Effect of malocclusion on jaw motor function and chewing in children: a systematic review

Systematic searches were conducted in MEDLINE (Ovid), Embase, and Web of Science Core Collection databases from inception until October 29, 2021. The search strategy (Supplementary file 1) was created using MeSH/Emtree terms with relevant free text terms, and truncated and/or combined with proximity operators, where appropriate. There were no search restrictions on date or type of publication, but only studies published in English were included. The database search was supplemented by a manual search of gray literature and Google Scholar using free text terms such as “chewing in children,” “bite force,” “electromyography,” and “jaw kinematics.” In addition, the reference lists of included studies were searched for any potentially eligible studies.

Duplicate studies were automatically removed from the search results using EndNote reference management software and were confirmed by a manual inspection. All shortlisted studies were then exported to a Microsoft Excel spreadsheet, where two independent authors (AA and NA) screened the titles/abstracts of the study list. Clinical studies that investigated the objective parameters of jaw motor function and mastication in children with malocclusion, with or without cross-comparison to the malocclusion-free group of children and/or adults, were included. However, studies on children with other orofacial anomalies or in a language other than English were excluded. All reviews (systematic or narrative), study protocols, letters to editor, opinion articles, commentaries, and case reports were also excluded. Based on the inclusion/exclusion criteria, studies were categorized as “included,” “excluded,” or “undecided” using a predefined scheme. Indecision about the inclusion of a study was resolved by joint discussion and/or consultation with a third author (AK), if necessary. The full texts of all included studies were obtained and carefully read by the two independent authors. The studies that met the inclusion/exclusion criteria were finally included in this systematic review. A clear reason was provided for the exclusion of a study, and disagreements about the inclusion/exclusion of a study were again resolved by joint discussion and/or consultation with a third author (if necessary).

The quality of evidence and risk of bias of the included studies were assessed using the Joanna Briggs Institute Critical Appraisal Tool. The purpose of this appraisal is to assess the methodological quality of individual studies and to determine the extent to which a particular study has addressed the possibility of bias in its design, conduct, and analysis. Two separate instruments related to methodological design (i.e., cross-sectional or prospective cohorts studies) were used [25]. The instrument/tool consists of several methodological questions on the representativeness of the study sample and inclusion criteria, the study setting and design, the validity and reliability of the exposure and outcomes, and the adequacy of the statistical analysis. A cumulative score was calculated for each study by adding the positive responses of the instrument questions. Each study was then classified as high quality (score between 100 and 80), moderate quality (score between 79 and 60), or low quality (score below 60) based on its cumulative score. Note that no study was excluded based on its quality score. Data extracted from the included studies encompassed demographics of study sample, type of malocclusion, orthodontic treatment/retention (if any), objective parameter for mastication measured, main outcomes, and study quality score. In addition, the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) tool [26] was used to assess the overall quality of the evidence of the impact/effect of orthodontic treatment on the selected parameters of jaw motor function and mastication.

There were eleven studies that investigated the MOBF in children with malocclusion [28‐31, 36, 37, 47‐49, 59, 70]. Specifically, three studies examined children aged 3–7 years [28, 36, 37], while the remaining studies examined older children [29‐31, 48, 59, 70]. MOBF was mostly compared between children with or without unilateral posterior crossbite (UPXB) [28, 30, 36, 37] or between children with or without anterior open bite (AOB) [36, 48]. Further, two studies compared MOBF between children with class I, class II, and/or class III malocclusion and children with normal occlusion [29, 70]. Three studies compared MOBF in children with class II division I malocclusion before and after functional orthodontic treatment [31, 47, 49], while one study investigated the effect of increased vertical face dimension on MOBF [59].

