Facial asymmetry and temporomandibular disorders: Results from twin study and Northern Finland Birth Cohort 1986

This doctoral dissertation demonstrates that chewing behavior may exert a greater influence on facial symmetry than genetic inheritance. Long-term unilateral mastication was found to be associated with alterations in mandibular volume, suggesting that the masticatory system adapts to habitual patterns of use. Furthermore, the findings indicate that temporomandibular disorders (TMDs) are prevalent among Finnish young adults aged 33 to 35 years and are frequently associated with psychological well-being.

Analyses based on 3D facial models revealed that unilateral mastication is associated with structural changes in the mandible: mandibular volume is greater on the side contralateral to the preferred chewing side. These findings suggest that functional factors may play a more substantial role than genetic factors in the development of facial asymmetry. The study further demonstrated that females exhibited, on average, greater facial symmetry than males. In addition, the number of missing teeth was associated with chewing side preference: individuals tend to chew on the side with more teeth.

A cross-sectional study conducted at the University of Oulu further aimed to determine the prevalence of TMD in the Northern Finland Birth Cohort 1986. Nearly one-third of the participants reported at least one TMD symptom, most commonly facial pain. More than one-quarter of the study population received a TMD diagnosis, and myalgia was the most common diagnosis. TMD-related pain and corresponding diagnoses were observed approximately twice as often among women as among men and were significantly associated with symptoms of depression and anxiety, as well as with socioeconomic status.

For the purposes of this research, 3D soft-tissue facial images were acquired from Lithuanian same-sex twins and from 33– to 35-year-old participants of the Northern Finland Birth Cohort 1986. Participants’ preferred chewing side was also determined. The final study sample consisted of 212 twins and 1788 cohort participants, who additionally participated in medical and oral health examinations between 2019 and 2020.

These findings advance our understanding of how everyday functional behaviors, such as mastication, contribute to the facial structure. Since masticatory system pain appears to be common among young adults, healthcare services should focus on identifying individuals at risk while also recognizing the important role of psychosocial and sociodemographic factors in the overall picture.