**2. Components of the temporomandibular joint**

#### **2.1 Temporary component**

The temporal bone contributes three regions to the TMJ, the largest being the articular or mandibular fossa, a concave surface whose anterior limit is the articular eminence, and its posterior limit is the postglenoid process [2]. The glenoid fossa is wider mediolaterally than anteroposteriorly, its surface is thin, and it may be translucent in a dissected skull, showing that although the articular fossa contains the posterior edge of the disc and condyle, it's not a functionally resistant tension part [1, 7]. The second portion, the articular eminence, is a transverse bony prominence that continues mediolaterally across the articular surface, is generally thick, and serves as a major functional component of the TMJ. The third portion of the articular surface of the temporal bone is the preglenoid plane, a flattened area anterior to the eminence [2, 7].

#### **2.2 Mandibular component**

The mandibular portion that is part of the TMJ is the condyle, it's a paired structure that forms an angle of approximately 145° to 160° with each other. It normally has an elliptical shape and measures on average 20 mm mediolaterally (range 13 to 25 mm) and 10 mm anteroposteriorly (range 5.5 to 16 mm). The condyle tends to be rounded mediolaterally and convex anteroposteriorly. The size and shape of the condyle present large individual variations that may be relevant in terms of biomechanical load. In its medial portion below its articular surface is the pterygoid fovea, site of insertions of the lateral pterygoid muscle [2, 8].

#### **2.3 Cartilage and synovium**

Lining the inner face of the joint, there are two types of tissue: articular and synovial cartilage. The space bounded by these two structures is called the synovial cavity, which is filled with synovial fluid. The articular surfaces of the temporal bone and condyle are covered with dense articular fibrocartilage. This cover has the ability to regenerate and remodel under functional stress. Deep to the fibrocartilage of the condyle, there is a proliferative zone of cells that can become cartilage or bone tissue. Articular cartilage is composed of chondrocytes and an intercellular matrix of collagen fibers, water, and a nonfibrous tissue, filling material, called the ground substance. Chondrocytes are arranged in three layers characterized by different cell shapes. The superficial zone contains small flattened cells with their longitudinal axes parallel to the surface. In the middle zone the cells are larger and rounder and appear in columns perpendicular to the surface. The deep zone contains the largest cells and is divided by the Level mark; below which some degree of calcification occurs [2].

#### *Biomechanics of the Temporomandibular Joint DOI: http://dx.doi.org/10.5772/intechopen.103836*

Cartilage is nourished primarily by diffusion from synovial fluid. Collagen fibers are arranged in an interlocking meshwork of fibrils parallel to the joint surface, joining as bundles and descending to them junction in the calcified cartilage between the level marks. Functionally, these meshes provide a framework for the interstitial water and the essential substance to resist the compressive forces encountered in the load [2].

Articular cartilage contains a higher proportion of collagen fibers than other synovial joints. The fundamental substance contains a variety of plasma proteins, glucose, urea and salts, as well as proteoglycans, which are synthesized by the Golgi apparatus of chondrocytes. Proteoglycans are macromolecules that contain a protein core linked to chondroitin sulfate and keratan sulfate glycosaminoglycan chains. Proteoglycans are involved in the diffusion of nutrients and metabolic degradation. The ground substance allows the entry and exit of large amounts of water, allowing its characteristic functional elasticity in response to deformation and load [2, 8].

The lining of the capsule is the synovial membrane, a thin, smooth, richly vascular, and innervated membrane that contains no epithelium. Synovial cells have a phagocytic and secretory function and are believed to be the site of hyaluronic acid production. Synovial fluid is considered an ultrafiltrate of plasma which comes from two sources: the first, from plasma by dialysis, and the second, from the secretion of type A and B synoviocytes [1, 2]. Among its functions is the lubrication of the joint, phagocytosis of particles and nutrition of the articular cartilage. It contains a high concentration of hyaluronic acid. The proteins found in synovial fluid are identical to plasma proteins; however, it has a lower total protein content, a higher percentage of albumin, and a lower percentage of α −2-globulin.

