Posted by Sarah Schuman, M.S. CCC-SLP on 11/11/2021 to
General Medical and Dental Information
A Brief Guide on Mouth Opening and Trismus
What is MIO vs. ROM?
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.
The MIO rotation scale allows for quick measurement of the oral opening. The MIO scale includes general trismus ranges and a small opening scale.
Specifically designed to measure severely limited mouth opening, the ROM prong scale slips between the central incisors to get a more accurate measurement.
Both MIO and ROM scales include a lateral ruler to measure lateral
excursion of the mandible-- useful for determining unilateral restriction!
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