Situations where the extent of oral opening may be measured:
(1) disorders of the temporomandibular joint
(2) prior to tracheal intubation or endoscopy
(3) with scleroderma
(4) following burns of the face or other scarring conditions
One measurement of the oral opening is the distance between the bottom of the upper incisor and the top of the lower incisor with the patient asked to open her or his mouth as wide as possible (Pope, 1995).The oral aperture is the measurement in millimeters from the outer vermillion borders of the lips when the mouth is maximally opened (Furst, 1979). To measure the dimensions of the oral opening, the horizontal distance between the corners of the mouth (intercommissural distance) is also measured.
If the mouth opening is taken to be an ellipse, then the approximate circumference of the mouth opening =
= 2 *
π * SQRT((((vertical semiaxis)^2) + ((horizontal semiaxis)^2)) / 2)area of the mouth opening =
= π * (vertical semiaxis) * (horizontal semiaxis)
where:
• Semiaxis = half the length of the axis diameter; this is the radius in a circle.
Cephalometry can be used to measure the angle of the opening and the internal distances between bony landmarks.
Limitations on the inter-incisor distance:
(1) This may need to be estimated if the person is missing key incisors.
(2) The inter-gingival distance might be used in an edentulous person for most situations.
(3) A person with abnormal dentition may need to use estimates of incisor location for some situations.
