Bruxism and masticatory pattern: an understudied relationship

1.1. Chewing

Chewing is one of the most principal functions for the stomatognathic system. This consists of the act of biting, crushing and chewing food as homogeneously as possible using the upper teeth against the lower ones, mobilizing the food from the right side to the left side and vice versa as many times as possible, this is considered a complex neurological process that also depends on the development of the craniofacial complex, of the central nervous system and dental occlusion [3-5]. The masticatory function is characterized by generating motor responses, which are rhythmic and learned, combining both reflex and voluntary activities. Once started, it continues almost automatically, being voluntary the start and end of the activity and being intentionally accelerated, slowed down or stopped.

The afferent sensory information of the S.E is conducted trigeminally to various structures of the central nervous system (CNS). This sensory information stimulates various areas within the brain stem and neocortex, both areas involved in important higher functions. The neuronal stimulus generated by chewing (considering all its stages) adding the times a day that it is triggered, takes approximately one hour. During this time, the different structures of the stomatognathic system are used thoroughly, and both the quality of this function and the excitation it produces, ultimately depend on the nature of the food used, so it must always be a food that stimulates a high functional demand [5-7].

Chewing is considered as a learned orofacial function and of relevance for the harmonious development of the stomatognathic and craniofacial system. As an alternating bilateral pattern, it promotes the symmetry of the neuromusculature, a harmonious growth of the maxillary bone bases, and which means a balanced occlusion with physiological wear. When the masticatory pattern is not physiological or ideal, an alteration of the temporomandibular joint may occur, with simultaneous contacts of work, rolling and sliding [8, 9]. In addition, chewing is an oral function that is closely related to the physical, mental and social health throughout the life of the individual.

Chewing has been tried to bring to an objective and quantifiable plane for study and analysis. One of the ways to objectively measure chewing is to consider masticatory efficiency, bite strength and ability to mix food, or it can be subjectively considering the perception of the patient’s own masticatory capacity. From the point of view of orofacial motor skills, the evaluation of chewing considers different aspects, such as: number of masticatory cycles, masticatory pattern, masticatory time, among others masticatory cycles correspond to the rhythmic movement that is generated as a result of the combination of movements when performing the masticatory drop, where the occlusal characteristics are important, in fact one of the conditions of occlusal stability is bilateral chewing with protrusion during incision [8-10].

1.2. Masticatory pattern (Pm)

Chewing is a motor and sensory function of the stomatognathic system, rhythmic, coordinated and learned, that is, it is stimulated and fed back by the peripheral perceptions of the stomatognathic system itself, its relationship with food and the entire environment of the masticatory process, involving the other organs of the senses such as smell, vision, touch and postural system. From all these areas they leave, by afferent routes, carrying the information to the Central Nervous System, from which efferent pathways emerge that go to different effector organs (muscles, tendons, glands) [11, 12] (Fig. 1).

Fig. 1Diagram of nerve transmission pathways

Neurologically this sensory stimulation and feedback activates a set of neurons responsible for the rhythm and coordination of chewing, located in the brainstem called the central generator of masticatory patterns (GCPm). The GCPm is found at the ponto-bulbar junction of the brainstem, specifically in the medial bulbar reticular formation between the motor root of the trigeminal nerve and the olive inferior. By definition, a pattern-generating center could produce a rhythmic activity in the absence of peripheral or upper descending stimulation, but this does not mean that peripheral stimulation does not play an important role in shaping the motor responses of that generating center , on the contrary, there is a great variability between each masticatory cycle and this is because the neurons of this generating center are constantly subjected to a feedback mechanism, which is born from the information emanated of the proprioception in the stomatognathic system and the higher centers such as the cerebral cortex, amygdala, periaqueductal gray matter, among others. This is important in activities of the human life such as how to manage emotions, instincts, discrimination between food, taste, hunger, salivary secretion and swallowing [13] (Fig. 2).

Fig. 2Scheme of variability in GCPm by feedback

The masticatory pattern, masticatory type or preferred side of chewing is defined as the location of the food bolus and the route of mandibular displacements during the masticatory act [13]. When establishing a measurement of the number of masticatory cycles in percentage to the right and left or both sides, the following relationship is had between 50 % and 65 % on each side is considered alternating bilateral chewing, more than 65 % on both sides is considered simultaneous bilateral chewing or hinge, between 66 % and 75 % on the same side is considered unilateral preferential and more than 75 % on the same side is considered chronic unilateral chewing. For efficient chewing it is considered that the alternated bilateral pattern turns out to be the ideal physiologically, which would imply that any other pattern could be considered as a chewing dysfunction with alteration of the efficiency, time and capacity of the masticatory process [14, 15].

Patients with temporomandibular disorder (TMD) usually present an altered masticatory function, it is thought due to direct factors including tooth pain or loss of teeth, and indirect factors such as joint damage or hypermobility that can alter masticatory function. From a more functional perspective, it is logical to think that, for some reason, the masticatory pattern was altered or was never learned correctly, what eventually would cause a functional alteration in the temporomandibular joint, and if it holds up over time, can become a structural injury [16].

