Temporomandibular Joint Soft-Tissue Pathology, I: Disc Abnormalities

Temporomandibular Joint Soft-Tissue Pathology, I: Disc Abnormalities Francesco Molinari, MD,* Paolo Francesco Manicone, MD,† Luca Raffaelli, MD,† Renzo Raffaelli, MD,† Tommaso Pirronti, MD,* and Lorenzo Bonomo, MD*

The internal derangements are the most common noninﬂammatory abnormalities of the disc, observed even in asymptomatic subjects. Because the temporomandibular joint shows large adaptative and compensatory mechanisms over dysfunctional disc motion, these disorders may be asymptomatic or minimally evident for a long time. A careful clinical evaluation, reinforced by imaging ﬁndings, should help differentiate asymptomatic derangements from painful conditions that may require treatment. Semin Ultrasound CT MRI 28:192-204 © 2007 Elsevier Inc. All rights reserved.

he temporomandibular joint (TMJ) is a diarthrodial not compensated by the TMJ disc, the nonuniform distribu- Tjoint, formed by the squamous portion of the temporal tion of mechanical stress during jaw movements would be bone and the condyle of the mandible. These two osseous reflected directly over the articular surfaces, leading to carti- elements are enclosed into a fibrous capsule and articulate lage damage. Peak loads are normally absorbed by local de- with each other by an interposed disc of connective tissue formations of the disc, occurring in the contact areas with the (Figs. 1 and 2).1-3 The disc is fixed to the articular capsule and articular surfaces. As the movement of the TMJ proceeds, the lateral margins of the condyle. The joint cavity is therefore these deformations involve progressively different portions divided into an upper and a lower compartment. A synovial of the disc. The result is a dynamic structural adaptation that membrane lines the inner side of the capsule and disc, except spreads the mechanical stress over larger surfaces.5,6 The na- for the articulating surfaces. Synovial fluid produced by this ture and extent of this adaptative response depend on the lining membrane fills the joint compartments. biomechanical properties of the disc and the forces acting on The TMJ is responsible for all movements of the jaw, which its molecular structure. take place in different orthogonal planes and around multiple Several investigators have reported that the local concen- axes of rotation. In these movements, the articular disc plays tration of collagen, elastin and proteoglycans, and the orien- a major role in compensating the incongruities of the articu- tation of these molecules into the extracellular matrix vary in lar surfaces. In the mandibular opening-closing movement, the different portions of the disc.7-9 Because of the inhomo- for instance, the condylar head rotates and translates relative geneous distribution of macromolecules and fluid in the con- to the temporal bone with a simultaneous gliding of the disc nective matrix, the disc expresses region-specific viscoelastic (Fig. 3). Several theories have been proposed to explain the properties.10 Stiffness, strength, permeability, and other mechanism that coordinates the disc-condyle complex dur- physical properties vary from the anterior to the posterior 4 ing jaw movements. The biomechanical properties of the band and in both lateral zones. Thus, each region of the disc 5 disc are largely involved in this coordination. responds to mechanical loading with a specific type of local deformation. These plastic changes are considered adaptative Biomechanical Properties of the Disc responses to the different mechanical stresses experienced Mechanical loading of the TMJ occurs over highly incongru- locally (ie, compression, tension, or shear). They also ensure ent articular surfaces and limited contact areas. If loading was the physiological coordination of the disc-condyle complex during jaw movements. The equilibrium between mechanical forces acting in the *Department of Bioimaging and Radiological Sciences, Catholic University TMJ and adaptative responses of the disc may change during of Rome, Italy. life. Mechanical loading has a continuous influence on the †Institute of Clinical Dentistry, Catholic University of Rome, Italy. Address reprint requests to: Francesco Molinari, MD, Department of Bioim- composition and behavior of the disc. Although the mecha- aging and Radiological Sciences, Catholic University of Rome, L.go F. nism is not clear yet, a modulation of the synthesis of con- Vito n. 1, 00168 Rome, Italy. E-mail: [email protected]. nective tissue has been suggested.11-15 Similarly, age-related

