Definition
Midline deviation, measured in mm, represents the amount of deviation of the upper or lower midline in relation to mid-facial line (Brisman, 1980).
Cant of the midline, aka axial midline angulation, measured in angle, represents the deviation of the line connecting the midline papilla and the incisal edges of the incisors in relation to true vertical (Erum and Fida, 2008).
What are the prevalences of these problems?
These problem are not uncommon in particular among orthodontic patients. A study by Sheats et al. in 1998 that included 5500 American children aged between 9-15 years old with no previous orthodontic treatment showed that:
Upper midline to facial midline deviation affects 47% of the cohort
Lower midline to facial midline deviation affects 62% of the cohort
Upper and lower midline incoincidence affects 46% of the cohort
Facial asymmetry affects 6% of the cohort
Chin deviation affects 4% of the cohort
Nose deviation affects 3% of the cohort
For fun only
Hollywood stars are not immune to these problems like Tom Cruise
Even criminals like Pablo Escobar!
What did the evidence show?
Janson et al. in 2011 in their study concluded that a midline deviation more than 2.2 mm and 10 degrees of cant of the midline are clinically significant and noticeable by orthodontists and laypeople.
On the other hand, buccal corridor sizes and smile arc alone do not seem to affect smile attractiveness.