Mixed Dentition Analysis: Space and Arch Length

Rationale of Mixed Dentition Analysis

• You should compare the space available to the space required
• There are then 3 possible scenarios:
  • Space excess
  • OK
  • Space deficiency

Mixed Dentition Space Analyses

• Used to estimate the amount of spacing or crowding, for a patient, if the primary teeth were replaced by their successors when the analysis is performed
• They do not predict the amount of spacing or crowding two or three years later
• They do not foresee the amount of natural decrease in arch perimeter which may occur during the transitional period without the loss of teeth

Tooth Size Arch Length (Perimeter) Analysis

• Common factor in malocclusion is the discrepancy in size between the teeth and the space available in the dental arch
• Crowding is the most frequently and most obviously seen arch length discrepancy
• Spacing is the opposite problem to crowding, and is excess space
• Third space-related problem is discrepancy in size between the teeth of the upper and lower dental arch, which makes normal occlusion and contact relationship unattainable
• Includes:
  • Arch length discrepancy:
    • Nance analysis
    • Moyers analysis:
      • Mixed dentition
      • Estimates space availability, during the transition from mixed dentition to permanent dentition -Tooth size discrepancy:
    • Bolton analysis:
      • Permanent dentition
      • Estimates the overbite and overjet that will likely be obtained after treatment has finished
      • Detects of improper occlusion and contact relationship (discrepancy in size between the teeth of maxillary and mandibular arch)

Dental Arch Dimensions

• Widths are C and D
• Arch length (depth) is B
• The circumference (perimeter) is A

Nance Analysis

• Arch length (perimeter) analysis in mixed dentition
• A space analysis in the mixed dentition
• Predicts the mesio-distal widths of the unerupted canines and premolars by using radiographs
• Total widths of the erupted central and lateral incisors are summed
• Space required = space necessary for alignment
• Measure with brass wire the arch perimeter from the mesiobuccal surface of one permanent mandibular first molar around to the mesiobuccal surface of the other permanent mandibular first molar
• Space available for alignment
• Space required is the total sum of the mesio-distal widths of erupted and unerupted permanent teeth (only 5 -5)
• Space available is using brass wire to measure the arch perimeter from the distal of the primary 2nd molar to the distal surface of primary 2nd molar on the other side
• Space available – Space required = arch length discrepancy
• • sign means excess space
• • sign means crowding
• Measure the greatest mesiodistal diameter of permanent teeth already erupted
• Measure any unerupted tooth from accurate bitewings
• Transfer the divider tips to the line
• One should repeat this process until all the anterior teeth and bicuspids are measured in both arches
• First molars are not measured
• Summarise the total mesio-distal diameters of 5 -5
• Cut a length of 0.25 soft brass wire approximately 4 inches long
• Adapt the wire to the normal arch shape
• Mark the wire at the mesial contact points of the permanent first molars
• Cut the wire at the contact points, and straighten
• Measure the wire with a caliper or a ruler
• If the space available is greater than the tooth size, it's interpreted as excess arch length
• If the tooth size is greater than the space available, the difference will indicate the amount of crowding which exists

Interpretation

• If the space available is greater than the tooth size, excess arch length is inferred
• If the tooth size is greater than the space available, the difference will indicate the amount of crowding that exists

