III. Masticatory patterns of subjects in the
normal bite (NB) and crossbite (CB) groups on the affected side (AS) and unaffected side (US) NB group (n 5 14) Normal pattern Reversed pattern Crossover pattern
AS US 100% (14/14) 92.8% (13/14)
CB group (n 5 9) AS US 0% (0/9) 55.6% (5/9)
0% (0/14)
7.2% (1/14) 55.6% (5/9) 33.3% (3/9)
0% (0/14)
0% (0/14) 44.4% (4/9) 11.1% (1/9)
affected side for the NB group and the CB group. These findings indicated facial asymmetry (Tables I and II). Table III presents a tally of the chewing patterns observed among the patients. Patients in the NB group had a normal chewing pattern on the affected side 100% of the time and a normal chewing pattern on the unaffected side 92.8% of the time. One subject from the NB group had a reversed pattern of chewing on the affected side 0% of the time, and a reversed pattern of chewing on the unaffected side 7.2% of the time. No patient in the NB group had a crossover chewing pattern on either the affected or unaffected side. In contrast, patients in the CB group had a normal chewing pattern on the affected side 0% of the time and a normal chewing pattern on the unaffected side 55.6% of the time. The patients in the CB group had a reversed pattern of chewing on the affected side 55.6% of the time and a reversed pattern of chewing on the unaffected side 33.3% of the time. The patients in the CB group had a crossover chewing pattern on the affected side 44.4% of the time and a crossover chewing pattern on the unaffected side 11.1% of the time. Comparisons of the width of the maxillary and mandibular dentitions, palatal width, and axial
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Table IV. Axial inclinations of the maxillary and
mandibular first molars and arch widths of the first molar NB group (n 5 14) Mean Linear measurements (mm) U6-6CW 53.3 L6-6CW 45.8 U6-L6CW 7.5 U6GW 36.9 Angular measurements ( ) AIMx-AS 96.0 AIMx-US 90.8 AIMa-AS 64.8 AIMa-US 74.1
CB group (n 5 9)
SD
Mean
SD
t test
3.5 3.0 2.8 2.9
50.0 46.1 3.9 33.6
3.7 2.6 2.8 3.3
* NS
4.8 6.1 5.3 8.1
98.6 86.0 70.4 70.8
4.0 2.9 6.0 6.5
NS * * NS
y
*
NS, Not significant; NB, normal bite; CB, crossbite; U6-6CW, width of the maxillary dentition; L6-6CW, width of the mandibular dentition; U6-L6CW, width of the maxillary dentition and the width of the mandibular dentition; U6GW, palatal width of maxillary first molars; AIMx-AS, axial inclination of the maxillary teeth on the affected side; AIMx-US, axial inclination of the maxillary teeth on the unaffected side; AIMa-AS, axial inclination of the mandible teeth on the affected side; AIMa-US, axial inclination of the mandible teeth on the unaffected side. *P \0.05; yP \0.01.
inclinations of the maxillary and mandibular first molars are shown in Table IV. The width of the maxillary dentition and palatal width were significantly greater in the patients in the NB group than in the patients in the CB group. The patients in the NB group had a significantly greater difference between the width of the maxillary dentition and the width of the mandibular dentition compared with that of the patients in the CB group. Significant differences in the width of the mandibular dentition were not noted. Significant differences in the axial inclination of the maxillary teeth on the affected side were not noted. Compared with the patients in the NB group, those in the CB group had significantly less axial inclination of the maxillary teeth on the unaffected side. Additionally, patients in the NB group had significantly less axial inclination of the mandibular teeth on the affected side compared with patients in the CB group. Significant differences in the axial inclination of the mandibular teeth on the unaffected side were not noted. DISCUSSION
The coverage of the first molars on the frontal sections was classified into either 1 of 2 categories: normal coverage on the affected side (NB group) or evidence of a crossbite (CB group). Chewing patterns were also assessed. A previous study reported that chewing is regular and consistent with a normal occlusion,26 whereas
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another study reported that a typical chewing pattern in patients with normal occlusion is one where the mandible moves smoothly from centric occlusion, downward, and then laterally toward the chewing side or the nonchewing side before returning to centric occlusion along a convex path.17 Various studies have evaluated and classified the differences in chewing patterns. Shiga et al16 assessed the relationship between occlusal contact during lateral movements and chewing patterns, and classified them into 7 types. Proeschel27 examined the chewing patterns of normal persons and those of patients with Angle Class I or Class II malocclusion, and derived 8 classifications of chewing patterns. Yano et al4 and Tomoyose et al28 both examined the chewing patterns of subjects with mandibular asymmetry; Yano et al derived 5 classifications of chewing patterns, and Tomoyose et al derived 3 patterns. To ascertain the normal and particular patterns of chewing, we classified chewing patterns into 3 types (normal, reversed, or crossover). Patients who chewed in a normal pattern moved their mandibles downward from centric occlusion and then laterally toward the chewing side or the nonchewing side, before returning their mandibles to centric occlusion along a concave, convex, or linear path. Thus, the patients had a chewing pattern that was consistent with that of subjects with normal occlusion.16 Patients who chewed in a reversed pattern (ie, in reverse sequence) moved their mandible laterally first, before moving it downward, and then returning it to centric occlusion. This pattern is often noted in patients with a laterally deviated mandible or prognathism, where a molar crossbite is more likely to occur. Nakaminami et al3 reported that a reversed chewing pattern tends to be highly prevalent among patients with a molar crossbite. Patients in the NB group mostly had a normal chewing pattern; the molars on the affected and unaffected sides were normally covered. However, 1 subject in the NB group had a reversed chewing pattern on the unaffected side. When there is occlusal contact on the balancing side, chewing can proceed in reversed order (the mandible will first move laterally and then downward).16 With a molar crossbite, chewing can proceed in a reversed sequence so that it does not interfere with occlusion.27 Thus, the differences in occlusal contact may lead to different chewing patterns because of their presumed associations with one another. Similarly, occlusion on the unaffected side is unobstructed by the return of the mandible to its original position; this may potentially account for the reversed pattern of chewing. Patients in the CB group did not have a normal chewing pattern on the affected side; instead, they had a reversed pattern (55.6%) or a crossover pattern
