The British Orthodontic Society believes that, if treatment has to be rationed, then the IOTN is an objective and reliable way to select those children who will benefit most from treatment and is a fair way to prioritise limited NHS resources.
The accurate use of IOTN requires specialist training and the assessment of dental health need for orthodontics using the IOTN should take place in a specialist orthodontic practice.
A simplified summary of the IOTN assessment is provided below.
The first part of the IOTN is the Dental Health Component (DHC) which has 5 Grades.
Grade 1 is almost perfection
Grade 2 is for minor irregularities such as:
● slightly protruding upper front teeth
● slightly irregular teeth
● minor reversals of the normal relationship of upper and lower teeth which do not interfere with normal function.
Grade 3 is for greater irregularities which normally do not need treatment for health reasons.
● upper front teeth that protrude less than 4 mm more than normal
● reversals of the normal relationship of upper teeth which only interfere with normal function to a minor degree; by less that 2 mm.
● irregularity of teeth which are less than 4 mm out of line
● open bites of less that 4 mm
● deep bites with no functional problems
Grade 4 is for more severe degrees of irregularity and these do require treatment for health reasons.
● upper front teeth that protrude more than 6 mm
● reversals of the normal relationship of upper teeth which interfere with normal function greater than 2 mm
● lower front teeth that protrude in front of the upper more than 3.5 mm
● irregularity of teeth which are more than 4 mm out of line
● less than the normal number of teeth (missing teeth) where gaps need to be closed
● open bites of more than 4 mm
● deep bites with functional problems
● more than the normal number of teeth (supernumerary teeth)
Grade 5 is for severe dental health problems
● when teeth cannot come into the mouth normally because of obstruction by crowding, additional teeth or any other cause.
● a large number of missing teeth.
● upper front teeth that protrude more than 9 mm
● lower front teeth that protrude in front of the upper more than 3.5 mm and where there are functional difficulties too
● cranio-facial anomalies such as cleft lip and palate. 
The second part of the IOTN is the Aesthetic Component (AC).
The NHS does recognise that some children need and benefit from orthodontic treatment on the basis of poor aesthetics. The Aesthetic Component of the IOTN is a scale of 10 colour photographs showing different levels of dental attractiveness. The grading is made by the orthodontist matching the patient to these photographs. The photographs were arranged in order by a panel of lay persons.
Within the NHS if a patient in Dental Health category 3 has an Aesthetic Component rating of 6 or more NHS treatment is permissible.