Bony elements and articular disc
The TMJ is formed by the bony structures of the temporal bone and the mandible. A distinctive feature of this articulation is pronounced incongruity (mismatch in size and shape) of the bony elements, which is compensated for by the presence of an intra-articular disc.
- Head of the mandible (caput mandibulae): the ellipsoid extremity of the condylar process, covered superiorly with fibrous cartilage.
- Mandibular fossa (fossa mandibularis): a depression in the temporal bone. The petrotympanic fissure divides it into an anterior (intra-articular) and a posterior (extra-articular) part. Importantly, only the anterior region is covered with articular cartilage.
- Articular tubercle (tuberculum articulare): a cylindrical eminence whose anterior and posterior slopes serve as functional working surfaces.
- Articular disc (discus articularis): a biconcave plate of dense fibrous tissue that eliminates incongruity. Fusing with the capsule throughout its length, the disc divides the joint cavity into two completely isolated chambers: the superior (volume 1.0–1.5 ml) and the inferior (0.5–0.8 ml). This separation creates high compensatory capabilities: when one chamber is damaged, movements can be partially preserved through the other.
Features of the capsule and ligamentous apparatus
- Articular capsule (capsula articularis): an elastic yet very durable cone-shaped membrane (consisting of an outer fibrous and an inner endothelial layer). Unlike other joints, it does not rupture during dislocations. Anteriorly and laterally, the capsule has its greatest length, which is why anterior dislocations occur significantly more frequently than posterior ones. In the posterior region, between the capsule and the posterior pole of the disc, lies the bilaminar zone (retrodiscal pad), an area of loose connective tissue. When the mandibular head is displaced posteriorly, this zone is traumatized, leading to impaired trophism and degeneration of TMJ structures.
- TMJ ligaments: formed by inelastic fibrous tissue. The main clinical rule: after overstretching, the original length of the ligaments is never restored. They are classified into intracapsular ligaments (fixing the disc to the fossa and head), extracapsular ligaments (the lateral ligament is the only proper joint ligament), and accessory ligaments (sphenomandibular and stylomandibular).
Biomechanics and muscle control
TMJ stabilization depends entirely on dental arch occlusion (occlusion) and masticatory muscle tone. The muscles are divided into two functional groups:
- Jaw depressors (anterior group): suprahyoid muscles (displace the jaw downward) and infrahyoid muscles (fix the hyoid bone).
- Jaw elevators (posterior group): temporal muscle (m. temporalis), masseter muscle (m. masseter), medial and lateral pterygoid muscles.
A unique biomechanical role is played by the lateral pterygoid muscle (m. pterygoideus lateralis). Its superior head attaches to the anterior margin of the articular disc. When the disc returns to its place, this head performs a rare type of work — lengthening contraction. It slows down its stretching, smoothly controlling the return of the disc to the posterior slope of the tubercle as the mandibular head is displaced posteriorly. The tone of this muscle critically depends on dental occlusion.
Blood supply to the joint
Blood supply to the TMJ comes from the external carotid artery basin (branches of the superficial temporal and maxillary arteries). Blood vessels penetrate the joint, forming a rich arterial network.
Notably, the articular disc itself has blood vessels only at its margins (in the anterior and posterior thickened parts). The intermediate, thinnest zone of the disc is devoid of vessels and is nourished exclusively by diffusion — via lymph and tissue fluid from the synovial cavities. The venous drainage forms a prominent network that actively anastomoses with the veins of the middle ear.
Briefly: answers to topical questions
Which nerves innervate the temporomandibular joint?
Innervation of the temporomandibular joint is carried out predominantly by sensory branches of several nerves.
- Auriculotemporal nerve (n. auriculotemporalis) — is the main source, giving off 4–7 independent constant branches.
- Facial nerve (n. facialis) — acts as an additional source of innervation.
- Posterior deep temporal nerve — serves as an additional source.
- Mandibular nerve, third branch of the trigeminal nerve (n. mandibularis) — also belongs to additional sources.
Which anatomical structures pass through the bilaminar zone of the TMJ?
Blood vessels and nerves pass through the bilaminar zone (retrodiscal pad) of the temporomandibular joint. This anatomical structure itself is a loose connective tissue layer (intercellular space) shaped like a trapezoid. It connects the posterior section of the mandibular fossa and the posterior part of the articular surface of the mandibular head, located in the posterior region of the joint between the articular capsule and the posterior pole of the articular disc.
What types of movements are possible in the temporomandibular joint?
Movements in three mutually perpendicular planes are possible in the temporomandibular joint.
- Vertical (frontal) movements — depression and elevation of the mandible (up and down).
- Sagittal movements — protrusion and retraction of the mandible (forward and backward).
- Horizontal (transverse) movements — lateral jaw movements (side to side, right and left).
- Circular movements — a combination of these movements, ensuring a variety of mandibular positions.
Into which specific veins does venous drainage from the temporomandibular joint occur?
Venous drainage from the temporomandibular joint occurs through the veins of the external acoustic meatus and the pterygoid venous plexus. The final drainage of venous blood flows into the facial vein basin.
Which specific muscles make up the anterior group (mandibular depressors)?
The anterior group of muscles depressing the mandible includes suprahyoid and infrahyoid muscles, as well as tongue muscles.
- Digastric muscle (m. digastricus) — belongs to the suprahyoid group.
- Mylohyoid muscle (m. mylohyoideus) — belongs to the suprahyoid group.
- Geniohyoid muscle (m. geniohyoideus) — belongs to the suprahyoid group.
- Genioglossus muscle — participates in jaw depression.
- Hyoglossus muscle (m. hyoglossus) — participates in jaw depression.
Also, the infrahyoid group includes the sternohyoid (m. sternohyoideus), sternothyroid (m. sternothyroideus), thyrohyoid (m. thyrohyoideus), and omohyoid (m. omohyoideus) muscles.
To easily recall the suprahyoid muscles that depress the mandible (Mylohyoid, Geniohyoid, Digastric), use the mnemonic phrase: "Jaw Supported By Two Bellies".
Class and exam questions
What type of cartilage covers the articular surfaces of the TMJ?
Unlike most human joints, the surfaces of the TMJ are covered not with hyaline cartilage, but with a thinner and more fragile fibrous connective tissue cartilage.
Why do anterior TMJ dislocations occur most frequently?
This is directly related to the anatomy of the articular capsule: anteriorly and laterally, it has its greatest length and is thinner, creating a weak point during excessive movement of the mandibular head.
Do the joint ligaments recover after being stretched?
No. All TMJ ligaments consist of dense inelastic fibrous tissue. If they are overstretched during trauma, their initial length is never restored.
What else to study on this topic
- Topography of the petrotympanic fissure and its effect on the intracapsular and extracapsular compartments of the fossa
- Histological structure and trophic functions of the bilaminar zone
- Reciprocal functioning of the superior and inferior heads of the lateral pterygoid muscle
- Synchronous biomechanics of the suprahyoid and infrahyoid neck muscles during jaw movement
- Effect of dental arch occlusion on masticatory muscle tone and joint stability
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Other topics in "Chapter 13. Temporomandibular Joint Disorders (V.V. Bekreev)"
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