Biomechanics of Temporomandibular Joint

Temporomandibular Joint structure, articular surfaces, kinetics and kinematics of the TM joint. Applied anatomy of TMJ.

1. Biomechanics of Temporomandibular Joint T. Sunil Kumar Dept of Physiotherapy

2. INTRODUCTION The temporomandibular (TM) joint is unique in both structure and function. Structurally, the mandible is a horseshoe-shaped bone that articulates with the temporal bone at each posterior superior end and produces two distinct but highly interdependent articulations. Each TM joint contains a disc that separates the joint into upper and lower articulations. Functionally, mandibular movement involves concurrent movement in the four distinct joints, resulting in a complex structure that moves in all planes of motion to achieve normal function.

3. JOINT STRUCTURE Articular Structures Multiple bones merge to form the structure and contribute to the function of the TM joints. These bones include the mandible, maxillae, temporal, zygomatic, sphenoid, and hyoid bones. The proximal or stationary segment of the TM joint is the temporal bone. The condyles of the mandible sit in the mandibular fossa of the temporal bone. The mandibular fossa is located between the post-glenoid tubercle and the articular eminence of the temporal bone

4. Structurally, the individual TM joints are considered to be synovial joints formed by the condyle of the mandible inferiorly and the articular eminence of the temporal bone superiorly. T he condyle of the mandible sits in the mandibular fossa, but it is not at all appropriate as an articular surface. So, the articular eminence contains a major area of trabecular bone and serves as the primary articular surface for the mandibular condyle . Thus, functionally, the mandibular condyle articulates with the articular eminence of the temporal bone. The articular eminence and the condyle are both convex structures, resulting in an incongruent joint

5. The TM joint is classified as a synovial joint, although no hyaline cartilage covers the articular surfaces. The articular surfaces of the articular eminence and the mandibular condyle are covered with dense, avascular, collagenous tissue that contains some cartilaginous cells. Because some of the cells are cartilaginous, the covering is often referred to as fibrocartilage. The articular collagen fibers are aligned perpendicular to the bony surface in the deeper layers to withstand stresses. The presence of fibrocartilage rather than hyaline cartilage is significant because fibrocartilage can repair and remodel itself

6. Accessory Joint Structures The incongruence of the TM joint is addressed by a unique articular disc. A disc located within each TM joint separates the articulation into distinct superior and inferior joints with slightly different functions . Thus, mandibular motion involves the simultaneous movement of four divergent joints. The inferior TM joint is formed by the mandibular condyle and the inferior surface of the disc and functions as a simple hinge joint. The superior TM joint is larger than the inferior joint and is formed by the articular eminence of the temporal bone and the superior surface of the disc; it functions as a gliding joint.

7. The thickness of the articular disc varies from 2 mm anteriorly to 1 mm in the middle to 3 mm posteriorly. The purpose of the disc is to allow the convex surfaces of the articular eminence and the mandibular condyle to remain congruent throughout the range of motion of the TM joint in all planes. It increases stability, minimizes loss of mobility, reduces friction, and decreases biomechanical stress on the TM joint.

8. The disc within each TM joint has a complex set of attachments. The disc is firmly attached to the medial and lateral poles of the condyle of the mandible, but it is not attached to the TM joint capsule medially or laterally. These attachments allow the condyle to rotate freely on the disc in an anteroposterior direction. The disc is attached to the joint capsule anteriorly, as well as to the tendon of the lateral pterygoid muscle. The anterior attachments restrict posterior translation of the disc. Posteriorly, the disc is attached to a complex structure, collectively called the bilaminar retrodiscal pad.

9. The two bands (or laminae) of the bilaminar retrodiscal pad are both attached to the disc. The superior lamina is attached posteriorly to the tympanic plate (at the posterior mandibular fossa). The superior lamina consists of elastic fibers that allow the superior band to stretch. The superior lamina allows the disc to translate anteriorly along the articular eminence during mandibular depression; its elastic properties assist in repositioning the disc posteriorly during mandibular closing.

