IntroductionMandibular asymmetries are defined as dissimilarity between some corresponding parts in the two opposite sides of this bone [1]. A complete bilateral symmetry is rarely found in living organisms, in fact many studies highlight how a difference between the right and left side of the face is very common in the population around the world [2]. These minimal "natural" variations make mandibular asymmetry aesthetically acceptable; on the contrary, facial types with more asymmetrical component may have a larger proportion of aesthetic problems. From a therapeutic point of view, as in the past as today, the correction of these asymmetries are considered significant not only to obtain a balance of the aesthetic sets but also to prevent functional alterations in the cranium-cervical-mandibular district. In fact there was a common belief that asymmetry was a predisposing factor of dysfunctions in this area [3]. Some researchers think that the mandibular asymmetry represents a possible risk factor for developing of functional alterations [4,5]. Some scientific publications report that the facial asymmetry, due to mandibular deviation, has a correlation with Temporomandibular Disorders [6]. According to these researchers, temporomandibular joints development could be influenced by the displacement of the mandible and it could be a risk factor for disc-condyle incoordination [7,8]. Furthermore, many studies showed functional connection between the stomatognatic system and the cervical system so that the alterations of the "mandibular system" seem to influence also the postural stability [9]. To better understand if a mandibular asymmetry can be considered a predisposing factor to a malfunction of the Stomatognathic and Postural systems, a sample of young asymmetric patients was investigated with clinical and digital findings comparing the data with statistical analysis.
The range of mandibular asymmetry analyzed was > 4mm and < 10mm, because, as reported in the literature this alteration was considered significant for a "conservative ", not surgical, treatment under an aesthetical and functional profile [10-12].
Materials and MethodsThe study was approved by the Institutional Human Ethics Committee.
Sample assessment
Patients were selected from those come to the Department of Oral and Maxillo-facial Sciences ( division of Orthodontics and Gnatology) at Sapienza University of Rome ( Rome, Italy), between January 2016 and March 2017 (7 clinical evaluations a day per 5 days at week).
Inclusion criteria: malocclusion IOTN- Index of Orthodontic Treatment Need - 1 to 3; - mandibular asymmetry > 4mm and < 10mm (the literature reports that asymmetries less than 4 mm are to be considered without clinical significance and greater than 10mm surgical) [10-12]; - growing patients ( from 7 to 15 years old); general and dental good health.
Exclusion criteria: Adult patients; - Syndromic cases of asymmetry; - TMD symptomatology referred; previous trauma; - previous orthodontic treatment; - systemic diseases; - tumors; - acute and chronic muscular-skeletal disease; - total or partial lack of radiographic assessments required.
Clinical evaluation and cephalometric evaluation of radiograms were performed as a measure of mandibular asymmetry. The clinical protocol consisted in: anamnesis, extra oral and intraoral clinical evaluation. Anamnesis was conducted with medical questionnaire to investigate patients' general health status and to find familiar cases of asymmetry. The patients were observed in the frontal view and from top to bottom to evaluate the lines of symmetry on the coronal plane (orthogonal to the bi-pupillary line passing through glabella and the nose line) and on the vertical one (un-leveling of gonial angles), to analyze the mandibular bodies length and the direction of mandibular deviation. The intraoral examination consists in general dental, periodontal and orthodontic analysis.
Radiographic evaluation of the asymmetry range. The cephalometric analysis on Orthopantomography (OPT) was conducted using Haberts and Saglam methods [13,14] in order to compare the two right and left sides of the mandible in terms of height and length. The cephalometric analysis on Postero Anterior skull radiograph was conducted on the basis of the guidelines suggested by Silvestri [15]. This exam is based on transverse and vertical measurements of one-sided dento-facial structure in relation to reference planes and compared with the contralateral. The further evaluation of the "splitting" of mandibular bodies could also be seen on Lateral skull radiograph. Radiographic data have been used only for the evaluation of the asymmetry level, given that the sample is composed of healthy patients.
In order to evaluate the presence/absence of TMJ, Diagnostic Criteria for temporomandibular disorders were used [16-18]. They provide an assessment including the most common TMD conditions. They consist of a valid and reliable Axis I screening questionnaire for identifying pain-related TMD and Axis I diagnostic algorithms for pain-related TMD, as part of a comprehensive TMD taxonomic classification structure. The Axis I diagnostic criteria are collected from the specified examination protocol and from the self-report issues. Axis II has not been evaluated.
Thirty patients were included but only 18 (8 male and 10 female aged between 8 and 15 years) agreed to take part in the scientific protocol. 8 patients did not partecipate for lack of parents' availability, 6 patients because they reside outside the city. Every patient submitted the informed consent.
