Arch form and width / for orthodontists by Almuzian

University of Glasgow
Arch form and width
MOHAMMED ALMUZIAN
1/1/2013

Arch form and width
Arch form may be described as the arch formed by the buccal and facial surfaces
of the teeth when viewed from their occlusal surfaces.
Factors determine the arch forms
1. Ethnicity with underlying genetic basis that determines the basal bone which
accommodate teeth. In Caucasian population, 45% have ovoid, 45% tapered
and 10% square
2. Type of malocclusion, like in cl3 the majority are square form,
3. Musculature which adapt the above position to the new one
4. Environmental factors like in standing teeth, habits and crowding
5. Orthodontic treatment
Implications of the arch form and width in orthodontic
1. Dental aesthetics
2. Heath status of the periodontal ligament
3. Treatment planning (space available)
4. Treatment mechanics
5. bracket prescription
6. selection of wires
7. Stability and prognosis.
Types of the archform
1. Bonwill-Hawley Archform

 in 1955 Hawley proposed a geometric method for predetermining the dental
arches
 The ideal arch was based on an equilateral triangle with a base representing
the inter inter-condylar width.
 The lower anterior teeth were arranged on the arc of a circle with a radius
determined by the combined width of the lower incisors and canines, with
the premolars and molars aligned with the second and third molars turned
toward the center.
1. Catenary curve by Scher 1949
 It is a shape formed by a length of chain held at each end and allowed to
drop
 archform good as far back as first molars, but ignores narrowing of archform
over the second molars
1. Brader/Trifocal Ellipse , Brader 1972
 Similar to anterior segment of the Catenary curve but the posterior segments
taper inwards providing a narrower arch posteriorly
1. Conic Sections , by Currier 1969,
 the simplest family of plane curves after straight lines
 fits as well as any ideal arch, provided the second and third molars are not
included
1. Andrews 1973
he developed his 'ideal' archform based on a computer analysis of 120 non-
orthodontic 'norms'

2. Roth,
"Tru-Arch" (A company): broader width labially than Andrews' archform
3. Bennet,
"Euroarch (Precision Orthodontics): smaller version of Andrews' archform
4. MBT
Three archforms available in the MBT system (tapered, square, ovoid)
5. Computer prediction by BeGole 1979,
Various other archforms have been constructed using algebraic equations
6. individualized arch form
7. Lee et al, suggested that a range of archforms be used, identical in shape but
varying in size. A clear perspex sheet with archforms of various sizes (90%,
95%, 100% and 105%) should be placed over a model of the patient's lower
arch before the start of treatment to determine the most appropriate size of
archform to be used throughout treatment. It should be noted that the
archform is decided on the basis of where the archwire would lie in relation
to brackets on the teeth, not on the basis of the occlusal edges of the teeth.
It will frequently be found that the 100% archform is suitable for:
- Non-extraction cases with intercanine widths between 24mm
- Extraction cases with intercanine widths between 26mm
The 95% or 90% archforms should be used where the inter-canine width is
smaller, particularly in crowded extraction cases.

Feltonet al 1987 aboutarch form found that
1. There is no generalised archform
2. Archform should be tailored to the original arch shape of an individual
otherwise it will relapse as (Little 1999) mentioned.
3. No particular archform was the closest match for more than 20% of cases
and so the individual adjustment is necessary
Arch Width
Normal growth and development of the arch form and width
1. Arch dimensions change with growth.
2. It is therefore necessary to distinguish changes induced by appliance therapy
from those that occur from natural growth.
3. The average changes achieved in a sample reported by Moyers et al 1976
 The changes in width vary between males and females. The male have more
growth
 More growth in the upper than the lower arch. This growth occurs mainly
between the ages of 7 and 12 years of age and is approximately 2 mm in the
lower arch and 3 mm in the upper.
 After the age of 12, growth in arch width is seen only in males while the
female show constriction
 Changes in arch width may not be accompanied by changes in arch length;
there is a tendency toward a decrease in arch depth in the third and fourth
decades.

