Physiology and Biomechanics of the Temporomandibular Joint

Original Editor - Jess Bell based on the course by Victoria Reboredo
Top Contributors - Jess Bell and Kim Jackson

This article or area is currently under construction and may only be partially complete. Please come back soon to see the finished work! (28/06/2021)

Maintenance of Jaw Position

The maintenance of mandible position depends on mandible reflexes and the action of gravity. It is also affected by an individual’s position / posture and specific variations which allow functional jaw movements to occur.

The resting position of the jaw is called physiological non occlusion:

• In this position, the teeth are not in contact - they maintain a separation of around 2 mm, while the lips close the oral cavity without pressure
• The maintenance of this resting position is due to the jaw reflexes, as well as active and passive mechanisms.

Passive mechanisms:

• Passive tension of the elevators of the mandible and connective tissue

Active mechanisms:

• Peripheral afferents including: muscle proprioceptors, articular proprioceptors, periodontal mechanoreceptors, mechanoreceptors of the mucosa (i.e. gums, lips, tongue, palatal area)
• Central control: this includes the influence of the cortico-visual, limbic and fusimotor-extrapiramidal system

Thus, the limbic and visual systems are actively involved in maintaining the position of the jaw. These systems have been found to have an impact on the tone of the masticatory muscles. Thus, situations that involve emotional stress or visual alterations can change the tone of the jaw muscles and its position.

Mastication

Mastication makes up the major part of the oral processing of food and it is a necessary part of eating. It is essential in order to fragmentate food and for deglutition (swallowing) - i.e. it is the initial phase of the digestive process.

The process of mastication is controlled by the central pattern generator in the brainstem and other phases of swallowing. It occurs in the mouth with the help of the mandible and associated muscles.

For mastication to occur, various sensory information (smell, taste and touch) is required, as well as information from the tongue, palate, lips, masseters and salivary glands.  

A change in one or more of these elements can cause issues with mastication.

Mastication has been divided into four phases:

1. Pre-oral (anticipatory) phase:
   • This phase “is the interaction of pre-oral motor, cognitive, pyschosocial and somataesthetic elements which begin the swallowing process.”
   • Information about the food, which is obtained via the optic and olfactory nerves, is interpreted in the central nervous system and a swallowing plan is developed
   • This information includes smell and specific routines that show the feeding act is about to begin
   • The orofacial structures start to prepare to receive food - e.g. the taste buds start to generate saliva
2. Oral phase:
   • The oral phase begins when food enters the mouth
   • The lips close and and the tongue forms a seal to prevent the food (which is being transformed into a bolus) from falling out
   • The bolus is formed through the movement of the lips, jaw, cheeks and tongue - i.e.the food is cut, split and ground up
   • Once the bolus is safe to be swallowed, it is pushed backwards by the tongue It has been found that the mastication process and the formation of the bolus is influenced by the physical characteristics of the food. A study by Mishellany and colleagues found that individuals tend to achieve comparable bolus particle size, but that the amount of time and number of cycles to achieve this result varies between individuals. Thus, it appears that the size and distribution of the particles of the food influence the deglutition reflex. Once the bolus has formed, it goes to the pharynx via a tongue elevation movement. The oral phase can be affected by pathology of the TMJ. For some patients with TMJ dysfunction it will be difficult for them to open their mouths. This will cause issues with mastication and, therefore, the overall digestive process.
3. Pharangeal phase:
   • The pharyngeal phase refers to the movement of the bolus through the pharynx
   • During this phase, the airway is also protected from the bolus as well as the protection of the airway
   • The bolus moves from the base of the tongue to the wall of the posterior pharynx 4. Oesophaeal phase:
   • This phase begins once the bolus passes through the upper oesophageal  sphincter
   • Peristalsis pushes the bolus down to the stomach via the lower oesophageal sphincter

Speaking

Speaking is a complex, dynamic sensorimotor activity. It has been found that there is a connection between the intra-oral information and the oral and cervical muscles.

Torisu and colleagues found, for instance, that intra-oral stimulation can inhibit neck muscle activity, which indicates that there is a neural connection between the trigeminal region and the cervical region. While this modulation might be largely due to nociceptive afferent nerves, non-nociceptive fibres might also be involved.

As the authors note, this connection is significant because it may suggest that orofacial pain could have an impact on head, neck and shoulder activity.

Breathing

The upper airways mark the beginning of the path between the nasal fossa / cavity and the oral cavity:

• The upper airways begin at the nasal cavity, before moving to the nasopharynx and oropharynx, down to the larynx and the extrathoracic trachea

Breathing occurs simultaneously with all other oral activities. The respiratory pattern, must therefore, be coordinated with other functions (including eating) that also occur in the mouth / oral cavity.

When we breathe, air can enter through the nose or the mouth, but it always passes through the pharynx - During swallowing, the pharynx is used as a passage for food.