Figure 1 Schematic (A, C) and magnetic resonance T2*-weighted (B, D) images of the temporomandibular joint (TMJ), in the coronal (A, B) and sagittal (C, D) view. Two osseous elements form the TMJ: the squamous portion of the temporal bone (GF and E) and the condylar head of the mandible (Co). An articular capsule (JC) attaches to the condyle and temporal bone. In the closed-mouth position, the condyle (Co) is housed in the bony roof of the glenoid fossa (GF). The articular eminence (E) forms the anterior wall of the glenoid fossa and represents the load-bearing portion of the joint during function. The entire articular surface of the temporal bone (GF and E) is saddle shaped with a variable degree of convexity and concavity. The condylar head may also have a significant variation in size and form, with a rounded to flattened superior surface. An articular disc (*) is interposed between the articular surfaces of the TMJ. In the coronal view (A, B), the inferior, concave surface of the disc matches the articular surface of the condylar head. The superior surface of the disc is slightly convex, fitting the concave surface of the articular fossa. The articular disc is firmly attached to the medial and lateral poles of the condyle by the collateral, disco-mandibular ligaments (DML). These attachments increase the medial-lateral stability of the disc during condylar movements. Similarly, connective fibers inserted between the anterior and posterior margins of the disc and the capsule (1, 2) participate actively in stabilizing the disc-condyle complex during function. (Color version of figure is available online.)

changes of the mineral and macromolecular components of cur even on the osseous elements of the TMJ, because of the disc cause variations of its viscoelastic properties.16-19 uncompensated loading. Trauma and other pathological conditions usually pro- Such interactions and reciprocal influences, mediated by me- duce more extensive structural changes, involving also other chanical loading, are common among the components of the tissues of the TMJ. If these pathological changes do not in- TMJ and may reach considerable complexity. In fact, concurrent volve primarily the disc, they may still have influences on its with an altered disc function, histological changes have been biomechanical behavior.20 For instance, a pathologic agent observed in other elements of the TMJ. These changes will be may alter the molecular composition and the properties of mentioned in dealing with disc abnormalities. the articular cartilage, increasing the roughness of the articular surface.21 The higher shear stress experienced locally Disc Abnormalities: Internal Derangements may induce changes on the viscoelastic properties of the disc. This adaptative response compensates the new loading con- Definition and Pathophysiology ditions. However, if mechanical stress overcomes the adap- The internal derangements are the most common noninflam- tative capabilities of the disc, degenerative changes may oc- matory abnormalities of the TMJ, being observed even in 194 F. Molinari et al.

