Temporomandibular Disorder: Examining the Cause And Treatments
The muscles, nerves, and structures of the mouth and jaw play a significant role in daily life—eating, drinking, speaking, and so forth. Thus, when a patient experiences pain in this area, it can be especially debilitating.1
One of the most common causes of such pain and discomfort is temporomandibular disorder (TMD). It has been estimated that 25% of the general population has TMD—a group of pathologies that affect the masticatory muscles (myogenous) and/or the temporomandibular joint (TMJ) (arthrogenous).2 Disability related to this disorder also has been found to have a major influence on socioeconomic factors and is associated with decreased employment.3
This review examines the pathophysiology of TMD as well as the available treatment options, including pharmacotherapy, physical therapy, and osteopathic manipulative medicine (OMM).
Pathophysiology of TMD
The function of the temporomandibular area is deeply dependent on a balanced coordination of joint and myofascial components. There also are nociceptors present, which are very sensitive to any tip in this delicate balance that can cause the symptoms of TMD. These include deep and diffuse pain associated with jaw motion, headache, neck pain, earache, TMJ clicking, and limited jaw opening.4
Although the pathophysiology of TMD is still not well understood, the factors that lead to TMD pain are being elucidated.5 The primary cause of TMD pain often is related to joint degeneration, disk displacement, or inflammatory destruction due to a variety of conditions, including osteoarthritis and rheumatoid arthritis.6 The primary afferent nociceptors of the orofacial region are Aδ or C fibers.7 These nociceptors can be further subdivided into mechano-nociceptors (sensitive to mechanical stimuli), thermo-nociceptors (sensitive to heat or cold), and chemo-nociceptors (sensitive to chemicals).8 Noxious chemical and mechanical stimuli generally excite the TMJ, which suggests that the area is richly innervated by chemo- and mechano-nociceptors.9 These mechano-nociceptors fire with supranormal protrusion, lateral movement, and rotation of the jaw.10 This leads to the conclusion that TMD pain occurs because of excessive mechanical stress on the joint and related ligaments. Interestingly, individuals with significant anterior disk displacement still can have no evidence of pain.11
Biochemical influences play a large role in the transmission and modulation of TMD pain. In the periphery, there is a correlation between the expression of substance P and calcitonin gene-related peptide and patients with TMD pain.12 However, it is unclear whether this is a cause or an effect. The release of these pain-associated neuropeptides may be caused by the excessive stresses and in turn become the cause of the pain. Alternatively, these compounds, as potent vasodilators, may be a compensatory response to the relative hypoxia caused by the pain.13
In addition to these peripheral mechanisms, there also are biochemical influences within the central nervous system. In the brainstem, the primary afferent neurons terminate at the trigeminal spinal tract nucleus. This nucleus consists of three subnuclei: subnucleus oralis, subnucleus interpolaris, and subnucleus caudalis. The subnucleus caudalis serves as the main brainstem relay for nociception and is so structurally similar to the spinal dorsal horn, an important structure in spinal nociception, that it is often referred to as the trigeminal dorsal horn.14
The subnucleus caudalis contains receptors for many neuropeptides, including γ-aminobutyric acid (GABA), neurokinin-1, N-methyl-D-aspartate (NMDA), opioids, and glutamate. Injection of glutamate, in particular into rat TMJ, has been shown to cause sensitization of nociceptors in both the TMJ and the subnucleus caudalis as well as increase the receptive field of noxious stimuli.15 Furthermore, injection of glutamate in these rat models activates the mastication muscles of the jaw, thereby causing a protective jaw-clenching reflex that limits the size of the opening.16 If this protective reflex is excessive, it may cause more pain, which will further increase masticatory muscle tone, causing a vicious cycle of increased muscle tone and pain.17
There are aspects of TMD pain that are not easily explained through its direct pathophysiology. TMD pain has been associated with many psychological factors.18 Interestingly, TMD also seems to be associated with irritable bowel syndrome, another pain syndrome with a very different anatomic location. These associations suggest that chronic pain conditions such as TMD are related via a central mechanism, regardless of the site of pain.19 Recent studies also have suggested that there are genetic risk factors associated with TMD symptomatology, especially those related to estrogen.20 Estrogen status and chronic inflammation, through a common mitogen-activated protein kinase/extracellularly regulated kinase, enhance central nociception in TMD.21
Diagnosis of TMD
The diagnosis of TMD begins with a careful history and physical examination, which may include signs and symptoms such as jaw tenderness, muscle spasm, clicking, and misalignment or discomfort with opening of the mouth. These signs and symptoms are not specific for TMD; therefore, the clinician must rule out systemic mimickers such as infection, connective tissue disease, and temporal arteritis by ordering such laboratory tests as complete blood count, rheumatoid, antinuclear antibody, and erythrocyte sedimentation rates, respectively. Imaging studies including jaw x-ray, arthrography, magnetic resonance imaging (MRI), ultrasound, and computed tomography scan also may be useful both in excluding other causes of temporomandibular pain as well as diagnosing TMD. Unfortunately, because there are no specific symptoms, lab tests, or imaging studies, TMD is largely a clinical diagnosis of exclusion.
Management of TMD
The approach to TMD pain management is based on the severity and etiology of the disorder. In general, nonsurgical approaches are preferred—either pharmacotherapy or physical therapeutic manipulation. The goal of therapy is to decrease joint pain, muscle pain, and associated inflammation.10