Mouth opening is measured as a MIO or ROM distance. MIO (maximal interincisal
opening) is the distance between the central incisors when the mouth is fully open.
Average MIO for adults is approx. 50mm. By measuring MIO, a therapist can determine
if a patient is WNL or has trismus.ROM (Range-of-Motion) is the maximum distance
the mandible moves. ROM and MIO may be different, e.g. patients with an open bite.
What is Trismus?
Reduced oral opening with an MIO of 35mm or less. Trismus impacts daily functions--speaking, eating, oral hygiene--and causes difficulties with dental care and airway management. Causes include radiation, surgery, trauma and TMJ disorders. It is a common side-effect of HNC care.
The 3-Finger Diagnostic
A quick method to determine if the patient has a limited opening, while adjusting for size differences. This test is especially useful with children, who do not have standardized diagnostic criteria. Unrestricted opening should allow the patient to insert their index, middle and ring fingers, vertically aligned, between their central incisors. If a patient has difficulty inserting 2+ fingers, trismus can be inferred.