In healthy individuals, swallowing is dominant to respiration. Breathing stops briefly when an individual is swallowing. This is causes by:

• The physical closure of the airway by the lifting of the soft palate and tilting of the epiglottis
• Neural suppression of respiration, by the brainstem

In the resting phase, air can enter via the nasal or oral cavity, generating two different respiratory pattern options. These patterns need to coordinate with the rest of the oral cavity's physiological functions.

When the oral cavity is used for breathing, a degree of jaw opening is necessary. In order to achieve this, tone in the elevator muscles decreases, which allows air to circulate.

Temporomandibular Joint (TMJ) Biomechanics

The TMJ is a ginglymoarthrodial joint that allows the movements of rotation and translation in the sagittal plane.

It has four articular surfaces:

• Mandible condyle
• Temporal fossa
• Inferior and superior surfaces of the articular disc
  • The superior surface faces the temporal fossa
  • The inferior surface is in contact with the mandible condyle

When the mandible is at rest, the mouth is slightly open, so that the teeth are not in contact - i.e. non occlusion. This distance is measured between the superior and inferior incisors.

The situation of the condyle inside the joint is a controversial subject. The so-called “central position” is a theoretical concept. The jaw is suspended, supported by the muscles and other stabilising elements,such as ligaments and the articular capsule. For more information on the anatomy of the TMJ, please click here.

Movements of the TMJ

Jaw Opening

Jaw opening is divided into the following phases: WEB and HELLEND and BORDONI

1. Pure rotation of the condyles on their axis
   1. Most of this movement happens in the infra-meniscal space of the condylo-discal complex
   2. This is facilitated by the lateral pterygoid muscle (inferior part) mylohyoid, geniohyoid and digastric muscles
2. Translation of the condylo-disc complex forwards
   1. This movement happens mainly in the superior compartment of the disc-temporal complex
   2. The jaw opens to 40 to 50 mm
   3. The temporomandibular ligament helps to maintain stability to prevent the jaw dislocating forwards
   4. The lateral pterygoid muscle is involved in this action
      1. NB: This muscle has opposite functions - while its superior fascicle relaxes during opening, stabilising the anterior displacement of the disc, the inferior fascicle contracts and allows the movement of the condyle
3. The ligaments create stability at the end of the movement
   1. The disc and condyles move medially, and the collateral lateral ligaments on each side of the TMJ tighten
   2. At a certain point, the condylar disc complex is unable to move any further due to the tension in the ligaments and the joint capsule. At this point, it rotates on its own axis.

Jaw Closing

Jaw closing is divided into three phases. Jaw closure is associated with cervical extension. The elevator muscles of the jaw work against gravity.

1. Condylar rotation in the inferior posterior meniscus area - this is similar to jaw opening, but in the opposite direction
   1. This phase starts without any specific muscle action - rather it occurs due to the relaxation of the muscles involved in opening and the release of tension of the ligaments
2. Translation of the condylo-disc superior meniscal area
   1. The complex made by the condyle and disc moves posteriorly to the most posterior and superior part of the mandibular fossa
3. When the condyle has reached to the most posterior and superior part of the articular fossa, there is a rotation in the posterior direction of the condyle in the intrameniscal space - this ends with occlusal contact

From the front, the disposition of the articular fossae and the anterior convergence of its axis make the pressure at the end of the movement fall laterally on the condyle. In normal conditions, this causes a lateral displacement of the disc, causing tension in the medial collateral ligaments.

Protraction

Protraction occurs when the jaw moves forwards. In order to achieve this, the jaw needs to be slightly opened in order to avoid any interference from the teeth (i.e. occlusion). This opening causes anterior rotation in the sagittal plane.

In the second phase, there is condylar translation forwards and downwards. This is due to the disposition of the condylar fossa, which makes the condyle go down.

This movement occurs due to the coordinated action of the pterygoid muscles (both fascicles). As the opening does not progress in this movement, the jaw is stabilised by the contraction of the temporalis muscles.

Retraction

The opposite movements occur during retraction. The condyle translates backwards and upwards inside the articular fossa. This movement is activated by the temporalis muscle and the posterior belly of the digastric muscle. In the last phase of the retraction, there is a posterior rotation of the condyle at the intrameniscal level (i.e. the condylo-disc complex).

Lateral Movements

The condyles work together to achieve lateral movements of the jaw. When assessing lateral movement, it is necessary to differentiate one condyle from the other:

• The working side is the side that moves laterally when taking the chin as a reference
• The non working side is the side that moves towards the midline

In the working side, there is a rotation of the condyle over its own vertical axis, and a transversal displacement of about 0.9 mm. This movement is caused by the deep masseter muscle, and the medial and posterior fascicles of the temporalis muscle.

In the non working side, the condyle moves to the midline, going forwards and moving closer to the midline, plus moving transversally approximately 0.4 mm. In this case, the muscles activated are the lateral pterygoid (inferior fascicle) and the medial pterygoid.