Temporomandibular joint anatomy: Ultrasonographic appearances and sexual dimorphism

Temporomandibular joint (TMJ) dysfunction is common, with a greater prevalence in females. While magnetic resonance imaging (MRI) is commonly used for clinical investigation, ultrasonography represents a potential alternative in some clinical scenarios. We designed a protocol for ultrasonographic evaluation of the TMJ and assessed its reliability. Presentation was compared between the sexes to establish whether an anatomical dichotomy underlies the female preponderance of TMJ dysfunction.

Ultrasound imaging of the TMJ was carried out in the longitudinal and oblique planes. Standard images were produced using model skulls and healthy volunteers. Measurements were made between the temporal bone, mandibular condyle, joint capsule and overlying skin, as well as of condylar translation during mouth opening. Both joints were scanned in 50 healthy volunteers. Measurements were repeated to evaluate reliability. A novel classification system was used to assess lateral condylar morphology.

The protocol facilitated reliable visualization of key anatomical features of the TMJ (average intraclass correlation coefficient = 0.75, $$  = 5.4E-03). Distribution of condylar morphology differed between the sexes. The capsular-cutaneous distance ('joint depth') and condylar-temporal bone distance ('interarticular distance') were significantly greater in males than in females.

Ultrasonography provides reliable views of the TMJ in two planes: longitudinal and oblique. Observed sexual dimorphism in TMJ anatomy might be associated with the female preponderance of dysfunction. With a standardized scanning protocol, ultrasound could provide a rapid, cost-effective alternative to MRI as a point-of-care imaging tool in TMJ clinics.