Biomechanics of TMJ PDF

**BIOMECHANICS OF TMJ** **Introduction** The TMJ has three degrees of freedom, with each of the degrees of freedom being associated with a separate axis of rotation. Two primary arthrokinematic movements (rota­tion and anterior translation) occur at this joint around three planes: sagittal, horizo...

**BIOMECHANICS OF TMJ** **Introduction** The TMJ has three degrees of freedom, with each of the degrees of freedom being associated with a separate axis of rotation. Two primary arthrokinematic movements (rota­tion and anterior translation) occur at this joint around three planes: sagittal, horizontal, and frontal.^1^ Rotation occurs from the beginning to the midrange of movement. In addition to the rotational motions during mouth opening and closing and lateral deviations, movements at the TMJ involve arthrokinematic rolls and slides. Gliding, transla­tion, or sliding movements occur in the upper cavity, whereas rotation or hinged movement occurs in the lower cavity. The motions of protrusion and retrusion are planar glides. Thus, mouth opening, contralateral deviation, and protrusion all involve an anterior osteokinematic rotation of the mandible and an anterior, inferior, and lateral glide of the mandibular head and disk; and mouth closing, ipsilateral deviation, and retrusion all involve a posterior osteokinematic rotation of the mandible and an anterior, inferior, and lateral glide of the mandibular head and disk^.1^ The TM joint 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.AS a third-class lever, the TM joint is designed to maintain its structure in spite of significant forces acting on it. the articular surfaces are covered with a pseudofibrocartilage that has the ability to remodel and repair and thus is able to tolerate repeated, high-level stress. 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.^2^ **Occlusal Position** Occlusal positions are functional positions of the TMJ. The occlusal position is defined as the point at which contact between some or all of the teeth occurs. Under normal cir­cumstances, the upper molars rest directly on the lower molars and the upper incisors slightly override the lower incisors. The ideal position provides mutual protection of the anterior and posterior teeth, comfortable and painless mandibular function, and stability. The median occlusal position corresponds to the position in which all of the teeth are fully interdigitated and is considered the start position for all mandibular motions.^1^ **Osteokinematics** The primary osteokinematics of the mandible are most often described as protrusion and retrusion, lateral excursion, anddepression and elevation. Varying degrees of combined mandibular translation and rotation occur during all primary movements. These combined kinematics optimize the mechanical process of mastication.^3^ **Mouth Opening (mandibular depression)** Mouth opening occurs in a series of steps, In the erect position, the condyles begin to rotate anteriorly and translate inferiorly and laterally during the first 25 degrees of opening as the jaw opens. The upper head of the lateral pter­ygoid muscle and the anterior head of the digastric muscle draw the disk anteriorly and prepare for condylar rotation during movement. This initial condylar rotation occurs as the mandibular elevators (masseter, temporalis, and medial pterygoid muscles) gradually relax and lengthen, allowing gravity to depress the mandible. The direc­tions of the fibers of the lateral and medial temporomandibu­lar ligaments also keep the condyle from moving posteriorly. The fibrous capsule and parts of the temporomandibular ligament limit excessive lateral movement of the condyle. The rotation occurs through the two condylar heads between the articular disk and the condyle. As the mandible moves for­ward on opening, the disks move medially and posteriorly until the collateral ligaments and the lateral pterygoid stop their movement. During the last 15 degrees of opening, the rotation ceases due to tightening of the collateral ligaments, and is replaced by an anterior translation of the condyles. During this translation, the condyle and disk move together. The anterior translation, which is produced mainly by muscle contraction, serves to prevent mandibular encroachment of the anterior neck structures. Opening is also assisted by the other suprahyoid muscles. In extremely wide opening, such as that occurs with yawning, the functional joint contact is on the distal aspect of the condyle, and the anterior lateral aspect of the condyle contacts the posterior part of the masseter muscle. In this position, the soft-tissue structures are in a position of stretch, making them more prone to dysfunction.^1^ 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 mm24,29 or as much as 25 mm,14 resulting 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.