Biomechanics of the Temporomandibular Joint

Abstract

Biomechanics is the study of mechanics applied to living beings, it analyzes loads, stress, tension, movement, size, shape, and structure of the body. The temporomandibular joint in physiological states is subject to the interaction of various bone (jaw and temporal), nervous, cartilaginous, and muscular components. When there is an alteration in any of the components, normal biomechanics are affected. Knowing in detail how each element works individually and is essential for the diagnosis and treatment of the different pathologies of the temporomandibular joint. The reconstructive procedures must carefully assess all these factors to achieve long-term success. The purpose of this chapter is to analyze the temporomandibular joint encompassing anatomy, physiology with a biomechanical approach for its diagnosis and treatment.

1. Introduction

The temporomandibular joint (TMJ) can be classified by its function and by its anatomy. Functionally it is ginglymoarthrodial, a term derived from ginglymus, meaning a hinge joint, allowing movement only forwards and backwards in one plane, and arthrodial, meaning a joint allowing sliding movement of surfaces [1]. Anatomically, it is a diarthrodial joint, defined as the discontinuous articulation of two bones that allow freedom of movement. The movement of the TMJ is dictated by muscles and limited by ligaments, its capsule of fibrous connective tissue is innervated, vascularized and strongly attached to the joint surfaces. It is also a synovial joint, whose fluid acts as a joint lubricant and supplies its metabolic and nutritional needs [2]. When occluding the mandible, it will be subjected to loads, a unilateral occlusion will result in load peaks at the contralateral TMJ. In addition, the condyle is an adaptable and regenerative unit with the ability to maintain functions despite trauma and degenerative changes [3]. The TMJ is the only joint in the human body that houses a growth center, resulting in the perpetual need for the left and right joints to work coordinated [4].

Biomechanics is the study of mechanics applied to living beings, it analyzes loads, efforts, tension, movement, size, shape and structure of the body. The temporomandibular joint is subject to forces produced by the masticatory muscles and the occlusion stress that is supported by the teeth [3]. In addition, it analyzes and helps understand the interaction of form, function and mechanism of the temporomandibular disorders to prevent, diagnose and cure these disorders [5]. A total joint replacement should function as close to a healthy joint as possible. It must be able to withstand the same forces and must produce the same movements as a normal joint [6].

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].

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 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. 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].

6.3 Retrusion

Voluntary retrusion in mandibular closure is given by the contraction of the posterior fibers of the temporalis muscle, as well as by the suprahyoid and infrahoid muscle groups [15].

6.4 Protrusion

Protrusion without occlusal contact is the result of contraction of the lateral and medial pterygoids as well as the bilateral masseters [15].

6.5 Lateral movements

Lateral movement of the mandible without tooth contact is achieved primarily by contraction of the medial and posterior fibers of the ipsilateral temporalis muscle and by contralateral contraction of the lateral and medial pterygoid and anterior temporalis fibers. The suprahyoid muscles are active keeping the mandible slightly protruded and depressed [15].

7. Mandibular kinematics

Functionally, mandibular movements are complex with six degrees of possible movement, which occur as complex interrelated rotational and translational activities. They are possible thanks to the relationship of four different joints: lower and upper. Although the TMJ does not function independently of the other, a classification of isolated mandibular movements is necessary [11, 16].

7.1 Types of movements

Movements have been extensively studied at the level of the occlusal interface, being Ulf Posselt one of the first to describe motion in three dimensions. Condylar rotation and translation of the condyle-disc assembly, in most cases, begin simultaneously. On average, condylar rotation increases or decreases linearly by approximately 2°/mm of anterior or posterior translation during opening or closing, respectively [8, 16].

7.2 Rotational movement

Rotation occurs when the condyles rotate around a fixed point or axis during mandibular opening and closing. Rotational motion can occur in three reference planes: horizontal, vertical, and sagittal. Each of them occurs around a point called the axis [11].

• Horizontal orientation axis: opening and closing movement, referred to as a hinge, therefore it occurs around an axis called the hinge axis. It is considered the purest rotation movement [16].

• Vertical axis of rotation: Also called frontal axis. It occurs when one of the condyles moves anteriorly from the position of the terminal hinge axis with the vertical axis in the opposite condyle, which remains in said axis. This type of movement does not occur normally [16].

• Sagittal axis of rotation: Occurs when one of the condyles moves inferiorly while the other remains in the position of the terminal axis. This movement occurs in conjunction with other movements. Mathematical studies indicate that in this plane there is the same contact and muscle activity from one side to the other, so there are no alterations in dental occlusion that result in a joint without load [11, 16].

The amount of condylar rotation does not differ between men and women. A finding that contrasts with the greater maximum interincisal opening of men compared to women due to differences in jaw length. In fact, with the same degree of rotation, the greater the length of the mandible, the greater the opening of the mouth. Consequently, the degree of interincisal opening cannot be considered as a measure of joint mobility or laxity, unless corrected for mandibular size [8].