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(44.4%) of chewing. Patients in the CB group had a normal chewing pattern on the unaffected side (55.6%), a reversed pattern (33.3%), or a crossover pattern (11.1%). Tomonari et al5 examined the relationship between molar coverage and masticatory movement; they reported that the sequence of chewing was reversed in the CB group: ie, lateral movement followed by downward movement of the mandible. In our study, the presence of a molar crossbite interfered with the cusps of the molars on the affected side during chewing, hindering masticatory movement. This resulted in a reversed or a crossover chewing pattern with a shorter trajectory of chewing than the trajectory of chewing in a normal chewing pattern. A reversed or crossover pattern of chewing was noted with malocclusion of the unaffected side. This led to inconsistent chewing patterns on the unaffected sides. Lateral deviation of the mandible involves asymmetry of the rami or the body of the mandible and the mandibular condyles, and asymmetry of the temporomandibular joints and muscles.29 Thus, morphologic aspects, such as the asymmetry of the mandibular condyles, must be examined in future studies of patients with a lateral deviation of the mandible. In this study, the width of the maxillary dentition, palatal width, and the difference between the width of the maxillary dentition and the width of the mandibular dentition were significantly greater among the patients in the NB group than among those in the CB group. Authors of 1 study found that persons with a normal chewing pattern chew with grinding movements.30 Negishi et al31,32 reported that subjects with a normal chewing pattern chew with grinding movements, and that this aids in the lateral expansion of the maxillary midpalatal suture. Additionally, Cattaneo et al33 analyzed a finite element model of the adult maxilla and reported that stress was concentrated around the key ridge. Their findings were similar to those of Negishi et al.32 Furthermore, authors of a study reported that major changes in the development of the maxillary dental arch result from the lateral expansion of the palate and not from the axes of the teeth.22 Nonetheless, the expansion of the maxillary midpalatal suture occurs primarily during childhood.34 The patients in our study were adults, so the differences in the width of the maxillary dentition of the NB and CB groups had presumably resulted from the changes in the axial inclinations of the mandibular molars. Significant differences in the axial inclinations of the maxillary teeth on the affected side were not noted. The maxillary teeth on the affected sude tended to be buccally inclined in the patients in the CB group. A
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comparison indicated that the patients in the CB group had significantly less axial inclination of the maxillary teeth on the unaffected side compared with the patients in the NB group. It was also observed that the patients in the CB group had lingual inclinations of the maxillary teeth on the unaffected side, and that the patients in the NB group had significantly less axial inclination of the mandibular teeth on the affected side relative to the patients in the CB group. Compared with the patients in the CB group, the patients in the NB group had lingually inclined molars on the affected side. A comparison showed no significant differences in the axial inclination of the mandibular teeth on the unaffected side. The molars on the unaffected side tended to be buccally inclined in the patients in the NB group, compared with the patients in the CB group. In this study, a lateral deviation of the mandible caused the mandibular molars to be located more buccally with respect to the maxillary dentition. Studies of subjects with normal occulusion have reported that stress is primarily placed on the lingual aspects of the buccal cusps of the mandibular molars during normal chewing.35,36 A study of patients with lateral deviation of the mandible reported that the occlusal contact area was significantly greater on the affected side of the mouth.37 In subjects with normal occulusion, the buccal cusps of the mandibular molars serve as functional cusps, but a lateral deviation of the mandible may result in lingual cusps of the mandibular molars serving as functional cusps. In other words, normal masticatory movements occur, and the mandibular molars may be forced toward the tongue, presumably causing the mandibular molars on the affected side to be lingually inclined. Takada et al38 reported that intraoral pressure was significantly correlated with the inclination of the mandibular first molars in patients with facial asymmetry; they found that an imbalance in intraoral pressure distribution was related to the incidence of asymmetric mandibular dentition and dental compensation. A reversed or crossover chewing pattern with a shorter trajectory of chewing presumably places less stress on the lingual aspect of the mandibular dentition. Dental compensation can lead to anteroposterior incongruity in the bones of the jaw and can cause horizontal incongruity.39 In patients with facial asymmetry, dental compensation is correlated to buccolingual pressure on the affected and unaffected sides, the buccolingual position of the mandibular molars, and the inclination of those molars. Buccolingual pressure and asymmetry in the dental arch are reported to be related to the dental compensation of the molars.40 Ishikawa et al40,41 reported that dental compensation was involved in maintaining coverage of the molars.
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Chewing with grinding strokes is reported to cause the mandibular molars to incline to a more upright position.31 Additionally, masticatory movement and changes in the axial inclination are closely related. Thus, a normal chewing pattern may cause lingual inclination of the teeth on the affected side and lead to more consistent chewing patterns. CONCLUSIONS
Masticatory movement and axial inclination are closely related. In subjects with lateral deviation of the mandible and a normal chewing pattern, the width of the maxillary dentition can increase as a result of the changes (ie, the orientation) in the axes of the teeth. A normal chewing pattern may lead to lingual inclination of the mandibular molars on the affected side. REFERENCES 1. Severt TR, Proffit WR. The prevalence of facial asymmetry in the dentofacial deformities population at the University of North Carolina. Int J Adult Orthod Orthognath Surg 1997;12:171-6. 2. Sato S, Takamoto K, Goto M, Kamoi S, Suzuki Y. An approach to development of skeletal malocclusion with mandibular lateral displacement. J Jpn Kanagawa Soc 1990;25-1:93-8. 3. Nakaminami T, Nishio K, Miyauchi S, Maruyama T. The effect of posterior crossbite on stomatognathic function. J Jpn Soc Stomatognath Funct 1968;6:87-96. 4. Yano K, Kubota M, Shinohara C, Kanegae H, Shibasaki Y. The correlation between preferred chewing and the horizontal condylar angle in patients with mandibular asymmetry. Jpn Jaw Deform 2000;10:110-6. 5. Tomonari H, Ikemori T, Kubota T, Uehara S, Miyawaki S. First molar cross-bite is more closely associated with a reverse chewing cycle than anterior or premolar cross-bite during mastication. J Oral Rehabil 2014;41:890-6. 6. Nie O, Kanno Z, Tianmin X, Jiuxiang L, Soma K. Clinical study of frontal chewing patterns in various crossbite malocclusions. Am J Orthod Dentofacial Orthop 2010;138:323-9. 7. Piancino MG, Talpone F, Dalmasso P, Debernardi C, Lewin A, Bracco P. Reverse-sequencing chewing patterns before and after treatment of children with unilateral posterior crossbite. Eur J Orthod 2006;28:480-4. 8. Martin C, Alarcon JA, Palma JC. Kinesiographic study of the mandible in young patients with unilateral posterior cross-bite. Am J Orthod Dentofacial Orthop 2000;118:541-8. 9. Throckmorton GS, Buschang PH, Hayasaki H, Pinto AS. Changes in the masticatory cycle following treatment of posterior unilateral crossbite in children. Am J Orthod Dentofacial Orthop 2001;120:521-9. 10. Rilo B, da Silva JL, Mora MJ, Cadarso-Suarez C, Santana U. Unilateral posterior crossbite and mastication. Arch Oral Biol 2007;52:474-8. 11. Kwak YY, Jang I, Choi DS, Cha BK. Functional evaluation of orthopedic and orthodontic treatment in a patient with unilateral posterior crossbite and facial asymmetry. Korean J Orthod 2014;44: 143-53. 12. Uesugi S, Yonemitsu I, Kokai S, Takei M, Omura S, Ono T. Features in subjects with the frontal occlusal plane inclined toward the contralateral side of the mandibular deviation. Am J Orthod Dentofacial Orthop 2016;149:46-54.