1.3. Bruxism (Bx)

In 2017, the ninth edition of the glossary of clinical prosthodontic terms (GPT9) was published by the American Academy of Prosthodontics, where Bx is considered as parafunctional teeth grinding or clenching, a nonfunctional, rhythmic, spasmodic and involuntary oral habit [17, 18].

Bx is considered a parafunction of multifactorial origin, since neither its etiopathogenesis nor its pathophysiology is clear, being mainly regulated by the central nervous system (CNS) and influenced by peripheral factors. It is characterized by repetitive jaw movements with no apparent functional purpose, especially clenching and/or grinding of the teeth, severely damaging the S. It can cause extensive loss of dental tissue by attrition (bruxofacets) or abfraction, dental fractures, dental hypersensitivity, pain in the orofacial musculature, masticatory dysfunction and TMD, because the adaptive capacity of the E.S. is exceeded by the intensity of the forces exerted on its components [19-21].

The first cause of Bx remains unspecified, which has generated different theories regarding its origin and has been associated with multiple risk factors. The risk factors mentioned can be classified into 2 groups that can even be interacting: peripheral or morphological/anatomical factors, and central psychological/pathophysiological factors [22]. Dentoskeletal alterations correspond to peripheral, morphological or anatomical factors. Within this group we can mention the alterations in dental occlusion, in the presence of dentomaxillary anomalies and their relationship with the cranio-cervico-mandibular unit, a possibility based on the assumption that, in the absence of occlusal balance, the activation of periodontal receptors occurs, with a secondary response of reactive type. Peripheral alterations, more precisely morphological alterations, were considered for a long time as the main factor causing bruxism due to their prevalence, which was much higher in population groups with malocclusion (class II and III) than in comparable groups with normal occlusion, triggering an excessive activity of the mandibular muscles [23-26]. This led to the inference that the corrected or adjusted occlusal alteration was sufficient to cause the disappearance of teeth grinding. In this way the theory was established that through bruxism the organism tried to eliminate these occlusal interferences. Currently, clinicians have expanded the search for other causal factors, where dental occlusion yields its action to other areas of knowledge (psychological and pathophysiological factors). This demonstrates a tendency in bruxism research closer to a biomedical-biopsychosocial model/paradigm than only to occlusion, mainly because it has been observed that not everyone is involved bruxist presents occlusal interferences and not everyone with these interferences is a bruxist, but the same studies do not indicate anything about the masticatory pattern present in the patient with Bx [27-29] .The scientific literature has repeatedly shown that the theory of alterations in dental occlusion lacks scientific validity, possibly because of a problem of methodology rather than a problem of clinical outcomes [30-32].

In the various studies of patients with possible and probable bruxism or who present TMD, there is a total lack of analysis of the masticatory function and its neuromuscular pattern [33, 34], rather than the peripheral factors themselves, as a possible causal factor of Bx or how determinant a specific type of masticatory pattern may be on the type of occlusion, given that the neuromuscular activity demanded by the function is very complex and requires precise muscular coordination of agonists and antagonists, this has implications at the level of the CNS, neocortex and hypothalamus [35, 36], it should be remembered that thanks to neuromuscular activity the formation and remodeling of bone tissue began at the embryonic level, therefore a bone problem has its origin in a muscular problem and not the other way around.

Therefore, the present pilot study aims to determine the type of chewing pattern installed in bruxing patients and the possible existence of a relationship between the presence of bruxism, answering our research question: Is there a predominant chewing pattern in bruxing patients and, if so, what is it?

2.1. Masticatory evaluation protocol

The procedure for the clinical evaluation of the masticatory pattern was performed by a group of 5 calibrated evaluators under the guidance of an expert. All subjects were evaluated during the 1st semester of the year 2022, following the guidelines established by Genaro et al. in the MBGR orofacial myofunctional examination [25, 38, 39] (Fig. 3). Finally, the type of masticatory pattern was evaluated as bilateral alternating, bilateral simultaneous, unilateral preferential or chronic unilateral.

Fig. 3Evaluation of masticatory function (Genaro et al., 2009)

Video Recordings of three masticatory acts were made for each subject evaluated, using a cereal bar divided into 3 equal pieces of 2 cm², a videorecording of the mandibular dynamics of the PMFA was also made for each subject, with 3 photographs of the occlusion in MIC (frontal, right lateral and left lateral) and upper arch plus lower arch in occlusal view. In addition, a postural photograph of the subject is taken for subsequent postural analysis.

2.2. Statistical analysis

After conducting the evaluations of the subjects, the functional and PMFA recordings, mouth and postural photographs, completing the bruxism self-report questionnaires of Fonseca's TMD, we proceeded to identify the masticatory pattern present in each subject of the sample. The data were tabulated in a specific Excel spreadsheet and processed through descriptive and inferential statistics using the Office 365-2022 Excel system.