^2^ **Mouth Closing (Mandibular elevation)** Closing of the mouth involves a reversal of the move­ments described for mouth opening. The condyles translate posteriorly as a result of an interaction between the retract­ing portions of the masseter and temporalis muscle and the retracting portions of the mandibular depressors. As the condyles translate posteriorly and glide medially, they hinge on the disks. The disks then glide pos­teriorly and superiorly on the temporal bone along with the condyles (as a result of the actions of the masseter, medial pterygoid, and temporalis muscles). When the jaws are closed to maximal occlusal contact, the condyles contact the disks, and the disks contact the posterior slopes of the articu­lar tubercles and the glenoid fossae.^1^ **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.These two muscle segments derive from the masseter muscle and attach to the anterolateral portion of the disc. They counteract the medial pull of the anteromedially directed lateral pterygoid. 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 tobe positioned to assist with anterior translation; however, no activity is noted during mandibular depression. 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.^2^ **Protrusion** Protrusion is a forward movement of the mandible that occurs at the superior joint compartments, which consists of the disk and condyle moving downward, forward, and laterally. The muscles responsible for protrusion are the anterior fiber of the temporalis and the medial and lateral pterygoid muscles.^1^ Protrusion of the mandible occurs as it translates anteriorly without significant rotation.^3^ **Retrusion** Retrusion is a backward movement of the mandible, pro­duced by the posterior fiber of the temporalis and assisted by the suprahyoid muscles. The retrusive range is limited by the extensibility of the temporomandibular ligaments.^1^ 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.14 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. An estimated 3 mm of translation occurs during retrusion; however, this motion is rarely measured. **Lateral Excursion** If a protrusion movement occurs unilaterally, it is called a lat­eral excursion, or deviation. For example, if only the left TMJ protrudes, the jaw deviates to the right. Lateral movements of the mandible are the result of asym­metric muscle contractions. During a lateral excursion to the right, the condyle and the disk on the left side glide inferiorly, anteriorly, and laterally in the sagittal plane and medially in the horizontal plane along the articular eminence. The condyle and the disk on the right side rotate laterally on a sagittal plane and translate medially in the hori­zontal plane while remaining in the fossa.^1^  To accomplish lateral excursion, the ipsilateralmovement 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.^2^ 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.^2^ **The Close- and Open-Packed (Resting) Positions** The close-packed position of the TMJ is difficult to determine because the position of maximal muscle tightness is also the position of least joint surface congruity and vice versa. The most commonly cited close-packed position is with the teeth clenched. The open-packed position is with the mouth slightly open, the lips together, and the teeth not in contact. The signifi­cance of this position is that it permits the tissues of the sto­matognathic system to rest and undergo repair.^1^ **Capsular Pattern** The capsular pattern of the TMJ is a limitation of mouth opening. If one joint is more involved than the other, the jaw will laterally deviate to the same side during opening.^1^ **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 fossawith mandibular elevation. 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.^2^ 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. 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. Although the temporalis and lateral pterygoid muscles on the left appear to create opposite motions of the mandible, concomitant contractions of the right lateral pterygoid and right temporalis muscles function as a force couple. 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. Because the temporalis muscle is also an elevator of the mandible, this combination of muscular activity is particularly useful in chewing.^2^ **The medial pterygoid and masseter muscles** form a functional sling around the angle of the mandible. Simultaneous contractions of these muscles can exert a powerful biting force that is directed through the jaw and ultimately between the upper and lower molars. The maximal biting force in this region averages about 422 N (95 lb) in the adult, twice that generated between the incisors. Acting on the internal and external sides of the mandible, the masseter and medial pterygoid also produce an important sideto-side force between the upper and lower molars. As shown in, simultaneous contraction of the right medial pterygoid and left masseter produces left lateral deviation. Contraction of these muscles in this synergistic fashion can produce a very effective shear force between the molars and food, on both sides of the mouth. combined muscular action is very effective at grinding and crushing food before swallowing.