Pain part 6: temporomandibular disorders

Clinical assessment

Whilst we await development of a shorter DC/TMD it is worth noting the existence of the Clinical Examination Protocol (CEP-TMD). This has been shown to produce diagnoses as reliable as Axis 1 of the RDC/TMD and is much quicker to complete, making it more suitable for the routine clinical practice setting.²⁹ In a relatively short time dentists can learn to make reliable TMD diagnoses according to the key findings from the clinical examination (Table 1).

A video showing how to carry out the examination plus the accompanying clinical forms, which help with history-taking, examination, diagnosis and follow-up, can be found at www.ncl.ac.uk/dental/AppliedOcclusion/

Although this system does not provide a formal Axis 2 diagnosis, it does ask the patient questions that provide a helpful insight into this important aspect of the condition. Importantly, patients can complete the history questionnaire in the waiting room, which saves time during the appointment. It is important to note if patients have had multiple previous treatments, particularly if they have been unsuccessful. These patients are not well suited to management exclusively in general dental practice. It is also worth remembering that a proportion of TMD patients will have symptoms of persistent orofacial or dento-alveolar pain, which again are not easily managed exclusively in primary care.

Imaging

Imaging in the form of plain radiographs can have a role, for example, in excluding TMJ pathology with osseous changes (eg rheumatoid arthritis). Its use for diagnosis of TMDs is, however, otherwise limited,³¹ as many TMDs involve the soft tissues. Moreover, when osseous condylar changes are seen radiographically they often fail to correlate with the level of clinical symptoms, which may result in false positives being recorded.³¹ Computed and cone-beam tomography unsurprisingly demonstrate an excellent ability to detect osseous changes. Nevertheless, the question remains after these ionizing investigations whether or not any demonstrated changes are symptomatic, which can only be identified through history-taking and clinical examination.³²

Magnetic resonance imaging (MRI) is principally used to determine the position of the articular disc or explain the existence of a soft tissue swelling (eg a joint effusion). Despite being the ‘gold standard’ for imaging of the soft tissues of the TMJ,³³ it too suffers from false positives, especially in relation to disc position and configuration.³⁴ This may change, however, with further advancements in this form of imaging.

It is clear that diagnosis of TMDs is currently best made after careful history-taking and clinical examination, preferably with a standardized approach as used with the RDC/TMD or the CEP-TMD. Imaging should currently be considered as an adjunct to clinical diagnosis and is indicated only in a minority of presenting cases as there is currently no clear evidence as to when TMJ imaging is beneficial.³² In secondary care, imaging may be indicated where conservative management has failed (eg for a disc displacement without reduction) and a comprehensive diagnostic work-up is needed, particularly in the rare cases when surgery may be considered.

Conservative therapy for management of TMDs including pharmacological treatment

Conservative therapy includes cognitive behavioural therapy, physiotherapy, the use of intra-oral splints and pharmacological therapy.

It has been suggested that all TMD patients may benefit from cognitive behavioural therapy (CBT).³⁸ This may begin in an extremely simple form with the dentist reassuring the patient and providing an explanation of the problem and its aetiology, along with a management strategy. A recent review of studies of CBT used in the treatment of patients with TMDs in secondary care showed it could improve outcomes.³⁹ However, the best mode of delivery and its cost-effectiveness still needed to be determined.³⁹

In many musculoskeletal disorders, physiotherapy started early in the course of the condition may help to reduce symptoms. Completing an exercise programme is beneficial in patients with TMDs whose problems are largely myofascial in aetiology.^40,41 Home-exercise programmes (in conjunction with CBT) have been described which general dental practitioners can tailor to their patients' needs, depending on the TMD diagnosis⁴² (Table 2).

