Review on Mandibular Muscle Kinematics

The complexity of mandibular dynamics encourages constant research as a vehicle to improve oral health. The gold standard motion capture system might help us to understand its functioning and its relation to body position, aiming to perform an exhaustive bibliographic review in the Dentistry field. Six different electronic databases were used (Dentistry & Oral Sciences Source, Scopus, Web of Science, PubMed, CINAHL and SPORTDiscus) in April 2022. The selection criteria includes a biography, critical analysis, and the full text from 1984 to April 2022, based on the odontological gold standard, whether or not in combination with additional devices. Clinical cases, bibliographic reviews or meta-analysis and grey literature were excluded. The checklist of the critical assessment methodology by Joanna Brigs was used (JBI). After choosing scientific articles published in peer-reviewed journals, 23 out of 186 investigations were classified as eligible with a total of 384 participants. The issue being addressed is related to the speech properties, posture and body movement in relation to dento-oro-facial muscle and facial analysis, mandibular kinematics and mandibular dynamics during the mastication process. The markers arrangement depends on the dynamic to be analysed. From a physiologic and pathologic perspective, the applications of the optic system are relevant in Dentistry. The scarcity of literature obtained implies the need for future research.


Introduction
Biomechanics is the science that studies, analyses and interprets the activity performed by living things, in particular the human activity, aiming to collect data quantitatively on forces and the effects caused by its application over the musculoskeletal system during motor activity performance [1]. The research on the motor activity in human beings allows the system functional capacity that intervenes in the physical action to be understood in order to be able to categorize it as physiologic or pathologic, and to be capable of establishing and verifying biomedical therapeutic procedures [1][2][3]. In order to carry out this analysis, there are human motion capture systems that gather spatio-temporal information that is digitally represented and an optical system, which is the most appropriate for recording human activity [4]. The optical systems currently require the use of large laboratories equipped with a multicamera synchronized system consisting of two or more units located in different angles, from which the location and the space is calculated; the higher the number cameras used, the higher the measurement accuracy and reliability will be [5]. Within this motion capture technique based on images, we can include a technique based on the use of reflective passive markers of an infrared lamp that are located in three strategic anatomic landmarks where the three-dimensional location of these is calculated through triangulation [4,5].

Searching Equations Database Results
"vicon" Dentistry & Oral Sciences Source 4 (Dentistry OR stomatology OR "facial bones" OR "facial muscles" OR "mandible kinematics" OR "mandible dynamics" OR "oral function" OR "temporomandibular joint" OR "stomatognathic system" OR bite OR feeding OR jaw OR mandible OR lip OR mastication OR munch OR mouth OR oral OR tongue OR tooth) AND "vicon"

Selection Criteria
Scientific articles as well as doctoral theses, proceedings of scientific journals and books in any language that belong to the biomedicine field, from January 1984 to April 2022 were selected. The entire text from the article must be available either by public access or through the request for a library loan. Experimental research was also accepted, including observational and transversal studies, as well as case-control in human population, with all of them being based on the gold standard system, whether or not in combination with additional devices that were necessary to carry out the research (e.g., pressure sensors, force plate, electromyogram, video camera, microphone). Thematic studies outside of the odontological environment, clinical cases, bibliographical reviews or meta-analysis, grey literature as well as articles that had not resorted to the gold standard system as the main tool for human motion analysis were not included.

Selected Studies and Data Extraction
The search strategy consisted of the initial identification of Medical Heading Subjects (MeSH) and the Descriptors in Health Sciences, which were used together with the Boolean operators.
The PICO strategy of research was selected, from which the following question was raised: Which are the most relevant articles published in Dentistry through the use of the gold standard human motion capture system between 1984 and 2022?
During the first phase, the key words described in Table 1 were searched in the different data banks. Duplicate articles were identified and successively the remaining articles were analysed through the reading of the title, summary and methodology aiming to determine if they fulfil the inclusion/exclusion criteria. Finally, the reading of the entire text to verify the eligibility was performed and included in the bibliographical review. The same procedure that was performed by both investigators was used to determine the presence of bias according to Joanna Briggs Institute (JBI) checklists.

