**1. Introduction: temporomandibular joint disorders**

The temporomandibular joint (TMJ) is the only dynamic articulation of the head and present unique anatomical, structural, and biochemical characteristics. Up to 40–50% of the population suffers different pathologies of TMJ [1, 2] that requires therapeutic interventions by different medical and paramedical specialists and represents an increasing social and psychosocial impairment [3].

 TMJ disorders (TMD) are a class of degenerative musculoskeletal conditions associated with morphological and functional deformities, which clinically result in pain and TMJ dysfunctions (impairment in mastication, speech, and facial expression) (see for a review [4]). Moreover, when TMD affect young subjects during growth, it can cause asymmetry of the facial skeleton [5]. In agreement with the above definition, TMD comprise a heterogeneous group of pathologies involving the TMJ, the associated jaw muscles, or both [6]. Up to 40–50% of the population suffers TMD [2], and up to 70% of them suffer TMD directly related to the articular disc [1].

**Figure 1.** 

*Cone bean CT of the right adult TMJ in normal conditions, osteoarthritis, posttraumatic, and hemifacial microsomy. Images obtained from Instituto Asturiano de Odontolgía, Oviedo, Spain.* 

The etiology of TMD can be traumatic, inflammatory, and congenital [6]. However, the primary TMD are degenerative inflammatory or noninflammatory diseases, that is, osteoarthritis or arthrosis, respectively [6]. Typical osteoarthritic changes include alterations in shape and size of TMJ components (flattened fossa, reduced articular eminence, decreased condylar volume, and thickened disc), abrasion of articular cartilage, and thickening and remodeling of the subchondral bone that leads to morphological deformity and dysfunction (**Figure 1**) [4].
