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The Effects of Masticatory Movement on the Axial Inclination of the Molars after the Treatment of a Lateral Deviation of the Jaw

We analyzed the masticatory movement patterns of patients with skeletal Class I or III malocclusion who had lateral deviations of the jaw and were treated using orthognathic surgery. The aims of this study were (1) to compare the chewing patterns at initial examination(T1), after removal of each participantʼs orthodontic appliance(T2), and after ≥1 year of maintenance(T3), and(2)to analyze the effects of different chewing patterns on changes in the axial inclination of the first molars. A Gnatho-Hexagraph III was used to measure the chewing patterns(with six degrees of freedom)of 21 participants(mean age: 23.2±5.4 years)with a menton deviation≥4 mm from the midline. A three-dimensional digital scanner was used to measure the axial inclination of the maxillary and mandibular first molars. Participants with normal chewing patterns on both the affected and unaffected sides at T1 had the same pattern at T2 and T3. In addition, 50% of the participants with reversed chewing patterns on the affected side at T1 exhibited normal patterns at T2, while 33.3% retained their original reversed pattern. Compared to the participants with normal chewing patterns, those with reversed patterns showed a significantly greater change in the inclination of maxillary and mandibular first molars on the affected side between T2 and T3. Palatal, maxillary first molar, and mandibular first molar widths showed similar results. Moreover, participants with normal chewing patterns had few changes in the buccolingual molar inclination; hence, the orientation of the molars remained unchanged. The results showed that the axial inclination of the molars was associated with the masticatory movement pattern after orthognathic surgery, which affected molar occlusion. This study was approved by the Ethics Committee of Nihon Universityʼs School of Dentistry at Matsudo(approval no. EC16-15-004-2)


Introduction
A lateral deviation of the mandible leads to aesthetic problems and impaired mastication because it can result in a molar crossbite. Patients with a lateral deviation of the mandible have a characteristic masticatory movement (1)(2)(3), which can be corrected using orthognathic surgery (4,5). Previous studies on participants with lateral deviations of the mandible have identified characteristic patterns of masticatory movement (e. g., reversed and crossover chewing patterns)that indicate that the masticatory movement exhibited by these individuals is asymmetric. And occlusion. In addition, other studies showed that orthognathic surgery could result in the correction of the morphology of the mandible, thereby leading to improved function (11)(12)(13).
However, one study on the morphological changes before and after orthodontic surgery found that despite the morphological changes, the pattern of masticatory movement remained unchanged (1), and another study indicated that the pattern of masticatory movement was set at age 12-14 years and did not subsequently change (14). These findings indicate that morphological improvements do not necessarily lead to functional changes. However, few studies have carefully examined the patterns of masticatory movement and changes in the eruption of the molars.
We generated hypotheses regarding the potential changes in masticatory movement patterns among 21 patients following orthognathic surgery for lateral deviation of the jaw and during the retention period, as well as the potential contributory role of the masticatory movement patterns in the process of relapse.
We also aimed to ascertain the changes in the masticatory movement patterns and the effects of these patterns on the axial inclination of the first molars.
This study was approved by the Ethics Committee of Nihon Universityʼs School of Dentistry at Matsudo(approval no. EC16-15-004-2).

Participants
Twenty-one participants with lateral deviations of the mandible(6 male and 15 female patients; mean age: 23.2 ± 5.4 years)were recruited.
The frontal and lateral cephalograms of each potential participant were analyzed in accordance with the Steiner analysis ( Fig. 1) (15), and the inclusion criteria were as follows: (1) A menton deviation of ≥ 4 mm from the midline of the face(i.e., the straight line connecting the crista galli and anterior nasal spine); (2) Skeletal Class I or III malocclusion(i.e., -2°≤ A point- The exclusion criteria were as follows: (1) Temporomandibular joint pain or dysfunction (e. g., trismus); (2) Previous orthodontic treatment; (3) Use of dental prostheses; The assessments were as follows: (1) Masticatory movement patterns (Fig. 2).

Measurements of dentition molds
Measuring device: 3D Dental Scanner (Yasunaga Computer Systems Co., Inc., Fukui, Japan) Method: The items in Figs. 5 and 6 were measured using the method described by Eguchi et al. (19). Molds of the upper and lower dental arch were taken at T1, T2, and T3.
Three-dimensional (3D) data based on the molds were obtained using a 3D Dental Scanner(Yasunaga Computer Systems Co., Inc, Fukui, Japan). The data were analyzed using specialized software for analyzing 3D data (Body-Rugle, Medic).
The changes in each measurement (which are listed below) between T2 and T3 were calculated, and the Classification of chewing patterns Normal pattern: From centric occlusion (CO), the mandible moves downward and then laterally toward the chewing side or the non-chewing side before returning to CO along a concave, convex, or linear path Reversed pattern: The reverse of normal chewing; the mandible moves laterally first before moving downward and then returning to CO Crossover pattern: The mandible moves slightly laterally, downward, slightly laterally again, and then returns to CO All measurements in this study were made by the same individual. All measurements were made in duplicate, and measurement reproducibility was examined. The author reassessed a series of 20 subjects two months after the initial measurements were taken. The values showed good reproducibility.