Results
A total of 186 articles were found in the scientific-technical databases through the searching criteria previously described. Subsequently, the PRISMA 2020 flowchart was used for their selection (Figure 1). A total of 23 articles ( Table 2) that fulfilled the selection criteria were included in the systematic review. used for their selection (Figure 1). A total of 23 articles ( Table 2) that fulfilled the selection criteria were included in the systematic review.

Geographic Origin
Most of the studies that were included in this bibliographical review were developed in Europe (34.78%) and North America (USA) (30.43%), followed by Asia (21.74%), Oceania (8.70%) and South America (4.35%).

Temporality in Publications
From the 23 selected articles, the established range with regard to the dissemination of research in relation to the gold standard is 1992-2021. In the field of Dentistry, it has been over 15 years since the first publication was created for medical purposes, coming from Germany [38], the pioneering country in this sector. In almost 30 years, 22% of publications were released in 2015, with Germany [39][40][41] and United States [42,43] being the countries where these pieces of research were developed. From the initial publication, it took 10 years for other research studies about optical capture systems in 3D were made known. Subsequently, from 2003 to 2006, they have not been noticed in this respect. The countries where the research has been developed, apart from those already mentioned, are: France [44]; Taiwan [45][46][47]; New Zealand [48]; Brazil [49]; Australia [50]; Poland [51,52]; Japan [53,54]; and Spain [55].

Geographic Origin
Most of the studies that were included in this bibliographical review were developed in Europe (34.78%) and North America (USA) (30.43%), followed by Asia (21.74%), Oceania (8.70%) and South America (4.35%).

Temporality in Publications
From the 23 selected articles, the established range with regard to the dissemination of research in relation to the gold standard is 1992-2021. In the field of Dentistry, it has been over 15 years since the first publication was created for medical purposes, coming from Germany [38], the pioneering country in this sector. In almost 30 years, 22% of publications were released in 2015, with Germany [39][40][41] and United States [42,43] being the countries where these pieces of research were developed. From the initial publication, it took 10 years for other research studies about optical capture systems in 3D were made known. Subsequently, from 2003 to 2006, they have not been noticed in this respect. The countries where the research has been developed, apart from those already mentioned, are: France [44]; Taiwan [45][46][47]; New Zealand [48]; Brazil [49]; Australia [50]; Poland [51,52]; Japan [53,54]; and Spain [55].

Funding
Among the included publications, five of them (27.74%) had private funding [48,53,54,56,57], one (4.34%) obtained mixed funding [50] and the remaining 17 (65.22%) did not achieve a grant [38][39][40][41][42][43][44][45][46][47]49,51,55,[58][59][60].  All participants showed, in the jaw and lip, significant changes in motion features and significant changes in speech intelligibility in five of the six participants were associated This study provides evidence supporting the use of a treatment approach aligned with a dynamic systems theory to improve motor speech movement patterns and intelligibility in children with CP  The gait variables that statistically changed after surgery were: pelvic rotation, hip range motion in the sagittal plane, knee range motion in the sagittal plane, and operated side in a significant manner The primary gait deviations that happen after surgery and the compensatory mechanisms that arise later, depend on the ubication of the graft donor site.  The sample size that was used in publications was not very large, being a total of 384 participants, with an average size sample of 16.69 participants and a median of 16.

Characteristics of the Sample
The sample size that was used in publications was not very large, being a total of 384 participants, with an average size sample of 16.69 participants and a median of 16.
Only two studies with a basic theme of Dentistry excluded patients with abnormalities, traumas or maxillary surgeries during the last two years, and included those with orthodontic or orthopaedic treatment in progress and without mandibular blocks [39].

