This informal CPD article ‘TMJ Biomechanical Analysis – Part 1: Systemic Relationships, Hyoid Function and Dental Influence’, was provided by Dr. Mauro Lastrico, Physiotherapist at AIFiMM Formazione, an organisation recognised by the Italian Ministry of Health as an authorised CME provider. They offer organised training courses in the Mézières Method, a rehabilitative and postural approach.
This article is part of a series of contributions dedicated to applying the principles of physics to the musculoskeletal system. The first article, Clinical Assessment of Muscle Shortening [1], introduced the physical model of muscle shortening as viscoelastic deformation of connective components, distinguishing the mechanical behaviour of the contractile component from that of the connective component [1,11]. The contribution dedicated to the hyoid bone, Hyoid Bone Biomechanical Analysis [2], demonstrated how the hyoid bone, lacking articulations with other skeletal segments, represents a point of mechanical convergence of muscular forces, and how alterations in its position produce functional consequences on swallowing, phonation, respiration and cranio-cervical axes [2,6,14].
The present work applies these principles to the analysis of the temporomandibular joint (TMJ), demonstrating how articular dysfunction constitutes, in the majority of cases, the final manifestation of alterations originating in other districts [4,6,7]. The objective is to provide an interpretive tool that enables identification of the primary causes of cranio-cervico-mandibular disorders and coherent orientation of therapeutic strategy [3,23].
This work is the result of clinical collaboration and the teachings of Dr Piero Silvestrini Biavati, dental surgeon, gnathologist and posturologist. The concepts relating to gnathology and occlusion presented here derive from his clinical experience and interdisciplinary approach [3].
1. From Local Symptoms to Systemic Causes
Temporomandibular joint symptomatology manifests locally [4,28]. However, the application of the physical principles examined in previous contributions shows that TMJ dysfunction constitutes, in the majority of cases, the final manifestation of alterations originating in other districts [1,2,6].
Excluding specific pathologies such as rheumatoid arthritis and articular rheumatism, TMJ dysfunction is mechanically sustained at a distance [6,7]. For this reason it is more appropriate to speak of cranio-cervico-mandibular disorders, a term that reflects the systemic nature of these manifestations [3,6,7].
Biomechanical analysis enables identification of three pathways through which cranio-cervico-mandibular disorders may manifest [3].
1.1 First Pathway: Alteration of the Mandibulo-Cranial Skeletal Relationship
The first pathway originates from an alteration of the mandibulo-cranial skeletal relationship [3,4]. When a structural problem exists at the dental level, a muscular compensation is activated that determines mandibular deviation [3,30]. This deviation produces articular compression that generates TMJ symptomatology: the structural alteration, in this case occlusal, forces the musculoskeletal system to adapt [4,29].
1.2 Second Pathway: Sensory Input Disturbances
The second pathway originates from disturbances of sensory input, whether visual or auditory [26]. The altered sensory information determines cranio-cervico-scapular muscular compensations that modify the position of the vertebrae and the hyoid bone [2,6]. These modifications secondarily involve the TMJ muscles, determining the articular problems [6,7].
1.3 Third Pathway: Primary Muscle Shortenings
The third pathway originates directly within the muscular system. Cranio-cervico-scapular muscle shortenings, without identifiable structural or sensory causes, alter the position of the vertebrae and the hyoid bone [1,2]. As with the previous pathway, this determines secondary involvement of the TMJ muscles [1,22,24].
1.4 Primary and Secondary Shortenings: Therapeutic Implications
In the first two pathways, muscle shortenings are secondary: the muscular system adapts to problems of another nature. In the third pathway, shortenings are primary: the muscular system is the origin of the problem [1,23].
This distinction determines the strategy of therapeutic intervention [3,23]. If the shortening is secondary, muscular treatment may produce temporary improvements but recurrence is predictable as long as the primary cause is not removed. If the shortening is primary, direct work on the muscles represents the definitive intervention [1,24].
Shortening of mandibular closing muscles
2. Clinical Examples
A first example clarifies the principle. A patient presents with cervical pain. Objective examination reveals rotation with consequent convexity of the cervical vertebrae. Treatment of vertebral rotations through rebalancing of the muscular vectors of the scalenes and levator scapulae produces regression of symptomatology [1,12]. At subsequent sessions, however, symptoms and vertebral alterations are once again present.
