Anatomy and physiology of the masticatory system

Introduction

The ATM is a very special joint, since it is a bicondylar diarthrosis connecting the mandibular condyle with the mandibular fossa of the temporal bone, through a discal fibrocartilage that divides the joint into two compartments.

1. Situations

Located on the lateral part of the face, behind the facial mass, under the base of the skull, it is in front and below the external auditory canal, at the limits of the temporal region above, masseteric below, parotid and auricular behind.

2. Limits

The ATM is part of the condylar region with fictitious lower limits, consisting of the neck and the condyle which form the posterosuperior end of the ascending ramus of the mandible. It is limited:

Outside: by the anterior zygomatic tubercle.
Anteriorly: by the subtemporal plane of the greater wing of the sphenoid.
Inside: by the petrosphenoidal suture.
Behind: through the non-articular glenoid cavity.

3. Articular surfaces

The mandibular condyle is in contact with the articular disc, the whole resting on the posterior wall of the articular tubercle of the temporal bone.

3.1. Temporal surfaces

3.1.1 Temporal condyle

Almost transverse projection, located at the back, very slightly concave from inside to outside. Only the articular tubercle is lined with cartilage.

3.1.2. Glenoid cavity or mandibular fossa

Non-articular, this cavity is strongly concave from front to back, located on the lateral face of the temporal bone, just in front of the external acoustic meatus (EAM). This cavity is crossed by the Glaserian fissure which separates it into two parts, one anterior articular, the other posterior non-articular.

3.2. Mandibular Surface

3.2.1. Mandibular condyle or condylar process

Supported by a collar, is an ellipsoid projection whose large horizontal obliques backwards and inwards. The two axes of the mandibular condyles intersect on the median line forming between them an angle which varies from 130 to 140°. The articular surfaces are covered with a tissue

Fibrocartilaginous connective tissue.
A mossy ridge divides it into two slopes:

An anterior, convex slope articulates with its temporal counterpart;
A posterior slope constitutes the posterior border of the mandible.

3.2.1.1. Neck of the condyle

It joins the condyle to the ascending ramus. Flattened from the front and back, it comprises two faces, one anterior and the other posterior. Its antero-internal portion is depressed by an irregular fossa where the lateral pterygoid muscle and the pterygo-temporomandibular aponeurosis are inserted. The upper part of the crest of the neck ends with the medial tubercle.

4. Disc apparatus

Composed of the disc and its attachments, the disc apparatus contains the entire head of the mandibular condyle. The condyle and the disc apparatus then form a complex called the condylo-disc.

4.1. The disk

Is a biconcave fibrocartilaginous lens, thicker in its posterior part than in its anterior part. It is composed of a dense, avascular and non-innervated collagenous fibrous tissue, it is flexible and inextensible. The disc occupies the space left free between the articular surfaces (mandibular condyle and glenoid cavity), separating the joint into 2 compartments: a temporomandibular zone (supradiscal) and a meniscal-mandibular zone (infradiscal). The disc can be divided into three topological zones: the thickest posterior band, the intermediate zone and the thin anterior band and is thus divided into two parts: medial, central and lateral.

4.2. The bilaminar retrodiscal area

Also called retrodiscal tissue or bilaminar lamina, it inserts on the posterior edge of the disc. It occupies the space between the disc and the posterior wall of the joint capsule. This region is formed of a loose tissue rich in venous plexus (vascular knee) or “Zenker’s vascular cushion”.

4.3 . The prediscal lamina This is a richly vascularized and innervated tendinous area that receives two muscular insertions: – An anteromedial insertion, single and horizontal, formed by the fibers of the superior bundle of the lateral pterygoid muscle. This muscular insertion is the most important of the two. An anterolateral insertion, double and vertical, formed by fibers of the temporal muscle (deep) and the masseter muscle (superficial).

5. Means of unions

The articular surfaces described above are maintained in close contact essentially by the tone of the masticatory muscles and, secondarily, by a capsule.