10. The inferior lamina is attached to the neck of the condyle and is inelastic. The inferior lamina serves as a tether on the disc, limiting forward translation, but does not assist with repositioning the disc during mandibular closing. Neither of the laminae of the retrodiscal pad is under tension when the TM joint is at rest. Loose areolar connective tissue rich in arterial and neural supply is located between the two laminae

11. Capsule and Ligaments The elasticity of the joint capsule and ligaments determines the available motion at the TM joint in all planes. Motion can be enhanced or restricted depending on the flexibility of these structures. The portion of the capsule superior to the disc is quite lax, whereas the portion of the capsule inferior to the disc is taut. Consequently, the disc is more firmly attached to the condyle below and freer to move on the articular eminence above.

12. The capsule is thin and loose in its anterior, medial, and posterior aspects, but the lateral aspect is stronger and reinforced with long fibers (temporal bone to condyle). The lack of strength of the capsule anteriorly and the incongruence of the bony articular surfaces predisposes the joint to anterior dislocation of the mandibular condyle. The capsule is highly vascularized and innervated, which allows it to provide a great deal of information about position and movement of the TM joint.

13. The primary ligaments of the TM joint are the TM ligament, the stylomandibular ligament, and the sphenomandibular ligament . The TM ligament bis a strong ligament composed of two parts, an outer oblique element and an inner horizontal element. The outer oblique element attaches to the neck of the condyle and the articular eminence. It serves as a suspensory ligament and limits downward and posterior motion of the mandible, as well as limiting rotation of the condyle during mandibular depression.

14. The inner horizontal component of the ligament is attached to the lateral pole of the condyle and posterior portion of the disc and to the articular eminence. Its fibers are aligned horizontally to resist posterior motion of the condyle. Limiting the posterior translation of the condyle protects the retrodiscal pad. The primary function of the TM ligament is to stabilize the lateral portion of the capsule. Neither band of the TM ligament limits forward translation of the condyle or disc, but they do limit lateral displacement.

15. The stylomandibular ligament is the weakest of the three ligaments and is considered a thickened part of the parotid sheath joining the styloid process to the angle of the mandible. Some investigators have identified the function of this ligament as limiting the protrusion of the mandible, but others have stated that it has no known function. The sphenomandibular ligament is described as the “strong” ligament that is the “swinging hinge” from which the mandible is suspended.

16. Some investigators have stated that it serves to protect the mandible from excessive anterior translation. Others have stated that this ligament has no function. The sphenomandibular ligament attaches to the spine of the sphenoid bone and to the middle surface of the ramus of the mandible. Abe and colleagues stated that the sphenomandibular ligament also has continuity with the disc medially

17. Loughner and colleagues examined the structures surrounding the TM joint in 14 cadaver heads and found that the sphenomandibular ligament is not continuous with the medial capsule; rather, it is immediately adjacent to the capsule. These investigators concluded that since the sphenomandibular ligament does not attach to the medial joint capsule, this ligament has no functional significance for the biomechanics of the TM joint. However, they suggested that the sphenomandibular ligament serves as an accessory ligament and, in concert with the TM ligament, provides structural support for the TM joint

18. JOINT FUNCTION Joint Kinematics The TM joint is one of the most frequently used and mobile joints in the body. It is engaged during mastication, swallowing, and speaking. Most of the time, the TM joint movements occur without resistance from chewing or contact between the upper and lower teeth. However, as a third-class lever, the TM joint is designed to maintain its structure in spite of significant forces acting on it. As previously noted, the articular surfaces are covered with a pseudofibro cartilage that has the ability to remodel and repair and thus is able to tolerate repeated, high-level stress.

19. Mastication requires tremendous power, while speaking requires intricate fine motor control. The musculature is designed to accomplish both these tasks. Both osteokinematic and arthrokinematic movements are required for normal function of the TM joint. Osteokinematic motions include mandibular depression, elevation, protrusion, retrusion, and left and right lateral excursions. Arthrokinematic movements involve rolling, anterior glide, distraction, and lateral glide.