^3^ **Arthrokinematics** Movement of the mandible typically involves bilateral action of the TMJs. Abnormal function in one joint naturally interferes with the function of the other. Similar to the osteokinematics, the arthrokinematics of the TMJ normally involve a combination of rotation and translation. In general, during rotational movement the mandibular condyle rotates relative to the inferior surface of the disc, and during translational movement the mandibular condyle and disc slide essentially together. The disc usually moves in the direction of the translating condyle.^3^ **PROTRUSION AND RETRUSION** During protrusion and retrusion the mandibular condyle and disc translate anteriorly and posteriorly, respectively, relative to the fossa. Maximum condylar translation of about 1.25 cm (about 12 inch) has been measured in each direction in healthy adults.The condyle and disc follow the downward slope of the articular eminence. The mandible slides slightly downward during protrusion and slightly upward during retrusion. The path and extent of the movement usually vary depending on the degree of opening and closing of the mouth (described ahead).^3^ **LATERAL EXCURSION** Lateral excursion involves primarily a side-to-side translation of the condyle and disc within the fossa. Slight multiplanar rotations are typically combined with lateral excursion., shows lateral excursion combined with a slight horizontal plane rotation. The mandibular condyle on the side of the lateral excursion serves as a relatively fixed pivot point, allowing a slightly wider arc of rotation by the contralateral condyle.^3^ **DEPRESSION AND ELEVATION** Opening and closing of the mouth occur by depression and elevation of the mandible, respectively. During these movements, each TMJ experiences a combination of **rotation and translation** among the mandibular condyle, articular disc, and fossa. No other joint in the body experiences such a large proportion of translation and rotation. These complex arthrokinematics are a necessary mechanical component of mastication (grinding and crushing of food) and for speaking. Because rotation and translation occur simultaneously, the axis of rotation is constantly moving. In the ideal case the movements within both TMJs result in a maximal range of mouth opening with minimal stress placed on the articular surfaces. It is likely not possible to define a single rotation-to-translation ratio that describes the kinematics of the TMJ during opening and closing of the mouth. This ratio varies based on natural intersubject variability in movement strategy and on overall cranial-dental anatomy, including the shape of the articular discs and articular surfaces. The **early phase**, constituting the first **35% to 50%** of the range of motion, involves primarily rotation of the mandible relative to the cranium. As depicted in, the condyle rolls posteriorly within the concave inferior surface of the disc. (The direction of the roll is described relative to the rotation of a point on the ramus of the mandible). The **rolling motion** swings the body of the mandible inferiorly and posteriorly. The axis of rotation for this motion is not fixed but migrates within the vicinity of the mandibular neck and condyle. The rolling motion of the condyle stretches the oblique portion of the lateral ligament, which may help initiate the late phase of opening the mouth. The **late phase of opening** the mouth consists of the final **50% to 65%** of the total range of motion. This phase is marked by a gradual transition from primary rotation to primary translation. This transition can be readily appreciated by palpating the condyle of the mandible during the full opening of the mouth. The full amount of translation is large, about 1.5 to 2 cm in the adult. **During the translation**, the condyle and disc slide together in a forward and inferior direction against the slope of the articular eminence. At the end of opening, the axis of rotation shifts inferiorly. The exact point of the axis is difficult to define because it depends on the person's unique rotation-totranslation ratio. **At the later phase of opening**, the axis is usually located below the neck of the mandible. Full opening of the mouth maximally stretches and pulls the disc anteriorly. The extent of the forward translation (protrusion) is limited, in part, by tension in the stretched, elastic superior retrodiscal lamina. The intermediate region of the disc translates forward while remaining between the superior aspect of the condyle and the articular eminence. This placement of the disc maximizes joint congruency and reduces intra-articular stress. The arthrokinematics of **closing the mouth** occur in the reverse order of that described for opening. When the mouth is fully opened and prepared to close, tension in the superior retrodiscal lamina starts to retract the disc, helping to initiate the early translational phase of closing. The later phase is dominated by rotation of the condyle within the concavity of the disc, terminated when contact is made between the upper and lower teeth.