The instability of results indicates the existence of a triggering factor that continues to interfere with the muscular system [23,24]. The cervical muscles are not the primary cause but the pathway through which a problem of another nature manifests [1,2].
If, upon further diagnostic evaluation, a dental problem is identified, the asymmetric activation in shortening of the mandibular closing muscles at each swallowing will induce imbalance in shortening of the cranio-vertebro-hyoid muscles [2,3,6]. In this case, gnathological intervention on the teeth, by removing the structural cause of secondary muscle shortening, enables resolution of symptomatology [3]. Should the muscle shortenings have become chronic, it may be necessary, after resolving the dental problem, to intervene on the muscles as well, but correction of the vertebrae and hyoid bone will finally prove stable [1,11].
A second example concerns a patient with TMJ pain in whom stomatognathic, visual, auditory or other problems have been excluded. Objective examination reveals cranio-vertebro-hyoido-scapular skeletal misalignments in the sagittal and frontal planes, induced by primary muscular vectorial imbalances [1,2,12]: it is in this case that work on the muscles can produce resolution of symptomatology [1,25].
3. Structural Anatomy and Biomechanics
The temporomandibular joint has the distinctive characteristic of being a double articulation [3,4]. The disc, interposed between the condyle and the temporal bone, does not merely serve as a shock absorber but possesses a true articular function [4,30]. It is therefore a double articulation: condylo-discal and disco-temporal.
The physiological position of the mandibular condyle, with the teeth in contact, should not be within the temporal fossa but on the eminence [3,4]. Both ligaments and the muscles acting at this level contribute to articular stability [4,5,15].
During mouth opening, a roto-translatory movement occurs [4,5]. The condyle moves along the temporal eminence while the rotatory movement occurs in the opposite direction to the translatory movement [4,30].
The muscles acting on the joint are: the masseters, temporals and medial pterygoids with a closing function; the lateral pterygoids with a function of controlling positioning movements of the mandible and lateral movements; the suprahyoid and infrahyoid muscles for mandibular opening [2,4,5,15].
4. Physiology of Swallowing
Swallowing is an involuntary movement that occurs several times per minute [8,16]. The masticatory muscles contract bringing the teeth into contact [17]. If the dentition is correctly positioned, the masticatory muscles act with equal intensity on both sides, using the minimum force necessary, and tooth closure does not influence other body districts [3,8].
Physiologically, no connection should exist between dental occlusion and distant skeletal districts [3,6]. However, if the dentition is pathologically positioned, this connection may become activated [3,6,7].
4.1 Non-Linearity of the Occlusion–Structure Relationship
The manner in which even a modest occlusal imbalance can have repercussions on the entire body follows non-linear principles [3,9,27]. In linear mathematics, there is direct proportion between stimulus and effect. In non-linear mathematics, a small variation can produce significant effects [9,27].
From a linear perspective, occlusal imbalances should not determine displacement of body masses. From a non-linear perspective, this becomes possible [9,27]. Being a non-linear relationship, not all individuals with occlusal problems present alterations of articular axes or symptoms [3].
Relationship between maxilla and mandible
5. Occlusal Problems and Systemic Consequences
Angle’s classification [10] distinguishes three classes of occlusion based on the relationship between upper and lower molar teeth.
Class I represents the physiological relationship in which upper and lower teeth meet correctly [10].
In Class II, the relationship between maxilla and mandible is altered due to mandibular retrusion [3,10]. This may derive from a purely dental problem, when teeth are malpositioned on normal bony bases, or from a skeletal problem, when the mandible is underdeveloped relative to the maxilla or the maxilla is excessively developed [10]. In Class II, with the teeth in contact, ascending of the mandibular condyle towards the temporal fossa is frequently observed, with consequent compression of articular structures [3,4].
In Class III, the relationship is altered due to mandibular protrusion, which may be caused by excessive mandibular development or maxillary underdevelopment [3,10].
Pathological classes determine alterations in the distribution of muscular and articular forces that can propagate throughout the entire cranio-cervico-scapular system [2,3,6].