5.1. The joint capsule

It is a fibrous sleeve that is found all around the joint. The capsule is made up of vascularized and innervated connective tissue. This capsule has two insertion circumferences:

The upper temporal one;

The other lower than the neck-condyle junction.

5.2. The synovium

It lines the deep surface of the capsule. There are 2 synovial membranes, which correspond to the above and below the disc levels of the joint. The synovial fluid lubricates the joint and thus facilitates joint movements. It helps exchanges between the capillaries and the joint cavity and ensures the metabolic replacement of non-vascularized tissues (disc).

5.3. Ligaments

There are two types of ligaments depending on their anatomical position:

* intrinsic (articular) ligaments: There are two main ones, the lateral collateral ligament, the main means of union of the joint and the medial collateral ligament.

* accessory extrinsic (extra-articular) ligaments are fibrous bands with no regulatory role in TMJ movements.

5.4. Muscles of the temporomandibular joint

From a physiological point of view, it is necessary to mention the masticatory muscles which play the role of active ligaments. The symmetrical action of these different muscles involved in elevation, lowering, propulsion, retropulsion, achieves a continual balance of force which fixes the stability of the ATM. They are divided into two groups:

1. Elevator muscles of the mandible

The main muscles of mastication that act on the TMJ are the lateral and medial pterygoid, the masseter and the temporalis

2. Depressor muscles of the mandible

The digastric muscle, the mylohyoid muscle, the geniohyoid muscle and the stylohyoid muscle .

6. Vascularization, innervation and lymphatics

6.1. Vascularization

Many arteries, through their trunks or branches, contribute to the vascularization of the ATM, such as the superficial temporal, the middle deep temporal, the maxillary, the posterior auricular, the ascending pharyngeal.

6.2. Innervation

The ATM is innervated on the sensory level by the auriculotemporal nerve, a sensory branch of the mandibular nerve. On the motor level, the innervation of the ATM is provided mainly by the mandibular nerve and the facial nerve.

6.3. Lymphatics

The lymphatic vessels drain into the parotid and pretragal lymph nodes.

ATM relations with neighboring structures

– Laterally, the temporomandibular joint is covered by the parotid fascia and skin.

– In front, it is the pterygomandibular region. It responds to the insertion of the lateral pterygoid muscle, and, further away, to the masseter muscle, the tendon of the temporal muscle, the lingual and inferior alveolar nerves.

– Medially: It is related to the infra-temporal space.

– Behind: We encounter a parotid extension which can be interposed between the joint and the anterior osteocartilaginous wall of the meatus.

Physiology:

From an anatomical-functional point of view, the arrangement of the mandible in relation to the base of the skull and the maxilla allows us to define two types of closely intertwined relationship levels:

* Temporomandibular relationships: dependent on the articular surfaces present, they determine a maxillomandibular relationship.

* Interdental or dento-dental relationships (occlusion).

Classification of movements:

1. Movements along the vertical and sagittal plane:

In this plane, the lowering and raising movements of the mandible take place, which lead respectively to the opening and closing of the mouth (represented by the Posselt diagram).

These two movements occur by a simultaneous action of the two condyles around the same transverse axis in two sequences:

The initial phase: it corresponds to the rotation on site of the two mandibular condyles around their own axis, a movement very quickly limited by the narrowness of the glenoid cavity (20 mm).

The secondary phase: it is determined by the forward translation of the two mandibular condyles, they slide forward and downward against the temporal condylar slope, thus emerging from the glenoid cavity to come and place themselves above the temporal condyle.

The average opening ranges from 44 to 54 mm (including over-bite), or approximately the thickness of three fingers.

The propulsion amplitude is approximately 10 to 15 mm. It is measured from maximum intercuspation occlusion to maximum protrusion

Its normal value is 11 to 13 mm. For Farrar 7, the ratio between the amplitudes of diduction and mouth opening must be ¼.

2. Movements in the horizontal and anteroposterior direction:

In this plane, the propulsion and retropulsion movements of the mandible take place; movements obtained by simultaneous translation of the two mandibular condyles respectively forward and backward.