Studies on relapse in archform (Felton, Little and others)
1. Arch form changes: 65% of cases had a change in archform, and 65%
returned to their pre-treatment shape (Total relapse).
2. Arch width changes
1. Growth: There is no evidence that appliances can stimulate "growth"
beyond that which would normally occur.
2. Age: It seems logical to consider increasing arch size at a young age so that
skeletal, dental-alveolar, and muscular adaptations can occur before the
eruption of the permanent dentition.
3. Amount: Approximately 3 mm stable upper molar expansion can be
achieved and stable. Approximately 1 mm stable lower molar expansion can
be achieved and stable
4. Exceptional Local factors
 Buccally or lingually displaced canine can be repositioned to their normal
position without risking the stability of arch width changes.
 Deep-bite cases (such as Class 11/2 cases) in which lower canines have
inclined lingually in response to the palatal contour of the upper canines
(1974, Shapiro)
 Cases where rapid maxillary expansion is indicated in the upper arch and
this expansion is maintained post-treatment (Haas 1972, Sanstorm, 1998)
but to very limited extents.
1. Extraction effects:
 Non-extraction cases: The archform tended to expand in the intermolar and
interpremolar width.
 Extraction cases: The archform tended to contract in the intermolar and
interpremolar width.

 Arch expansion is more likely to be stable in the absence of extractions and
is most effective in the posterior region.
1. Change in the post retention phase:
 Large individual variation in the stability of archform posttreatment
 Pretreatment archforms appear to be the best guide to future stability
 Greater the treatment change associated with greater postretention change
 The mandibular intercanine width tended to relapsed to the original
 The non-extraction cases did not show significant relapse, the inter first
premolar width in particular being stable with expansion
 In the extraction cases, the intercanine width was much more prone to
relapse after retention if the pre-treatment dimension had been increased
during treatment.
1. Evidences:
The paper by Burke et al 1998 confirms
 The overall message from the orthodontic literature that if arch form is
changed during orthodontic treatment, in many cases there will be a
tendency for relapse to the original dimensions. This is particularly true of
inter-canine width.
 Changes in inter-molar width seem to be more stable.
 Another point is that there is a great variation in the arch form which need to
be customized
 In Caucasian population, 45% have ovoid, 45% tapered and 10% square
MBT arch-form philosophy

1. The Tapered Arch Form
Indication
 Patients with narrow tapered arch forms.
 Gingival recession in the cuspid and bicuspid regions. This situation occurs
most frequently in adult orthodontic cases.
 Also, cases with tapered arch forms undergoing partial treatment in one
arch only benefit from this arch form, so that no expansion occurs in the
treated arch.
1. The Square Arch Form
Indication
 Cases with broad arch forms.
 In cases that require buccal uprighting and expansion of the arch.
 If over expansion has been achieved.
1. The Ovoid Arch Form
Indication
 This arch form has been used in the majority of treated cases.
 During initial archwire stages like when using multistrand wires, .014
and .016 stainless steel round wires, and all Nitinol Heat-Activated
nickel titanium wires.
 When using .018, .020 round stainless steel wires and rectangular stainless
steel wires (wires that significantly influence arch form) one of the above
three arch forms is selected.

Systematic method to selectthe AW form
1. Initial light wire can be used in any form, it has little influence because of
their low force and they used for short period (015 or 017 multistand, 012,
014 NITI, 016HANT or 014 SS).
2. Intermediate AW (mean any AW stiffer than above) should be customized
by: Using clear template to select the arch form from original SM or
individual patient form (IAF) technique
3. Rigid working AW (19*25 SS) should be customized using clear template to
select the arch form from original SM or individual patient form (IAF)
technique then shaping the upper arch 3mm wider than lower one
Individual patient form (IAF) technique:
1. After the rectangular HANT stage a wax template is moulded over the lower
arch to record the indentations of the brackets
2. The .019/.025 stainless steel archwire is bent to the indentations in the wax
bite
3. The wire is then compared with the starting lower model, or a Xerox copy of
the model, to ensure that it closely resembles the overall starting shape.
4. The wire is then checked for symmetry on a template.
5. Finally, a Xerox copy of the wire is made and stored in the patient notes.
6. This is the patient's IAF.
7. The uppers in a form which is 3 mm wider.
8. Archwire coordination is important throughout treatment, especially with
the heavier round wires and the .019/.025 rectangular stainless
There are some cases that will require arch form modification from the normal
IAF and the usual upper/lower archwire coordination.
1. Modification due to posterior torque considerations