Children with UPXB [28, 36, 37] or with AOB [36] in the primary dentition had similar MOBF compared to children with normal occlusion. During the mixed/permanent dentition stage, the magnitude of MOBF increased in children with malocclusion [29, 30, 70], similar to a previous meta-analysis of children with normal occlusion [23]. Children with UPXB appear to have similar MOBF on their crossbite and non-crossbite sides [30] but displayed a significantly lower MOBF than crossbite-free children [28, 30, 37], while children with AOB had a similar MOBF to AOB-free controls [48]. The MOBF in children with a class II/1 malocclusion increased 1 to 2 years before orthodontic functional treatment [49] but decreased during treatment [47, 49]. After 1 year of retention, the MOBF in children with class II/1 increased and reached bite force values before functional orthodontic treatment [47]. When MOBF in children with class I or class II/1 malocclusion was compared with children with normal occlusion, it was found to be lower [29, 31]. This trend remained until after the malocclusion was orthodontically corrected with functional appliances [31]. No relationship was observed between the MOBF and vertical facial dimension [59] or between MOBF and gonial angle or masseter muscle thickness [31], but children with a class II/1 malocclusion who had the lowest MOBF showed more favorable treatment outcomes than children with a higher MOBF [49]. The overall quality of evidence regarding the effect of orthodontic treatment on MOBF in children with Cl II/1 malocclusion is “moderate” (Supplementary file 3) due to the lack of control groups in two studies [47, 49].

Figure 2 shows the pooled MOBF from seven studies [28, 30, 31, 36, 37, 48, 59] of children with normal occlusion compared to children with malocclusion during primary and mixed/permanent dentition stages. The relative difference of MOBF between children with normal occlusion and children with malocclusion was low (− 4%) during the primary dentition. During mixed/permanent dentition, both groups showed an increase in MOBF, but the magnitude of increase for the control group (38%) was twice that of the malocclusion group (17%), resulting in a greater relative difference between the two groups during mixed/permanent dentition (10%).

The EMG activity of the masticatory muscles was investigated in 30 studies, mostly in children in mixed/permanent dentition stage [32, 33, 35, 38‐40, 48, 50, 52‐58, 60‐67, 76‐82]. The EMG activity of masseter and anterior temporalis muscles was investigated in relation to the Dental Health Component of the Index of Orthodontic Treatment Need (IOTN-DHC) [77], or compared between children with or without AOB [33, 48, 50, 82], with or without an anterior deep bite [60], or with or without teeth crowding [66]. Furthermore, the EMG activity of masseter and/or temporalis muscles was investigated in children with class I [32, 64], class II [32, 64, 65], or class III malocclusions [32, 76] or with or without incompetent lips [35]. The masseter and temporalis EMG activity was also compared before and after functional orthodontic treatment of children with class II/1 [57, 61, 79], class II/2 [81], or class III malocclusion [80]. Studies also examined the EMG activity of the masseter and temporalis muscles of children with untreated unilateral/bilateral buccal crossbite [39, 53, 54, 56, 62, 63, 78] or before and after orthodontic crossbite treatment with quad-helix [38, 52], Rapid Maxillary Expansion (RME) [40, 58, 67] or “function generating bite” functional appliance [55], and/or after 6 months of RME treatment retention [58, 67].

The EMG activity among the studies was investigated during mandibular resting [32, 35, 38, 48, 50, 52‐54, 56, 60‐62, 64, 65, 77, 78, 80, 81], lateral excursion [77], protrusion [77], maximal clenching [33, 38, 48, 50, 52‐54, 56‐58, 60‐62, 64, 66, 67, 76, 77, 79‐82], or chewing behaviors [33, 38‐40, 48, 55, 58, 63, 65, 66, 76‐78, 82].

Similar EMG activity of masseter and temporalis muscles was observed in children with no, few, or slight-to-borderline orthodontic treatment needs [77], in children with or without teeth crowding [66], or in children with or without deep bite [60]. Lip incompetency in adolescents resulted in lower EMG resting activity of anterior temporalis compared to adolescents with competent lips [35]. For children with AOB, one study showed that they had lower EMG activity of masseter and anterior temporalis muscles compared to AOB-free children during chewing hard food [33], while another study did not [48]. Furthermore, two studies showed that children with skeletal AOB showed lower clenching and chewing EMG activities of masseter and temporalis muscles compared to AOB-free children [50, 82] and/or children with dentoalveolar AOB [82]. Both AOB-free children and children with dentoalveolar AOB were able to adapt their EMG activity during clenching or chewing activities, while children with skeletal AOB did not [82]. Orthodontic myofunctional therapy of children with skeletal AOB increased their EMG activity of masseter and anterior temporalis muscles during maximal clenching [50].

