Why should orthodontists deal with the temporomandibular joint?

The correct functioning of the temporomandibular joint (TMJ) is of paramount importance for patients’ well‐being and oral health‐related quality of life (Chanthavisouk et al., 2022; Häggman‐Henrikson et al., 2022; John, 2020). Orthodontists routinely work on dental occlusion, and occlusal changes may also result from a number of TMJ pathologies. A deep understanding of the relationship between occlusion and TMJ problems is therefore crucial for orthodontists (Michelotti and Iodice, 2010).

The collective term temporomandibular disorders (TMDs) includes a set of heterogeneous conditions that affect the masticatory muscles, the TMJ, and the surrounding tissues and structures. The most common conditions are classified in the diagnostic criteria for temporomandibular disorders (DC/TMD) and are characterized by regional acute or persistent pain in the facial and/or preauricular areas, limitation of or interference with jaw function, and/or TMJ noises during jaw movements (Schiffman et al., 2014). However, other possible conditions might affect the TMJ. These are classified in the expanded taxonomy of TMDs and include joint diseases, fractures, and congenital/developmental disorders (Peck et al., 2014).

Some occlusal features, such as unilateral crossbite and overjet larger than 7 mm, have been historically considered risk factors for TMDs (Pullinger et al., 1988); nowadays, however, the role of occlusal factors is not supported by strong evidence and it is considered irrelevant within the etiological framework of the biopsychosocial model for TMDs (Manfredini et al., 2017; Olliver et al., 2020; Suvinen et al., 2005).

Changes in the occlusal relationship may result from changes in TMJ morphology, which are associated with degenerative joint disease, neoplasm, or fractures. It is therefore important that orthodontists have the diagnostic skills that are necessary to assess TMJ problems (Caldas et al., 2016). Finally, orthodontists were also blamed for provoking TMDs, and for this it is important to know what the effects of orthodontic treatment on the TMJ could be (Al‐Moraissi et al., 2017; Owen, 1988).

Anatomy of the temporomandibular joint

The TMJ is a double bicondylar diarthrosis. The mandibular condyle seats in the glenoid fossa and articulates with the articular eminence of the temporal bone. Unlike other joints, articular surfaces are covered with fibrocartilaginous tissue instead of hyaline cartilage, and there is a fibrous disc between the two articular surfaces. The joint cavity contains synovial fluid. The articular disc divides the joint into two compartments: an upper space called the temporo‐discal space, and a lower space called the condylar‐discal space.

The mandibular condyle is the most superior part of the condylar process along the condylar neck. It presents an ovoid form with rounded medial and lateral poles. The condyle antero‐posterior axis is approximately 1 cm, while the latero‐medial axis is around 2 cm; the major axes of the two condyles converge at the foramen magnum at an angle of 145°. The antero‐superior portion of the condyle that articulates with the disc and with the tubercular eminence is covered with fibrocartilage and serves as the area under the heaviest load among all the body joints (50–80 kpa; Koolstra, 2002).

The glenoid fossa is part of the squamous portion of the temporal bone. It is a very thin, concave bony layer that extends forward with the temporal tubercle, giving to the temporal articular surface the classic “saddle” morphology. The posterior part of the temporal tubercle is called the eminence plane and changes its inclination throughout life. In newborns the eminence plane is almost flat, but with functional loading and changes in dentition it becomes steeper, up to a 45° slope during adulthood. However, with aging it changes again to a flat plane. The eminence plane is covered with fibrocartilage, but not the posterior part of the glenoid fossa, as this is not an articular surface.

The mandibular condyle is surrounded by the fibrous joint capsule, which has a protective and proprioceptive role. It extends from the perimeter of the cranial articular surface to the condylar neck of the mandible. The inner part of the joint capsule is a synovial membrane producing fluid for lubrification and trophism of the joint. The outer part of the joint capsule, instead, is made of collagen and elastic fibers, which allow a degree of movement freedom to the joint, but at the same time also restrict excessive excursions. Indeed, the joint capsule limits both the forward translation of the condyle and in part its distraction and posterior movement. The ligaments stabilizing the TMJ are the temporomandibular ligament, the stylomandibular ligament, and the sphenomandibular ligament.