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13. Damstra J, Fourie Z, Ren Y. Evaluation and comparison of posteroanterior cephalograms and cone-beam computed tomography images for the detection of mandibular asymmetry. Eur J Orthod 2013;35:45-50. 14. Janson G, de Lima KJ, Woodside DG, Metaxas A, de Freitas MR, Henriques JF. Class II subdivision malocclusion types and evaluation of their asymmetries. Am J Orthod Dentofacial Orthop 2007; 131:57-66. 15. Hayashi R, Kawamura A, Kasai K. Relationship between masticatory function, dental arch width, and bucco-lingual inclination of the first molars. Orthod Waves Jpn Ed 2006;65:120-6. 16. Shiga H, Kobayashi Y, Arakawa I, Yokoyama M, Tanaka A. Relationship between pattern of masticatory path and state of lateral occlusal contact. J Oral Rehabil 2009;36:250-6. 17. Akiyama H, Shiga F, Kobayashi Y. The analysis of masticatory movements—frontal patterns of chewing path of incisor point in normal subjects. J Jpn Prosthodont Soc 1991;35:609-21. 18. Proschel P, Hofmann M. Frontal chewing patterns of the incisor point and their dependence on resistance of food and type of occlusion. J Prosthet Dent 1988;59:617-24. 19. Eguchi S, Townsend GC, Toby H, Kasai K. Genetic and environmental contributions to variation in the inclination of human mandibular molars. Orthod Waves 2004;63:95-100. 20. Oliveira NL, Da Silveira AC, Kusnoto B, Viana G. Three-dimensional assessment of the maxilla: a comparison of 2 kinds of palatal expanders. Am J Orthod Dentofacial Orthop 2004; 126:354-62. 21. Veli I, Yuksel B, Uysal T. Longitudinal evaluation of dental arch asymmetry in Class II subdivision malocclusion with 3dimensional digital models. Am J Orthod Dentofacial Orthop 2014;145:763-70. 22. Okano M. A growth pattern of bucco-lingual inclination and arch width of first molar—a comparison with Japanese and 2 groups of South Pacific populations. Orthod Waves Jpn Ed 2006;65:112-21. 23. Langberg BJ, Arai K, Miner RM. Transverse skeletal and dental asymmetry in adults with unilateral posterior crossbite. Am J Orthod Dentofacial Orthop 2005;127:6-16. 24. Masuoka N, Muramatsu A, Ariji Y, Nawa H, Goto S, Ariji E. Discriminative thresholds of cephalometric indexes in the subjective evaluation of facial asymmetry. Am J Orthod Dentofacial Orthop 2007; 131:609-13. 25. Ricketts RM. The value of cephalometrics and computerized technology. Angle Orthod 1972;42:179-99. 26. Jemt T, Hedegard B. Reproducibility of chewing rhythm and of mandibular displacements during chewing. J Oral Rehabil 1982; 9:531-7. 27. Proeschel PA. Chewing patterns in subjects with normal occlusion and with malocclusions. Semin Orthod 2006;12:138-49. 28. Tomoyose Y, Bandai H, Sugawara J, Mitani H. Characteristics of chewing path in skeletal Class III patients with mandibular asymmetry. Orthod Waves 2002;61:376-91. 29. Pirttiniemi P, Kantomaa T. Relation of glenoid fossa morphology to mandibulofacial asymmetry, studied in dry human Lapp skulls. Acta Odontol Scand 1992;50:235-43. 30. Watt DM, MacGregor AR. Designing complete dentures. 1st ed. Philadelphia: W. B. Saunders; 1976. p. 152-5. 31. Negishi S, Hayashi R, Saitoh K, Kasai K. The influence of masticatory exercises using hard chewing gum on chewing pattern and first molar of mixed dentition. Orthod Waves Jpn Ed 2010;69: 156-62. 32. Negishi S, Hayashi R, Nakagawa A, Murata Y, Kasai K. A discussion of masticatory force mechanics on the median palatine suture of
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34. 35.
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children: analysis involving a 3-dimensional finite element technique. Orthod Waves Jpn Ed 2013;72:164-72. Cattaneo PM, Dalstra M, Melsen B. The transfer of occlusal forces through the maxillary molars: a finite element study. Am J Orthod Dentofacial Orthop 2003;123:367-73. Enlow DH, Moyers RE. Handbook of facial growth. 2nd ed. London: United Kingdom; 1982. p. 154-77. Wang MQ, Zhang M, Zhang JH. Photoelastic study of the effects of occlusal surface morphology on tooth apical stress from vertical bite forces. J Contemp Dent Pract 2004;5: 74-93. Dejak B, Młotkowski A, Romanowicz M. Finite element analysis of stresses in molars during clenching and mastication. J Prosthet Dent 2003;90:591-7. Yoshida Y, Goto T, Yoshiura K. The balance of occlusal force, occlusal contact area and occlusal pressure in patients with
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mandibular deviation. Jpn Soc Oral Diagn Oral Med 2006;19: 1-6. Takada J, Ono T, Miyamato J, Yokota T, Moriyama K. Association between intraoral pressure and molar position and inclination in subject with facial asymmetry. Eur J Orthod 2010;9: 243-9. Kim SJ, Kim KH, Yu HS, Baik HS. Dentoalveolar compensation according to skeletal discrepancy and overjet in skeletal Class III patients. Am J Orthod Dentofacial Orthop 2014;145: 317-24. Ishikawa H, Nakamura S, Iwasaki H, Kitazawa S, Tsukada H, Sato Y. Dentoalveolar compensation related to variations in sagittal jaw relationships. Angle Orthod 1999;69:534-8. Ishikawa H, Nakamura S, Iwasaki H, Kitazawa S, Tsukada H, Chu S. Dentoalveolar compensation in negative overjet cases. Angle Orthod 2000;70:145-8.