Figure 2 Schematic (A, C) and magnetic resonance T2*-weighted (B, D, E) images of the articular disc of the temporomandibular joint in the sagittal view, in closed- (A, B, D) and open-mouth position (E). The articular disc or meniscus is a biconcave (bow tie) or sigmoid-shaped structure, made up of fibrous connective tissue with varying amounts of fibrocartilage, interposed between the temporal bone and the mandible (see also Fig. 1). The disc is divided into three parts: an anterior band (AB), a thinner avascular intermediate zone (IZ), and a posterior band (PB). The thickness of the disc in these three parts differs in the medial-lateral direction, according to a differential loading pattern. Site-specific mechanical loads also influence the molecular composition of the disc (the distribution of proteoglycans, cartilage cells, etc.). The AB prolongs anteriorly into connective fibers (AP), which insert on the anterior margin of the preglenoid plane. Medially, some of the fibers of the AP are continuous with the superior head of the lateral pterygoid muscle and probably with the fibers of the masseter and temporal muscles. The PB continues posteriorly into a richly innervated and vascularized attachment, the retrodiscal tissue (RT). The RT is a fibroelastic structure covered by a synovial layer and can be divided into temporal, intermediate, and condylar layers or laminae. The temporal lamina or superior retrodiscal lamina (1) attaches to the most posterior areas of the glenoid fossa. The inferior lamina (2) is attached to the posterior region of condylar neck. Between these two laminae is a loose connective tissue (intermediate layer, 3) innervated by sensory fibers. The shape of the disc and its contact areas with the articular surfaces continuously change during jaw movement (C) to adapt to the articular surfaces of the mandible and temporal bone. The volume of the posterior attachment increases significantly when the disc-condyle complex moves anteriorly (D and E), because the stretching of the elastic fibers promotes the inflow of blood into the vessels-rich intermediate layer of the RT. (Color version of figure is available online.) asymptomatic subjects. The term derangement refers to an Degenerative changes of the TMJ may also occur because of alteration in the normal pathways of motion of the TMJ that mechanical derangements of the disc. largely involves the function of the articular disc. Therefore, Many etiologic factors have been proposed to explain disc these alterations have been also referred to as disc derange- derangements. Traumatic events may cause stretching, tear- ments. They differ from degeneration because the quality and ing, or rupture of the disc, lateral ligament, or capsule. When structure of the TMJ tissues are not necessarily altered. The bleeding occurs, fibrotic or hyperplastic intra-articular reac- inflammatory and degenerative disorders of the joint are clas- tions may lead to restricted mobility and pain.26 Less obvious sified in a different pathological group (ie, osteoarthritis). injuries of the TMJ may also cause soft-tissue responses and However, much overlap in the clinical course of these two lead to permanent intra-articular changes, with long-term disorders has been reported.22 The articular surfaces may be effects on the disc function. However, several studies have interested by degeneration with a normally located disc.23-25 failed to confirm these theories.27,28 TMJ soft-tissue pathology 195

Figure 3 Schematic representation of the TMJ during physiologic opening-closing movement of the mandible (from a to l). Jaw opening occurs with a rotation and anterior translation of the condyle of the mandible (a to c). The condyle and disc move forward simultaneously (disc-condyle complex). At maximal jaw opening (g), the condyle reaches the preglenoid plane, beyond the apex of the articular eminence. In this position, the condylar surface contacts the anterior band and the intermediate zone of the disc, whereas the posterior band is stretched over the articular eminence. At the same time, the temporal layer of the retrodiscal tissue is pressed against the fossa, whereas the condylar layer loosens (e to i). During jaw closing (h to l), the movement of the disc-condyle complex is the exact reverse of the opening one. The lateral movements of the mandible are rare during chewing. They are detected more frequently during parafunction (eg, tooth grinding).

A laxity of the ligaments of the TMJ has also been related to ments.38 Morphologic changes of LPM, such as hypertrophy, disc derangements.29-31 However, the prevalence in the gen- atrophy, or contracture, have been found in patients with eral population of disc derangements appears not to match anterior disc displacement without reduction.39 A higher with that of joint laxity. Controversial data have been also propensity toward anterior disc displacement has also been published on this issue.32 found in subjects in which the LPM attaches to the disc but Bruxism has been also reported as a potential cause of disc not to the condyle.40 derangements, since compressive overloading may alter the Independently from the etiologic factors and concurrent connective tissue of the TMJ.33,34 Bruxism is also frequently unfavorable conditions, disc derangements occur when the associated with various temporomandibular disorders. How- function of the disc is compromised. An initial adaptative ever, a causal relationship between this nonphysiologic be- response, triggered by overloading, induces structural havior and disc derangements has not yet been proven.35 changes in the TMJ. This is a slow but continuous process of Changes in the composition of the synovial fluid may increase modeling that involves all the elements of the TMJ, within the intra-articular friction, leading to unstable disc motion.36,37 tissue-specific limits. Although physiological changes occur These biochemical changes may also affect the joint lubrication in the disc, its ability to remodel is lower than that of other and nutritional requirements of the articular surfaces. tissues of the TMJ, such as the capsule, capsular ligaments, An improper activity of the lateral pterygoid muscle (LPM) and retrodiscal tissues.41 Decreased vascularity and extensive during TMJ motion has been also related to disc derange- fibrous transformation have been reported in the retrodiscal 196 F. Molinari et al.

