Temporomandibular joint (TMJ)
The temporomandibular joint (TMJ) is a paired articulation connecting the mandible to the cranium. It is combined (right and left joints always function simultaneously; isolated movement is impossible) and complex (contains a cartilaginous intra-articular disc).
The joint is formed by three main bony structures: the mandibular head, the mandibular fossa of the temporal bone, and the articular tubercle. The intra-articular disc attaches to the capsule along its entire perimeter and divides the joint cavity into two isolated spaces:
- Upper compartment (between the fossa and the disc). Movements along the sagittal axis occur here—forward and backward displacement of the mandible. During this, the condyle along with the disc moves onto the articular tubercle.
- Lower compartment (between the disc and the condyle). Responsible for depression and elevation of the mandible. Movement follows the frontal axis through the rotation of the condyle directly within the fossa.
The joint is reinforced by the lateral, styromandibular, and sphenomandibular ligaments.
Biomechanics of mouth opening and muscular apparatus
Mandibular depression is provided by four muscular structures: the mylohyoid muscle (forming the floor of the oral cavity), the stylohyoid muscle, as well as the anterior and posterior bellies of the digastric muscle.
The process of mouth opening is not a simple bone depression, but a complex biomechanical chain of events in the TMJ, divided into three stages:
- Initial stage: the mandibular condyles perform a rotational movement around the transverse axis.
- Intermediate stage: the articular condyles glide downward and forward along the slope of the articular tubercle, reaching its crest.
- Maximum opening: the condyles perform additional rotation and are securely fixed at the anterior edge of the articular tubercle.
Lateral movements and occlusal contacts
When the mandible shifts to the right or left, both compartments of the TMJ are engaged. On the rotation side (working side), the condyle remains in the fossa, rotating around the vertical axis. On the opposite (balancing) side, the condyle along with the disc shifts onto the articular tubercle—downward, forward, and slightly inward.
This pathway is called the lateral condylar path. Anatomically, it is formed by the medial and upper walls of the articular fossa, as well as the slope of the articular tubercle. The angle between this pathway and the sagittal plane is known as the Bennett angle.
During lateral excursions, two types of physiological tooth contacts are distinguished:
- Canine guidance (canine protection) — only antagonist canines on the working side come into contact, while all other teeth are disoccluded at this moment.
- Group function (group guidance) — an entire group of teeth on the working side is in contact.
Root instrumentation
Knowledge of jaw anatomy is critical in periodontology, particularly when scaling and root planing with specific curettes. Their working ends are designed taking into account the anatomical position of teeth within the upper and lower jaws.
- Langer curettes have a strict anatomical adaptation. Instruments for the anterior group (incisors and canines) are universal for both jaws. However, separate modifications exist for premolars and molars: one is adapted exclusively for the maxilla, and the other for the mandible.
- Columbia curettes are classified differently. One modification universally suits incisors, canines, and premolars on both jaws. For maxillary and mandibular molars, another unified instrument modification is used.
To memorize the function of the TMJ ligamentous apparatus, imagine a swing: the styromandibular and sphenomandibular ligaments form a distinct anatomical loop that securely "suspends" the mandible from the cranial bones.
Class and exam questions
Which muscles depress the mandible?
This process involves the mylohyoid and stylohyoid muscles, as well as the anterior and posterior bellies of the digastric muscle.
What is the Bennett angle?
This is the angle of the lateral condylar path. It is formed between the direction of condylar displacement on the balancing side and the sagittal plane. Normally, it equals 15–17°.
What is the difference between canine guidance and group function?
In canine guidance during lateral mandibular movement, only antagonist canines contact. In group function, an entire group of teeth on the working side comes into contact.
What happens in the TMJ during maximum mouth opening?
The mandibular condyles perform an additional rotational movement and are fixed at the anterior edge of the articular tubercle.
What else to study on this topic
- Structure and function of the TMJ intra-articular disc
- Detailed classification of periodontal Langer and Columbia curettes
- Anatomical structures forming the lateral condylar path
- The role of the articular tubercle in mandibular biomechanics
- Mechanism of the balancing and working sides during mastication
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