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Relationship between molar occlusion and masticatory movement in lateral deviation of the mandible Yuji Suzuki,a Katsuhiko Saitoh,a Ryutaroh Imamura,a Kaori Ishii,a Shinichi Negishi,a Ryuichi Imamura,b Masaru Yamaguchi,a and Kazutaka Kasaia Matsudo, Chiba, Japan
Introduction: The relationship between molar occlusion and chewing patterns was examined in subjects with laterally deviated mandibles. Methods: Twenty-three patients with mandibular deviation from the midline (4 mm or more) and skeletal Class I (0 #ANB #4 ) were divided into 2 groups: normal bite and crossbite. The chewing pattern was classified as normal, reversed, or crossover. Results: The normal bite group had a normal chewing pattern on the affected side 100% of the time and a reversed chewing pattern on the affected and unaffected sides 0% and 7.2% of the time, respectively. Additionally, the normal bite group showed no evidence of a crossover chewing pattern and also had significantly less axial inclination of the mandibular teeth on the affected side compared with the crossbite group; lingual inclination was also evident. The crossbite group had a normal chewing pattern on the affected and unaffected sides 0% and 55.6% of the time, respectively, and reversed and crossover chewing patterns on the affected side 55.6% and 44.4% of the time, respectively. Conclusions: A normal chewing pattern tends to result in lingual axial inclination of the mandibular molars on the affected side, as well as a more consistent chewing pattern. (Am J Orthod Dentofacial Orthop 2017;151:1139-47)
L
ateral deviation of the mandible in a person with facial asymmetry can readily lead to esthetic and functional impairment. In a study of 1460 patients with a dentofacial deformity, Severt et al1 found that 34% had facial asymmetry. They reported that 74% of the patients with facial asymmetry had lateral deviation of the mandible. Additionally, lateral deviation of the mandible presents a number of problems, such as differences in the inclination of the occlusal plane, incongruity of the length of the body or rami of the mandible, and telescopic occlusion or crossbite of the molars. Sato et al2 reported that craniofacial development and occlusion are closely related factors, leading to lateral deviation of the mandible and skeletal crossbite.
From the School of Dentistry, Nihon University, Matsudo, Chiba, Japan. a Department of Orthodontics. b Department of Maxillofacial Orthodontics. All authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest, and none were reported. Address correspondence to: Yuji Suzuki, Department of Orthodontics, Nihon University School of Dentistry, Matsudo, Chiba, Japan 271-8587; e-mail, yuji. [email protected]. Submitted, July 2016; revised and accepted, November 2016. 0889-5406/$36.00 Ó 2017 by the American Association of Orthodontists. All rights reserved. http://dx.doi.org/10.1016/j.ajodo.2016.11.023
Nakaminami et al3 reported that subjects with lateral deviation of the mandible have a shorter trajectory of chewing than do those with normal occlusion, and that those with lateral deviation of the mandible often have a linear path of masticatory movement. These findings indicate that a lateral deviation of the mandible causes masticatory movements uncharacteristic of that seen in normal occlusion, and this chewing motion may affect stomatognathic function.2,3 In some instances, lateral deviation of the mandible may cause a crossbite in the molars on 1 side of the mouth. If a molar crossbite is evident, then chewing proceeds in a reverse sequence (the mandible moves laterally and then downward), as reported in several studies.3-10 Nonetheless, a study has reported that treatment can normalize the chewing pattern (the mandible will move downward and then laterally) in children with a reverse sequence from a crossbite.11 However, no authors have fully examined the relationship between masticatory movement in laterally deviated mandibles and axial inclination of the teeth. In this study, we aimed to examine the relationship between the presence or absence of molar crossbite and molar occlusion and chewing patterns in patients with a lateral deviation of the mandible. 1139
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Fig 1. Reference points and measurement points on a frontal cephalogram: ANS, Anterior nasal spine; deviation in mental spine, vertical distance between VRL and Me; GO, shifted-side gonion; GO0 , nonshifted side gonion; LO, latero-orbitale (right side); LO0 , latero-orbitale (left side); Me, mental spine; Mo, molar; NC, maximum thinning of the crista galli; occlusal plane, angle formed by Mo-Mo’ from x-axis; VRL, a straight line passing through NC and orthogonal to a horizontal reference line.
Fig 2. Reference points and measurement points on a frontal cephalogram: Col-Go, Distance between condylar (Col) and GO; Col’-Go’, distance between Col’ and GO’; Col-Me, distance between Col and Me; Col’-Me, distance between Col’ and Me; GO-Me, distance between GO and Me; GO0 -Me, distance between GO0 and Me; VRL, a straight line passing through NC and orthogonal to a horizontal reference line.
This study was approved by the ethics committee of Nihon University's School of Dentistry at Matsudo, Chiba, Japan (number EC16-14-042-1).
the maxillary alveolar base; this resulted in a cohort of 23 patients. Additionally, the measurement points and the items measured were selected based on studies by Uesugi et al,12 Damstra et al,13 and Janson et al14 (Figs 1 and 2). The exclusion criteria were the following: (1) patients with temporomandibular joint pain or temporomandibular joint dysfunction (eg, trismus); (2) patients who had previously received orthodontic treatment; (3) patients with dental prostheses, dental caries, or missing teeth; (4) patients with a congenital deformity or syndrome, or previous trauma; and (5) ptients with a functional crossbite. If the maxillary and mandibular first molars on the affected side (the side to which the mandible was deviating) were in normal occlusion on the frontal sections, the subject would be deemed as having a normal bite (14 patients with a normal bite [NB group]: 2 men, 12 women; mean age, 19.5 6 4.5 years). If a crossbite was evident on the affected side, the patient would be deemed as having a crossbite (9 patients with a cross-bite [CB group]: 1 man, 8 women; mean age, 25.1 6 5.1 years).