Nance Analysis Disadvantage

• One disadvantage of the Nance Analysis is the possible teeth distortion

Moyers Analysis

• Introduced by Robert E. Moyers
  • Center for Human Growth and Development
  • University of Michigan, Ann Arbor
• This mixed dentition analysis aims to evaluate the amount of space available in the arch for succeeding permanent teeth and necessary occlusal adjustments
• Used to derive the sizes of unerupted canines and premolars from the knowledge of the sizes of permanent teeth already erupted (Probability Tables)
• Mandibular incisors are chosen for measurements:
  • Early in the mixed dentition
  • Measurable with ease and accuracy
  • There's a high correlation between the sizes of the mandibular incisors and the combined sizes of the canines and premolars in either arch
  • Little tooth size and shape variations in comparison with maxillary incisors
  • Very rarely congenitally missing
• Advantages of Moyers Analysis:
  • Minimal systematic errors
  • Equal reliability by the beginner or the expert (it does not require sophisticated clinical judgment)
  • It's not time-consuming
  • No need for special equipment or radiograph
  • Dental casts (advised) or in the mouth can be used
  • Used for both maxillary and mandibular arch measurements
  • Space Available:
    • Set the Boyle gauge to a value equal to the sum of the widths of right central and lateral incisors
    • Place one point of the gauge at the midline between centrals and let the other point lie along the arch perimeter on the right side and mark it with a pencil
    • Repeat this process on the left side
    • Measure the distance from the point marked on the model to the mesial surface of 1st permanent molar (Space available for 345)
  • Measure the width of each lower incisor
  • Record the widths on the chart
• Lower incisors (mandible) are measured to:
  • Establish the space available for the lower unerupted canines, first, and second premolars
  • Predict the size of the lower unerupted canines, first, and second premolars
• Upper incisors (maxilla) measures are used to:
• Establish the space available for the upper unerupted canines, first, and second premolars
• Lower incisors are measured
  • Predict the size of the upper unerupted canines, first, and second premolars
• Maxillary incisors are NOT used for predictive procedures:
  • Too much variability in size and shape
  • Poor correlation with other groups of teeth

Bolton Analysis (1952)

• This analysis is based on the ratio between the sums of the mesiodistal widths of maxillary and mandibular teeth derived from 55 patients with perfect Class I occlusion
• Estimates the overbite and overjet that will likely be obtained after treatment is finished
• Estimates the effects of extractions on posterior occlusion and incisor relationship
• Identifies occlusal discrepancies produced by interarch tooth size incompatibilities
• Performed on 55 cases with excellent Class I occlusion:
  • 44 orthodontically treated
  • 11 untreated with full dentition
• Teeth measured by:
  • Mesiodistal widths of 12 maxillary teeth
  • Mesiodistal widths of 12 mandibular teeth
  • Overbite (in terms of %)
  • Overjet (in terms of mm)
• Measure the greatest mesio – distal width of each tooth with dividers
• Transfer the divider tips to the lines beginning at the midline
• Continue on both sides of the midline until the first molars are measured
• Anterior ratio
  • 3-3/U 3-3 X 100
• Overall ratio
  • L 6-6/U 6-6 X 100
• Total size discrepancies have been conventionally described as relative excess in tooth structure in one arch
  • <77.2% - excess in U3-3

  • 77.2% - excess in L3-3

  • < 91.3% - excess in U6-6

  • 91.3% - excess in L6-6

• OB (overbite): 31.3% / OJ (overjet): 0.74mm
• Anterior Ratio
  • Range: 74.5 – 80.4
  • Mean: 77.2
  • SD: 1.65
• Overall Ratio
  • Range: 87.5 – 94.8
  • Mean: 91.3
  • SD: 1.91

Managing Bolton Size Discrepancies in the Anterior Segment

• For Generalized large mandibular teeth relative to maxillary anterior teeth:
  • Reduce the width of the lower incisors (interproximal stripping – not more than 2 to 2.5 mm) + increase lingual crown torque (crown inclination)
  • Extraction of one mandibular incisor
• For Generalized small maxillary anterior teeth relative to mandibular anterior teeth:
  • Mild discrepancy: increase the distal root tip (increase the angulation of the teeth)
  • Significant discrepancy: combined orthodontic restorative technique (i.e: veneers, proximal build up)
• Small maxillary lateral incisors:
  • Restoration (Veneers, proximal build ups)

Mixed dentition analysis compares available space to required space, identifying space excess, adequacy, or deficiency. It estimates spacing or crowding upon primary teeth replacement but does not predict future changes or natural arch perimeter decreases. Tooth size-arch length discrepancies, like crowding or spacing, are key factors in malocclusion.