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. 



OraStretch® MIO Scales

(In Disposable Paper or Reusable Plastic)

The MIO rotation scale allows for quick measurement of the oral opening. The MIO scale includes general trismus ranges and a small opening scale.

OraStretch® ROM Scales

(In Disposable Paper Only)

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! 


For More Detailed Use & Instructions
                                    Click here!


Lindsay Wachowiak

Date 9/1/2022

Denese McAdam

Date 2/10/2023

Allen Bradberry

Date 2/24/2023

Royce Gragg

Date 2/25/2023

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