MATERIAL AND METHODS
Patients with a dentofacial deformity who visited the Department of Orthodontics at the Nihon University's hospital within the past 8 years were considered for enrollment. On a frontal cephalogram, a straight line connecting the crista galli and the anterior nasal spine served as the midline of the face; a straight line connecting the anterior nasal spine and the mental spine of the mandible served as the midline of the mental spine.12 Candidates for enrollment were patients with a lateral deviation of the mandible, which was defined as a deviation of 4 mm or more from the midline of the face to the mental spine. Patients with a lateral deviation of the mandible, who were also classified as skeletal Class I (0 #ANB #4 ), were included. A model of the maxillary and mandibular dentitions in occlusion was used to determine whether the mandibular molars were located buccally with respect to
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Fig 3. Masticatory pattern classification, described as follows. CO, Centric occlusion. In the normal pattern, from centric occlusion, the mandible moves downward and then laterally toward the chewing side or the nonchewing side, before returning to centric occlusion along a concave, convex, or linear path. In the reverse pattern, the reverse of normal chewing, the mandible moves laterally first before moving downward and then returning to centric occlusion. In the crossover pattern, the mandible moves slightly laterally, downward, slightly laterally again, and then returns to centric occlusion.
The Gnatho-Hexagraph II (GC Corporation, Tokyo Japan) was used to measure masticatory movement. A mandibular clutch was placed on the subject's mandibular anterior teeth. The patient was then instructed to relax in a seated position; the patient's head was not immobilized. Once the Frankfort plane was horizontal, a head frame and a facebow were attached. The upper margin of the external acoustic meatus on each side constituted a reference plane. The upper margin of each external acoustic meatus and the inferior margin of the left orbit constituted the Frankfort horizontal plane. The mandibular condyles, the mesiobuccal cusp of the mandibular first molars, and the point of contact between the mandibular central incisors served as measurement points.15 The patients were instructed to freely chew the chewing gum. Once the gum softened, the subject was instructed to start at the maximal intercuspal position and chew the gum on each side for 30 seconds. This masticatory movement was recorded. The test food used was a piece (1.5 g) of normal chewing gum (100% xylitol chewing gum; Oral Care, Tokyo, Japan). Masticatory movement was analyzed at the mandibular incisal point, and a total of 10 chewing strokes (strokes 5-14) on the dominant chewing side were analyzed.6,16 The pattern of masticatory movement was analyzed using the software that came with the equipment to measure jaw movement. Gum was chewed on the affected and unaffected sides for 30 seconds each. The chewing pattern was based on a total of 10 chewing strokes (strokes 5-14). The chewing pattern was classified as normal (from centric occlusion, the mandible moves downward and
then laterally toward the chewing side or the nonchewing side, before returning to centric occlusion along a concave, convex, or linear path), reversed (the reverse of normal chewing; the mandible moves laterally first before moving downward and then returning to centric occlusion), or crossover (the mandible moves slightly laterally, downward, slightly laterally again, and then returns to centric occlusion) (Fig 3).17,18 The items in Figures 4 and 5 were measured using the method of Eguchi et al.19 To model the dentition before orthodontic treatment, the width of the maxillary dentition, the width of the mandibular dentition, the palatal width at the maxillary first molars, the axial inclination of the maxillary first molars, and the axial inclination of the mandibular first molars were measured.20-23 A 3-dimensional scanner (Yasunaga Computer Systems, Co., Inc., Fukui, Japan) was used to obtain 3-dimensional data on the dentition, and the data were analyzed using 3-dimensional analysis software (Medic Engineering Corporation; Kyoto, Japan). The width of the maxillary dentition, width of the mandibular dentition, palatal width at the maxillary first molars, and buccolingual inclination of the maxillary and mandibular first molars were measured.22 A plane passing through the incisal ridge and the cusps of the second premolars and first molars served as the maxillary reference plane. A straight line passing through the cusps of the second premolars and the first molars served as the x-axis, a straight line orthogonal to the x-axis served as the y-axis, and a straight line orthogonal to the intersection of the x-axis and y-axis in the reference plane served as
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Fig 4. Maxillary and mandibular dentition measurements: the distance between the mesial cusps of the maxillary first molars (U6-6CW), the maximum distance from the lingual groove of the maxillary first molars to the neck of the tooth (U6GW), and the distance between the mesial cusps of the mandibular first molars (L6-6CW).
Fig 5. Axial inclinations of the maxillary and mandibular first molars: a straight line passed through the transition point from the groove on the buccal aspect of the first molar to the buccal groove on the occlusal surface, and through to the transition point from the groove on the lingual aspect to the lingual groove on the occlusal surface. The angle formed by this straight line and the z-axis served as the axial inclination of the teeth, yielding an axial inclination on the affected side (AI-AS) and an axial inclination on the unaffected side (AI-US).
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Table I. Results of the measurement points on the
lateral and frontal cephalograms
Table II. Results of the measurement points on the lateral and frontal cephalograms
NB group (n 5 14) CB group (n 5 9) ANB ( ) Occlusal plane ( ) Deviation in mental spine (mm)
Mean 1.9 2.5 7.3
SD 1.0 2.1 2.2
Mean 2.0 2.9 8.9
SD 1.3 2.4 4.2
t test NS NS NS
NS, Not significant.