Study designAfter the first screening, the 18 patients recruited for the study were examined. Clinical and anamnestic DC/TMD evaluations [16-18], performed during the preliminary phase, were useful to highlight eventual functional alterations in the Stomatognatic system. The examination was comprehensive of muscular pain analysis (myalgia and myofascial pain), during jaw movements and occurring with provocating test, applying manual pressure, in particular to masseter and temporalis muscles. Also other muscles and tendon insertions, such as frontalis m., pterygoideus lateralis m., pterygoideus medialis m., trapezius m., sternocleidomastoideus m., coronoid process and mastoid processes were evaluated. Moreover intra-articular disorders and the type of mandibular movement (such as the presence of limitations/ abnormal excursion/ deflections) were analyzed. The clinical evaluation of trigger points, joint excursion and pain provocation of the cervical spine has also been done.
In order to evaluate some possible positional head anomalies due to visual defects, patients' parents had to fill in an orthoptic questionnaire edited by our equipe on the basis of the principal ocular defects and to test specific "behaviors" occurring in the growing patients.
Electromyography of the masseter muscles and the anterior bundle of the temporal muscle with BTS TMJOINT were applied. The following indexes were considered: POC = muscular activity index with a percentage report of every muscle (normal range 83ASIM = asymmetry index. Evaluation of balanced muscular activation between both sides (normal range % -10 and +10)
TORS = activation of couple of muscles who expresses mandibular rotation on the transversal plane (normal range % 90 -100%). BAR = muscular center of gravity.
The assessment of muscular balance and adaptability to a "minor" skeletal asymmetry is been evaluated with TORS and ASIM index; the evaluation of occluso-muscular center of gravity is been evaluated with the POC index.
Tek-Scan III software 8.0 Windows PC for the analysis of the occlusal center of gravity and masticatory load discrepancies. Although T-scan is usually tested in patients with permanent dentition without dental loss, the evaluation is evenly been done.
A statokinesigram, which represents the projection to the ground of the center of gravity and pressure, for the evaluation of plantar support forefoot / hindfoot and load distribution between the left and right side has been conducted with the stabilometric platform DIASU.
A rasterstereography (Formetric3D) for qualitative evaluation of the spinal column anomalies has been performed.
The following indexes were considered: Pelvic Torsion: normal range (0-1, 9 degree- according to Harzmann) [19]
Antero-posterior bend VP-DM* (individual range)
Lateral bend VP-DM* (individual range)
Lateral deviation VP-DM* (0-5mm according to Harzmann) [19]
Kyphotic Angle: normal range (47-50degree according to Harzmann) [19]
Pelvic rotation: (individual range)
Pelvic inclination: (0-4 mm according to Harzmann) [19] VPDM.
VP is the prominent vertebrae at the cervical level and DM is the middle point of the straight line joining the left and right lumbar dimples.
Analysis of the dataAll data were analyzed by means of descriptive percentages, mean, median and standard deviation systems. Afterwards, then results emerging from the Stomathognatic system evaluation were cross-linked with the postural data through quantitative and qualitative statistics value, c2 and T-student test were used.
Based on the electromyographic evaluation, the sample was divided in two "groups": 1.patients with significant ASIM index, 2. patients with normal ASIM. This further detailed analysis is been done to better understand the relationship between asymmetry and functional anomalies.
The same operator, previously calibrated, carried out all the clinical and instrumental evaluations. Another operator controlled all the data to verify their reliability.
ResultsIn view of the great amount of data emerging from research, the results were divided into three sections, a) results emerged from the clinical evaluation, b) results emerged from second level examinations, c) results emerged from quantitative cross-linked examinations. Records without any statistical significance have not been reported.
Clinical resultsClinical evaluation results, as reported, include number and type of muscles positive to the palpation test, trigger points and excursion of cervical rachis, orthoptist questionnaire answers (Table 1).
second level examinations results
Analysis of TORS, POC and ASIM index (EMG scores). Assessment of muscular balance and adaptability to skeletal asymmetries. The 83% of patients (15 patients) had a TORS index score out of normal range that confirmed the presence of a significant mandibular deviation on the transversal plane and the presence of peculiar occlusal patterns, related to the skeletal structure. For what concerning the side of mandibular deviation, the TORS index scores of these patients are compatible with the clinical and radiographic data.