20. Mandibular Depression and Elevation Mandibular depression and elevation are fundamental components of mastication. Under normal circumstances, the motions of mandibular depression and elevation are relatively symmetrical, with each TM joint following a similar pattern. To accomplish mandibular depression and elevation, the mandibular condyle must roll and glide. The literature is contradictory as to whether the rolling and gliding occur sequentially or concurrently. However, the literature is consistent in what rolling and gliding occur.

21. During rotation, the mandibular condyle spins relative to the inferior surface of the disc in the lower joint. During translation, the mandibular condyl e and disc glide together as a condyle-disc complex along the articular eminence. Translation occurs in the upper joint between the disc and the articular eminence

22. Normal mandibular depression range of motion is 40 to 50 mm when measured between the incisal edges of the upper and lower front teeth. Mastication requires approximately 18 mm of mandibular depression. Rolling occurs predominantly during the initial phase of mandibular depression with as little as 11 mm or as much as 25 mm, r esulting from rotation of the condyle on the disc. The remaining motion results primarily from anterior translation of the condyle-disc complex along the articular eminence. The shape of the condylar head and the steepness of the articular eminence positively correlate with the amount of rotation.

23. Both the shape of the condylar head and the steepness of the articular eminence can be asymmetrical from one TM joint to the other, thus affecting the symmetry of motion. As a quick screen, the clinician may use the adult knuckles (proximal interphalangeal joints) to assess the degree of mandibular depression. Two knuckles placed between the upper and lower incisors is considered functional, while three knuckles is considered normal. Gravity assists with mandibular depression.

24. The mandibular elevators are thought to provide eccentric control of mandibular depression, although their contribution is unclear. Mandibular elevation is the reverse of mandibular depression. The mandibular condyle rotates posteriorly on the disc in the lower joint, and the condyle-disc complex translates posteriorly in the upper joint

25. Control of the Disc During Mandibular Elevation and Depression Active and passive control is exerted on the articular disc during mandibular depression and elevation. Passive control occurs through the capsuloligamentous attachments of the disc to the condyle. The lateral pterygoid muscle attaches to the anterior portion of the disc, producing active control, although evidence suggests that this attachment may not be consistently present. Bell proposed two other muscle segments that may assist with maintaining disc position during active movement.

26. During mandibular depression, the medial and lateral attachments of the disc to the condyle limit the motion between the disc and condyle to rotation. As the condyle translates, the biconcave shape of the disc causes it to track with the condyle without any additional active or passive assistance. However, the inferior retrodiscal lamina limits forward excursion of the disc. The superior portion of the lateral pterygoid muscle attaches to the disc and appears to be positioned to assist with anterior translation; however, no activity is noted during mandibular depression

27. During mandibular elevation, the elastic character of the superior retrodiscal lamina applies a posterior distractive force on the disc. In addition, the superior portion of the lateral pterygoid demonstrates activity that is assumed to eccentrically control the posterior movement of the disc, while maintaining the disc in an anterior position until the mandibular condyle completes posterior rotation to the normal resting position. Abe and colleagues suggested that the sphenomandibular ligament also assists this action. Again, the medial and lateral attachments of the disc to the condyle limit the motion to rotation of the disc around the condyle

28. Mandibular Protrusion and Retrusion Mandibular protrusion and retrusion occur in the upper TM joint. The condyle-disc complex translates in an anterior inferior direction, following the downward slope of the articular eminence, during protrusion and returns along a posterior superior path. Rotation is not present during protrusion and retrusion. The teeth are separated during these motions. Ideally, the lower teeth should surpass the upper teeth several millimeters; however, protrusion is considered adequate when the upper and lower front incisal edges touch.

