Arch Forms 1-Ortho / orthodontic courses by Indian dental academy

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INDIAN DENTAL ACADEMY

CONTENTS:1. Introduction

2. Importance of Arch Forms

3. Normal growth & Development of Arches

4. Different concepts of Arch Forms

5. Relapse Tendency of Arch Forms

6. Practical solutions to maintain Arch Forms

7. Recent Developments

8. Conclusion


INTRODUCTION:

Many orthodontists have sought to find a universal ideal arch

form and although most studies have used similar materials- a

collection of untreated samples, there has been very little

agreement about the natural shape of this ideal arch.

Arch form may be described as the arch formed by the

buccal and facial surfaces of the teeth when viewed from their

occlusal surfaces. To the orthodontist, the shape of this arch

form holds the key to the final result of any case. Which arch

form do we follow? , has been the central question that has been

raised time and again and has haunted the orthodontist.


A review of literature shows the following assumptions:

• Algebraic or geometric formula

• The arches may be ellipsoidal, hyperbolic,

parabolic, square, omega etc.

• All ideal arch forms are of the same shape

and differ only in size

• Ideal arches are symmetrical

The basic principle of archform in orthodontic treatment should be

preserved, that would place the teeth in a position of maximum

stability www.indiandentalacademy.com

IMPORTANCE OF ARCH FORMS:1) STABILITY: Relationship between the arch form and stability cannot be ignored Joondeph & Reidel in one of their nine theorems for stability have stressed on the need to maintain the existing arch form, particularly in the mandibular arch for stability

2) OCCLUSION:Unless the teeth are aligned in a proper arch form in both upper and lower arches, the occlusion will not be normal. Angle(1907) emphasised this with his concept of Line of Occlusion.

3) ESTHETICS:Primary reason for the patient to take treatment. Teeth arranged in proper arch form, will improve smile value as proposed by Sarver(2003).www.indiandentalacademy.com

NORMAL GROWTH & DEVELOPMENT OF ARCHES

Arch dimensions change with growth hence it is important to distinguish changes induced by appliance therapy and by growth.

Acc. To Scott(1967) arch form is determined prior to muscular development and is independent of functional activity of the oral musculature.

Moorrees(1969) pointed out that considerable individual variation in arch form will occur with normal growth, with a tendency toward an increase in inter-molar width during change over from deciduous to permanent dentition.


A review of the change in arch form show the following findings:

male arches grow wider that female arches

Lower intercanine width increases significantly in changeover dentition but doesn’t change in permanent dentition after 12 years.

Upper and lower inter-molar widths increase spontaneously to a considerable extent between 7-18 years especially in males.

Little change in arch width in premolar region after 12 years

Changes in arch width may not be accompanied by changes in arch length. Tendency towards a decrease in arch depth in the 33rd and 4th decades.


DIFFERENT CONCEPTS OF ARCH FORM:1. BONWILL CONCEPT

2. BONWILL HAWLEY CONCEPT

3. ANGLES LINE OF OCCLUSION

4. APICAL BASE CONCEPT

5. CATERNARY ARCH FORM

6. BRADER ARCH FORM

7. ROCKY MOUNTAIN DATA SYSTEM

8. ROTH’S TRU ARCH FORM

9. RICKETTS PENTA MORPHIC ARCH FORM

10. MATHEMATIC & GEOMETRIC MODELS FOR ARCH FORMS

11.ARCH FOR DETERMINATION USING CONE BEAM COMPUTED TOMOGRAPHY


BONWILL’S CONCEPT OF ARCH FORM:Developed certain postulates for artificial dentures in 1885

He noted the tripod shape of the mandible is formed by an equilateral triangle, with its base between the condyles and the apex between the central incisors. length of each side approx 4 inches.

He emphasized “ the human body is in perfect consonance with geometry, physics and mechanics. If nature is given a fair chance to right herself, she will return to normal standard of mathematical and mechanical precision, to do otherwise would anihilate creation”


BONWILL & HAWLEY ARCH FORM:

Hawley in 1905, modified Bonwill’s concept.

He recommended thet the combined widths of the 6 anterior teeth serve as the radius of a circle and the teeth be placed on that circle. From this circle he constructed and equilateral triangle with the base representing the intercondylar width.