In addition to alterations related to dental class, three further occlusal problems can determine systemic skeletal alterations [3].
5.1 Difference in Tooth Length (Pre-contact)
When teeth of different lengths are present within a dental arch, during mouth closure the masticatory muscles will act asymmetrically and with intensity exceeding that physiologically necessary [3,15].
The mandibular condyle on the side of the shorter teeth, in order to permit their contact, must position itself beyond the physiological position, ascending towards the temporal fossa. The mandible thus performs a torsional movement [3,4,30].
Within the mandibular fossa there are numerous receptors. Their compression can trigger painful symptomatology localised to the TMJ, the ear or the head [4,28,29].
The asymmetric and excessive muscular activation determines the involvement of other muscular districts through the action of the hyoid muscles [2,14]. The cervical vertebrae lose their symmetry, the shoulder may become elevated and, if the process persists over time, a complex series of skeletal alterations of the entire body may result [1,2,6,22].
5.2 Excessive Freeway Space
At rest, with the masticatory muscles relaxed as occurs between one swallowing and the next, the posterior teeth should not be in contact but should present a physiological freeway space of approximately 2 millimetres [3,4].
When the freeway space is excessive, for example due to teeth that are overall too short, the masticatory muscles would need to be perpetually in tension to maintain the correct space [3]. To avoid this continuous effort, the muscles anterior to the cervical column, taking a fixed point on the third thoracic vertebra, displace the entire head forwards, with the synergistic assistance of the scalenes [2,6,13].
The dental arches are brought closer together, relieving the work of the masticatory muscles. However, the anterior displacement of the head modifies the body’s centre of gravity and, to prevent loss of equilibrium, the muscular districts below are activated, modifying the course of the entire vertebral sinusoid and physiological skeletal relationships [1,2,12,26].
5.3 Decreased or Absent Freeway Space
This is the opposite problem: the muscles posterior to the cervical column are activated to flex the cranium posteriorly and permit mouth opening, distancing the dental arches and relieving the work on the hyoid muscles [2,3,6].
The overall centre of gravity of the body undergoes displacement. The muscles below are activated to maintain equilibrium, acting on the entire vertebral column and altering the verticality of individual body segment centres of mass [1,12,26].
All the imbalances analysed may in turn generate, through secondary muscle shortenings, the onset of orthopaedic pathologies — low back pain, cervical pain — that may be defined as secondary to a primary pathological involvement of the stomatognathic apparatus [1,3,13].
6. Gnathological Intervention
When differential analysis identifies a problem of stomatognathic origin, the primary intervention falls within the dental domain [3,4].
The bite splint represents the initial tool [3]. It prevents the patient’s habitual pathological occlusion by reprogramming the mandibular movement at each swallowing, correcting the positioning of the temporomandibular joint [3,4]. Subsequently, the appropriateness of definitive dental intervention to stabilise results is evaluated [3].
The alteration of occlusal structure, by determining secondary muscle shortenings, produces skeletal alterations that could not be achieved through work on the muscular system alone [1,3]. This does not necessarily mean that dental work corrects all skeletal parameters or achieves total remission of symptoms, particularly if expressed in body regions distant from the TMJ [3,24].
It means that to achieve further improvement of skeletal relationships it may be necessary to add work on muscular rebalancing, but only after removing the causes triggering secondary muscle shortening [1,3,23]. Otherwise, improvement could neither be perfected nor stabilised [1].
The mechanisms analysed can also operate in the opposite direction: a muscular imbalance originating from other body districts may determine, through the mechanisms of muscular interconnection described in the analysis of the hyoid bone [2], occlusal problems that will generate condylo-menisco-temporal conflicts [3,6,22].
Conclusions – Part 1
The temporomandibular joint should therefore be interpreted within a broader biomechanical network rather than as an isolated anatomical district. Hyoid position, swallowing mechanics and dental relationships may all influence mandibular balance and contribute to the development or persistence of dysfunction. This systemic perspective provides the basis for a more accurate clinical interpretation of TMJ disorders. The specific muscular mechanisms acting on the joint, the conditions that may lead to locking, and the clinical tests that help identify the dominant dysfunction will be examined in Part 2.
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