Figure 4 Disc derangements: schematic representation of the normal position of the articular disc (A to D) and main directions of dislocation (E to H). The images refer to the right TMJ in rest position (closed-mouth). In the sagittal view (A), the posterior band of the disc is normally located within a 10-degree angle counterclockwise from the vertical line passing through the axis of rotation of the condylar head. Anterior displacement (E) should be considered when the posterior band locates anteriorly to the area enclosed in this 10-degree-angle range. Posterior displacement (F) occurs when, in the sagittal view, the posterior band falls into a 10-degree angle clockwise from the vertical line. Lateral or medial displacements occur when, in the coronal view, the corresponding lateral or medial attachments of the disc and its borders bulge from the borders of the condylar head (G and H, from the normal position in C and D). These unidirectional displacements are often associated. The posterior displacement is rare either unidirectionally or as a part of a multidirectional displacement. The oblique orientation of the lateral pterygoid muscle and the angulation of the condyle tend to direct most meniscal displacements in an anterior-medial path.

tissue for continuous compression and shear.42-44 These ad- Classiﬁcation and Clinical Course aptative changes can also have mechanical implications on Different criteria have been used to classify disc derange- the behavior of the articular disc. However, as long as the ments. A common approach, used in reporting magnetic res- system preserves the ability to adapt to the new functional onance imaging examinations of the TMJ, is to assess the status, the altered mechanical loading is compensated by the direction of disc displacement, which may be anterior, me- structural modeling of the TMJ. Although the coordination of dial, lateral, or even posterior (Fig. 4). Multidirectional dis- the disc-condyle complex may be lost, in this stage the pa- placements are considered more frequently than unidirec- tient is usually asymptomatic. tional ones (Fig. 5). Posterior derangements are rare.45,46 The The exposure to excessive or prolonged loading may over- oblique orientation of the lateral pterygoid muscle and the come the biomechanical limits of the TMJ. If modeling is angulation of the condyle direct most meniscal displace- unable to restore a functional equilibrium, the tissues of the ments in an anterior-medial path. TMJ can still show compensatory mechanisms to prevent or Anterior disc derangements are grouped into four cate- limit potential damages. When the reserve of adaptative and gories based on the degree of dislocation, reversibility dur- compensatory responses is exhausted, the changes taking ing the opening-closing movement, and changes in disc place in the TMJ are known as regressive modeling (ie, mal- shape (Table 1 and Figs. 6 and 7). In the early stage, joint adaptation).3 At this stage, decompensated and destructive noise or dysfunction is not evident. However, at mouth morphologic changes are usually revealed with pain and opening, the patients may feel a slight catching sensation. other clinically evident signs and symptoms. With time, the This may be the earliest sign that a change in the frictional clinical manifestations may become less obvious because a properties of the joint has occurred. Therefore, this stage late-stage response allows for functioning, even when the of derangement has been referred to as TMJ disc incoor- TMJ is altered. dination. TMJ soft-tissue pathology 197

Figure 5 Anterior-lateral displacement demonstrated by magnetic resonance fast spin-echo DP-weighted images in the sagittal (A to E) and coronal (F to L) planes in closed-mouth position. Sagittal sections in the ﬁrst line show the displacement of the articular disc, which bulges anteriorly and laterally against the joint capsule (black arrowheads). The lateral component of the displacement is better recognized in the coronal sections in the second line (white arrowheads). The morphology of the disc is irregular and wavy. The disc also shows a tendency to bend and fold over the lateral aspect of the joint capsule (B and I). Multidirectional displacements are more common than unidirectional ones. A careful analysis of all sagittal and coronal images of the MR examination is mandatory to deﬁne the direction of disc derangement.