Themes
Different topics were studied, all of them were related to the stomatognathic system in a direct or indirect way, which remains the most studied topic (43.48%) the speech properties, especially pronunciation [42,43,[45][46][47]50,52,56,57,60]. Dento-oro-facial level changes that influence posture and body movement [39][40][41]44,51] were also a reason for study (21.74%), as well as facial muscle analysis (17.39%) [38,49,53,54], mandibular dynamics during chewing (8.695%) [48,59] and mandibular kinematics in laterality and protrusion movements (8.695%) [55,58]. The selected research studies that were published in different journals and themes can be seen in Table 3. Only two studies with a basic theme of Dentistry excluded patients with abnormalities, traumas or maxillary surgeries during the last two years, and included those with orthodontic or orthopaedic treatment in progress and without mandibular blocks [39].

Relationship between Articles
The data that was used in one of the studies belonged to a previous research [56]; the authors of two publications shared their experimentation subjects in their studies [40,41] and another study [57] included a section of the sample that was used in other research [43]. Finally, three research studies aimed to continue previous scientific articles [55][56][57].

Area of Research
All the research studies included in the current article were selected according to structural involvement related to the stomatognathic system (Table 4). Table 4. Areas of research of the included studies and facial landmarks that have been used.

Stüssi et al. (1992) [38]
To assess the results achieved after a dynamic reconstruction of paralysis of the facial nerve, measuring facial distances and the length of buccinator muscles during contraction, also reflecting its speed and fatigue To study the kinematic pattern of the golf swing, and the analysis of the temporomandibular joint, using jaw repositioning devices in order to cover occlusal surfaces and to increase 3 mm the vertical dimension To study facial, labial and mandibular dynamics referring above all to muscle contraction in these areas To conduct research on the coordination between mandibular and labial dynamics during speech To collect labial and mandibular kinematic data generated during the speech process  To determine the kinematics in several joints of human anatomy, among them the ankle, knee and hip joint, the electromyographic activity of the muscles of the lower limbs during bipedal stance and on one leg in centric relation and maximum intercuspation

Marker Arrangements on Head and Facies
Different research studies suggest very similar locations in the category of stomatognathic system and in areas around them

Discussion
This is the first systematic review performed over the application of the gold standard motion capture system in the Dentistry field. In the search for literature only 23 articles were found in relation with stomatognathic system, from which all of them were included in this literature review aiming to cope with as much existent information as possible over the use of referential optical system applied to dental sciences.

Discussion
This is the first systematic review performed over the application of the gold standard motion capture system in the Dentistry field. In the search for literature only 23 articles were found in relation with stomatognathic system, from which all of them were included in this literature review aiming to cope with as much existent information as possible over the use of referential optical system applied to dental sciences.
Research studies that focus on speech [42,43,[45][46][47]50,52,56,57,60] have been selected not only as this is a part of the stomatognathic apparatus, but also to study the mandibular and labial dynamic.

Quality Evidence
For the critical assessment of the methodological quality in the included research studies in the present review and in order to determine the presence of bias, the Joanna Briggs Institute (JBI) [61] checklist was selected for observational cross-sectional studies. The JBI scale assigns a rating from zero to nine points to the selected articles. The majority of these publications obtained scores over six points, indicating an optimal methodological quality in their studies [40,41,[43][44][45][46][47][50][51][52][53][56][57][58]60]; seven publications [39,42,48,49,54,55,59] obtained a middle level score due to the inaccuracy of inclusion criteria as well as the absence of a comparison group and the sample size; the worst score was obtained by one article [38], the score below four points being the only choice due to sample size, the participants, data collection and their analysis.
The sampling frame (First point of the JBI checklist) seemed dubious in all the articles due to the lack of an explanation and also due to its own contextualization. Additionally, almost all of the studies presented a short or deficient sample size (point number three in the JBI scale) where the number of participants was between one and 39. As a consequence, studies could be qualified as inappropriate due to the inability to detect significant differences in order to obtain conclusive results [62].
Twelve studies did not present a comparison group (fifth point of JBI checklist), which may increase the presence of bias causing the obtained results to be interpreted in a wrong way as a result of the absence of a control group [63].