The number of leukocytes is less than 200 per cubic millimeter and less than 25% of these cells are polymorphonuclear. Only a small amount of synovial fluid, usually less than 2 ml, is present within the healthy TMJ [2].

#### **2.4 The articular disc**

Its biconcave in shape with a length of approximately 12 mm and a width of 16 mm. It is firmly attached to the lateral and medial poles of the condyle [9]. made up of dense fibrous connective tissue and is not vascularized or innervated, an adaptation that allows it to resist pressure, is composed of densely organized collagen fibers, high molecular weight proteoglycans, elastic fibers, and cells ranging from fibrocytes to chondrocytes. Collagen is mainly made up of types I and II. The fibers have a typical pattern of distribution in the intermediate zone, oriented sagittally and parallel to the disc surface. Most of these fibers continue into the anterior and posterior bands to intertwine or continue with the oriented collagen fibers transversely and vertically of these bands or pass through the entire bands to continue towards the anterior and posterior disc attachments. Vertically and transversely oriented fibers are more pronounced in the anterior and posterior band. In the intermediate part there is weaker cross-linking of the collagen bundles, which makes this area less resistant to mediolateral shear stresses [8].

Anatomically the disc can be divided into three regions in a sagittal section: an anterior portion (about 2 mm), posterior portion (about 3 mm), and a middle portion of 1 mm. The anterior portion of the disc consists of a layer of fibroelastic fascia (upper) and a fibrous layer (lower). The disc is flexible and adapts to the demands of the joint surfaces, joining the capsule anteriorly, posteriorly, medially, and laterally [2, 7]. It's bounded inferiorly by the articular surface of the mandibular condyle and laterally and medially by the synovial membrane. It divides the inferior and superior joint

compartment into two spaces. The inferior joint space contains approximately 0.9 ml of synovial fluid, while the superior joint space contains approximately 1.2 ml [9].

Articular disc has been shown to have region- and direction-dependent variations in biomechanical response. Female joint discs tend to be stiffer and relax less than male discs, suggesting a possible etiologic factor in the development and progression of temporomandibular disorders, and the higher prevalence among women [10].

The presence of a fibrocartilaginous disc in the joint prevents peak loads because it has the capacity to deform and adapt to the joint surfaces. These deformations ensure that the loads are absorbed and distributed over larger contact areas. In addition, the shape of the disc and the location of the contact zones continuously change during mandibular movement to adapt to the articulating surfaces. As a result, there will be a change in the magnitude and location of the deformations [11].

#### **2.5 Retrodiscal tissue**

The retrodiscal area is called the bilaminar zone because it consists of two laminae separated by loose connective tissue made up of elastic fibers, blood vessels, lymphatics, nerves, and adipose tissue. The inferior lamina inserts into the periosteum of the condyle approximately 8 to 10 mm below the condylar apex. The lamina consists of thick fibers that originate from almost the entire height of the posterior band and lacks elastic fibers. The lamina stretches with occlusion and bends as the condyle rotates into the mandibular opening. It is believed to serve as a control ligament to prevent extreme rotation of the disc at the condyle in rotational movements [2, 8]. On the other hand, the upper lamina inserts into the periosteum of the fossa anterior to the squamotympanic and petrotympanic fissures, is thinner than the lower lamina and contains thinner collagen fibers. It has elastic fibers and collagen fibers that fold in the occluded position and stretch during opening or protrusion, allowing the disc to slide anteriorly. The position of the disc is ensured by the lateral and posterior inferior ligaments [8].

