MODEL ANALYSIS
Study casts :
are essential diagnostic aids
The study models provide a three dimensional view of the maxillary and mandibular
dental arches. They represent the teeth and their supporting tissues and also the
relationship between the upper and lower dental arches in centric occlusion.
Dr. Azzam Aljapri
1- CAREY'S ANALYSIS
Many malocclusions occur as a result of discrepancy between arch length and tooth material
It is performed on the lower cast.
The same analysis on upper cast is called Arch Perimeter Analysis
- Determination of arch length :
The arch length anterior to the first permanent molars is measured using a soft brass wire.
The wire is placed contacting the mesial surface of the first permanent molar of one side and
along the anteriors and is continued on the opposite side in the same way up to themesial
surface of the opposite first permanent
- Determination of tooth material :
The mesio-distal width of the teeth anterior to the first molars (second premolar to second
premolar) is measured and summed up. The mesio-distal width is measured from the mesial
to the distal anatomic contact points of each tooth
Inference
- If the discrepancy is between 0 to 2.5 mm proximal stripping can be carried out to reduce
the tooth material
- If the discrepancy is between 2.5 to 5 mm it indicates the need to extract the second
premolars
- If the discrepancy of more than 5 mm indicates the need to extract the first premolars
Dr. Azzam Aljapri
2- ASHLEY HOWE'S ANALYSIS
Ashley Howe considered tooth crowding to be due to deficiency in arch width rather than
arch length.
- Determination of total tooth material (T.T.M.)
The mesio-distal width of all the teeth mesial to the second permanent molars is measured
with the help of dividers
and all the values are summed up. This value is called the total tooth material. The mesio-
distal width is measured from mesial to the second permanent molars
- Determination of premolar diameter (P.M.D.)
The premolar diameter refers to the arch width from the tip of the buccal cusp of one first
premolar to the tip of buccal cusp of the opposite first premolar
- Determination of premolar basal arch width (P.M.B.A.W.)
The measurement of the width from canine fossa of one side to the other gives us the width
of the dental arch at the apical base or the junction between the basal bone and alveolar
process
The canine fossa is found distal to canine eminence.
Inference:
The PM.B.A. W. and the PM.D. are compared
- If the PM.B.A.W. is greater than the PM.D then it is an indication that arch expansion is
possible.
- If on the other hand the PM.B.A.W is less than PM.D., then arch expansion is not possible
According to Ashley Howe, to achieve normal occlusion with a full complement of teeth the
basal arch width at the premolar region should be 44% the sum of mesio-distal widths of all
the teeth mesial to the second permanent molar (total tooth material)
This ratio (expressed as %) between the apical base width at the premolar region and the
total tooth material is called the premolar basal arch width percentage. It is determined
using the formula :
PM.B.A.W. %= PM.B.A.W. x 100 T.T.M
A. If PM.B.A.W. % is 37% or less, it indicates a need for extraction
B. If PM.B.A.W. % is 44% or more, the case can possibly be treated without extracting any
teeth
C. If PM.B.A. W. % is 37 - 44%, the case is referred to as a borderline case
Dr. Azzam Aljapri
3- PONTS ANALYSIS
Ponts in 1909 presented a system whereby the measurement of the four maxillary incisors
automatically established the width of the arch in the premolar and molar regions.
Ponts analysis helps in:
A. Determining whether the dental arch is narrow or is normal
B. Determining the need for lateral arch expansion
C. Determining how much expansion is possible at the premolar and molar regions
Dr. Azzam Aljapri
- Determination of sum of incisors (S.I.)
The mesio-distal width of the 4 maxillary incisors is measured and the values summed up.
This value is called sum of incisors ( S.I )
- Determination of measured premolar value (M.P.V.)
The width of the arch in the premolar region from the distal pit of one upper first premolar
to the distal pit of the opposite first premolar is measured. It is called the measured
premolar value
- Determination of measured molar value (M.M.V.)
The width of the arch in the molar region from the mesial pit of one upper first molar to the
mesial pit of the opposite first molar is measured. This value is called the measured molar
value
- Determination of calculated premolar value (C.P.V.)
Calculated premolar value or the expected arch width in the premolar region is determined
by the formula: S.1. x 100 / 80
- Determination of calculated molar value (C.M.V.)
Calculated molar value or the expected arch width in the molar region is determined by the
formula: S.I. x 100 /64
Inference:
If measured value is less than calculated value it indicates the need for expansion.
Dr. Azzam Aljapri
