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Orthodontic assessment of degree of dysgnathia and treatment outcome (PAR index)

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PAR index, Richmond, Manchester

The PAR index is a screening instrument with which malocclusion can be assessed at any time during diagnosis and treatment. It is used to ‘measure’ malocclusion objectively and compare treatment outcomes scientifically.

The index was formulated by a group of 10 experienced orthodontists working with Dr. Richmond (Manchester) in 6 sessions. Over 200 pre- and post-treatment plaster models were used in the study. Scores are awarded according to different criteria, the sum of which described the degree of dysgnathia. The higher the score, the more severe the malocclusion.

Degree of malocclusion
Score 0 orthognathic situation
Score rarely over 50 high degree of irregularity

The difference between the scores before and after treatment shows the improvement produced by the orthodontic treatment.

Components of the PAR index
1. Upper and lower anterior segment
2. left and right buccal occlusion
3. Overjet
4. Overbite
5. Centreline
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1. Upper and lower anterior segment (contact point displacements)

  • The anterior segments of the maxilla and mandible are included in the assessment.
  • Anterior segment (definition): mesial contact point 13 (or 33) as far as the mesial contact point of 23 (or 43)
  • Features assessed: crowding, gaps, impacted teeth
  • The contact point displacements are measured as the shortest distance between the contact points of the adjacent teeth parallel to the occlusal plane
  • The greater the contact point displacements, the higher the score
  • Missing teeth are not taken into account.
  • Impaction of a tooth (definition): If the distance between the two teeth adjacent to the impacted tooth is ≤ 4 mm, 'impaction' is assumed.
Contact point displacements Score
0 mm to 1 mm 0
1,1 mm to 2 mm 1
2,1 mm to 4 mm 2
4,1 mm to 8 mm 3
> 8 mm 4
Impacted teeth (gap ≤ 4 mm) 5

2. Left and right buccal occlusion

  • The occlusion of the teeth is assessed from the canine to the last molar
  • Occlusion is assessed in all three dimensions.
  • Erupting teeth and deciduous teeth in infraocclusion are not assessed.
  • Three values are obtained for each side, i.e. right and left, and these 6 values are added.
Sagittal Vertical Transverse
Good occlusion (exactly class I, II or III) 0 No open bite 0 No crossbite 0
Displacement less than half a premolar width 1 Lateral open bite (more than 2 mm at at least 2 teeth) 1 Crossbite tendency 1
Displacement more than half a premolar width(cusp-cusp configuration) 2     Single tooth crossbite 2
        More than one tooth in crossbite 3
        More than one tooth in buccal or lingual nonocclusion 4

3. Overjet

  • All incisors are included in the assessment.
  • The most anteriorly protruding incisal edge is assessed.
  • The measurement is made with the ruler parallel to the occlusion plane (in radial direction relative to the dental arch).
  • Canine crossbites are also included in the assessment of overjet.
  • Example: it is not unusual to see two upper lateral incisors in crossbite with increased overjet of the central incisors (in this situation, for example, the score would be 1 with an overjet of 4 mm and 3 for the crossbite, giving a total of 4 points)
Overjet Anterior crossbite
0-3 mm 0 No crossbite 0
3,1-5 mm 1 One or more teeth in edge-toedge occlusion 1
5,1-7 mm 2 A single tooth in crossbite 2
7,1-9 mm 3 Two teeth in crossbite 3
Greater than 9 mm 4 More than two teeth in crossbite 4

4. Overbite

  • The most severe overbite of the 4 incisors is assessed.
  • The 'covering' of the lower incisors or degree of open bite (from upper incisal margin to lower incisal margin) is measured.
Open bite Overbite
No open bite 0 ≤ one third 'covering' of the lower incisors 0
Open bite ≤ 1 mm 1 between one third and two thirds covering 1
Open bite 1.1 mm to 2 mm 2 more than two thirds covering 2
Open bite 2.1 mm to 4 mm 3 covering of the entire tooth crowns or more 3
open bite ≥ 4 mm 4    

5. Centerline

  • Transverse distance between the upper and lower dental centerline
Centerline assessment
Matching or displacement by up to ¼ the width of the lower central incisor (i.e. approximately ≤ 1.5 mm) 0
Displacement by 1/4 to 1/2 the width of the lower central incisor 1
Displacement by more than half the width of the lower central incisor 2

PAR index ruler