During propulsion the mandibular condyles slide forward and downward against the temporal condylar slope, and fall into place under the temporal condyles.

3. Movements along the horizontal and transverse plane:

In this plane the movements of diduction or laterality are carried out (Gothic arch of Gysi).

These movements cause the mandible to move towards a given side through the combination of two actions:

– Forward and inward translation of the contralateral mandibular condyle (non-working condyle);

– And the passive rotation of the homo-lateral condyle around its vertical axis (working condyle).

– the movement made by the latter is called Bennett movement (03mm).

The National College of Occlusodontics (NCO) defines Centric Relation Occlusion (CRO) as:

“The highest reference condylar situation, achieving a simultaneous and transversely stabilized bilateral condylo-disco-temporal coaptation, suggested and obtained by non-forced control, reiterative in a given time and for a given body posture and recordable from a mandibular rotation movement without dental contact.”

In fact, we speak of the “highest” condylar position and not the “most backward” as was used in the past. Higher because a more posterior or even more anterior position would cause a lowering of the condyle.

“Coaptation” because we are looking for interposition and wedging of the disc with the two condyles, and “simultaneous” because the situation must be the same on the left as on the right.

“Suggested and … not forced” because the guidance of the movement must not be imposed but taught to the patient through successive opening and closing movements in rotation.

This situation must be “reiterative”, that is to say that there is the possibility of reproducibility.

“without dental contact” because no contact should influence the condylar position. This is a reference located outside the dental arches.

This definition is only valid for a healthy temporomandibular joint.

ATM and occlusion:

The resting position: the mandible is slightly lowered; the condyle-disc assembly is blocked against the articular eminence of the temporal bone and oriented in an antero-superior direction.

The position of maximum intercuspation: the disc is located in an oblique plane downward and forward between the temporal eminence and the condylar head.

The centric relation: Is a reference position for joint balance.

It is currently considered to be the situation of bilateral, high, simultaneous condylo-disco-temporal coaptation, recordable from a rotational movement.

Compensation curves: the Spee curve and the Wilson curve.

Diagnostic assessment:

1. Anamnesis:

This is a crucial time in developing the diagnosis.
The time of onset of the symptoms – mainly painful – is often decisive, because it is linked, for example, to dental treatment or a stressful situation.
The characteristics of painful manifestations or joint noises will be sought. Personal and family history – essentially osteoarticular – must be noted.

2. Clinical picture:

The clinical semiology is dominated by pain and by direct and indirect signs of a disturbance of joint dynamics,
Many secondary manifestations also constitute warning signs of joint pathology.
Palpation should be thorough and explore both the joint structures and the masticatory muscles;
Disturbances of joint dynamics will be sought by inspection of mandibular movements and by palpation of joint and muscle structures.
An intraoral examination will complete the assessment: classic study of the dental organs and their periodontium, looking for lesions secondary to functional disorders or occlusal overloads, and gnathological study by analysis at rest and during different movements, the extent of which will be measured precisely.
3. Temporomandibular imaging:
3.1. Orthopantomogram:
This examination allows bilateral visualization of the temporomandibular bone structures, the alveolar processes, the maxillary bases and finally the teeth.
But the joint space is the least well highlighted entity.

3.2. Specific X-rays:
Schuller’s incidence:

Explore condylar profiles and interarticular space.

Limitations: these incidences do not highlight the lateral regions (most often affected by arthritic changes) of the mandibular condyle.

The lateral teleradiography:

It gives a parasitic superposition on both sides, which is not very interesting.

Low face incidence:

giving an image of the two condyles: cases of asymmetries.