The evaluation of the activation of each muscle with the POC index showed in all these patients the predominance of temporalis muscle in the same side of mandibular deviation, as well as the cross-activation of the contralateral masseter. In a physiological muscular behavior, the vector moment generated by opposite muscles pairs is equal to 0. In cases of mandibular asymmetry, "muscular vector moment" could be the expression of mandibular deviation (as showed with the TORS index) versus the temporalis muscle side (as showed with the POC index). TORS index also expresses the predominance of anterior occlusal contact in the side of mandibular deviation or lack of posterior occlusal contacts.
On the contrary, the evaluation of the muscular functionality and balance with the ASIM index revealed how only the 39% of the patients (7 patients) with a clinical and radiographic mandibular asymmetry had an abnormal score. These patients with significant ASIM index were also analyzed with the T-SCAN to compare the different muscular work on both side and the distribution of occlusal contacts and load. The results were discordant and irregular because of the limits linked to the device not calibrated on mixed dentition (Table 2).
Evaluation of muscular Center of Gravity (CG) position. Ten patients showed the center of gravity in anterior position while eight in posterior one. The evaluation of the CG, as well as in the literature, has led to these results [20]. As the POC index showed, an anterior occlusal CG is related to the main activity of the anterior temporalis muscle bundle; a posterior occlusal CG is related to the main activity of the masseters. Occlusions characterized by a posterior CG are the most functional from the biomechanical point of view because it keeps the condylar load within normal limits and the lever arm in the sagittal plane is contained within physiological limits. The opposite situation happens in cases of occlusal anterior CG [21] (Table 2).
At the stabilometric platform qualitative analysis, only 33% of patients had a load discrepancy between right and left side. 50% of patients highlighted a posterior center of gravity, 17 % an anterior one and the 33% centered (Table 2).
Formetric qualitative analysis, comparison between the direction of mandibular deviation and related postural changes. Almost the totality of the patients showed the contralateral foot opening to the direction of the mandibular deviation. In 7 patients (38%) the pelvis was tilted and had a torsion contralateral to the direction of mandibular deviation, in 3 patients (17%) was ipsilateral, 7 patients have not shown any anomaly. In 8 patients (44%) the VPDM lateral deviation at the thoracic level was contralateral to the direction of mandibular deviation. 66% of patients (12 patients) had a scoliotic postural behavior (Table 3).
cross-linked examination analysisQuantitative statistical analysis to evaluate the correspondence between mandibular asymmetry and potential change of postural scheme. We have compared the patients who have an initial imbalance of muscular and occlusal assessment and who have not, cross-linking the data obtained from stabilometry platform and EMG. The c2 test was used. The c2 value (0,37) of the postural examination showed that there wasn't any significant difference between the frequencies of the two samples. The Formetric and EMG scores were compared using T-student test. No significant results occurred from each parameter of the Formetric evaluation. Therefore the relationship between the mandibular asymmetry and postural changes was not found.
DiscussionThe idea, which still influences clinician's mind, that the mandibular asymmetry represents locus minoris resistentiae for the development of a correct functionality of the stomathognatic and the correlated systems [4,5], was moving over time. The intermediate level, between 4 and 10 mm, of mandibular asymmetry was analyzed in patients without any other specific disease. In fact the type of asymmetry investigated is not associated to syndromic growth anomalies, tumors, inflammatory or traumatic process. It is already known that these specific pathological alterations have "dysfunctional aspects" due to the greater asymmetrical component, involving also other facial/cranial structures, and the systemic implications [22].
The perception of the asymmetry is more of 6mm for common people, around 4mm for the orthodontist, under this threshold it has no clinical significance, while over 9mm surgical treatments was indicated [10-12]. According to some authors [4,5,23], the asymmetry not only gives an aesthetic discomfort but also it may be involved in functional mechanisms. Other authors didn't report cases of mandibular asymmetry with a significant functional impact [14]. Because of these dissonant data, this research protocol was performed to observe the features of the asymmetrical patients from a functional (muscular, articular) point of view in both the Stomatognathic and Spine system.
In order to assess it, DC/TMD have been considered for the screening of the intensity and sites of perceived pain. Confirming the criteria used for the screening, in the sample nobody reported pain during functional movements or palpation test, but only mild discomfort, not considered as a dysfunction. Furthermore, the emerging data are overlapping with the natural variability present in the non-asymmetric population.
Cervical trigger point test highlights the positivity mainly for the SCM. In view of the functional characteristics of this muscle [24], the single data cannot be directly and exclusively attributed to the mandibular asymmetry, because there are too many variables that can cause such a response. It is customary to note that children take on spoiled habits when performing homework or sports, habits considered by the neuromuscular system to be normal. Another reason may be the presence of refractive defects, emerged from the Orthoptic questionnaire, that it is compatible with this clinical context [25].
