CHAPTER 7 Structure and Function of the Temporomandibular Joint Hyoid bone


Digastric muscle (anterior belly)


Thyrohyoid muscle


Omohyoid muscle


(superior belly)


Sternohyoid muscle


Mylohyoid muscle


Stylohyoid muscle


Digastric muscle (posterior belly)


Sternohyoid and omohyoid muscles (cut)


Thyrohyoid muscle


Sternothyroid muscle


139


Figure 7.11 The suprahyoid and infrahyoid muscle groups associated with the TM joint. (From Levangie P, Norkin C. Joint Structure and Function: A Comprehensive Analysis, 5th ed. Philadelphia, PA: F. A. Davis Company, 2011: p. 220, with permission.)


Omohyoid muscle


Sternohyoid muscle (cut)


Clinical Connection 7.1


Faulty forward head posture has been associated with symptoms affecting the head and neck, includ- ing headache, cervical spine dysfunction, and TM joint dysfunction. When the head and neck are protruded forward and the shoulders are rounded, adaptive shortening occurs in the suboccipital cervical extensors and the anterior chest tissues. The overlengthened deep cervical fl exors, thoracic extensors, and scapular retractors become weak and ineffective. This abnormal posture has an adverse effect on the kinematics of the TM joint. Postural exercises have been shown to result in signifi cant improvements in pain and mouth opening in indi- viduals with TM joint dysfunction.


pterygoid and the secondary muscles are activated (e.g., if the teeth get “stuck” in a chewy food and need added muscle strength to pull the jaw away from the food to open the mouth). The lateral pterygoid consists of fi bers that run horizontally, attaching to the sphenoid bone, and posteriorly, attaching to the mandible, the articular disc, and the joint capsule. Although there is some controversy in the literature, it is believed that the superior fi bers pro- trude the mandible, and the inferior fi bers depress and protrude the mandible. Bilateral contraction of the mas- seter, temporalis, and medial pterygoid muscles elevates the mandible. Mandibular protrusion is accomplished by bilateral action of the masseter and the medial and lateral pterygoid muscles. Retrusion occurs through activation of the posterior fi bers of the temporalis muscles and the anterior portion of the digastric muscle.


SECONDARY MUSCLES


(Fig. 7.10B) attaches on the zygomatic arch and the angle and ramus of the mandible. The medial pterygoid (Fig. 7.10C) is located on the inside of the mandible and is similar to the masseter in size and fi ber alignment. It attaches superiorly to the lateral pterygoid plate on the sphenoid bone. The inferior attachment is on the internal surface of the mandibular ramus near its angle. Low-level activity of the temporalis keeps the mouth in a resting position with the lips together and the teeth slightly apart. As the mouth opens, this muscle activ- ity decreases because gravity is usually suffi cient to pull the mandible down. With resisted opening, the lateral


The secondary muscles of mastication and other mouth movements are divided into two groups depending on their relationship to the hyoid bone—the suprahyoid group and the infrahyoid group (Fig. 7.11). Both groups are involved with tongue movements and movements that occur during speech and swallowing. The infrahyoid muscles are the omohyoid, sternohyoid, sternothyroid, and thyrohyoid muscles. They stabilize the hyoid bone, allowing the suprahyoid muscles to assist with depres- sion. The suprahyoid muscles—the digastric, geniohyoid, mylohyoid, and stylohyoid—are involved with mandibu- lar depression. The digastric muscle is mainly associated with mandibular depression (Fig. 7.12). Table 7.1 sum- marizes the synergistic action of the primary and second- ary muscles at the TM joint.


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