System Application
Vicon is still one of the main optoelectronic motion capture tridimensional systems based on markers [6], and traditionally has been used for the analysis of biomechanics, gait and robotics [64]. It is a precision system that analyses motion through video and it is controlled by a computer allowing the analysis of the dysfunctions of the human corporal dynamics [65]. The first research studies based on the use of the gold standard were focused on the clinical analysis of motion and human gait [66][67][68][69][70]. Over the years the application of this system has given rise to numerous investigations in the field of biomedicine, as well as the studies performed on Dentistry, research studies that link Dentistry with other sciences and studies that analyse elements of the stomatognathic system.
Knowledge of masticatory mandibular dynamics with the gold standard system is in service of prosthodontics, allowing implants design, crowns, and fixed partial dentures, in addition to restoring therapeutics in order to identify eccentric premature occlusal contacts during the mastication process [55], and this contributes to maintaining the coordination and health of dento-oro-facial system [71].
The method is useful for behavioural sciences, biomechanics and the food industry [48]. It can contribute to developing therapeutic protocols targeted to improve motor speech centre [50], and as a result the existing methods are not an ultimate solution in view of these types of problems [72].
Research has shown that using a repositioning splint during gait increases the symmetry of run pattern and it could help prevent injuries and improve performance [39]. Therefore, as certain authors show [33], the occlusion in maximum intercuspation and centric relation are related to functional body posture.
The occlusal plane inclination influences the kinematics of the masticatory cycle, limiting the jaw movement during closing [59]. Recently, particular attention has been attached to mandibular kinematics with the gold standard system in order to design mandibular advancement devices that allow greater opening ranges and thereby favouring the treatment of obstructive sleep apnoea [55]. Current devices are manufactured without taking into consideration anatomical features of the temporomandibular joint and without individualizing mandibular morphology of patients. As a consequence, in order to avoid a mandibular retrusion in certain areas where the upper respiratory tract could become narrowed, the individualization of the case is necessary as not all patients move their lower jaw equally [73].

System Calibration
At the start of the study, and before initiating the record, it is recommended [39,48,49] to calibrate the capture system and for the participants to undergo a previous test. Ward et al. [50] reflect the importance of the responsible investigator to be qualified and trained from a methodological approach. After having informed the participants, the markers are properly placed, and cameras record the image from these markers while they are connected to a device, which synchronizes its functions and transmits these images to a particular computer and software. Some authors calibrate the position for each marker from a reference cube of 24 × 24 cm with eight spherical markers: head, forehand, nose, chin, lips (commissures, Cupid arc, middle and lower point) [50] and even 10 markers [42].

Individual Placement
Researchers recommend that the subject should be in an upright position to achieve the most reliable calculation of the spatial coordination [38,46,48,53,54] or erect and with arms at rest [40], except for when the research focused on the study of the four main jaw positions (centric relation, centric occlusion, miocentric and maximum intercuspation) and its relationship with the gait. In this particular case, a 15 m walkway is installed, which is visualized by eight infrared cameras [39]. At times, when the body kinematics is analysed the body mass index of the participants is registered [40].