The resting EMG activity of masseter muscle showed an age-related increase in children with class I malocclusion but not with class II and III malocclusions [32]. The resting and clenching EMG activities between untreated children with class I and class II malocclusions yielded no differences in two studies [61, 64], but they were lower in children with class II malocclusion in one study [81]. Furthermore, the chewing EMG activity was lower in children with class II/1 [65] but not with class III malocclusion [76] compared to malocclusion-free children. Contradicting results were reported on the changes in EMG activity in relation to orthodontic treatment of children with malocclusions. While others found that resting [61, 79, 80] or clenching [79, 80] EMG activities of masseter and temporalis muscles in children with class I, class II/1, or class III malocclusions were stable, regardless of treatment type. Others found that functional orthodontic treatment of children with class II/1 and/or II/2 malocclusions resulted in an increase [57, 81] or decrease [61] in the masseter and temporalis muscle EMG activity during mandibular clenching, which reached control levels after treatment [57, 81].

No sex-related differences were observed in children with functional UPXB at mandibular resting, but at mandibular clenching, boys showed more EMG activity [53]. Compared to crossbite-free children, children with UPXB displayed similar masseter and anterior temporalis EMG activity during mandibular resting in four studies [38, 54, 56, 78], while one study showed lower resting EMG activity for the masseter but not the anterior temporalis muscle in children with UPXB than controls [62]. During chewing activity, limited differences in masticatory muscle EMG activity exist between children with or without bilateral or UPXB [38, 40, 58, 63, 78]. Contradicting results were observed during mandibular clenching. Three studies showed lower EMG activity in UPXB children than controls for the masseter muscle only on the crossbite side [38, 54] or for both the anterior temporalis and masseter EMG activity [62], while another study showed higher anterior temporalis EMG activity in only right-sided crossbite children compared to controls [56].

The bilateral EMG activation in children with UPXB compared to controls at mandibular rest was highly asymmetric in one study [52], but not in another study [56]. During clenching, children with UPXB showed symmetric EMG activity in one study [54] but two studies showed higher EMG asymmetry [52, 56], which decreases to normal control levels after orthodontic treatment [52]. During chewing, two studies reported higher masseter and/or anterior temporalis EMG asymmetry in children with UPXB compared to controls [39, 55], while three studies did not [58, 62, 63]. Moreover, the combined EMG activity of masseter and anterior temporalis muscles in children with UPXB decreased after orthodontic treatment with RME [40, 58, 67] but increased after 3–6 months of retention to resume normal levels [40, 55, 58]. Figure 3 shows the normalized EMG activity of masseter and anterior temporalis muscles in children with UPXB during the mixed/permanent dentition compared to crossbite-free children [38, 54, 56, 62, 63, 78] during mandibular resting, chewing and maximal clenching behaviors. It is evident that no difference exists on EMG activity between the two groups during mandibular resting, but perhaps a small difference is observed during chewing. During maximal clenching, however, a clear EMG difference is observed, particularly on the masseter muscle, where children with UPXB showed lower EMG activity than crossbite-free children, corroborating the observed results of MOBF (Fig. 1).

Similar to MOBF, the craniofacial parameters, such as gonial angle, showed no clear correlation to the EMG activity of masticatory muscles [60, 64, 65]. Furthermore, according to GRADE assessment, the quality of evidence regarding the effect of orthodontic treatment on EMG activity was “moderate” and “very low” for children with UPXB and Cl I, II, and III malocclusions, respectively (Supplementary file 4).

The jaw kinematics during chewing, such as number or duration of chewing cycles, jaw chewing pattern, or jaw lateral and vertical displacement were investigated in 16 studies [33, 38, 41‐44, 51, 55, 63, 65, 67‐69, 71‐73]. Children with primary dentition were investigated in two studies [44, 68], while the remaining studies investigated children with mixed/permanent dentition.

The majority of the studies compared the jaw kinematics during chewing between children with untreated UPXB and crossbite-free children [41, 43, 44, 63], before and after orthodontic treatment [38, 42, 55, 67, 69, 73], or after a short-term retention [38, 51, 67]. The jaw kinematics during chewing were also compared between children with or without AOB [33], between children with or without class II/1 malocclusion [65], between children with various types of malocclusions compared to malocclusion-free children [72], or between children with or without anterior crossbite before [68, 71] and/or after orthodontic treatment [71].

