Coincidence of the Cranial Base and Soft Tissue Facial Midlines

Objectives: This longitudinal study uses cone beam computerized tomography (CBCT) to investigate midline, facial, cranial base, cervical, and rotational asymmetry to the cranial base midline (CBML) in 120 patients in a general dental population in Australia. Many dental patients require alterations to the habitual midline mandibular position during treatment. Little is known about facial midline asymmetry in the general population.

Methods: A 0.4voxel, 20 second CBCT scan was taken with soft tissue midline points marked.  The CBML was drawn from foramen cecum to basion, and extended onto the coronal scan. Measurements were made from soft tissue, skeletal midline points, and various cranial base, cervical, and facial skeletal points to the CBML. Midline coincidence was accepted if points fell within 0.5mm of the left or right of the cranial base midline. Correlations were made using the Pearson correlation coefficient (r) between midline, facial, cervical, cranial base asymmetry.

Results: The frontonasal suture (0.22mm) was the most reliable skeletal indicator of the CBML. There were no soft tissue facial points that were reliable midline markers of the CBML. The maxillary midline frenum (0.87mm) was not coincident with the CBML, but was coincident with the buccal maxillary midline suture (0.49mm). The mandibular midline frenum was not a reliable midline marker of the mandible at the genial foramen (0.84mm).

Conclusions: Frontonasal suture and Posterior nasal spine are reliable skeletal midline markers. There are no clinically reliable soft tissue markers of the cranial base midline. The maxillary frenum is a reliable midline marker of the maxilla, but the mandibular midline frenum is not a reliable midline marker of the mandible.  The cervical midline points had no correlations with any facial skeletal or soft tissue midline point.