UG Model analysis Ashley Howe's index

ASHLEY  HOWE’S  ANALYSIS

Parameters

·      TTM - Refers to sum of the mesiodistal width of the teeth from first molar to first molar

·      Premolar width (PMD) - Arch width is measured from the buccal cusp tips of the first premolar on one side to the buccal cusp tip on other side

·      PMBAW (premolar basal arch width) -  measured from the root apices of the first premolar on one side to the first premolar on other side

FIG 20  : Placement of dividers distal to the canine eminence to measure PMBAW

·      

Canine Fossa width - Measurement of width from canine fossa (immediate distal to canine root eminence) of one side to the other (Fig 20) gives us the width of the dental arch at the apical base or the junction between the basal bone & alveolar process

•       If canine fossa is not clear,  measurement is made from a point that is 8 mm below the crest of interdental papilla distal to canine

Following measurements have to be obtained

1.    Percentage of PMD to TTM

                  PMD X 100

                      TTM

2.    Percentage of PMBAW to TTM

                 PMBAW X 100

                         TTM

       

       Inference           

  If PMBAW > PMD :-

   it is an indication that basal arch is sufficient to allow expansion

  If PMD > PMBAW :-

    Can be three possibilities

   1) Contraindicated for expansion

   2) Move teeth distally

   3) Extract some teeth

•       According to Ashley Howe, to achieve normal occlusion with a full complement of teeth PMBAW should be 44% of TTM

•       Therefore if PMBAW X 100/TTM is:

   a) if 37% or less             case requires extraction   

   b) if 44% or more             treatment by non-extraction

   c) if between 37-44%             (border line case) may/may not require extraction                 

UG Model Analysis Pont's and Linder Harth index

Pont’s Index

Pont’s index helps in:

1.    Determining whether the dental arch is narrow or normal

3.    Determining how much expansion is possible at premolar and molar regions

The greatest width of incisors is measured with calipers recorded on a line, & their sum is recorded in millimeters

              This is termed as “sum of incisors” (SI)

Calculated premolar value (CPV)

The required arch width in the premolar region is calculated by formula:-

                 SI X 100

                     80

Calculated molar value (CMV)

The required arch width in the molar region is calculated by formula:-

                 SI X 100

                     64

Measured premolar value (MPV)

The distance between the upper right first premolar & upper left first premolar (that is the distal end of the occlusal grove) is recorded (Fig 17) & called as “measured premolar value” (MPV)

FIG 17 : Measurement of premolar width (MPV)

Measured Molar value (MMV)

The distance between the upper right first molar & upper left first molar (that is the mesial pit on the occlusal surface in maxillary arch ) is recorded (Fig 18) & is termed as “measured molar value” (MMV)

FIG 18 : Measurement of molar width (MMV)

Inference                    

Difference between the measured & calculated values indicate expansion or contraction of arches. The amount of contraction determines the needs for expansion

  1) If measured value is less, expansion is required

  2) If measured value is more, inference of expanded arch is drown and further expansion cannot be done.

Linder Harth Index

It is very similar to Pont’s analysis however he made a variation in the formula to determine the calculated premolar value as per the population sample.

·      Calculated premolar value:-

                    SI X 100

        85                      

·      Calculated molar value:-

        SI X 100

             65                                                                                                                               

UG Model Analysis Measurement of crowding and spacing

Measurement of Crowding

Crowding of the teeth can be measured in 4 segments for e.g. for the upper arch measurements are taken from distal of 15 to mesial of 13, mesial of 13 to mesial of 11, mesial of 11 to mesial of 23 and mesial of 23 to distal of 25.

FIG 15: Measurement of crowding (Measure along the alveolar ridge from contact point to contact point in segments with a divider)

As can be seen, when we go from one segment to other, a common point is considered between two segments to prevent missing out the crowding present between two segments. In each of these segments the combined mesiodistal measurements of the teeth in the segment are added up and from it, the actual space available is subtracted which will give us the individual crowding present in each segment. The sum of the crowding measured in the 4 segments is taken as the total crowding.