No differences were observed on number of chewing cycles between children with or without class II/1 malocclusion [65]. Children with AOB while eating hard food showed shorter total chewing time and jaw closing duration with a narrower chewing cycle width than their controls [33]. Children with true anterior crossbite in the primary dentition chewed with a greater jaw sagittal opening angle than crossbite-free children [68]. Furthermore, children with UPXB in the primary dentition displayed wider jaw closing angle with higher frequency of reverse chewing cycle, particularly on the crossbite side, than crossbite-free children [44]. During the mixed/permanent dentition, the chewing cycle duration and speed in children with UPXB were similar to crossbite-free controls in two studies [55, 63], while it was slower in one study [42]. Yet their jaw closing angle remained wider than the controls, particularly on the crossbite side [55], which was restored after orthodontic intervention [55]. Furthermore, children with UPXB compared to controls [42, 43, 51, 55, 72, 73] or compared to children with anterior crossbite [43] displayed higher frequency of reverse chewing cycle, particularly on the crossbite side. However, the occurrence of reverse chewing cycle after orthodontic intervention became similar to crossbite-free controls [55, 73]. Furthermore, children with various types of malocclusions, especially who suffer from functional displacements [72] or who had anterior crossbite [71], showed a greater variability in jaw chewing pattern compared to controls. The variability of jaw chewing pattern decreased after orthodontic treatment in children with anterior crossbite and became similar to controls [71] but remained higher than the controls in children with UPXB [69]. The lateral and vertical jaw amplitude during chewing was similar in children with UPXB and crossbite-free children [41]. Another study showed that the lateral and vertical amplitude of the jaw during chewing was shorter in children with UPXB compared to crossbite-free children, which increased significantly after orthodontic treatment and remained after 1 year of retention [38]. While another study found that children with UPXB chewed with shorter vertical amplitude on the crossbite side and longer on the non-crossbite side than the crossbite-free controls, which resume normal control levels after orthodontic intervention [42]. RME treatment of children with UPXB led to an increase in maximum mouth opening [67]. Furthermore, according to GRADE assessment, the quality of evidence regarding the effect of orthodontic treatment on jaw kinematics in children with UPXB and AXB was “moderate” and “low,” respectively (Supplementary file 5).

Eight studies investigated the chewing efficiency by measuring the food trituration and mixing ability in children with malocclusion [22, 27, 34, 45, 46, 70, 74, 75]. While one study included children in the primary dentition [46], the majority of the studies included children in the mixed/permanent dentition [22, 27, 34, 45, 70, 74, 75]. Five studies used artificial test food models as a form of silicon-based tablets (i.e., CutterSil, Optocal or Optosil tablets) [22, 27, 34, 45, 46], one study used peanuts and carrots [70], and two studies investigated food mixing ability using colorimetric capsules, which were freely masticated and their content were analyzed with a spectrophotometer [74, 75].

Various types of malocclusions were also investigated. The masticatory efficiency of children with normal occlusion was compared to children with class I, II, or III malocclusions [22, 27, 34, 45, 70], to children with AOB [46, 74, 75], or with anterior/posterior crossbite [46].

During primary dentition, the habitual chewing of children with AOB compared to children with UPXB showed no differences in the size or number of test food particles [46]. However, children with normal occlusion, compared to the two malocclusion groups, chewed the test food into greater number of particles with a smaller size [46]. Using similar colorimetric method, one study showed reduced mixing ability in children with AOB in the mixed/permanent dentition compared to AOB-free children [74], while another study did not [75]. The median particle size of both natural and artificial test food was markedly reduced with the increase in age, where children with class I and II malocclusions, except class III malocclusion, showed similar median particle size compared to children with normal occlusion [27, 34, 70].

In counter to these studies, it was shown that adolescent girls (11 to 15 years) with class II malocclusion, compared to adolescent girls with normal occlusion, had reduced chewing efficiency [45]. Similar result trend was observed after a 2-year follow-up of adolescent girls (11 to 15 years) with or without malocclusion [22]. Apart from the age-related “maturity” of the chewing efficiency in girls with or without malocclusion, it was shown that girls who have class II malocclusion, which was corrected orthodontically, showed similar chewing efficacy to girls with untreated class II malocclusion. However, both the groups of adolescent girls showed reduced chewing efficiency compared to girls with normal occlusion [22]. In addition, GRADE assessment indicated that the quality of evidence concerning the effect of orthodontic treatment on chewing efficiency for children with Cl II malocclusion was “low” (Supplementary file 6).