Intra-oral splint therapy can also reduce TMD symptoms^43,44 and can work effectively in general dental practice.^45,46 The two most commonly used splints in the management of TMDs are: soft splints or stabilization splints. Soft splints are usually made of flexible polyvinyl of 2 mm thickness and are full coverage splints.⁴⁷ Stabilization splints (also known as Michigan or Tanner appliances, depending on whether they are fitted to the upper or lower teeth) are also full coverage splints, most commonly made of hard acrylic.⁴⁸ Stabilization splints involve creating ‘occlusal stability’ by producing even contacts throughout the arch on closing in ‘centric relation’, and canine or anterior guidance in lateral excursions, with removal of all interferences⁴⁸ (Figure 1). It should be remembered that centric relation is defined as a position of health and cannot be recorded in many TMD patients.⁴⁹ Therefore, stabilization splints often require multiple adjustments to achieve stability.

There is some evidence to suggest that, for the majority of patients, stabilization splints offer no additional benefit over soft splints,⁵⁰ or splints with no occlusal surface,⁴⁵ in reducing TMD signs or symptoms. In routine cases it would therefore seem sensible to construct a simple, less expensive soft splint in the first instance to see how the patient responds. We review such treatment after three months. Approximately 10–15% of our TMD patients are provided with stabilization splints (based on our splint production numbers).

Generally, we recommend patients wear splints only for sleeping,⁵¹ although a few patients with occlusal instability wear their stabilization splint full-time with frequent adjustment needed by the dentist, usually on a two-week basis, until a stable occlusion has been created on the splint. Fricton et al, in a meta-analysis of splint studies, have emphasized the importance of accurate splint adjustment in securing any advantage of using this type of appliance.⁵² To avoid patients becoming over-reliant on the splint they should be encouraged to wean themselves off daily use within six months. In those few patients who need to persist with daily or full-time wear it is important that the splint is kept under close review.

We advise caution in the use of partial coverage splints as unwanted occlusal changes may occur, particularly with prolonged wear when the splint may act as a Dahl appliance.⁵³ A similar consideration applies to anterior repositioning splints which are sometimes used to treat symptomatic TMJ clicking.⁵² With prolonged wear these appliances can cause posterior open bites in some patients.

Where dentists choose to prescribe partial coverage splints they need to ensure that patients are carefully followed up.⁵⁴ However, the NTI TSS splint can sometimes be useful for short-term use in patients with acute muscle symptoms, where it would otherwise be difficult to record an impression for a full coverage splint because of limited mouth opening. In such situations, it is important that the dentist makes a correct diagnosis – limited opening caused by a disc displacement without reduction is unlikely to be helped by such an approach. Where a disc displacement without reduction is diagnosed or suspected, dentists are advised to seek early help in management, unless they have training in manipulating the mandible to release the displaced disc. However, with many patients the disc remains displaced forwards or forwards and medially but, over time, opening improves and discomfort decreases in response to exercises and other conservative measures.⁵⁵ Only very occasionally is surgery needed, usually in the least invasive manner of arthrocentesis or arthroscopy.

In cases where a TMD patient fails to respond sufficiently to conservative treatment, or during episodes of acute pain, medication prescribed in conjunction with the patient's general medical practitioner can be a useful adjunct in management. Many different classes of medication have been used to target the pain caused in differing TMDs, including:

The evidence base for the effectiveness of the varied potential pharmacological interventions for pain caused by TMDs is limited and continued research in this field is required.^56.57 A reasonable approach to managing TMDs at the initial point of contact may be:

Patients suffering from TMDs should always be reviewed and followed up carefully once treatment is instituted to ensure that progress is as would be expected.

Irreversible therapy for TMD management

The main types of irreversible therapy which have in the past been proposed are occlusal adjustment and surgical intervention.

There is no evidence that occlusal adjustment is more or less effective than a placebo in treating TMD problems and, as it is irreversible, it is not indicated as a first line treatment.⁵⁸ In certain circumstances though, it may seem sensible to undertake an occlusal adjustment, for example if the patient's TMD pain began after placement of a restoration and this restoration is shown to be causing an occlusal interference.⁷ On rare occasions, occlusal adjustment may be required to establish occlusal stability and comfort following stabilization splint treatment. Occlusal equilibration, which can involve sometimes extensive adjustment of multiple teeth, needs to be approached with particular caution as a partial equilibration can result in worsening of a patient's symptoms. Dentists undertaking such work need to be suitably trained and skilled in this procedure.