Within the two articular surfaces the TMJ presents the articular disc, which is described as a biconcave fibrous lamina that can be subdivided into three zones: anteroinferior, intermediate, and posterior. The intermediate zone is the thinnest part of the disc, while the posterior zone is the thickest at around 3 mm. The part that bears the highest load is the intermediate part, which is formed by collagen fibers. The articular disc is extremely viscoelastic, due to the presence of elastic fibers that allow shape variation and exchange of fluids (blood, synovial fluid, water) during functional and postural activities (Fanghänel and Gedrange, 2007). The disc is avascular and inserts anteriorly to the articular capsule, where it is partially connected to the tendon of the lateral pterygoid muscle; laterally and medially the condylar disc ligaments (polar ligaments) connect the disc to the condyle. When the disc is in a “physiological position” relative to the condyle, the intermediate zone is facing the tubercular eminence, while the posterior zone is on the top of the condyle in the middle of the glenoid fossa (Figure 21.1; Academy of Prosthodontics, 2017). The disc presents several receptors (i.e. Pacinian corpuscles, Golgi tendon organs, and muscle Ruffini endings) that increase their density from the disc center to its periphery, and provide feedback during mastication together with the periodontal receptors (Wink et al., 1992).

Posteriorly to the disc there is a retrodiscal space or bilaminar zone formed by two laminae separated by an interlaminar space, made of connective and fat tissues, which contain nerves and the retrodiscal venous plexus. The retrodiscal space also contains branches of the deep auricular, anterior tympanic, and middle meningeal arteries, of the auriculotemporal nerve, and of the temporomandibular veins. The role of the retrodiscal space is to protect the posterior part of the glenoid fossa during jaw closure, and to provide the blood supply to the TMJs.

Should orthodontists care about condylar position?

Since the early 1970s it has been hypothesized that a malocclusion can have a negative impact on the physiological condylar–disc relationship, which in turn can result in the onset of TMDs (Roth, 1973, 1981a).

Nowadays, several occlusal features like Class II Division 2, deep bite, discrepancies between centric occlusion and centric relation, and occlusal interferences are still considered risk factors for TMDs altering the physiological condylar–disc relationship. In order to prevent or treat TMDs, Roth’s philosophy has emphasized the search for an optimal condyle position as one main goal of orthodontic treatment. This position was determined using a particular bite registration (power centric) followed by articulator mounting of the patient’s dental casts (Roth, 1973, 1981a, 1981b; Roth and Rolfs, 1981). However, the existence of an ideal centric relation (CR), with condyles centered in the glenoid fossa, has been questioned (Rinchuse, 2021). The term CR refers to a reproducible relationship between the condyle and the glenoid fossa, independently of the tooth contacts. First, in the 1970s, it was defined as a retruded postero‐superior condyle position; in the 1980s it changed to the antero‐superior position of the condyle (Rinchuse, 2021). The operational criteria to identify CR are not only difficult to define, but they also appear to lack reliability and validity (Kandasamy et al., 2013). Indeed, in a study conducted on magnetic resonance to assess the validity of three different methods to find the CR, neither the maximum intercuspation, not the retruded CR, nor the power‐centric bite registration was able to achieve the CR (Kandasamy et al., 2013).