the z-axis. A plane passing through the incisal ridge of the mandibular central incisors and the cusps of the mandibular deciduous second molars (second premolars), and the cusps on the crown of the first molar served as the mandibular reference plane. A straight line was passed through the transition point from the groove on the buccal aspect of the first molar to the buccal groove on the occlusal surface, and through the transition point from the groove on the lingual aspect to the lingual groove on the occlusal surface. The angle formed by this straight line and the z-axis served as the axial inclination of the teeth, yielding an axial inclination on the affected side, and an axial inclination on the unaffected side. Statistical analysis
The percentages of chewing patterns on the affected and unaffected sides were calculated. The chewing patterns were normally distributed in each group. The Student t test was used to test for significant differences between the groups for the widths of the maxillary and mandibular dentitions, palatal widths, and axial inclinations of the maxillary and mandibular first molars on the affected and unaffected sides. RESULTS
Measurements from lateral cephalograms are shown in Tables I and II. Authors of previous studies have measured the distances between condylar and mental spine, condylar and gonion, and gonion and mental spine on the affected and unaffected sides according to lateral cephalograms to identify facial asymmetry.12-14,24 We analyzed lateral cephalograms in accordance with the analysis of Ricketts.25 The deviations in mental spine were 7.3 6 2.2 mm (mean) for patients in the NB group and 8.9 6 4.2 mm (mean) for patients in the CB group. Significant differences in the deviation in mental spine were not noted. Similarly, significant differences in the occlusal plane angle were not noted. The values for condylarmental spine, condylar-gonion, and gonion-mental spine were greater on the unaffected side compared with the
NB group (n 5 14) Col-Me (mm) Col-Go (mm) Go-Me (mm) CB group (n 5 9) Col-Me (mm) Col-Go (mm) Go-Me (mm)
Affected side
Unaffected side
Mean
SD
Mean
SD
t test
112.1 60.9 60.8
9.8 5.3 6.7
117.6 64.1 65.0
9.5 4.0 8.5
NS NS NS
110.1 59.5 58.4
6.0 5.2 4.4
118.6 65.4 63.1
5.6 4.0 6.5
y
* NS
NS, Not significant; Col-Me, distance between Col and Me; Col-Go, distance between Col and GO; Go-Me, distance between GO and Me. *P \0.05; yP \0.01.
Table III. Masticatory patterns of subjects in the
normal bite (NB) and crossbite (CB) groups on the affected side (AS) and unaffected side (US) NB group (n 5 14) Normal pattern Reversed pattern Crossover pattern
AS US 100% (14/14) 92.8% (13/14)
CB group (n 5 9) AS US 0% (0/9) 55.6% (5/9)
0% (0/14)
7.2% (1/14) 55.6% (5/9) 33.3% (3/9)
0% (0/14)
0% (0/14) 44.4% (4/9) 11.1% (1/9)
affected side for the NB group and the CB group. These findings indicated facial asymmetry (Tables I and II). Table III presents a tally of the chewing patterns observed among the patients. Patients in the NB group had a normal chewing pattern on the affected side 100% of the time and a normal chewing pattern on the unaffected side 92.8% of the time. One subject from the NB group had a reversed pattern of chewing on the affected side 0% of the time, and a reversed pattern of chewing on the unaffected side 7.2% of the time. No patient in the NB group had a crossover chewing pattern on either the affected or unaffected side. In contrast, patients in the CB group had a normal chewing pattern on the affected side 0% of the time and a normal chewing pattern on the unaffected side 55.6% of the time. The patients in the CB group had a reversed pattern of chewing on the affected side 55.6% of the time and a reversed pattern of chewing on the unaffected side 33.3% of the time. The patients in the CB group had a crossover chewing pattern on the affected side 44.4% of the time and a crossover chewing pattern on the unaffected side 11.1% of the time. Comparisons of the width of the maxillary and mandibular dentitions, palatal width, and axial
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Table IV. Axial inclinations of the maxillary and
mandibular first molars and arch widths of the first molar NB group (n 5 14) Mean Linear measurements (mm) U6-6CW 53.3 L6-6CW 45.8 U6-L6CW 7.5 U6GW 36.9 Angular measurements ( ) AIMx-AS 96.0 AIMx-US 90.8 AIMa-AS 64.8 AIMa-US 74.1
CB group (n 5 9)
SD
Mean
SD
t test
3.5 3.0 2.8 2.9
50.0 46.1 3.9 33.6
3.7 2.6 2.8 3.3
* NS
4.8 6.1 5.3 8.1
98.6 86.0 70.4 70.8
4.0 2.9 6.0 6.5
NS * * NS
y
*
NS, Not significant; NB, normal bite; CB, crossbite; U6-6CW, width of the maxillary dentition; L6-6CW, width of the mandibular dentition; U6-L6CW, width of the maxillary dentition and the width of the mandibular dentition; U6GW, palatal width of maxillary first molars; AIMx-AS, axial inclination of the maxillary teeth on the affected side; AIMx-US, axial inclination of the maxillary teeth on the unaffected side; AIMa-AS, axial inclination of the mandible teeth on the affected side; AIMa-US, axial inclination of the mandible teeth on the unaffected side. *P \0.05; yP \0.01.
inclinations of the maxillary and mandibular first molars are shown in Table IV. The width of the maxillary dentition and palatal width were significantly greater in the patients in the NB group than in the patients in the CB group. The patients in the NB group had a significantly greater difference between the width of the maxillary dentition and the width of the mandibular dentition compared with that of the patients in the CB group. Significant differences in the width of the mandibular dentition were not noted. Significant differences in the axial inclination of the maxillary teeth on the affected side were not noted. Compared with the patients in the NB group, those in the CB group had significantly less axial inclination of the maxillary teeth on the unaffected side. Additionally, patients in the NB group had significantly less axial inclination of the mandibular teeth on the affected side compared with patients in the CB group. Significant differences in the axial inclination of the mandibular teeth on the unaffected side were not noted. DISCUSSION
The coverage of the first molars on the frontal sections was classified into either 1 of 2 categories: normal coverage on the affected side (NB group) or evidence of a crossbite (CB group). Chewing patterns were also assessed. A previous study reported that chewing is regular and consistent with a normal occlusion,26 whereas