In the next stage, the articular disc has slipped forward in an only condylar rotation is allowed. On the other hand, adher- anterior-medial position. In rest position and in centric occlu- ences also limit the mobility of the disc (ie, “stuck,” “fixed,” or sion, the posterior band of the disc is located behind the apex of “frozen disc”).38 In addition, late-stage changes in disc morpho- the condylar head. Mouth opening occurs with a clicking or logic and magnetic resonance signal become more evident. A popping sound, because the posterior band of the disc slips back biconvex, rounded, irregular, or flat disc usually indicates more over the condylar head. As a result, in the open-mouth position advanced disease.47-49 Tear and perforations of the disc may also the intermediate zone of the disc will be placed correctly be- occur.38 tween the condylar head and the eminence of the temporal The progression from the subclinical or clinical manifestation bone. Because the opening movement relocates the disc in the towards the late stages of disc derangements has been confirmed joint, this stage is referred to as disc displacement with reduc- by retrospective and cross-sectional studies.50,51 Lundh et al52 tion. Occasionally, a second clicking sound is heard during reported that 9% of reducing disc derangements progressed to mouth closure (“reciprocal click”), because the posterior band of nonreducing ones within 3 years. However, reducing disc dis- the disc slips forward off the condyle. Other clicking sounds can placements can also remain constant for many years, suggesting also be produced by irregularities or defects in the surface of the that the clicking joint does not necessarily progress to locking disc or by changes in the convexity of the condyle and/or artic- derangement.52-56 ular eminence. These sounds are usually less obvious than those The morphologic and functional changes of the TMJ usu- caused by anterior disc displacement. They are also found at the ally correlate well with the clinical signs and symptoms that same point of the TMJ translator movement rather than at dif- characterize each stage. However, because altered loading ferent points, as occurs with reciprocal clicking. induces structural changes in the TMJ tissues (ie, remodel- In the third category of internal derangement, a greater ing), large inconsistencies may also be found between imag- degree of anterior displacement of the disc is found. The disc ing findings and clinical manifestations of disc derangement also acts as an obstacle, preventing the condyle to overcome (Fig. 8). the posterior band when mouth opening is attempted. In this condition the joint appears as “locked.” Clicking sounds are not heard. This stage is referred to as disc displacement with- Clinical Assessment of out reduction or closed lock. Temporomandibular Disorders The fourth category is also characterized by a limitation of and Diagnostic Protocols mouth opening. However this limitation may not be caused by disc displacement. The disc may be in a normal position but Disc derangements may have clinical manifestations sim- advanced degenerative changes have occurred. Adherences are ilar to those of other disorders of the facial region. Pain and usually found with the disc and the articular eminence, so that jaw dysfunction can be related to toothache, pericoronitis, 198 F. Molinari et al.

Table 1 Classification of Disc Derangements Clinical Stages Type Clinical Hallmarks Imaging Features (MRI) Stage I Incoordination ● Catching sensation during mouth opening ● None (incoordination) ● No joint noise ● No pain, joint tenderness ● No opening limitation Stage II Displacement with ● Asymptomatic except for the clicking or ● Disc displacement in (intermittent reduction popping sound centric occlusion locking) ● Episodes of limited mouth opening that last ● Normally located disc in for various lengths of time open-mouth position ● In the late stage, presence of intermittent locking ● Joint pain with increasing function ● Association with joint tenderness on lateral palpation ● Lateral deviation of the mandible in unilateral condition ● Headache and possible muscle pain related to protective splinting of the mandible ● “Hitting an obstruction” when opening is attempted ● Obstruction may disappear spontaneously or manipulating the mandible beyond the interference Stage III Displacement ● Clicking noises disappear ● Disc displacement in (closed lock) without ● Referred history of clicking or popping both centric occlusion reduction ● Inability to open mouth widely and maximal open-mouth ● Localized pain in the TMJ increasing with positions attempted mouth opening and chewing ● Limited condylar ● Joint tenderness on lateral palpation translation ● Deviation of the mandible to the affected ● Morphological side with mouth opening pathologic changes of disc (rounded, irregular disc, etc.) Stage IV (disc Stretched ● Restricted mouth opening ● Stuck or fixed disc adhesion) retrodiscal ● Absence of pain unless in voluntary mouth ● Disc perforation (difficult tissue loses its opening or chewing with stretching of joint to assess) elasticity, thins, capsule and perforates. Progression to osteoarthrosis MRI, magnetic resonance imaging.