7.3 Translational motion

Translation can be defined as a movement in which every point of the object t simultaneously has the same speed and direction. In the masticatory system, it occurs when the mandible protrudes. During normal movements, rotation and translation occur simultaneously, as the mandible rotates in one or more axes, each of the axes is changing orientation in space [16].

The total movement of the mandible does not consist only of rotation and translation. Side-to-side or eccentric bodily movement of the mandible and rotation and translation of the joints indicate that the mandible acts as a free-moving or floating; structure. Controlled by pairs of complementary and opposing functional muscle groups that gradually exert impulse force with numerous force vectors, the three-dimensional movement of the mandible with a dual-operation joint system is unlike any other orthopedic system in the body [17].

Classical records analyzed mandibular movements in terms of their geometry, using mechanical systems. Posselt designed an instrument called a gnatho-tensiometer, which could record border movements in all three planes, obtaining the Posselt diagram. Currently, technology has made it possible to improve position tracking techniques and thus be able to analyze mandibular kinematics with high spatial and temporal resolution (Figure 1) [18].

Figure 1.

Posselt diagram.

7.4 Temporomandibular joint and its relationship with occlusion

Movement is not only guided by the shape of the bones, muscles, and ligaments, but also by the occlusion of the teeth [1]. The Glossary of Prosthodontic Terms defines occlusion as the static relationship between the chewing surfaces of the maxillary and mandibular teeth. Dental contact has to be studied from a functional perspective and a more adequate definition of occlusion would be the biological and dynamic relationship of the components of the masticatory system that determines dental relationships [19].

Occlusion comprises a wide range of topics, the biomechanics of occlusal contact between two teeth with different cusp inclinations form a complex system [16]. From a clinical point of view, TMJ changes including intracapsular exudate and joint tissue loss can result in occlusal changes such as anterior or posterior open bites. It is important to mention that a particular occlusal scheme is not a determinant of disease. There is no evidence to suggest that one scheme predominates over another. Group functions compared to canine guides cause less condylar displacement, this displacement is small and has no clinical significance [19].

The range of vertical movement is dictated by anterior determinants such as overbite and posterior determinants such as TMJ condylar guidance. From a biomechanical point of view, anterior versus posterior determinants have a greater influence on tooth contact due to their proximity to the teeth. On the other hand, the condylar guide will influence when the molars are in contact or close to contact during mandibular movements [19].

7.5 Loads at the TMJ

Studies about whether the TMJ is subjected to load has been the subject of discussion for many years. Brehnan et al. in 1981 was able to corroborate in his studies carried out on monkeys that there is a load in the TMJ. It’s accepted that mechanical loading is essential for growth [11]. During the natural function of the joint, a combination of compressive, tensile, and shear loads occur [5]. The efforts produced by the loads will generate a deformation which can be quantified by determining the change between the original length with the final length of a structure, this deformation is expressed as a percentage, there are two types of deformation: elastic one in which eliminating the force the material recovers its original dimension, while plastic deformation is one in which the original dimension is not recovered. The elastic limit es the yield point beyond which permanent deformation occurs and the tissue does not return to its original shape. Ultimate strength is the stress a tissue can withstand, and breaking strength is the stress at which the tissue breaks (Figure 2) [20].

Figure 2.

Graph shows that the elastic limit and the maximun resistance.

The value of the maximum resistance of the disc depends on the direction of the applied stress and the region where it is applied. For example, the ultimate strength of the intermediate zone of the disc is 37.4 MPa (1 MPa = 106N/m2) when a tensile stress is applied anteroposteriorly, while it is 1.6 MPa when the application of stress is medio-lateral [11].

During compressive loading the disk becomes smaller, during tensile loading, it is stretched in the direction of loading, during shear loading, one edge of the disk surface moves parallel to the adjacent surface (Figure 3) [16]. Therefore, an unloaded TMJ may show degenerative changes, which may lead to impaired masticatory function. However, an excessive load that exceeds the adaptive capacity can also lead to degradation of the joint structure [11]. If the surfaces of the condyle or fossa have significant bony irregularities, the distribution of force over an even smaller square area of the joint can make these ratios more diverse and destructive. Otherwise, an aging dysfunctional disc/capsule does not have the necessary viscoelastic properties to meet the functional demands of the TMJ [17].

Figure 3.

Different types of load over disc. A. Normal state. B. Tension. C. Compression D. shear.

Any surgical procedure must restore functional congruence between all four joint surfaces. Any intervention must limit the instability of the joint to eliminate the progressive influence of torque and shear at the lateral attachment of the disc/capsule to the mandibular condyle. Currently, no synthetic or biological material meets the viscoelastic properties disk/capsule Knowledge of biomechanics will guide the clinician in making decisions for the surgical treatment of TMJ.