The loose tissue of the retrocondylar space compensates for pressure changes that arise when the retrocondylar space expands during translation. The loose fibroelastic structure allows the blood vessels to expand, causing the posterior superior lamina to press against the fossa and the posterior inferior lamina to fold superiorly. The blood vessels are connected with the pterygoid venous plexus located anteromedially to the condyle. Therefore, during opening, blood drains backwards and laterally to fill the enlarged space behind the condyle, and upon closing, it is pushed into the pterygoid plexus [8].

#### **2.6 Ligaments**

They are composed of collagen and act predominantly as restraints on movement of the condyle and disc. Three ligaments can be considered main: collateral, capsular and temporomandibular ligaments. Other ligaments such as the sphenomandibular, stylomandibular, pterygomandibular, and Pinto ligaments are considered accessory ligaments because they serve to some extent as passive restrictors in mandibular movement [2, 7].

#### *2.6.1 Collateral or discal ligaments*

They are short paired structures that span each joint, they attach superiorly to the temporal bone along the rim of the glenoid fossa and articular eminence, and

*Biomechanics of the Temporomandibular Joint DOI: http://dx.doi.org/10.5772/intechopen.103836*

inferiorly to the neck of the condyle along the rim of the articular facet. It surrounds the joint spaces and the disc, being attached anteriorly and posteriorly, as well as medially and laterally. The function is to resist medial, lateral and inferior forces, thus maintaining the attachment of the disc to the condyle. This offers protection in extreme movements, a secondary function is to contain the synovial fluid within the superior and inferior joint spaces [2, 7].

#### *2.6.2 Temporomandibular (lateral) ligaments*

They are found on the lateral aspect of each TMJ or temporomandibular joint. They are individual structures that function in pairs with the corresponding ligament in the opposite TMJ. It can be separated into two different parts, which have different functions. The external oblique part descends from the external aspect of the articular tubercle of the zygomatic process and inferiorly to the external posterior surface of the condylar neck. It limits the amount of inferior distraction that the condyle can have in translation and rotation movements. The internal horizontal part also arises from the external surface of the articular tubercle, just medial to the origin of the external oblique part of the ligament, and runs horizontally posteriorly to join the lateral pole of the condyle and the posterior pole of the disc. The function of the inner portion is to limit the posterior movement of the condyle, particularly during rotational movements, for example when the mandible moves laterally in masticatory function [2, 7].

#### *2.6.3 Sphenomandibular ligament*

It is a remnant of Merckel's cartilage. It originates from the sphenoid spine and on its way to the mandible inserts into the medial wall of the TMJ joint capsule. It continues its descent to attach to the lingula of the mandible as well as to the lower part of the medial side of the condylar neck. Its main function is to protect the TMJ of an excessive translation of the condyle, after 10 degrees of opening of the mouth, also functions as a point of rotation during the activation of the lateral pterygoid muscle [2, 7].

#### *2.6.4 Stylomandibular ligament*

The stylomandibular ligament arises from the styloid process to the posterior margin of the mandible or the angle of the mandible. It is considered a thickening of the deep cervical fascia. Its function is to limit the excessive protrusion of the mandible [2, 7].

#### *2.6.5 Pterygomandibular ligament*

The pterygomandibular ligament or raphe (PTML) is a thickening of the oropharyngeal fascia. It arises from the apex of the hamulus of the internal pterygoid plane of the skull to the posterior zone of the retromolar trigone of the mandible, limiting its movements [2, 7].

#### *2.6.6 Pinto or malleomandibular or discomalleolar ligament*

It has two parts: The first part refers to the middle ear involving the malleus in relation to the anterior ligament of the malleus; the second, the portion of the joint capsule of the TMJ, in contact with the retrodiscal tissues. The functions are two. In the TMJ it protects the synovial membrane with respect to the tensions of the structures surrounding and in the middle ear, would seem to control or influence the appropriate pressure for this area of the ear [2, 7].