Statistical analysis
The changes in the masticatory movement patterns were analyzed using McNemarʼs test. The differences in the axial inclination of the maxillary and mandibular first molars and width of the dentition between cases involving normal, crossover, and reversed chewing patterns were analyzed using Kruskal-Wallis and Mann-Whitney U tests. There was a significant difference between T1 and T2 in the masticatory movement patterns on the affected sides(p = 0.03). There were no significant differences in the patterns between T1 and T2 on the unaffected sides. In addition, there was no significant difference between T1 and T2 in the pattern of masticatory-movement patterns on the affected and unaffected sides. Only one participant had a chewing pattern at T2 that differed from that at T3; the participant had a reversed chewing pattern at T2 that changed to a crossover chewing pattern.
At T1, 16(76.1%), two(9.5%), and three(14.2%)out of The changes in the inclination of the maxillary and mandibular teeth as a result of the same masticatory movement pattern on the affected side between T2 and T3 are shown in Fig. 6 (a). The statistical significance of the changes in the inclination of the teeth was tested. The changes in the inclination of the teeth were significantly greater for participants with reversed chewing patterns compared to those with normal or crossover chewing patterns. The changes in the inclination of the maxillary and mandibular teeth as a result of the same masticatory movement pattern on the unaffected side between T2 and T3 are shown in Fig. 6 (b). There were no significant differences in the inclination of the teeth between the three groups of participants with different chewing patterns.
Changes in the width of the dentition as a result of the same masticatory movement pattern on the affected side at T2 and

T3
The changes in the width of the dentition as a result of the same masticatory movement pattern on the affected side at T2 and T3 are shown in Fig. 7. The participants with reversed chewing patterns had significantly greater changes in the width of their dental arch(U6-6CW, L6-6CW, and U6GW) compared to those with normal or crossover chewing patterns.

Masticatory movement patterns
This study examined 1)Changes masticatory movement patterns in morphological improvement and retention period, 2)Changes in the patterns of masticatory movement and the effects of these patterns on the axial inclination of the molars.
In result, it became clear that the chewing movement In participants with normal chewing patterns, normal alignment of the molars on the affected and unaffected sides was often observed, and the masticatory movement patterns did not change between T1, T2, and T3. Therefore, the normal chewing pattern was the most consistent of the three patterns of masticatory movement.
In a crossover chewing pattern, during masticatory movement, the mandible moves slightly laterally, downward, slightly laterally again, and then returns to centric occlusion. However, as a result of the lateral deviations of the mandible, some of the participants had crossover chewing patterns despite not having temporomandibular joint pain or dysfunction(e.g., trismus).
The results show that over 50% of the participants with reversed chewing patterns at T1 had crossbites. This supports the findings of a previous study that showed that molar crossbites can cause occlusal interference, thereby hindering masticatory movement (8). Moreover, a study of individuals with reversed chewing patterns(i.e., the mandible moves laterally and then downward)but without molar crossbites indicated that they each had a crossbite involving the anterior teeth and supernumerary teeth in the premolar region, which caused the reversed chewing pattern (8).
These findings all indicate that occlusal interference can lead to a reversed chewing pattern.
There was a significant difference between T1 and T2 in the variation of the masticatory movement pattern on the affected sides. Among participants with reversed chewing patterns on the affected side T1, 50% had normal chewing patterns on the affected side by T2. Studies of children with crossbites have found that after their crossbites were corrected, each child had a more normal chewing pattern(i. e., the mandible moved downward and then laterally) (25,26). In addition, a study on orthognathic surgery for adults with skeletal Class III malocclusions found that the proportion of adults who had reversed chewing patterns decreased after surgery (27). Changes in the axial inclination of the maxillary and mandibular teeth The affected side was more susceptible than the unaffected side to the effects of different masticatory movement patterns on the inclination of the teeth. In addition, reversed and crossover chewing patterns resulted in greater changes in the inclination of the maxillary and mandibular teeth compared to normal chewing patterns.
After maintenance for ≥ 1 year, the inclination of the molars changed as a result of the masticatory movement patterns.
A study on masticatory movements and changes in molar inclination showed that chewing with grinding movements causes the mandibular molars to incline more buccally (31), which indicates that masticatory movement is closely related to changes in the axial inclination of the teeth.
Therefore, for individuals with normal chewing patterns that involve grinding movements, the molar inclination of the teeth changes very little after orthognathic surgery.
Changes in the width of the maxillary and mandibular dental arch The maxillary and mandibular molars did not revert back to form crossbites in any of the participants by T3. However, there were greater changes in the width of the maxillary dental arch in participants with reversed chewing patterns compared to those with normal or crossover chewing patterns, indicating that regression was more likely in those with reversed chewing patterns. Negishi et al. (31)reported that chewing with grinding movements caused the mandibular molars to incline more buccally and their width to expand. It is likely that this phenomenon affected the width of the maxillary and mandibular dental arch and the axial inclination of the teeth at T2 and T3.
Masticatory movement was classified into three patterns: normal, crossover, and reversed chewing. Fewer changes in the molar inclination and width of the maxillary and mandibular teeth were observed at T2 and T3 among the participants with normal chewing patterns compared to those with the other two chewing patterns. Therefore, the molar inclination and width of the maxillary and mandibular teeth were more consistent among the participants with normal chewing patterns compared to those with the other patterns. Reversed chewing patterns also resulted in substantial changes in the molar inclination and width of themaxillary and mandibular teeth during maintenance, and tended to lead to regression.