29. Protrusion is an important component necessary for maximal mandibular depression. Retrusion is an important component of mandibular elevation from a maximally depressed mandible. Control of the Disc During Mandibular Protrusion and Retrusion During protrusion, the posterior attachments of the disc (the bilaminar retrodiscal tissue) stretch 6 to 9 mm to allow completion of the motion. The degree of retrusion is limited by tension in the TM ligament as well as by compression of the soft tissue in the retrodiscal area between the condyle and the posterior glenoid spine.

30. An estimated 3 mm of translation occurs during retrusion; however, this motion is rarely measured Mandibular Lateral Excursion Lateral excursion involves moving the mandible to the left and to the right. The degree of lateral excursion considered normal for an adult is 8 to 11 mm. One functional screen to estimate whether this motion is normal is to observe whether the mandible can move the full width of one of the central incisors in each direction.

31. Active lateral excursion is described as contralateral (the opposite side) or ipsilateral (the same side) relative to the primary muscle action. To accomplish lateral excursion, the ipsilateral mandibular condyle spins around a vertical axis within the mandibular fossa, while the contralateral mandibular condyle translates anteriorly along the articular eminence. A slight degree of spin and lateral glide of the contralateral mandibular condyle is necessary to achieve maximal lateral excursion.

32. Another normal asymmetrical movement of the TM joint involves rotating one condyle around an anteroposterior axis while the other condyle depresses. This movement results in a frontal plane motion of the mandible, with the chin moving downward and deviating from the midline toward the condyle that is spinning. These motions are generally combined into one complex motion used for chewing and grinding food

33. Deviations and deflections may be noted during osteokinematic movements of the mandible. A deviation is a motion that produces an “S” curve as the mandible moves away from the midline during mandibular depression or protrusion and returns to midline by the end of the movement. A deflection is a motion that creates a “C” curve, with the mandible moving away from midline during mandibular depression or protrusion but not returning to midline by the end of the movement. Deviations and deflections may result from mandibular condyle head shapes differing from right to left. If no other signs or symptoms accompany these asymmetries, then deviation or deflection is considered inconsequential.

34. Muscles Primary Muscles The muscles acting on the TM joint are divided into primary and secondary muscle groups. The primary muscles include the temporalis , masseter , lateral pterygoid , and medial pterygoid . The temporalis is a flat, fan-shaped muscle that is wide at the proximal portion and narrow at the inferior portion.

35. The superior fibers attach to the cranium while the inferior fibers attach to the coronoid process and the anterior edge and medial surface of the ramus of the mandible. The temporalis fills the concavity of the temporal fossa and can be palpated easily over the temporal bone. The masseter is a thick, powerful muscle with its superior attachments on the zygomatic arch and zygomatic bone and its inferior attachment on the external surface of the ramus of the mandible.

36. The lateral pterygoid consists of superior and inferior segments that travel in a horizontal direction and combine posteriorly to attach to the neck of the mandible, the articular disc, and the joint capsule. The medial pterygoid parallels the masseter in line of force and size. The superior fibers attach to the medial surface of the lateral pterygoid plate on the sphenoid bone, and the inferior attachment is on the internal surface of the ramus near the angle of the mandible

37. Secondary Muscles The secondary muscles are smaller than the primary muscles and consist of the suprahyoid and infrahyoid groups . The digastric, geniohyoid, mylohyoid, and stylohyoid comprise the suprahyoid group. The infrahyoid group includes the omohyoid, sternohyoid, sternothyroid, and thyrohyoid muscles. The suprahyoid muscles assist with mandibular depression, while the infrahyoid muscles are responsible for stabilizing the hyoid.

38. Both the suprahyoid and infrahyoid muscle groups are involved in speech, tongue movements, and swallowing. The digastric muscle is predominantly responsible for mandibular depression. The hyoid bone has to be stabilized for the digastric muscle to depress the mandible. This stabilization is provided by the infrahyoid muscles.

39. Coordinated Muscle Actions Mandibular depression occurs from the concentric action of the bilateral digastric muscles in conjunction with the inferior portion of the lateral pterygoid muscles. Mandibular elevation results from the collective concentric action of the bilateral masseter, temporalis, and medial pterygoid muscles. The bilateral superior lateral pterygoid muscles eccentrically control the TM discs as the mandibular condyles relocate into the mandibular fossa with mandibular elevation.