### **3. Irrigation**

The vascular supply of the TMJ arises mainly from branches of the superficial temporal artery, the maxillary artery, and the masseteric artery. All arteries within a radius of 3 cm contribute to the vascularization of the TMJ through the appearance of secondary capillaries that branch to surround the joint capsule [12]. Venous drainage occurs through the pterygoid plexus in the retrodiscal area, which alternately fills and empties in protrusion and retrusion movements, respectively, to subsequently communicate with the internal maxillary vein, the sphenopalatine vein, the medial meningeal veins, the deep temporal veins, the masseteric veins and the inferior alveolar vein [7].

Lymphatic drainage is not always easy to describe because, in the case of TMJ disease, the lymph nodes may increase in number. Generally, the lymphatic system that drains the TMJ comes from the area of the submandibular triangle [7].

### **4. Innervation**

The TMJ has several proprioceptive receptors, particularly in the parenchyma of the articular disc: Golgi—Mazzoni and Ruffini; Myelinated and unmyelinated nerve fibers are innervated primarily by the auriculotemporal nerve posteriorly, the masseteric nerve anteriorly, the posterior deep temporal nerve anteromedially, and the branch of the TMJ arising directly from the mandibular nerve anteriorly. The middle part, although there are variations in these innervation pathways [13].

### **5. Muscles**

Classically, four masticatory muscles are described: temporal, masseter, lateral and medial pterygoid, although the supra and infrahyoid muscles also participate in mandibular movements [14].

#### **5.1 Temporal muscle**

The function of the temporalis muscle is to elevate the mandible for closure. It is not a power muscle. Contractions of the middle and posterior portions of the muscle contribute to retrusion of the mandible, and a small degree of unilateral contraction of the temporal bone assists in deviation of the mandible to the ipsilateral side [14].

#### **5.2 Masseter muscle**

Both the superficial and deep parts of the masseter muscle are powerful elevators of the jaw, but they function independently and reciprocally in some movements.

*Biomechanics of the Temporomandibular Joint DOI: http://dx.doi.org/10.5772/intechopen.103836*

The deep layer of the masseter is not active during protrusive movements and is always active during forced retrusion, whereas the superficial portion is active during protrusion and is inactive during retrusion. Similarly, the deep masseter is active in ipsilateral movements but does not function in contralateral movements, while the superficial masseter is active during contralateral movements but not in ipsilateral movements [14].

#### **5.3 Medial pterygoid muscle**

The primary function of the medial pterygoid is elevation of the mandible, but it also has a limited role in unilateral protrusion in synergism with the lateral pterygoid to promote rotation to the opposite side [14].

#### **5.4 Lateral pterygoid muscle**

It has two portions that can be considered two functionally distinct muscles. The main function of the lower head is protrusive and contralateral movement. When the two inferior bundles contract, the condyle is pulled forward and below the articular eminence, with the disc moving passively with the condylar head. This movement contributes to the opening of the oral cavity. When the inferior head works unilaterally, it produces a contralateral movement of the mandible. The function of the superior bundles is predominantly involved with the closing and retrusion movements [14].

#### **5.5 Supra and infrahyoid muscles**

This group of muscles is formed by 4 suprahyoid pairs that are digastric, mylohyoid, stylohyoid and geniohyoid and 4 infrahyoid pairs that are sternohyoid, omohyoid, sternothyroid and thyrohyoid whose function in mandibular movements is to fix or move the hyoid [14].

## **6. Mandibular movements and muscle activity**

Mandibular movement during function and parafunction involves complex neuromuscular patterns originating and modifying from central and peripheral origin. The ATM contributes about 2000 movements per day [11, 15].

#### **6.1 Jaw opening**

The active muscles are the digastric, mylohyoid, and geniohyoid. There is no activity in the temporal when there is a slow opening and the mandible is in maximum opening, although some activity can occur in the medial pterygoid [15].

#### **6.2 Jaw closure**

There is no temporary activity during mandibular closure as long as there is no contact with the teeth. The elevation without contact is given by the masseter and medial pterygoid [15].