^3^ Although the muscular action is not completely understood or agreed upon, the superior head of the lateral pterygoid is likely active eccentrically during closing of the mouth. Eccentric activation exerts a forward tension on the disc and neck of the mandible.^3^  **MASTICATION** Chewing is a complex rhythm of mandibular movement, powered by coordinated activity of the muscles of mastication,facial expression, and tongue.^4^ Mandibular Motion during Chewing: A single chewing stroke consists of one loop of mandibular depression, lateral deviation, and elevation. A frontal plane view reveals that the mandible typically follows a path along the midline of the body during depression. As elevation begins, the path of the mandible deviates laterally and returns to midline as mandibular depression begins again. In the rest position, the upper row of teeth typically does not contact the lower teeth. When the mandible is elevated in the sagittal plane from this position, the teeth of the lower row make only slight contact with the upper row because the mandibular teeth are arranged in a narrower arc than the maxillary teeth. To maximize contact between upper and lower teeth, a necessity to grind food, the mandible deviates laterally as it elevates to the maxillary teeth. there are two distinct phases of food preparation by the teeth. The crushing phase occurs as the food is compressed between the maxillary teeth and the teeth on the elevating mandible. This phase ends with maximum mandibular elevation. When elevation is complete, contact between the rows of teeth persists as the teeth slide on each other to achieve the intercuspal position in which contact between the molars on one side of the mouth is maximal. The gliding between the rows of teeth constitutes the grinding phase of mastication. This phase is characterized by plane motion of the mandible, with little or no additional elevation.^4^ **Muscle Activity during Mastication** The act of chewing typically occurs on one side of the mouth at a time. The side on which the actual chewing occurs is known as the working side, while the opposite side is known as the balancing side. These functions are to Move the mandible in the masticatory path Stabilize the balancing side of the mandible Maintain appropriate alignment between the disc and the mandibular condyle Control the location of the food to optimize mastication same time, the infrahyoid muscles contract, fixing the hyoid bone. As a result, the suprahyoid muscles contribute to mouth opening. Also during the opening phase of mastication, the lateral pterygoid muscle contracts, particularly the inferior head, producing the anterior translation of the mandible that accompanies mandibular depression. Mandibular depression and protrusion are followed by lateral deviation, elevation, and retrusion of the mandible for crushing and grinding. These motions occur with the mandibular elevators, the masseter, medial pterygoid, and temporalis muscles as well as the lateral pterygoid. Lateral deviation occurs with contraction of the ipsilateral masseter and temporalis and the contralateral medial and lateral pterygoid. The temporalis also produces retrusion. The crushing phase consists of active mandibular elevation and, therefore, the muscle contractions within this phase are primarily concentric. Grinding occurs with little or no additional elevation, so contraction of the mandibular elevators during this phase is primarily isometric. The moment arms for the mandibular elevators increase as the mouth moves from the open toward the closed position. The moment arms are maximum at approximately the point at which the mandible is positioned to grind the food, thus optimizing the moments the muscles can generate to chew the food.