maxillary sinusitis, earache, salivary gland pathosis, tem- muscle stiffness; reduced motion of the mandible).57 In poral arteritis, neuralgias, and tension-type headache. All the same group (MPD) are included regional problems of these conditions should be excluded when assessing pa- the muscles, such as myositis, myospasm, local myalgia, tients with clinical suspicion of disc derangements. In ad- myofibrotic contracture, as well as the centrally mediated dition, two other temporomandibular disorders (TMDs) chronic muscle pain. Systemic disorders, such as fibromy- must be considered in the differential diagnosis of symp- algia, may also have considerable overlap in clinical fea- tomatic disc derangements: myofascial pain and dysfunc- tures with MPD.58 tion (MPD), and painful inflammatory or degenerative The diagnosis of painful conditions of the temporoman- conditions of the TMJ. dibular region requires a careful evaluation of the history MPD differs from primary TMJ disorders, because in the of the patient (dental, medical, and psycho-social data) former pain originates from the masticatory muscles. Myo- and a detailed examination of signs and symptoms. Clin- fascial pain is the most common TMD. It is characterized ical assessment should be always performed before imag- by a dull ache in the TMJ region that increases during ing (Fig. 9A to F). If the most important symptom reported function, with other possible ancillary findings (ie, ten- by the patient is pain, its characteristics should be assessed sion-type headache, earache, or toothache; a sensation of as part of a routine diagnostic protocol. When the loca- TMJ soft-tissue pathology 199

Figure 6 Schematic representation of disc position and morphology during the stages of anterior derangement. In a normal subject (A to C), the disc adapts to the articular surfaces of the TMJ, and it glides during jaw opening between the temporal bone and the condylar head. Simultaneously, the condylar head rotates and translates in the anterior direction. The intermediate zone maintains a consistent relationship with the condyle and temporal eminence. Stage 2, open or intermittent lock (D to F). Anterior disc displacement is found in the closed-mouth position. Posterior band reduces behind the condyle on partial opening and appears normal at full open-mouth position. Stage 3-4, closed lock (G to I). Anterior disc displacement does not disappear during jaw opening. Severe anterior displacement of disc prevents forward and downward motion of mandible. Stage 1 is not displayed in the drawing (see the text for further details). tion, intensity, quality, duration, modifiers, chronicity of remains largely unknown. Therefore, therapy is largely de- pain, and associated symptoms suggest a potential masti- pendent on the initial clinical assessment of the patient. catory muscle disorder, a panoramic radiograph should be When signs and symptoms are correctly interpreted, the ap- first obtained to exclude possible dental, periodontal, or plication of research-based therapeutic guidelines can lead to other problems of the oral region. If the patient’s history treatment success. According to literature, a positive out- and clinical findings suggest an intracapsular joint prob- come can be achieved by therapy in 75 to 90% of pa- lem, the assessment of the TMJ should be performed using tients.59-64 MRI. Such an imaging tool can be used concurrently to A wide consensus has been reached through the years on exclude some causes of muscular problems (ie, focal my- considering conservative and reversible approaches as first-line ositis, abscess, muscle atrophy, etc.) and local diseases of therapy of symptomatic disc derangement.65-69 Included in this the oral region, providing helpful information in the dif- group are various medications, such as nonsteroidal anti-inflam- 38 ferential diagnosis of TMDs. matory drugs and muscle relaxant, oral appliances, home care procedures, and cognitive-behavioral information program. When these approaches fail to produce clinical improvements Therapeutic Outlines on painful dysfunctional conditions of the TMJ, surgical proce- Although genetic, biochemical, and histological aspects of dures (ie, arthrocentesis, arthroscopic surgery, discoplasty, and the TMDs have been studied, the etiology of these disorders discectomy) may be indicated. Several case-based studies have 200 F. Molinari et al. TMJ soft-tissue pathology 201