Surgical treatment of TMDs include procedures such as arthrocentesis, arthroscopy, open joint procedures and joint replacement. The current evidence for using these procedures in TMD patients is poor.⁵⁸

Historically, orthodontic treatment was thought to cause TMDs in some patients. Orthodontic treatment has also been used in TMD management, based on the outmoded belief that malocclusions cause TMDs. A recent systematic review has refuted that there is any evidence base for either of these assumptions.⁵⁹

Traumatic aetiology of TMDs

Evidence in the form of high ‘odds ratios’ is now emerging that TMD sufferers are more likely to have a positive history of trauma compared with non-TMD sufferers.¹³ Such trauma may result from external injury, yawning and prolonged opening, as may occur during dental treatment. It may also occur as a result of a variety of parafunctional activity (eg as assessed by the Oral Behaviours Check List¹³), but parafunctional behaviour may be difficult to measure as it may occur whilst the patient is asleep.

Free radical damage of the TMJ

Recent research suggests that free radicals may be released in the joint capsule as a result of parafunctional stresses.⁶⁰ If these free radicals build up excessively they can exert a degenerative effect on the TMJ tissues. Such a build-up may result from a defective scavenger system for free radicals. The research breakthrough is that the patient's genotype may determine how effectively the scavenger system works.⁶⁰

Peripheral and central sensitization

Another significant research finding relates to peripheral and central sensitization, which has been extensively studied in other parts of the body. These two effects effectively amplify the sensation of pain through mechanisms acting along the neuronal track to the brain (peripheral sensitization) or within the brain itself (central sensitization).

A neuropeptide, ‘calcitonin gene-related peptide’ (CGRP), known to be influential in such sensitization, has been demonstrated to cause cellular changes inside the TMJ capsule, which promote and maintain peripheral sensitization. Indeed, elevated levels of CGRP are known to be indicative of mobility impairment and pain in the temporomandibular complex.⁶¹ An exciting development would be if CGRP could be targeted for future therapy.⁶² Widespread pain, outside of the orofacial region, can also accompany the more localized TMDs and recent research has identified several pro-inflammatory cytokines and anti-inflammatory cytokines associated with the presentation of widespread pain alongside TMDs⁶³ and these also could offer potential therapeutic targets.

Cortical changes in TMD and during splint wear

Research on cerebral constitution (eg using functional magnetic resonance imaging, fMRI, which measures changes in blood supply to brain regions) has highlighted the degree of neuroplasticity inherent in sufferers of TMDs. These patients show cortical changes related to pain duration, pain unpleasantness, and pain intensity.⁶⁴

fMRI studies on the cerebral representation of the TMJs in pain-free controls fitted with stabilization splints have shown decreased activation within affective and sensorimotor areas of the brain, thereby for the first time giving some indication of a potential central mode of action of stabilization splints.⁶⁵

TMDs and genetic polymorphisms

A polymorphism is a variation in DNA sequence, which is relatively common within the population. Genomic research linking TMDs to polymorphisms in the genes, which encode a number of enzymes, involved in tissue maintenance and neuronal transmission has emerged,^66,67,68 including catechol-O-methyltransferase (COMT). COMT degrades catecholamines after they have been released at neuronal synapses, so patients with deficient levels of the enzyme have a lower pain threshold and are more likely to develop TMD. The COMT polymorphism effect has recently been shown to be modified by lifetime incidence of depressive symptoms,⁶⁹ thereby beginning to help explain the association between chronicity of TMDs and depression.⁷⁰ All of these polymorphisms may, in the future, help tailor management to the individual's genetic make-up.