Schuller incidence:

Profile teleradiography:

Low face incidence:

Hirtz incidence:

Diagnostic assessment:

3.3. Tomographies:
Shows only bone condition.
The position of the disc is deduced from the comparison of images obtained with the mouth closed or open.
The disc can be made visible by injecting intra-articular contrast agent.
3.4. Arthrography:
The indications for this technique are essentially the demonstration of disco-condylar dysfunction and disc damage.
3.5. The scanner:
Allows the bone and tissue elements of the region examined to appear from a single section and to have a direct image of the disc. But it has no indication for the study of the disco-ligament system, it allows the appreciation of the bone constituents
3.6. Cone beam tomography:
Has the advantage of delivering minimal irradiation doses with limitation of the irradiation field and presents the possibility of having more precise images and better resolution than the CT and 2D and 3D reconstructions.
3.7. Magnetic resonance imaging:
Is currently the most suitable examination to show in an atraumatic, painless and radiation-free way, the disc, its tissue environment, and also to give an increasingly precise image of the bone condition.
3.8. Scintigraphic exploration:
With intravenous injection of 20 mCi of technetium 99 methyldiphosphonate, a nonspecific image of the vascularization of the temporomandibular structures can be obtained immediately.
Three hours after the injection, a scintillation camera can detect the radioactivity emitted by the technetium molecules attached to the hydroxyapatite crystals, replacing the calcium. This establishes the mapping of osteoblastic activity,
The potential advantage of this examination lies in the possibility of early detection of bone alterations not yet detectable radiologically.
Apart from the detection of intra-articular inflammatory pathologies, this technique should mainly be used to detect tumor pathologies (especially metastatic).

3.9. Arthroscopy:
For the ATM, small diameter arthroscopes are used, coupled with a luminous optical fiber and a camera, allowing the joint parts to be monitored live or on a video screen.
3.10. Ultrasound:
The use of high frequency probe (at least 7.5 MHz) is necessary.
The articular disc is identified by a hypoechoic image surrounded by two hyperechoic lines, located at the upper edge of the condyle.

4. Biological table:
4.1. Cytobacteriology:
Joint puncture may be useful in cases of septic arthritis : bacteriological analysis of the fluid most often shows the presence of Staphylococcus aureus. Streptococci, pneumococci, coliforms and bacteroids have also been found.
At the same time, this puncture can be used to wash the joint and inject medication into it.
4.2. Cytopuncture and synovial biopsy: can also be informative but used exceptionally given the small quantity of synovial fluid.
4.3. non-specific biological tests:
VS CRP – blood count – serology – Urea – crea – blood sugar….

5. Other additional examinations:
Axiography: Allows condylar movements to be recorded in 3D, and compared over space and time.
EMG (electromyographic): Analyze the symmetrical or non-symmetrical aspect of muscle contractions. Check the quality of restoration work by comparing the results before and after treatment.
Sonography: records temporomandibular vibrations using suitable equipment. Allows you to keep track of these joint noises and their evolution over time, and to clearly specify their chronology of appearance in relation to the movement.

Thermography: it highlights changes in skin temperature, sometimes correlated with anomalies in the underlying structures.
Occlusal Contact Analyzers: Allows you to record the chronology, pressure and distribution of occlusal contact points and keep track of these occlusal contact points and their evolution over time.
Mandibular movement analyzer (mandibulography): Allows recording of mandibular kinematics. Magnetic waves or fields are captured by a receiving antenna (or camera) placed on the face. This detection device transmits signals to a computer which allows a graphical representation of these movements, as a function of time, speed and acceleration.

Conclusion

The temporomandibular joint is a very complex joint, the elements and functioning of which are still incompletely understood to this day.
This articulation involves a set of neuromuscular and articular systems which mean that the slightest pathology at these levels can cause serious physical, psychological and sometimes even aesthetic disturbances.
Today, the study of the temporomandibular joint is even considered in a more global framework; it is increasingly often integrated into the framework of studies of balance and posture.

Cracked teeth can be healed with modern techniques.
Gum disease can be prevented with proper brushing.
Dental implants integrate with the bone for a long-lasting solution.
Yellowed teeth can be brightened with professional whitening.
Dental X-rays reveal problems that are invisible to the naked eye.
Sensitive teeth benefit from specific toothpastes.
A diet low in sugar protects against cavities.