40. The other mandibular motions of protrusion, retrusion, and lateral deviation are produced by the same muscles that elevate and depress the mandible, but in different sequences. Mandibular protrusion is produced by the bilateral action of the masseter, medial pterygoid, and lateral pterygoid muscles. Retrusion is generated through the bilateral action of the posterior fibers of the temporalis muscles, with assistance from the anterior portion of the digastric muscle. Lateral deviation of the mandible is produced by the unilateral action of a selected set of these muscles.

41. The medial and lateral pterygoid muscles each deviate the mandible to the opposite side. The temporalis muscle can deviate the mandible to the same side. The lateral pterygoid muscle is attached to the medial pole of the condyle and pulls the condyle forward. The temporalis muscle on the ipsilateral side is attached to the coronoid process and pulls it posteriorly. Together these muscles effectively spin the condyle to create deviation of the mandible to the left.

42. Relationship to the Cervical Spine and Posture The cervical spine and TM joint are intimately connected. A biomechanical relationship exists between the position of the head, the cervical spine, and the dentofacial structures. The attachments of the primary and secondary muscles provide strong evidence of the relationship among the TM joint, cervical spine, throat, clavicle, and scapula. The impact of posture on the TM joint becomes apparent once the attachments of the musculature are examined.

43. Given their attachments, muscles acting on the mandible may also impact the atlanto -occipital joint and cervical spine. Head and neck position may affect tension in the cervical muscles, which may in turn influence the position or function of the mandible. Correct posture minimizes the forces produced by the cervical spine extensors as well as the other cervical muscles necessary to support the weight of the head. Over time, improper posture can lead to adaptive shortening or lengthening of the muscles around the head, cervical spine, and upper quarter.

44. COMMON IMPAIRMENTS AND PATHOLOGIES Mechanical stress is the most critical factor in the multifactorial Etiology. Dysfunction of either the muscles or the joint structure generally is at fault. Most clients with TM dysfunction will not fit into a specific category of dysfunction classification, which creates a clinical challenge. Additionally, only 20% to 30% of individuals with internal derangement of the TM joint develop symptomatic joints. These symptoms may progress or resolve spontaneously

45. Age-Related Changes in the TM Joint The aging process affects the joints of the human body. The TM joint is no exception. However, degenerative changes are not always the result of the normal aging process. Degenerative changes may occur from a preexisting dysfunction. Furthermore, degenerative changes do not necessarily indicate disability. Inflammatory Conditions Inflammatory conditions of the TM joint include capsulitis and synovitis. Capsulitis involves inflammation of the joint capsule, and synovitis is characterized by fluctuating edema caused by effusion within the synovial membrane of the TM joint.

46. Individuals with inflammatory conditions experience pain and inflammation within the joint complex, which may diminish mandibular depression. Osseous Mobility Conditions Osseous mobility disorders of the TM joint complex include joint hypermobility and dislocation. Many similarities are noted in the client history and clinical findings for these two conditions. Hypermobility, or excessive motion, of the TM joint is a common phenomenon found in both symptomatic and nonsymptomatic populations.

47. Capsular Fibrosis Unresolved or chronic inflammation of the TM joint capsule stimulates overproduction of fibrous connective tissue, which creates capsular fibrosis of the TM joint complex. The resultant fibrosis causes progressive damage and loss of tissue function. Articular Disc Displacement Articular disc displacement occurs when the articular disc subluxes beyond the articular eminence. Two conditions can result: disc displacement with reduction and disc displacement without reduction.

48. Without intervention, disc displacement with reduction often advances to disc displacement without reduction. Degenerative Conditions Two degenerative conditions may affect the TM joint: osteoarthritis and rheumatoid arthritis. Hertling and Kessler stated that 80% to 90% of the population older than 60 years have some symptoms of osteoarthritis in the TM joint.

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