Temporomandibular Joint (TMJ) Anatomy
The Temporomandibular Joint (TMJ) is a ginglymoarthroidial joint, which is also diarthroidal, in that it translates and rotates throughout function. It is comprised of the following structural components:
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The posterior slope of the eminentia
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The mandibular condyle
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The articular disc
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The synovial tissue
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The vascular and innervated retrodiscal tissue, which includes the posterior attachment of the articular disc to both the mandible, and to the temporal bone
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The posterior attachment tissue, which is vascular and innervated
The articular disc is a biconcave fibro-cartilaginous capsule that normally rests between the head of the mandibular condyle and the articular eminence of the temporal bone. The disc is attached to the lateral and medial poles of the condyle, while also being elastically attached posteriorly, to both the temporal bone and the mandible. Anteriorly, the disc is attached to a few fibers from the superior belly of the lateral pterygoid muscle, which assists in translating both the disc and the condyle forward during mandibular excursions (Figure 1).
Figure 1. A lateral cross-sectional view through the right Temporomandibular Joint showing: A-posterior slope of eminentia with typical convex contour; B-mandibular condyle; C- biconcave articular disc fitted between a convex eminentia and a convex condylar head; D-synovial tissue; E-retrodiscal tissue including the posterior attachment of articular disc and temporal bone; F-posterior attachment tissue which is innervated and vascular; G-superior belly of lateral pterygoid muscle with fibers attaching to medial aspect of articular disc; H-inferior belly of lateral pterygoid muscle attaching to condyle.
Through normal excursive movements of the mandible, the Temporomandibular Joint is required to both rotate and translate. A healthy joint will perform these without giving off any sounds or vibrations throughout its entire Range of Motion (Ishigaki, Bessette, & Maruyama, 1993).
In a healthy occlusion, during function there exists a synergistic coordination in the firing and relaxation of the opposing elevator and depressor muscles (Higashi, 1989). Masticatory muscle coordination results from the sensory input of the teeth, the muscles themselves, the Periodontal Ligaments, the tongue, and the oral soft tissues. The input from these organs creates a pattern of muscle activity, or muscle engrams, that are founded within a pool of neurons located within the brainstem’s, Central Pattern Generator (CPG) (Lund, 1991; Levy, 2009). If there is any malocclusion, it will reduce the coordinated firing of these muscles (Higashi, 1989) causing the vectoring of forces upon the posterior aspect of the disc (Radu, Marandici & Hottel, 2004; Wang, Sun, Yu, Liu, Jiao, Wang, Liu, & He, 2012).
Any degradation of the posterior aspect of the disc eventually diminishes its normal, self-centering capability. This begins as disc laxity, which allows the disc to partially displace under function, and then possibly to fully displace, over time. When the articular disk reduces and then displaces during mandibular opening and closing movements, the disc gives off a vibration, which can be audible, that indicates there is a disruption of the normal, smooth motion of the structures within the Temporomandibular Joint (Findlay & Kilpatrick, 1960).
If the displaced condition deteriorates further over time, it is likely the disc will become permanently displaced, whereby undesirable forces from the condyle will be placed upon the disc’s posterior tissue attachment. This structure is the nutrient supply for the disk and synovial tissue, which is highly vascularized and innervated. Applied pressure from the condyle typically denervates and devascularizes this tissue over time, which then either forms a connective tissue “pseudo-disk”, or ultimately perforates. A perforation allows the condylar bone and temporal bone to have direct contact (Rohlin, Westesson, & Eriksson, 1985). This bone-to-bone contact will give off a distinctive vibration pattern that repeats each time the mandible opens and closes (Rohlin, Westesson & Eriksson, 1985).
Additionally, a chronic anteriorly displaced disc, as in a Piper Classification IVb, can place pressure on the branches of the inferior alveolar artery, which are the primary blood supply for the condylar head and mandibular bone marrow (Wysocki, Reymond, Krasucki, 2012). Restricted blood flow can then cause necrosis of the condylar head, and exacerbate structural Temporomandibular Joint degeneration.
Joint Vibration Analysis technology (JVA) is capable of detecting a variety of these internal Temporomandibular Joint anomalies, by using accelerometers that accurately measure the vibrations made by the disk and condyle during repeated mandibular opening and closing (Figures 2 and 3). The bilateral accelerometer design detects how much of a vibration’s energy transfers from the ipsilateral to the contralateral Temporomandibular Joint, which reveals to the clinician to which direction the disc is displacing. Less than 20 percent of the energy transferring to the opposite side indicates an anterior displacement, 20-60% Indicates an anterior-medial displacement, greater than 60% energy transfer indicates a medial displacement (Radke & Kull, 2012).
Figure 2. As the patient closes their mandible (A), the articular disc displaces anteriorly (B), which creates a vibration detected by Joint Vibration Analysis technology (C)
Figure 3. As the patient opens their mandible (A), the displaced articular disc reduces (B), which creates a compressed high amplitude waveform indicative of Disc Displacement with Reduction (DDR), as detected by JVA (C)
A normal Temporomandibular Joint does not create any sizable vibrations when operating under routine function (Mazzetto, Hotta, Carrasco, & Mazzetto, 2008). This generates a JVA trace without appreciable detected vibrations (Ishigaki, Bessette, & Maruyama, 1994). If there is dysfunction present within the TM Joint, vibrations will appear within the JVA trace. These detected vibrations have been extensively studied, to determine the companion structural deformity that each vibration represents, such that over the past few decades, these studies indicate that vibration patterns have distinct diagnostic value for the health or pathology of the Temporomandibular Joint structures (Huang, Lin, & Li, 2011; Ishigaki, Bessette, & Maruyama, 1993a; Ishigaki, Bessette, & Maruyama, 1993b; Olivieri, Garcia, Paiva, & Stevens, 1999; Goiato, Garcia, dos Santos, & Pesqueira, 2010; Owen, 1996; Christensen & Orloff, 1992).