Temporomandibular Joint: Understanding Its Primary and Secondary Movements

What are the basic types of movement by the temporomandibular joint?

The temporomandibular joint (TMJ) is a complex, biaxial synovial joint that allows for a combination of hinge and gliding movements, facilitating essential functions such as chewing, speaking, and swallowing. Its primary movements include depression and elevation, complemented by protrusion, retrusion, and lateral excursion.

Introduction to the Temporomandibular Joint (TMJ)

The temporomandibular joint (TMJ) is the articulation between the condyle of the mandible (lower jawbone) and the temporal bone of the skull. Specifically, the mandibular condyle articulates with the mandibular fossa and articular eminence of the temporal bone. Unique among human joints, the TMJ is classified as a ginglymoarthrodial joint, meaning it combines characteristics of both a hinge (ginglymoid) joint, allowing for rotational movement, and a gliding (arthrodial) joint, enabling translational movement.

A crucial component of the TMJ is the articular disc, a biconcave fibrocartilaginous structure that divides the joint into two separate synovial cavities: the superior and inferior joint spaces. This disc allows for the distinct rotational and translational movements, enhancing congruence between the articulating surfaces and distributing forces during jaw function.

Primary Movements of the TMJ

The two most fundamental movements of the TMJ are those involved in opening and closing the mouth.

• Depression (Opening the Mouth)

  • This movement involves the lowering of the mandible, increasing the space between the upper and lower jaws. It's a complex action that occurs in two distinct phases:
    • Phase 1: Rotation: During the initial 20-25mm of mouth opening, the mandibular condyles primarily rotate posteriorly within the inferior joint space, around a horizontal axis.
    • Phase 2: Translation (Gliding): For further opening, the condyles and the articular disc translate anteriorly and inferiorly along the articular eminences of the temporal bone, within the superior joint space.
  • Key Muscles Involved: The primary muscles responsible for jaw depression are the lateral pterygoid (inferior head) and the suprahyoid muscles (digastric, geniohyoid, mylohyoid), which pull the mandible downwards. The infrahyoid muscles stabilize the hyoid bone during this action.

• Elevation (Closing the Mouth)

  • This is the powerful action of raising the mandible to close the mouth, bringing the teeth into occlusion. It is essentially the reverse of depression, with the condyles first translating posteriorly and superiorly, followed by posterior rotation.
  • Key Muscles Involved: The strongest muscles of mastication are responsible for elevation: the masseter, temporalis, and medial pterygoid. These muscles work synergistically to generate significant force for biting and chewing.

Secondary Movements of the TMJ

Beyond simple opening and closing, the TMJ allows for more nuanced movements essential for complex oral functions like chewing and speaking.

• Protrusion (Protracting the Jaw)

  • This movement involves moving the mandible directly forward, causing the lower incisors to extend anterior to the upper incisors.
  • Key Muscles Involved: Primarily the bilateral contraction of the lateral pterygoid muscles (superior and inferior heads), assisted by the medial pterygoid muscles.

• Retrusion (Retracting the Jaw)

  • This is the opposite of protrusion, pulling the mandible directly backward from a protruded position, or slightly backward from a neutral position.
  • Key Muscles Involved: The posterior fibers of the temporalis muscle are the main retractors, assisted by some of the suprahyoid muscles (e.g., digastric).

• Lateral Excursion (Side-to-Side Movement)

  • This movement, also known as lateral deviation or grinding, involves moving the mandible from side to side. It is crucial for the grinding action during mastication.
  • Key Muscles Involved: This movement is achieved by the unilateral contraction of the medial and lateral pterygoid muscles on one side, which pulls the jaw to the opposite side. For example, contraction of the right lateral and medial pterygoids will move the jaw to the left. The temporalis and masseter muscles on the opposite side stabilize and guide the movement.

The Importance of TMJ Health and Function

The intricate interplay of these movements allows for the precise and coordinated actions required for daily activities. Any disruption to the normal mechanics of the TMJ, whether due to muscle imbalance, disc displacement, or structural changes, can lead to temporomandibular joint dysfunction (TMD), manifesting as pain, clicking, limited range of motion, or headaches. Understanding these basic movements is foundational for diagnosing and treating such conditions, as well as for optimizing overall oral and craniocervical health.

Conclusion

The temporomandibular joint is a marvel of biomechanical engineering, facilitating a wide range of movements critical for survival and communication. From the fundamental actions of depression and elevation to the more nuanced movements of protrusion, retrusion, and lateral excursion, each motion is orchestrated by a precise collaboration of muscles and joint structures. A thorough understanding of these movements is paramount for anyone involved in health, fitness, or rehabilitation, underscoring the TMJ's vital role in the kinetic chain of the head and neck.