^4^ the motions of the TMJs during mastication and the muscles primarily responsible for these motions: Depression: digastric, mylohyoid, and geniohyoid muscles Protrusion: lateral pterygoid muscle Elevation: masseter, temporalis, and medial pterygoid muscles Lateral deviation: masseter and temporalis on the ipsilateral side and medial and lateral pterygoid on the contralateral side Retrusion: temporalis muscle **STABILIZATION OF THE BALANCING SIDE OF THE MANDIBLE** Forceful contraction of the mandibular elevators on the working side produces lateral deviation toward the working side and tends to produce a rotation of the mandible toward the chewing side about an anterior posterior axis. This rotation tends to distract the TMJ on the balancing side and to compress the TMJ on the working side. The mandibular elevators on the balancing side of the mandible contract with the contralateral elevators to stabilize the mandible during the crushing and grinding phases. The activity of the muscles on the balancing side adds to the bite force and also stabilizes the mandible to maintain the bite location on the teeth. At the same time the bolus on the teeth of the chewing side tends to distract the TMJ on the chewing (working) side and narrows the joint space on the opposite (balancing) side This tilt of the mandible helps to explain the large compressive forces that occur on the balancing side. The harder the food is to chew, the more compression occurs on the balancing side.^4^ **MAINTAIN APPROPRIATE ALIGNMENT BETWEEN** **THE DISC AND MANDIBULAR CONDYLE** In chewing, the mandible is cyclically opening and closing, requiring repeated anterior and posterior gliding of the mandibular condyle. The intraarticular disc also translates to stay with the head of the mandible and maximize congruencybetween mandible and temporal bone. The lateral pterygoid plays a critical role in stabilizing the disc and maintaining its alignment on the mandible as well as in protruding the mandible.^4^ **CONTROL FOOD LOCATION** Regardless of the integrity of the primary muscles of mastication, effective chewing requires that the food be located appropriately between the teeth during the crushing and grinding phases. In addition, the food, moistened by saliva, requires kneading to form a bolus that can be swallowed safely. The buccinator and the intrinsic and extrinsic muscles of the tongue perform this function in mastication. The buccinator is the only muscle that can regulate the lateral part of the cheek between the mandible and maxilla The buccinator is essential in preventing food from becoming trapped in the buccal space, between the cheek and the teeth.^4^ **TWO-DIMENSIONAL ANALYSIS** OF THE FORCES IN THE TMJ COMPLEX Although the TMJ exhibits motion through three planes, most of the motion of the mandible occurs in the sagittal plane. **Bite Force** The bite force is greatly influenced by the location of the bite. It is generally agreed that bite forces are greatest when the bite occurs close to the first molar and are least when it occurs at the incisors. The decrease in bite force during an incisal bite appears to result from a decrease in the muscles' mechanical advantage and probable inhibition of the temporalis muscle needed to maintain the protruded position. Reported peak bite forces on the molars range from approximately 500 N to almost 1,000 N (112--225 lb).^4^ Clinical Relevance **Joint Reaction Forces** the factors that influence the validity of the calculations, including the location, magnitude, and direction of the bite force as well as theassumptions made regarding the moment arm and crosssectional diameters of the active muscles. Actual calculations of peak joint reaction forces on a mandibular condyle are available from only a few studies and range from approximately 400 N to approximately 1,100 N (90--250 lb). Although it is generally accepted that the balancing side of the mandible sustains significant loads during bite, only one known study compares the loads on the balancing and working sides, suggesting that the balancing side of the temporomandibular complex sustains approximately twice the load sustained by the working or chewing side. Additional studies report that the joint space is narrower on the balancing side during mastication, supporting the view that the balancing side sustains more compression during chewing than the working side. Some studies examine the stress (force/area) applied to the mandible and intraarticular disc and report that during bite the anterior aspect of the mandibular condyle and neck sustain compressive loads while the posterior aspect and the articular surface of the temporal bone sustain both compressive and tensile loads. The intraarticular discreportedly sustains large stresses in the lateral aspect of the intermediate zone.^4^ **References** 1\. Dutton M. Dutton\'s orthopaedic: Examination, evaluation and intervention, fifth edition. 5th ed. Columbus, OH: McGraw-Hill Education; 2020:1267-69. 2\. Levangie PK, Norkin CC. Joint structure and function: A comprehensive analysis. 5th ed. F.A. Davis Company, 2014:217-21. 3\. Neumann DA. Kinesiology of the musculoskeletal system: Foundations for rehabilitation. 3rd ed. St. Louis, MO: Mosby, 2016:443-45,449-51. 4\. Oatis C. Kinesiology: The mechanics and pathomechanics of human movement. 2nd ed. Lippincott Williams & Wilkins; 2011:446-470.