Finally, as mentioned before, the relationship between glenoid fossa and mandibular condyle is continuously changing throughout life due to ongoing remodeling. The most significant changes occur during growth with eruption of the permanent dentition and growth at the condyle and the tubercular eminence. Also during adulthood there are small but continual changes in the relationship between glenoid fossa and mandibular condyle, as a result of the physiological aging of the stomatognathic system (tooth wear, TMJ remodeling, muscular changes; Ingervall, 1968). Considering these aspects, it is difficult to assume that this relationship would remain stable over the years, and that the maximum intercuspation (MI) will always coincide with the CR, as indicated as primary goal of so‐called orthodontic gnathology (CR–MI coincidence). However, it has been considered that, although the physiological relationship between articular disc and condyle exists and presents the posterior band covering the head of the condyle and the intermediate band between the condyle and the articular eminence (Academy of Prosthodontics, 2017), this relationship changes frequently both within and between individuals. Indeed, the position of the condyle in the fossa is influenced by the circadian variation of muscle tone and many other factors such as parafunction, stress levels, tongue posture, and hydration of the disc, among others (Michelotti et al., 1997).

Furthermore, the need for CR–MI coincidence as prevention or treatment of TMDs is not supported by scientific evidence (Greene and Manfredini, 2020). First, the CR appears to be an unreliable and unachievable position; second, the transfer of the CR to the articulators may be flawed; and finally, there are no studies supporting the notion that CR is more common in healthy subjects or less common in TMD patients (Bonilla‐Aragon et al., 1999; Lelis et al., 2015; Paknahad et al., 2015). The current evidence indicates that the position of the TMJ in healthy subjects is variable, ranging from retruded to centered and anterior, hence it can be speculated that an ideal three‐dimensional position of the condyle that can be used to prevent or treat TMDs does not exist (Bean and Thomas, 1987; Kircos et al., 1987; Türp et al., 2016). Finally, current evidence suggests that there is a range of acceptable condylar positions for all individuals, and that usually most patients adapt well to changes in this position (Zonnenberg et al., 2021). The condyle–fossa relation is strongly influenced by anatomical variation of the fossa, varying from narrow and deep to wide and shallow. Asymptomatic patients may present three possible scenarios of condyle–fossa relation: a concentric position of the condyle in the fossa, a posterior position of the condyle in the fossa, or an anterior position of the condyle in the fossa, but with an average ratio of fossa depth to fossa width. In contrast, patients with disc displacement more commonly present a wider and shallower fossa or a very narrow posterior joint space associated with a narrow or deep fossa, depending on the type of disc displacement (Pullinger, 2013). Hence, anatomical factors seem to have a more important role than condylar position in the onset of TMDs. When considering that it is not possible to change the morphology of the TMJs, one aim of orthodontic treatment should be to reach an acceptable maximal intercuspation position, without sliding that is clinically significant, independently of the CR position (Kandasamy et al., 2018).

Temporomandibular joint disorders

TMJ problems are routinely encountered by orthodontists in daily practice, and therefore it is important to have a correct diagnosis and management of these conditions. TMJ signs and symptoms include pain (arthralgia), condylar disc incoordination (disc displacement), and anatomical or degenerative changes (arthrosis, systemic arthritis, and growth disturbances; Greene et al., 2017). In general, the management of signs and symptoms of TMDs, such as pain or dysfunction, should include reversible therapies based on the biopsychosocial model (Sharma et al., 2020; Suvinen et al., 2005). The biopsychosocial model suggests biological, clinical, and behavioral characteristics of the patient as factors involved in the onset, maintenance, and remission of TMDs (Sharma et al., 2020). Hence, the focus of occlusion as a risk factor for TMD has shifted, and other putative factors have been identified, such as genetic predisposition, central neural system pain control mechanisms, psychosocial status, and parafunctions, with all of these playing an important role in TMD evolution (Slade et al., 2016). Based on current evidence, changing the occlusion to treat TMD is not recommended (Kandasamy et al., 2018). Conversely, reversible treatments should be preferred, also because of large fluctuations in the signs and symptoms of TMDs. Symptoms may spontaneously decrease or even disappear without any treatment; hence, conservative management is recommended including cognitive behavioral therapies, biofeedback, oral occlusal appliances, physical therapies, and pharmacological agents (Al‐Moraissi et al., 2020, 2022). Epidemiological studies indicate an increased prevalence of TMJ disorders, such as clicking sounds, in the age range between 15 and 25 years or younger (Magnusson et al., 1994; Rauch et al., 2020); this age range corresponds to the time when adolescents or young adults more frequently seek orthodontic treatment. Considering that the average duration of orthodontic treatment is around 20 months, it is very likely that TMDs occur during or after the orthodontic treatment and patients may assume that they causally relate to that treatment (Michelotti and Iodice, 2010).