Regarding the results obtained from the second level examinations, the EMG scores have allowed to express some clinical considerations. 15 patients (88%) showed the correspondence between the verse of the mandibular symphysis deviation and the predominant activity of both the anterior bundle of the omolateral temporalis and the contralateral masseter. An asymmetrical mandible presented a long side (balancing side) where the masseter has the dominant activity and a short side (working side) where the temporalis activity is prevalent. According to EMG scores, in a deviated mandible, there is a "crossed muscular pair's vector moment", given by temporalis muscle on one side and masseter muscle on the opposite one. The verse of mandibular deviation has a correspondence with the dominant temporalis muscle side. On the sagittal plane, this could express a retro positioning of the condyle of this side and, when the resulting muscular center of gravity was anterior, it could increase the articular load. Even though the patients have not reported any symptom from a clinical point of view, this may be a possible risk factor for the condylar-disc incoordination, as confirmed in the literature [26,27].
The clinical evaluation showed, without any statistically meaning, a similar scheme for the trapezius and scalene muscles. This data was well matched with what occurred from the trigger points at the cervical level. Otherwise, in literature, as reported by Korbmacher [28], there was a less evidence of relationship between mandibular position and vertebral posture in regions below the cervical spine [29,30]. Therefore, these results suggested the clinicians to take into account this particular muscular behavior in both systems when patients with asymmetry have been analyzed.
Even if the presence of an anterior CG should be considered as a biomechanical risk factor for eventual joint and muscular dysfunction [2,20,21], no significant data emerged; in fact 10 patients (55%) have an anterior center of gravity and 8 (45%) a posterior one. These results follow the natural distribution of the healthy population.
Analyzing with the EMG a sample of patients homogeneous for skeletal asymmetry, the expected results should have showed a similar homogeneity under a functional profile, instead, only the half of the patients has a significant ASIM index. This apparently contradictory evidence indicated that a bone asymmetry not necessarily produced a functional altered behavior.
For what concerning postural qualitative analysis, data emerging from the stabilometry showed that only 33% of the entire sample (6 patients) had a load discrepancy between right and left side. It would seem that the asymmetry of postural load was not necessarily evident in patients with mandibular asymmetry. On the contrary, the data emerged from the qualitative analysis of the Formetric scores, showed that all patients presented the same limited anomalies, among which the presence of a scoliotic postural behavior contralateral to the direction of mandibular deviation. The analysis of these latter results needs further detailed studies also by other specialists. On the basis of the ASIM index scores, the sample of patients was divided in two groups (1.patients with significant ASIM index and 2.patients with ASIM index normal). The comparison was made in order to notice an eventual difference in postural behavior between these two groups. The analysis has not carried out statistically significant results. In the light of the data emerging from the sample examined, the hypothesis that a mandibular asymmetry could produce a postural anomaly, cannot be accepted.
No clinical and digital data supported the hypothesis that the "minor" mandibular asymmetry in growing patients necessarily lead to a muscular functional imbalance. The only data that could be considered a potentially critical factor for a development of a TMJ dysfunction was the retrusive position of the short side of the asymmetrical mandible [31,32]. A further observation to underline is the extreme adaptability and plasticity present in the growing patients that could underestimate some possible functional anomalies.
Conclusions
The present examination was conducted to achieve a greater knowledge about the possible functional alterations that a mandibular asymmetry could produce in both the stomatognatic and postural system. The results suggest that a minor mandibular asymmetry should not be always considered as a locus minoris resistentiae in the functional development of these systems. Although it cannot be ruled out that some anomalies may occur, at the time of evaluation, no patient showed any specific functional alterations. As mentioned above, a condyle retrusion of the shortest side of asymmetric mandibular should be considered as a possible cause of TMJ instability particularly in growing age patients [27,32]. Despite the negative results, a great care was recommended for the clinicians who intercept mandibular asymmetry in these patients; because it must also considered that, the extreme plasticity of the systems can make asymptomatic some negative compensations. For what concerning the postural development, no significance associations with mandibular asymmetry were found from a statistical point of view. Considered the complexity of the topic, further researches are requested.
Statement of Interests
The authors certify that they have NO affiliations with or involvement in any organization or entity with any financial interest (such as honoraria; educational grants; participation in speakers' bureaus; membership, employment, consultancies, stock ownership, or other equity interest; and expert testimony or patent-licensing arrangements), or non-financial interest (such as personal or professional relationships, affiliations, knowledge or beliefs) in the subject matter or materials discussed in this manuscript.