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 and bracket prescription or selection of wires

5. 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.

Felton et al 1987 about arch 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 select the 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 expansion the 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 consideration during 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.