Interdental spacing

Interdental spacing is measured with a transparent scale between all the teeth mesial to 6. (Fig 16)

FIG 16 : Measurement of interdental spacing

A commonly made mistake in cases of palatally or buccally blocked out tooth is calculating the actual space available for the tooth as spacing (Fig 17)

FIG 17

UG Model Analysis Measurement of Proclination

Measurement of proclination by direct method

The model to be analyzed is kept on the table. A set square is used so that the edge of the set square is perpendicular to the floor and touches the labial surface of the most proclined teeth. The proclination is measured from the deepest point in the sulcus to the edge of the set square. (Fig 12)

FIG 12 : Measurement of proclination of incisors

In the upper arch, the anteriors are ideally slightly proclined, so, whatever is the proclination value, 2mm (normal proclination of upper incisor) is subtracted and the remaining value is the proclination to be corrected. Retroclined anteriors are measured as negative proclination (Fig 13)

FIG 13 : Negative proclination measurement in case of retroclined incisors

In contrast with the upper, the lower teeth are supposed to be upright on the basal bone therefore there is no subtraction in the lower arch.

We have to remember that the proclination value that we get is the value on one side of the arch. So, to get the total proclined value, we have to multiply by 2 if the proclination is equal on both sides. In some cases the two central incisors have different proclination values, in such cases we have to take the mean of the values. (Fig 14)

FIG 14 : Measurement of proclination of differently inclined incisors

There, will also be some cases where only one central incisor is proclined and other teeth are normally inclined where we have to calculate it as crowding.

UG Model Analysis Calculation of TTM and Sum of Incisors

The width and the position of the teeth in all the three planes can be tabulated in the following manner (Table 1)

On the basis of the above measurements we come to certain values like

TTM (Total tooth material) – Sum of the measured mesiodistal widths of all teeth. Maxillary and Mandibular TTM are measured separately. 

The normal TTM for the maxillary arch is 91.7 mm and for the mandibular arch is 84mm, these are the values closest to Indian population.

SI (Sum of Incisors) - Sum of mesiodistal widths of upper and lower incisors are measured separately. The normal SI for the maxillary arch is 31.7 mm and for the mandibular arch is 24mm.

Some inferences can be obtained on the basis of these values

·      TTM more than normal indicates wider teeth and less than normal indicates narrower teeth, to be entered in the table as +ve or –ve respectively.                                      

·      SI more than normal indicates wider incisors and less than normal indicates narrower incisors, to be entered in the table as +ve or –ve respectively.

·      If SI is normal and TTM is more it indicates wider posterior teeth and if TTM is less indicates  narrower posterior teeth

·      If TTM is normal and SI is more it indicates that the posterior teeth are narrower and if SI is less it indicates posterior teeth are narrower.

The significance of this reading is that if the SI is more, and then only in such cases slenderisation can be used as a method of gaining space.

UG Model Analysis Position of teeth

Position of the teeth

The position of teeth can be described in three planes of space

·      A-P plane  - whether the teeth are proclined,retroclined or normally inclined (Fig 5,6)

FIG 5 : Occlusal view of retroclined teeth  (Upper right central and lateral incisor)

FIG 6 :  Lateral View of proclined incisor (upper left lateral) and retroclined incisors (the remaining incisors)

Vertical plane - whether the teeth are Supraerupted or infraerupted. (Fig 7, Fig 8)

FIG 7: Infraerupted second bicuspid

FIG 8 : Supraerupted lower incisors, lower left canine may be considered as infraerupted

Transverse plane- Whether the posterior teeth are buccally placed or palatally placed. 

(Fig 9)

FIG 9 : Occlusal view showing buccally placed lower right first bicuspid and labially placed lower right central incisor and lingually placed lower left incisors

Whether any  teeth are rotated .i.e. mesiopalatal/distobuccal or mesiolingual/ distopalatal rotations (Fig 10)

FIG 10 : Mesiolingually/Distolabially rotated lower central incisors (Mesial is lingual)/Distal is labial)

Angulation of the anteriors  i.e mesially tipped or distally tipped also need to be mentioned in the transverse plane. (Fig 11)

FIG 11 : Mesially tipped central incisors.

UG Model Analysis Tooth width measurement

The mesiodistal dimension of the teeth is measured. The teeth are measured mesiodistally at the maximum interproximal width or for ideally aligned teeth it would be at the contact point. Digital calipers or a Boley gauge is used for measurement. (Figs 1,2,3,4)

FIG 1 : Using dividers to measure width of tooth in a case of anterior crowding






FIG 2 : Using digital calipers for measuring tooth width

FIG 3 : Measuring tooth width in spacing

FIG 4: Measuring tooth width in crowding

The distal most tooth to be included for measurement in model analysis is the permanent 1^st molar. Permanent 2^nd and 3^rd molars are not included in model analysis.