The additional buccal root torque in the upper molar brackets tends to narrow the
upper arch, and the progressive buccal crown torques in the lower posterior
brackets tends to upright the lower molar teeth and widens the lower arch. The
combined effect of these appliance features can be a tendency towards molar
crossbite in some cases. When this is observed, the posterior segment of the upper
archwire can be widened to 5 mm wider than the lower archwire in the molar
regions.
2. Modification after maxillary expansion
After the upper arch has been expanded with a rapid maxillary expander or a
quadhelix two things can occur. First, the lower arch compensates by uprighting
buccally, and second, the upper arch tends to relapse. To manage these effects,
the lower arch can be widened by using a wider arch form (usually one size wider
- for example from tapered to ovoid) and the upper arch expansion can be held
with a correspondingly wider arch form.
3. Asymmetries
In cases where it is clear that the patient has an arch asymmetry, and there are
many such cases, the archwires later in the treatment may be modified to assist
correction of the asymmetry
Techniques for expansionthe arch using arch wire
1. Wide 0.019*0.025 SS arch wire
There is a correct technique for archwire expansion. If the wire is bent to expand
its width, it is important to make sure it is not over expanded and thus distorted
from the arch form. When the ends of the expanded wire are held, and pressed
back towards the chosen arch form (IAF), the wire should match that shape. If
over expanded or incorrectly expanded, it will not match the chosen arch form

(IAF) when the ends are pressed towards it, and this will cause problems due to
narrowing or widening of the inter-canine width.
2. Upper arch expansion with a jockey wire
There are limits to the expansion force which can be delivered by one .019/.025
rectangular wire during routine treatment. If necessary, particularly near the end
of treatment, a little more expansion force can be achieved by using a 'jockey
arch' . This is merely a second archwire, also expanded, and tied in place over the
normal archwire. The jockey arch may be of .019/.025 rectangular steel, or of
heavier round steel wire. If the upper first molars carry headgear tubes, it can be
convenient to end the jockey archwire in those tubes. It is helpful if the
normal .019/.025 wire has buccal root torque in the molar region to attempt
bodily movement of molars and avoid tipping. It is important to have adequate
bone width to achieve upper molar expansion
MBT archwire considerationduring settling stage
1. Typically, a .014 or .016 round I IANT wire is used in the lower arch,
coordinated to the IAF for the patient. In the upper arch, a .014 round
sectional wire can be placed from lateral incisor to lateral incisor. These
wires can be accompanied by the use of vertical triangular elastics where
settling needs to occur.
2. Patients can be seen at approximately 2-week intervals during the settling
phase. Elastics can be worn fulltime for the first 2 weeks, then at night for a
period of 2 weeks, if settling is adequate. Debanding can then be scheduled.
3. Some variations to this general settling technique are as follows:
o If cuspids were labially displaced in the upper arch, the sectional wire
in the upper anterior segment can be extended to the cuspids to hold
them in position.

o If diastemas were present in the upper and lower anterior segments,
these areas should be tied together lightly with elastic thread or
ligature wires
o If teeth have been extracted, figure-8 ligature wires should be placed
across the extraction sites to hold them closed.
o If palatal expansion was carried out, a small removable palatal plate,
with .018 wires extending inter-proximally in the gingival areas, can
be used to maintain expansion during the settling phase
o if the original malocclusion was moderate to severe Class 11/1
malocclusions a full upper .014 archwire can be used in settling and
this wire can be bent back behind the most distal molars. This
controls the overjet, but inhibits settling of the posterior teeth
somewhat. Archwire bends may therefore be placed where individual
teeth need to settle.
o If it is intended that settling may take longer than approximately 6
weeks, it is beneficial to leave the lower rectangular steel wire in
position during this extended settling phase. This will help to
maintain lower arch form.

Arch form and width / for orthodontists by Almuzian

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