This informal CPD article ‘TMJ Biomechanical Analysis – Part 2: Muscular Mechanisms, Joint Locking and Clinical Tests’, 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 continues the biomechanical analysis of TMJ dysfunction, focusing on muscular control, locking mechanisms and clinical assessment.
7. Analysis of the TMJ Muscular System
The muscles acting on the temporomandibular joint are: masseters, temporals and medial pterygoids with a closing function; lateral pterygoids with a function of controlling mandibular positioning movements and lateral movements; suprahyoid and infrahyoid muscles for mandibular opening [2,4,5,15].
The closing muscles are vectorially dominant over the other muscle groups [3,5,15].
7.1 Function of the Lateral Pterygoids
The lateral pterygoids insert on both the condylar neck and the articular disc [4,5,15]. During swallowing, through proprioceptive information from dental contacts and in particular from the canine guidance, they memorise and automate with great precision the mandibular closing position, without the need to consciously seek dental contact [3,4,5].
During masticatory movements they control lateral mandibular displacements [4,5]. During physiological mouth opening, the hyoid muscles determine the descent of the condyle along the temporal eminence in roto-translation; the lateral pterygoids maintain contact between the condyle and the articular disc [4,5,14,21].
7.2 The Mechanism of Temporomandibular Locking
When the closing muscles are in shortening — whether due to an intrinsic muscular problem or a dental problem — mouth opening becomes limited or impossible (temporomandibular locking), as the closing muscles are vectorially dominant over the hyoid muscles [1,3,15].
However, since mouth opening is a hegemonic function, a compensatory strategy is employed that involves the use of the lateral pterygoids in partial or total substitution of the hyoid muscles [3,5]. The lateral pterygoids, rather than limiting themselves to a control function, take the cranial insertions as a fixed point and the condylar insertions as the mobile point, pulling the condyles forwards and determining a sudden, forced opening of the mandible [3,4].
The opening occurs in subluxation, in the direction of the temporal crest. If the crest is exceeded, dislocation occurs [4,5]. Subluxation therefore represents the secondary product of the strategies employed to resolve the primary problem, namely the entrapment of the mandibular condyle by the closing muscles [3,4].
8. Clinical Tests
Three clinical tests are described that provide information on both the structural and muscular components of the TMJ [3,25].
8.1 Closing Test
The operator places the little fingers in the patient’s external auditory canals while the patient maintains the mouth open. Upon requesting closure, the operator may perceive skeletal contact of the condyles ascending towards the temporal fossa [3,4].
Under physiological conditions, the operator’s finger, with the teeth in occlusion, should not detect any contact with the mandibular condyle [3]. Contact with the condyle may sometimes be a light touch, but at times the pressure can become significant. If the condyle overrides the operator’s finger, this signals that the condyle positions itself in the fossa at each swallowing [3,4].
Ascending of the condyle towards the temporal fossa is always determined by dental problems [3,4,10].
To obtain further information on the muscular coordination of the closing muscles, the operator takes manual contact with the patient’s temporal and masseter muscles and asks the patient to rhythmically clench the mandible [3,25]. Observation is directed towards detecting simultaneity and equal intensity of muscular contraction on both sides. Should asymmetric contraction be detected in terms of activation timing or intensity, this may be the expression of a muscular problem, whether primary or secondary to a dental problem [3,15].
8.2 Opening Test
The operator places the hands on the external aspect of the joint and asks the patient to open the mouth in order to perceive the mandibular movement [3,4,5].
If at a given moment the movement undergoes an acceleration, this signals the intervention of the lateral pterygoids in substitution of the hyoid muscles to resolve a locking problem [3]. The earlier the acceleration occurs, the more severe the problem induced by shortening of the closing muscles [1,3].
Locking may occur both in the presence and in the absence of occlusal problems [3,4].
During opening, endo-articular sounds may also be detected, the most significant of which is the click, which reveals condylo-discal incoordination [4,28,29]. This occurs when the condyle positions itself in the fossa, losing total or partial contact with the articular disc [4]. During mouth opening, the mandible encounters the resistance of the disc and when recapture occurs a sharp sound is perceived. Condylo-discal incoordination is also caused by ascending of the condyle into the temporal fossa, and ascending is in turn determined by a gnathological problem [3,4].
Posterior dental arches
8.3 Test for Dental Interference on the Muscular System
A spacer of approximately 2–3 millimetres is positioned between the posterior dental arches, simulating the presence of a bite splint [3]. The spacer may be crafted manually or, for greater test accuracy, a diagnostic hydraulic bite splint with communicating vessels may be used, consisting of two reservoirs containing physiological solution connected by a small tube [3].
The bite splint, by preventing the patient’s habitual dental occlusion, reprogrammes the mandibular movement and, if a hydraulic bite is used, reprogramming occurs at each swallowing since the fluid, moving from one reservoir to the other, fills dental asymmetries and it is more likely that occlusion occurs in muscular equilibrium [3].
With the bite splint in the mouth, worn for a period of 15–20 minutes, three outcomes may be distinguished based on the body’s responses [3].
Negative bite. No evident variation in skeletal body relationships. This is the physiological condition: the teeth do not interfere with the myofascial system [3].
Positive bite with correction. Evident improvement of skeletal relationships and verticality, accompanied by subjective sensations of well-being and/or appreciable reduction of symptomatology. In this case the teeth interfere with the myofascial system and referral to the specialist is indicated [3].
Positive bite with aggravation. This outcome signals that dental position is not the primary problem but that, at the present time, intervention on gnathological problems would create an additional disturbing element [3]. The therapeutic strategy is to work on the myofascial system or to investigate other possible sources of the patient’s symptomatology [1,3,24]. After a period of treatment, the bite test is repeated and, if the therapies undertaken have been effective, a change in responses will be observed, shifting either towards the first outcome (no correlation between teeth and underlying structures) or towards the second (gnathological problems are present and, having previously addressed other priorities, can now be treated) [3].
9. Indicative Symptomatology
Certain symptoms may be indicative of skeletal problems deriving from the stomatognathic circuit with consequent TMJ involvement [3,4,28]: ear pain (in the absence of otorhinolaryngological problems), articular sounds and difficulty in opening and/or closing the mouth, headache, cervicobrachialgia (with anterior or posterior head displacement), lumbar and/or dorsal pain (with anterior or posterior head displacement) [2,3,6].
Conclusions
The temporomandibular joint, in the majority of cases, does not represent the site of a primary pathology but the point at which alterations originating in other districts converge and manifest [1,2,3,6]. Biomechanical analysis identifies three pathways of development of cranio-cervico-mandibular disorders: structural dental alterations, sensory input disturbances and primary muscle shortenings. The distinction between primary and secondary shortenings orients the therapeutic strategy [1,3,23].
The hyoid bone, as described in the contribution dedicated to its biomechanical analysis [2], functions as a point of mechanical redistribution of tensions between the stomatognathic district and the cranio-cervico-scapular system, accounting for the propagation of alterations from one district to another [2,6,14,22].
The closing muscles are vectorially dominant over the hyoid muscles, and their shortening determines the mechanism of temporomandibular locking [1,3,15]. Subluxation, frequently observed in clinical practice, does not represent a primary cause but the consequence of the compensatory strategy implemented by the lateral pterygoids to resolve the locking [3,4].
The clinical tests proposed enable distinction between dental interference that is present, absent or contraindicated at the current time, thereby orienting the sequence of therapeutic intervention [3]. Interdisciplinary collaboration between physiotherapist and gnathologist represents the necessary condition for effective management of cranio-cervico-mandibular disorders [3,7,23].
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