Average Tooth Measurements Anterior to First Permanent Molar Most tooth mass during mixed dentition!!
Occlusal Changes in the Mixed Dentition Flush terminal plane may lead to: –Class I (with late mesial shift) –Class II –End to end Mesial step may lead to: –Class I (normally) –Class III Distal step leads to Class II
Flush Terminal Plane FTP Class II Class I (desirable) ETE
Mesial Step Mesial Step Class III Class I (desirable)
Class II division 1, transverse dimension Changes in arch length and width follow same general pattern as normals Class II, div 1 have relative constriction of the maxillary arch Constriction present at three stages of dental development, and does not self correct. Bishara, et al, Longitudinal comparisons of dental arch changes in normal and untreated Class II, division 1 subjects and their clinical implications AJO, November 1996.
Leeway Space Sum of (c-d-e) > (3-4-5) This allows more space for 3-4-5 This "leeway space" averages 1.7 mm. in the mandibular buccal segment; 0.9 in the maxillary buccal segment
Late Mesial Shift Refers to mandibular permanent molar moving mesially Good news - if permanent molars are ETE, late mesial shift allows mandibular molar to move into a Class I occlusion Bad news - the above reduces arch length
Late Mesial Shift (cont.) FTP Class I Molar moves into this space...
Incisor Liability Permanent incisors are larger than primary incisors This difference in size is termed "incisor liability" How does the body create enough room for the larger, permanent incisors?
Overcoming Incisor Liability Interdental spacing of primary incisors Intercanine arch width growth Labial positioning of the permanent incisors Favorable size ratio between the primary and permanent incisors
Overcoming Incisor Liability Interdental spacing of primary incisors Good interdental spacing of primary incisors allows for better alignment of the larger permanent incisors.
Overcoming Incisor Liability Labial positioning of the permanent incisors –permanent incisors erupt to a more labial position –permanent incisors are angled more labially –the above creates more arch length
Overcoming Incisor Liability Favorable size ratio between the primary and permanent incisors –size ratio between the primary and permanent incisors may be favorable or unfavorable –favorable: large primary, small permanent –unfavorable: small primary, large permanent
Mixed Dentition Analysis Why: predict amount of crowding after permanent teeth come in When: during mixed dentition What: boley guage, models, prediction table Where: your office Who: YOU!!!
MDA Methodology Measure mesial-distal diameter of the mandibular incisors and sum Measure space available for mandibular incisors Subtract #1 from #2; negative number indicates crowding in the incisor region
MDA Methodology (cont.) Measure space available for 3-4-5 on each side of the arch Calculate from prediction table the size of 3-4-5 Subtract #2 from #1 on each side
MDA Methodology (cont.) At this point, you should have three numbers: –number for incisor crowding or excess space –number for right buccal segment crowding or excess space –number for left buccal segment crowding or excess space Add the three numbers; negative = crowding, positive = space
REMEMBER! MDA Methodology (maxillary arch) Use mandibular incisors to predict the size of the maxillary 3-4-5 Do everything else the same as described for mandibular teeth
MDA Example LeftIncisorsRight Space Available 19.8 mm Tooth Size 23.0 mm Difference Measure 21-12, measure space available for 21-12
MDA Example LeftIncisorsRight Space Available 19.8 mm. Tooth Size 23.0 mm. Difference -3.2 mm. Subtract tooth size from space available
MDA Example LeftIncisorsRight Space Available 20.1 mm.19.8 mm.19.5 mm. Tooth Size 23.0 mm. Difference -3.2 mm. Measure space available for 3-4-5 on each side
MDA Example LeftIncisorsRight Space Available 20.1 mm.19.8 mm.19.5 mm. Tooth Size22.2 mm.23.0 mm.22.2 mm. Difference -3.2 mm. Calculate size of 3-4-5 from prediction table
MDA Example LeftIncisorsRight Space Available 20.1 mm.19.8 mm.19.5 mm. Tooth Size22.2 mm.23.0 mm.22.2 mm. Difference-2.1 mm-3.2 mm.-2.7 mm. Subtract tooth size from space available on left and right
MDA Example LeftIncisorsRight Space Available 20.1 mm.19.8 mm.19.5 mm. Tooth Size22.2 mm.23.0 mm.22.2 mm. Difference-2.1 mm-3.2 mm.-2.7 mm. Total difference is -8.0 mm.
Allowance for Late Mesial Shift If permanent molars are end to end, mandibular molar should move mesially to obtain Class I interdigitation. This will decrease the arch length available for 3-4-5. Figure 1.7 mm. per side with ETE relationship.
Other Factors Influencing Amount of Predicted Crowding Anterior-posterior position of incisors Position of molars Curve of Spee