From NHS webpage
“A rating system known as the Index of Orthodontic Treatment Need (IOTN) is used to assess your eligibility for NHS treatment. The British Orthodontic Society (BOS) website has more information about the different grades used for the IOTN.
NHS treatment is available for grade 4 and grade 5 cases. Grade 3 cases are usually judged on an individual basis. Treatment may also be made available if the appearance of a person's teeth, jaw or face is of concern.”
www.bos.org.uk/Public-Patients/Orthodontics-for-children-teens/Fact-File-FAQ/What-Is-The-IOTN
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.