Figure 8 Example from a patient with inconsistent clinical and MRI findings of anterior disc derangement. In the closed-mouth position (A), the T2*-weighted MR image suggests anterior displacement of the articular disc, whose posterior band is positioned beyond the condylar head (large arrow). In the open-mouth position (B to C), the two spatially adjacent T2*-weighted MR images confirm a nonreducing disc displacement, as the disc is located anteriorly to the articular eminence and the condylar head (small arrows). The shape of the disc is deeply altered and its structure is twisted. Concurrently, the roundness of the condylar head is reduced and, just below its articular surface, MR signal alterations can be found. Based on these MRI findings, a stage IV disc derangement might be suspected, because the displacement was not reduced by jaw opening. However, the patient had been clinically classified as a stage II disc derangement, because he was asymptomatic and had referred only occasional episodes of limited mouth opening and intermittent locking. Physical examination had shown normal opening range of the mouth. This may be sensed from the MR images in the open-mouth position (B, C) as the condylar head appears beyond the articular eminence. These clinical-radiological inconsistencies can be explained by the ability of the TMJ tissues to remodel under unfavorable loading conditions (see the text for further details). shown that surgical management may be effective. However, to repositioning appliance has deeper effects on the position increase the chances of treatment success in patients not respon- of maximum intercuspation and may cause a permanent sive to conservative therapy, the least invasive procedure that occlusal change.71,72 The theory behind the use of this can be effective should be first proposed. appliance is to shift the mandibular condyles in a more Oral appliances (Fig. 9G and H) are used in patients forward position and, therefore, to allow for recapturing of with symptomatic disc derangement for stabilizing or re- a displaced disc.73 This appliance is often used in patients positioning the occlusal maxillomandibular relationship. with painful clicking or intermittent locking. However, The stabilizing approach is reversible. It produces changes the results of this strategy may not be as good as expected. in the occlusal behavior of the patient, by inducing con- Long-term case report studies have suggested that TMJ sciousness of any oral parafunction. Therefore, it reduces clicking may recur over time and, even without audible the chance of further wear, chipping, or cracking of the joint sounds, disc position can remain altered.74-77 teeth. In unstable occlusion caused by absence of multiple posterior contacts bilaterally, stabilization restores the au- Conclusions tomatism of normal occlusion. It is also indicated in patients suffering from bruxism and for managing symptoms Under nonphysiologic loading, the various structures of the associated with TMJ disc derangement.70 The mandibular TMJ show adaptative and compensatory responses to prevent

Figure 7 Fast spin-echo DP-weighted MR images in closed (B, D, F) and open (A, C, E) mouth position, from three subjects. Images in the first line indicate the normal position of the articular disc in an asymptomatic subject (A, B). Images in the second line (C, D) were obtained from a nurse exposed to face trauma at work. Immediately after the accident, she reported lateral deviation of the jaw and transient difficulty in mouth opening. Reciprocal clicking sounds from the TMJ of the affected side occurred after she forced the jaw open. The MR examination revealed anterior displacement of the articular disc in centric occlusion (D), with reduction in full open-mouth position (C). Images in the third line (E, F) are relative to a young woman with a 2-month history of bilateral TMJ pain and limited mouth opening, after repeated episodes of jaw blockage in open-mouth position. In resting position (F), the articular disc appears dislocated anteriorly. The superior contour of the disc is also slightly concave, retaining a small fluid collection in the upper joint compartment. In the open-mouth position (E), the condylar head remains almost stuck in the glenoid fossa, suggesting a limitation in the forward translation of the condyle itself. The disc is bent inferiorly. The fluid in the upper joint cavity collects into the increased concavity. The condylar head does not pass over the posterior band of the disc, which acts as an obstacle to the movement of the TMJ. 202 F. Molinari et al.