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another study reported that a typical chewing pattern in patients with normal occlusion is one where the mandible moves smoothly from centric occlusion, downward, and then laterally toward the chewing side or the nonchewing side before returning to centric occlusion along a convex path.17 Various studies have evaluated and classified the differences in chewing patterns. Shiga et al16 assessed the relationship between occlusal contact during lateral movements and chewing patterns, and classified them into 7 types. Proeschel27 examined the chewing patterns of normal persons and those of patients with Angle Class I or Class II malocclusion, and derived 8 classifications of chewing patterns. Yano et al4 and Tomoyose et al28 both examined the chewing patterns of subjects with mandibular asymmetry; Yano et al derived 5 classifications of chewing patterns, and Tomoyose et al derived 3 patterns. To ascertain the normal and particular patterns of chewing, we classified chewing patterns into 3 types (normal, reversed, or crossover). Patients who chewed in a normal pattern moved their mandibles downward from centric occlusion and then laterally toward the chewing side or the nonchewing side, before returning their mandibles to centric occlusion along a concave, convex, or linear path. Thus, the patients had a chewing pattern that was consistent with that of subjects with normal occlusion.16 Patients who chewed in a reversed pattern (ie, in reverse sequence) moved their mandible laterally first, before moving it downward, and then returning it to centric occlusion. This pattern is often noted in patients with a laterally deviated mandible or prognathism, where a molar crossbite is more likely to occur. Nakaminami et al3 reported that a reversed chewing pattern tends to be highly prevalent among patients with a molar crossbite. Patients in the NB group mostly had a normal chewing pattern; the molars on the affected and unaffected sides were normally covered. However, 1 subject in the NB group had a reversed chewing pattern on the unaffected side. When there is occlusal contact on the balancing side, chewing can proceed in reversed order (the mandible will first move laterally and then downward).16 With a molar crossbite, chewing can proceed in a reversed sequence so that it does not interfere with occlusion.27 Thus, the differences in occlusal contact may lead to different chewing patterns because of their presumed associations with one another. Similarly, occlusion on the unaffected side is unobstructed by the return of the mandible to its original position; this may potentially account for the reversed pattern of chewing. Patients in the CB group did not have a normal chewing pattern on the affected side; instead, they had a reversed pattern (55.6%) or a crossover pattern
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(44.4%) of chewing. Patients in the CB group had a normal chewing pattern on the unaffected side (55.6%), a reversed pattern (33.3%), or a crossover pattern (11.1%). Tomonari et al5 examined the relationship between molar coverage and masticatory movement; they reported that the sequence of chewing was reversed in the CB group: ie, lateral movement followed by downward movement of the mandible. In our study, the presence of a molar crossbite interfered with the cusps of the molars on the affected side during chewing, hindering masticatory movement. This resulted in a reversed or a crossover chewing pattern with a shorter trajectory of chewing than the trajectory of chewing in a normal chewing pattern. A reversed or crossover pattern of chewing was noted with malocclusion of the unaffected side. This led to inconsistent chewing patterns on the unaffected sides. Lateral deviation of the mandible involves asymmetry of the rami or the body of the mandible and the mandibular condyles, and asymmetry of the temporomandibular joints and muscles.29 Thus, morphologic aspects, such as the asymmetry of the mandibular condyles, must be examined in future studies of patients with a lateral deviation of the mandible. In this study, the width of the maxillary dentition, palatal width, and the difference between the width of the maxillary dentition and the width of the mandibular dentition were significantly greater among the patients in the NB group than among those in the CB group. Authors of 1 study found that persons with a normal chewing pattern chew with grinding movements.30 Negishi et al31,32 reported that subjects with a normal chewing pattern chew with grinding movements, and that this aids in the lateral expansion of the maxillary midpalatal suture. Additionally, Cattaneo et al33 analyzed a finite element model of the adult maxilla and reported that stress was concentrated around the key ridge. Their findings were similar to those of Negishi et al.32 Furthermore, authors of a study reported that major changes in the development of the maxillary dental arch result from the lateral expansion of the palate and not from the axes of the teeth.22 Nonetheless, the expansion of the maxillary midpalatal suture occurs primarily during childhood.34 The patients in our study were adults, so the differences in the width of the maxillary dentition of the NB and CB groups had presumably resulted from the changes in the axial inclinations of the mandibular molars. Significant differences in the axial inclinations of the maxillary teeth on the affected side were not noted. The maxillary teeth on the affected sude tended to be buccally inclined in the patients in the CB group. A
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comparison indicated that the patients in the CB group had significantly less axial inclination of the maxillary teeth on the unaffected side compared with the patients in the NB group. It was also observed that the patients in the CB group had lingual inclinations of the maxillary teeth on the unaffected side, and that the patients in the NB group had significantly less axial inclination of the mandibular teeth on the affected side relative to the patients in the CB group. Compared with the patients in the CB group, the patients in the NB group had lingually inclined molars on the affected side. A comparison showed no significant differences in the axial inclination of the mandibular teeth on the unaffected side. The molars on the unaffected side tended to be buccally inclined in the patients in the NB group, compared with the patients in the CB group. In this study, a lateral deviation of the mandible caused the mandibular molars to be located more buccally with respect to the maxillary dentition. Studies of subjects with normal occulusion have reported that stress is primarily placed on the lingual aspects of the buccal cusps of the mandibular molars during normal chewing.35,36 A study of patients with lateral deviation of the mandible reported that the occlusal contact area was significantly greater on the affected side of the mouth.37 In subjects with normal occulusion, the buccal cusps of the mandibular molars serve as functional cusps, but a lateral deviation of the mandible may result in lingual cusps of the mandibular molars serving as functional cusps. In other words, normal masticatory movements occur, and the mandibular molars may be forced toward the tongue, presumably causing the mandibular molars on the affected side to be lingually inclined. Takada et al38 reported that intraoral pressure was significantly correlated with the inclination of the mandibular first molars in patients with facial asymmetry; they found that an imbalance in intraoral pressure distribution was related to the incidence of asymmetric mandibular dentition and dental compensation. A reversed or crossover chewing pattern with a shorter trajectory of chewing presumably places less stress on the lingual aspect of the mandibular dentition. Dental compensation can lead to anteroposterior incongruity in the bones of the jaw and can cause horizontal incongruity.39 In patients with facial asymmetry, dental compensation is correlated to buccolingual pressure on the affected and unaffected sides, the buccolingual position of the mandibular molars, and the inclination of those molars. Buccolingual pressure and asymmetry in the dental arch are reported to be related to the dental compensation of the molars.40 Ishikawa et al40,41 reported that dental compensation was involved in maintaining coverage of the molars.