Markers and Placement Selection
It is crucial to consider a proper selection of size [74] and marker locations in order for an optimal analysis of the structures to be conducted [75]. Other motion capture systems without markers have obtained mixed results in the location of the areas in the human body that are to be analysed, compared to the gold standard system having variations from 2 cm to 8 cm, increasing this discrepancy during motion [74,76].
The researchers encourage the installation of cameras around the capture space at the working room, aiming to avoid hidden markers. Although the gold standard system supplies hemispherical markers of 14.5 and 9.5 mm, they are not appropriate for light reflection in small areas such as the face and eyelids. As a consequence, spherical markers [54] have been created with a 4 mm diameter covered with reflective tape [49]. Some researchers also use reflective markers of 3 mm [42,43,56,58,60] in the upper lip, lower lip, corners of the mouth, the right side jaw, the left side jaw, and the middle jaw. The OptiTrack markers have semispherical shape and they are covered by a reflective sheet and are also covered by a cosmetic glue that allows easy application and removal [52]. Although for the recording of facial motions the use of small markers is recommended [76], the use of big markers increases the precision of the data registered [74].
The kinematic data are processed with a kind of software that does not need a facial model. Some authors considered strategic points of the face for 11 markers: central point of forehead close to the hairline, supraciliary, malar regions that coincide with eye canthus, pars centralis in both upper eyelids, labial commissure and chin [49]. The nasion marker and the one on the chin show the vertical movement and the head rotation [56,58].
Some other authors aiming to register the facial dynamics placed a total of 44 markers, 18 of which corresponded to the stomatognathic area. Subsequently, they corrected the number, and reduced it to 28, since this number is sufficient to register the facial dynamics [52]. In order to register lingual dynamics (protrusion and literalities), some researchers place eight reflective markers of 3 mm in the facies, three in the jaw (middle line and mental protuberance and on both sides of this) and five referential markers (middle line and on both sides of frontal) in the nasion and nasal appendix. In addition, one marker is placed on the tip of the tongue using denture adhesive [58].
With the objective of studying kinematics of the buccinator muscle, its length and contractibility, they are used when placed in the corners of the mouth and jugal areas and it is registered together with other facial points with four cameras stacked, one above another, on the same vertical axis of the right and left cheek, respectively, allowing the complete detection of the six markers, with two of them in the area outlined above [38].
By studying the masticatory kinematics and occlusal morphology, six reflective markers were secured to the skin using double sided adhesive tape, onto the surface, osteological referential points, pogonion, nasion, right and left lateral condylion, right and left gonion. According to this researcher, the position of the marker during the register, and also due to the skin displacement, could provoke a slight variation although it may only be a minor one [59].
One of the gold standard motion capture system models has been designed to automatically track the movements through contrast detection between a circular marker and surrounding pixels. For that purpose, an adjacent light source for each camera has been used and retro reflective markers (bright white against a darker environment) [50].

Auxiliary Devices for the Markers' Placement
Some researchers use 10 mm markers that are attached to the temple of a spectacle frame as a reference coordinate system [54] as well as taking measurements with miniature pressure sensors in order to register the labial seal [53], which are water resistant [54].
Some authors use markers with a 25.4 mm diameter fixed on the skin using double sided adhesive tape [44]. Among other points, the tragion point is registered so as to determine angulations at that level, while the participants wear a splint or replacement mandibular devices to determine if the mandibular position together with other joints are affected by the speed of the golf ball. In the current study none of participants show clinical or historical evidence of the temporomandibular joint dysfunction or the myofascial pain dysfunction syndrome or occlusal alteration [44].
The motor control of speech in a given population can be studied [45,46] through reflective markers with a 0.6 diameter [46] over an adjustable plastic facial mask with straps, which cover the upper half of patient: pars anterior head, preauricularis and nasal area. Registering labial dynamics and buccal opening speed with markers in commissures in the upper and lower lip [45][46][47]. The mandibular and lingual dynamics record is as important as recording the movement markers in the right, left and middle forehand, in the nasion and in the nose, as these are referential markers in order to take into account head rotation [43,60].
With regards to the analysis of masticatory trajectories, there is another alternative, which consists of designing a special device for monitoring the mandibular movement. Eight cameras arranged in a semicircle record six reflective markers, three of them are placed at the nasal bridge and two of them on the forehand [48]. Due to the fact that the skin has minimal movement during mastication, the rest are arranged over the dental device that has been designed. The participant must be in occlusion to be able to chew food with different textures later [48].
In addition, it is registered together with kinematic corporal mandibular dynamics, and the researchers use up to 13 cameras and whole-body markers of Plug-In-Gait, also registering electromyographic activity of the rectus femoris muscle, medial vastus muscle, biceps femoris, tibialis anterior, the soleus muscle, the medial gastrocnemius and the masseter [40]. Two mandibular positions were registered; the centric relation and the maximal intercuspation [40]. If in addition to the kinematics of the trunk and head, the postural stability is analysed, some researchers are supported with the use of a force platform [41].