How should orthodontists manage a patient with signs and symptoms of TMD before or during treatment (Figure 21.2)? In general, before the orthodontic treatment it is important to screen the patient for TMD, and DC/TMD is a good instrument for identifying TMD (Schiffman et al., 2014). If a patient presents signs or symptoms of TMD before starting orthodontic treatment, the first step is to make a diagnosis, considering possible differential diagnoses, and then, in the case of TMD, start conservative therapy such as pharmacotherapy, counseling, behavioral therapy, home exercises, physical therapy, and/or occlusal appliances to manage signs and symptoms. In general, the orthodontic treatment should not be initiated if TMD pain is significant (Michelotti and Iodice, 2010). Moreover, patients with a positive history of TMD might be more prone to developing TMD during the treatment, or sometimes TMD might occur for the first time during the orthodontic treatment. In this case, the first step is to make a correct diagnosis, temporarily suspend or reduce the active orthodontic phases (e.g. intermaxillary elastics), and then manage the signs and symptoms until remission. When the patient is pain free or symptoms become very mild, orthodontic treatment can be continued as previously planned or, if necessary, modified according to the patient’s condition. Furthermore, the doctor should emphasize that TMDs are highly prevalent in the general population and present a multifactorial etiology, and evidence does not support a possible cause–effect relationship between orthodontic treatment and TMD (Michelotti and Iodice, 2010).

There is no convincing evidence supporting the idea that occlusion or malocclusion or orthodontic treatment might cause TMDs; but some TMDs, such as osteoarthrosis or arthralgia, might cause occlusal alterations like open bite, crossbite, or Class II malocclusion. Hence, an appropriate TMJ examination before starting orthodontic treatment is needed and could affect treatment planning, which should be tailored to the patient’s conditions and expectations (Caldas et al., 2016). In the next section we discuss the most common TMDs that occur before, during, or after orthodontic treatment (joint pain and disc displacement), and then we consider local or systemic conditions that may affect the TMJs and also have an impact on dental occlusion.

Joint pain: Arthralgia

Arthralgia is defined as joint pain that is affected by jaw movement, function, or parafunction, and replication of this pain occurs with provocation testing of the TMJs.

The DC/TMD provides a flow diagram for the diagnosis of arthralgia and its diagnostic accuracy (Table 21.1; Schiffman et al., 2014). When arthralgia is present, the pain is reported to be directly in front of the ear, the lateral pole of the condyle is usually tender to palpation, and the pain is usually constant and increased by jaw movements.

Arthralgia might be a symptom linked to several conditions, such as inflammation of different components of the TMJ (ligaments, retrodiscal tissue, bone, fibrocartilage), and it is often present in the case of arthritis. Arthritis is defined as an inflammation or infection associated with edema, erythema, and/or increased temperature over the affected joint, and it includes inflammation not only of the bone structures but also other TMJ structures such as synovia (synovitis) or capsule (capsulitis), and retrodiscal structures (retrodiscitis). The majority of inflammatory conditions affecting the joint are secondary to macro‐ or microtrauma to the tissues within the joint, such as a bump to the chin (microtrauma), onychophagia, tooth grinding, or tooth clenching (microtrauma; Sierwald et al., 2015; Slade et al., 2016). Furthermore, internal derangements of the TMJ (disc displacement with or without reduction) might also be a cause of arthralgia due to a possible inflammation of the retrodiscal tissue (Naeije et al., 2013). Retrodiscal tissue is not as able to adsorb the stress as the articular disc, hence the load present on this area will provoke inflammation and pain. In this case, the patient usually reports trauma and/or joint sounds and/or limitation of mouth opening, and pain is modified during jaw movements.