Temporomandibular Disorder (TMD) is the main cause of pain of non-dental origin in the oro-facial region including head, face and related structures. The aetiology and the pathophysiology of TMD is poorly understood. It is generally accepted that the aetiology is multifactorial, involving a large number of direct and indirect causal factors. Among such factors, occlusion is frequently cited as one of the major aetiological factors causing TMD. It is well known from epidemiologic studies that TMD-related signs and symptoms, particularly temporomandibular joint (TMJ) sounds, are frequently found in children and adolescents and show increased prevalence among subjects between 15 and 45 years old. Aesthetic awareness, the development of new aesthetic orthodontic techniques and the possibility of improving prosthetic rehabilitation has increased the number of adults seeking orthodontic treatment. The shift in patient age also has increased the likelihood of patients presenting with signs and symptoms of TMD. Because orthodontic treatment lasts around 2 years, orthodontic patients may complain about TMD during or after treatment and orthodontists may be blamed for causing TMD by unsatisfied patients. This hypothesis of causality has led to legal problems for dentists and orthodontists. For these reasons, the interest in the relationship between occlusal factors, orthodontic treatment and TMD has grown and many studies have been conducted. Indeed, claims that orthodontic treatment may cause or cure TMD should be supported by good evidence. Hence, the aim of this article is to critically review evidence for a possible association between malocclusion, orthodontic treatment and TMD.

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To evaluate the prevalence of temporomandibular disorder pain (TMD‐pain), temporomandibular joint (TMJ) noises, oral behaviours in an Italian adult population sample, their possible association with gender, oral behaviours, self‐reported facial trauma and orthodontic treatment. Subjects older than 18 years were recruited from general population in public spaces during their daily life. A specific questionnaire was developed to collect data on TMD‐pain, TMJ noises, oral behaviours, orthodontic treatment and facial trauma. A total of 4299 subjects were included in the study. The most common symptom in the sample was TMJ clicking (30.7%), followed by TMD‐pain (16.3%) and TMJ crepitus (10.3%). Oral behaviours were reported in 29% of the sample; 43.6% of the sample reported a previous or ongoing orthodontic treatment. TMD‐pain and TMJ clicking were significantly associated to gender, oral behaviours and a positive history of previous facial trauma. Crepitus was significantly associated to oral behaviours, facial trauma and higher age. Ongoing orthodontic treatment was significantly associated to TMD‐pain and TMJ sounds. In a general Italian adult population sample, TMD‐pain is associated to female gender and is less prevalent than TMJ clicking. TMDs are associated to trauma and oral behaviours.

The relationship between unilateral posterior crossbite and skeletal asymmetry is still unresolved. To date, most of the studies available report a skeletal asymmetric growth. EMG activity of masticatory muscles is different between crossbite and non-crossbite sides. Subjects with UPCB show smaller bite force than non-crossbite subjects. There is no consistency of studies reporting masticatory muscle thickness asymmetry in UPCB subjects. UPCB is associated to an increase in the reverse chewing cycle. The literature available on the subject is of medium-low scientific and methodological quality, irrespective of the association reported. Further investigations with higher sample size, well-defined diagnostic criteria, rigorous scientific methodologies, and long-term control are needed.

It is not possible to establish an association between posterior crossbite, muscle pain, and disc displacement because the distribution of the studies supporting or not supporting the association is similar. The consequences of posterior crossbite on the development of TMDs deserve further investigations, with high sample size, well-defined diagnostic criteria, and rigorous scientific methodologies. Finally, long-term controlled studies are needed to identify posterior crossbite as a possible risk factor for TMDs.