Master Record
The marker mapping is determined by the kinematics features of the tissue to be studied [52]. Due to the great complexity of the stomatognathic system, the ideal mapping in virtual reality is quite difficult, as a result the temporary loss in the visibility fields of the camera, or by the excessive proximity of the neighbour markers, resulting in temporary data loss [52]. Some authors maintain that anatomical features, capacity for expression and intense facial hair can interfere in standard mapping of the markers [52].
Occasionally, it is recommended to adhere the markers to the skin through the use of a degreasing substance [49].
Binh et al. [77] suggest a mapping of markers for the study of facial and mandibular dynamics. In order to obtain varied and relevant information in terms of investigation, although there are templates in the literature in marker distribution [77], the authors call for the creation of a master record, from which markers are tested and defined to finally decide the number and location for further research. It is important to emphasize that a greater number of markers allows the improvement of the nuances of imitation, but it also increases the probability of having mistakes or fading.

Limitations
Limitations exist in the present systematic review. Despite the fact that there is a bias to a large extent in some of the studies that were included due to the scarcity of the literature published in the Dentistry field, all the articles found by the researchers were included. Potential for bias was evaluated through methodological quality assessment of the articles through JBI protocol by means of its critical assessment guide.
Several problems were encountered in the methodological design in the majority of the studies, such as the contextualization of the sampling frame, as well as some deficiencies with regards to the determination of sample power being observed. There is only an article that talks of the need of training of researchers in using the optical system in question [50] and a minority of the calibration of the said technology prior to it being used [48][49][50]52]. In 2010, talks of the concordance intra-radar and inter-radar [46] and subsequently the concordance was mentioned in only one study [59]. As a result we assume that from a methodological perspective, when drafting the information regarding concordances and training, some of the studies have ignored or obviated these aspects 60].

Future Research
As a result of the scarcity of studies found, it is suggested one considers future studies through the gold standard system in the field of pure dental sciences. Therefore, such research can provide relevant data for better understanding of kinetics and mandibular kinematics in isolation and its relation to the kinematics of other body systems. On the question of the notorious influence between the static and dynamic relationship between the maxillary and lower jaw with regard to the rest of body structures [78], however, at the present time there is insufficient data. As a result, we believe that this optical capture system could encourage development in this field since the scientific literature on the matter at this point is insufficient.

Conclusions
This review identifies 23 articles that addressed issues related to Dentistry in a direct or indirect way. The studied topics were: • the study of the speech properties (43.48%) • dento-oro-facial level changes that influence posture and body movement (21.74%) • facial muscle analysis (17.39%) • mandibular dynamics during chewing (8.695%) • mandibular kinematics in laterality and protrusion movements (8.695%) Five research studies were found that focused exclusively on Dental Sciences, five research studies related the field of Dentistry to other sciences, and the remaining research studies analysed the elements related to the stomatognathic system in an indirect way. From a physiological and pathological perspective, the applications of the optic system in Dentistry are especially relevant.
The markers' arrangement depends on the dynamic to be analysed, and that is the great variability in anatomical references.
It can be concluded that the scarcity of literature on the gold standard motion capture system in the field of Dentistry, and the methodological errors that have been found concerning some research studies, shows the need for future investigative studies to be conducted in this area with this kind of advanced technology.