Figure 9 Phases of clinical examination of a patient with suspect of a TMJ disorder (A to F) and conservative therapeutic planning with oral appliance on the maxillary arch (G to H). In each patient, face symmetry (A) should be first evaluated, noting developmental abnormalities, trauma, and swelling of external tissues. Similarly, the symmetry of the opening movement of the mouth (B) should be assessed. Side deviations at jaw opening may suggest unilateral disc derangements. The range of motion in the vertical plane (C) in normal subjects is 40 Ϯ 3 mm for men and slightly less for women. If the interincisal distance differs largely between maximum-unassisted opening and maximum-assisted opening, a muscle based limitation should be suspected. In the horizontal plane, lateral and protrusive jaw movement and their possible limitations should be assessed. Joint sounds should be evaluated with palpation (D) and auscultation (F) during jaw movements. However, clicking is unspecific, being heard also in asymptomatic subjects. Muscle palpation, performed both from common external approach (D) and from intraoral manipulation (E), might reveal muscle tension and alterations of muscular trophism. Finally, intraoral examination should consider possible odon- togenic causes of patient’s orofacial pain or parafunctional habits that have special relevance to myofascial pain. Oral appliance of the upper maxillary bone (G, H). (Color version of figure is available online.) permanent tissue damages and allow for function. Decom- 5. Tanaka E, van Eijden T: Biomechanical behavior of the temporoman- pensation and degenerative changes may occur when TMJ dibular joint disc. Crit Rev Oral Biol Med 14:138-150, 2003 6. Scapino RP, Canham PB, Finlay HM, et al: The behaviour of collagen modeling is unable to restore a biomechanical equilibrium. fibres in stress relaxation and stress distribution in the jaw-joint disc of Disc derangements may lead to altered loading and regressive rabbits. Arch Oral Biol 41:1039-1052, 1996 changes in the TMJ. However, because of the adaptative ca- 7. Teng S, Xu Y, Cheng M, et al: Biomechanical properties and collagen pacity of the joint, these changes may be asymptomatic or fiber orientation of temporomandibular joint discs in dogs: 2. Tensile minimally evident for a long time. A careful clinical evalua- mechanical properties of the discs. J Craniomandib Disord 5:107-114, 1991 tion, reinforced by imaging findings, should help distinguish 8. Mills DK, Fiandaca DJ, Scapino RP: Morphologic, microscopic, and asymptomatic disc derangements from pathologic conditions immunohistochemical investigations into the function of the primate that may require treatment. TMJ disc. J Orofac Pain 8:136-154, 1994 9. Minarelli AM, Del Santo M Jr, Liberti EA: The structure of the human temporomandibular joint disc: a scanning electron microscopy study. J References Orofac Pain 11:95-100, 1997 1. Haskin CL, Milam SB, Cameron IL: Pathogenesis of degenerative joint 10. del Pozo R, Tanaka E, Tanaka M, et al: The regional difference of disease in the human temporomandibular joint. Crit Rev Oral Biol Med viscoelastic property of bovine temporomandibular joint disc in com- 6:248-27, 1995 pressive stress-relaxation. Med Eng Phys 24:165-71, 2002 2. Oberg T, Carlsson GE, Fajers CM: The temporomandibular joint. A 11. Carvalho RS, Yen EH, Suga DM: Glycosaminoglycan synthesis in the rat morphologic study on a human autopsy material. Acta Odontol Scand articular disk in response to mechanical stress. Am J Orthod Dentofa- 29:349-34, 1971 cial Orthop 107:401-410, 1995 3. Laskin DM, Greene CS, Hylander WL: Temporomandibular Disorders: 12. Sindelar BJ, Evanko SP, Alonzo T, et al: Effects of intraoral splint wear An Evidence-Based Approach to diagnosis and treatment. Hanover on proteoglycans in the temporomandibular joint disc. Arch Biochem Park, IL, Quintessence Publishing Co., Inc., 2006 Biophys 379:64-70, 2000 4. Scapino R: Morphology and mechanism of the jaw joint, in McNeill C 13. Kuboki T, Shinoda M, Orsini MG, et al: Viscoelastic properties of the (ed). Science and Practice of Occlusion. Chicago, IL, Quintessence pig temporomandibular joint articular soft tissues of the condyle and Publishing Co., Inc., 1997, pp 23-40 disc. J Dent Res 76:1760-1769, 1997 TMJ soft-tissue pathology 203

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