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Chewing with grinding strokes is reported to cause the mandibular molars to incline to a more upright position.31 Additionally, masticatory movement and changes in the axial inclination are closely related. Thus, a normal chewing pattern may cause lingual inclination of the teeth on the affected side and lead to more consistent chewing patterns. CONCLUSIONS
Masticatory movement and axial inclination are closely related. In subjects with lateral deviation of the mandible and a normal chewing pattern, the width of the maxillary dentition can increase as a result of the changes (ie, the orientation) in the axes of the teeth. A normal chewing pattern may lead to lingual inclination of the mandibular molars on the affected side. REFERENCES 1. Severt TR, Proffit WR. The prevalence of facial asymmetry in the dentofacial deformities population at the University of North Carolina. Int J Adult Orthod Orthognath Surg 1997;12:171-6. 2. Sato S, Takamoto K, Goto M, Kamoi S, Suzuki Y. An approach to development of skeletal malocclusion with mandibular lateral displacement. J Jpn Kanagawa Soc 1990;25-1:93-8. 3. Nakaminami T, Nishio K, Miyauchi S, Maruyama T. The effect of posterior crossbite on stomatognathic function. J Jpn Soc Stomatognath Funct 1968;6:87-96. 4. Yano K, Kubota M, Shinohara C, Kanegae H, Shibasaki Y. The correlation between preferred chewing and the horizontal condylar angle in patients with mandibular asymmetry. Jpn Jaw Deform 2000;10:110-6. 5. Tomonari H, Ikemori T, Kubota T, Uehara S, Miyawaki S. First molar cross-bite is more closely associated with a reverse chewing cycle than anterior or premolar cross-bite during mastication. J Oral Rehabil 2014;41:890-6. 6. Nie O, Kanno Z, Tianmin X, Jiuxiang L, Soma K. Clinical study of frontal chewing patterns in various crossbite malocclusions. Am J Orthod Dentofacial Orthop 2010;138:323-9. 7. Piancino MG, Talpone F, Dalmasso P, Debernardi C, Lewin A, Bracco P. Reverse-sequencing chewing patterns before and after treatment of children with unilateral posterior crossbite. Eur J Orthod 2006;28:480-4. 8. Martin C, Alarcon JA, Palma JC. Kinesiographic study of the mandible in young patients with unilateral posterior cross-bite. Am J Orthod Dentofacial Orthop 2000;118:541-8. 9. Throckmorton GS, Buschang PH, Hayasaki H, Pinto AS. Changes in the masticatory cycle following treatment of posterior unilateral crossbite in children. Am J Orthod Dentofacial Orthop 2001;120:521-9. 10. Rilo B, da Silva JL, Mora MJ, Cadarso-Suarez C, Santana U. Unilateral posterior crossbite and mastication. Arch Oral Biol 2007;52:474-8. 11. Kwak YY, Jang I, Choi DS, Cha BK. Functional evaluation of orthopedic and orthodontic treatment in a patient with unilateral posterior crossbite and facial asymmetry. Korean J Orthod 2014;44: 143-53. 12. Uesugi S, Yonemitsu I, Kokai S, Takei M, Omura S, Ono T. Features in subjects with the frontal occlusal plane inclined toward the contralateral side of the mandibular deviation. Am J Orthod Dentofacial Orthop 2016;149:46-54.
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13. Damstra J, Fourie Z, Ren Y. Evaluation and comparison of posteroanterior cephalograms and cone-beam computed tomography images for the detection of mandibular asymmetry. Eur J Orthod 2013;35:45-50. 14. Janson G, de Lima KJ, Woodside DG, Metaxas A, de Freitas MR, Henriques JF. Class II subdivision malocclusion types and evaluation of their asymmetries. Am J Orthod Dentofacial Orthop 2007; 131:57-66. 15. Hayashi R, Kawamura A, Kasai K. Relationship between masticatory function, dental arch width, and bucco-lingual inclination of the first molars. Orthod Waves Jpn Ed 2006;65:120-6. 16. Shiga H, Kobayashi Y, Arakawa I, Yokoyama M, Tanaka A. Relationship between pattern of masticatory path and state of lateral occlusal contact. J Oral Rehabil 2009;36:250-6. 17. Akiyama H, Shiga F, Kobayashi Y. The analysis of masticatory movements—frontal patterns of chewing path of incisor point in normal subjects. J Jpn Prosthodont Soc 1991;35:609-21. 18. Proschel P, Hofmann M. Frontal chewing patterns of the incisor point and their dependence on resistance of food and type of occlusion. J Prosthet Dent 1988;59:617-24. 19. Eguchi S, Townsend GC, Toby H, Kasai K. Genetic and environmental contributions to variation in the inclination of human mandibular molars. Orthod Waves 2004;63:95-100. 20. Oliveira NL, Da Silveira AC, Kusnoto B, Viana G. Three-dimensional assessment of the maxilla: a comparison of 2 kinds of palatal expanders. Am J Orthod Dentofacial Orthop 2004; 126:354-62. 21. Veli I, Yuksel B, Uysal T. Longitudinal evaluation of dental arch asymmetry in Class II subdivision malocclusion with 3dimensional digital models. Am J Orthod Dentofacial Orthop 2014;145:763-70. 22. Okano M. A growth pattern of bucco-lingual inclination and arch width of first molar—a comparison with Japanese and 2 groups of South Pacific populations. Orthod Waves Jpn Ed 2006;65:112-21. 23. Langberg BJ, Arai K, Miner RM. Transverse skeletal and dental asymmetry in adults with unilateral posterior crossbite. Am J Orthod Dentofacial Orthop 2005;127:6-16. 24. Masuoka N, Muramatsu A, Ariji Y, Nawa H, Goto S, Ariji E. Discriminative thresholds of cephalometric indexes in the subjective evaluation of facial asymmetry. Am J Orthod Dentofacial Orthop 2007; 131:609-13. 25. Ricketts RM. The value of cephalometrics and computerized technology. Angle Orthod 1972;42:179-99. 26. Jemt T, Hedegard B. Reproducibility of chewing rhythm and of mandibular displacements during chewing. J Oral Rehabil 1982; 9:531-7. 27. Proeschel PA. Chewing patterns in subjects with normal occlusion and with malocclusions. Semin Orthod 2006;12:138-49. 28. Tomoyose Y, Bandai H, Sugawara J, Mitani H. Characteristics of chewing path in skeletal Class III patients with mandibular asymmetry. Orthod Waves 2002;61:376-91. 29. Pirttiniemi P, Kantomaa T. Relation of glenoid fossa morphology to mandibulofacial asymmetry, studied in dry human Lapp skulls. Acta Odontol Scand 1992;50:235-43. 30. Watt DM, MacGregor AR. Designing complete dentures. 1st ed. Philadelphia: W. B. Saunders; 1976. p. 152-5. 31. Negishi S, Hayashi R, Saitoh K, Kasai K. The influence of masticatory exercises using hard chewing gum on chewing pattern and first molar of mixed dentition. Orthod Waves Jpn Ed 2010;69: 156-62. 32. Negishi S, Hayashi R, Nakagawa A, Murata Y, Kasai K. A discussion of masticatory force mechanics on the median palatine suture of
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