The radius of each arch varied depending on size of teeth, so the arch

dimensions differed as a function of tooth size but the arch form was

constant.

This was used as a guide for establishing arch form


ANGLE’S LINE OF OCCLUSION

Angle in 1906,described the LINE OF OCCLUSION – “he line of greatest normal occlusal contact” . But

in 1907, he rediscribed it as “ the line with which in form and in position according to type, the teeth must be in harmony if in normal occlusion.”

Ricketts(1997) redefined the line of occlusion to its

contemporary definition – “A distinctively individual line at the inciso-buccal contact, with a location, position & form to which the teeth must conform to be in normal occlusion”


APICAL BASE CONCEPT:

Was proposed by Lundstorm. He highlighted the need to consider

the apical base when determining the arch form for the patient.

“Orthodontic experiments showed that a normal occlusion attained

by mechanical treatment is not necessarily accompanied by a

development of apical base in harmony with the position of the teeth,

with the result that the occlusion cannot be maintained.”

“Occlusion doesn’t control form and amount of apical base

development but apical base is capable of affecting the dental

occlusion”


CATERNARY ARCH FORM:

Caternary Arch Form

Concept first proposed by David Musich & James Ackerman(1973). To measure the arch perimeter they used an instrument that was a modified Boley Guage with a chain incorporated in it - CATANOMETER

Schulhoff(1997) used the same concept to describe the lower arch. Caternary curve is the shape that the loop of a chain would take if it were suspended from 2 hooks. Shape of the curve depends on the length of the chain and the distance between the hooks.


When the width across the first molars is used to establish the

posterior attachments, a caternary curve fits the dental arch form

nicely for most individuals. Preformed archwires based on

average intermolar dimensions.

Bruide & Lilley found that the shape of basic bony arch at 9.5 weeks I.U , was

caternary design.

Caternary curve was made popular by work of McConail & Scher, who felt that

from an engineering and biological point of view, the caternary curve was the

simplest curve possible and could be easily explained mathematically


BRADER ARCH FORM

Brader in 1971, presented a mathematical model of dental arch form at the annual session of A.A.O for which he won Milo Hellman Research Award Of Special Merit.

He proposed that the arch form was a trifocal ellipse, which was based on the findings of Proffit, Norton & Winders. The trifocal ellipse was patterned after the shape of an egg- extremely resistant to collapse & produced stable arch form.


Traditionally, it was believed that the tongue pressure and the lip and cheek pressure was equal in magnitude and opposite in direction. This was disproved by Lear & Moorrees(1969). They found the tongue pressure was always more than the lip and cheek pressure. Also verified the time- pressure equilibrium hypothesis.

Not only pressure but duration of pressure should also be considered.


Therefore Brader hypothesized the arch form was a

Trifocal Ellipse and PR=C Where,

P = Pressure

R = Radius of curvature of ellipse curve at the pressure site

C = Mathematical ConstantHe also took data from Winders study and calculated the pressure exerted at different regions of the arch

BUCCOLABIAL P R C

MOLARS 4 28 112.0

PREMOLAR 4.9 23 112.7

CANINE 6.9 16.3 112.5

INCISOR 11.3 10 113

LINGUAL P R C

MOLAR 9.2 12.2 112.2

INCISORS 15.2 7.5 112.5

PR=C, was applied and was noted that the product of P and R was the samewww.indiandentalacademy.com

Thus the equation expressed the most fundamental association between forces and shape and revealed an inverse relation between force and curvature.

Then to find the tension exerted by the lips and cheeks, he used the Laplace Formula for elastic container,

Pi=Pe+T(1/R+ 1/R’), where; Pi= internal pressure

Pe= external forces

T= tension of elastic envelope

R= radius in horizontal plane

R’= radius in vertical plane

Pe=0, since atmospheric pressure is equal on both sides

R’ not considered as its contribution not known and may be of very small magnitude.

Therefore, Pi=T/R T=PiR. Since T=C,

Thus, the dental arch remains in a state of equilibrium coz the product of P & R on the lingual side (C) is always equal and opposite to the product of P & R on facial side (T).


Clinical Implications Of PR=C

1) Growth of Dental Arches

Brader suggested that the dental arches grow as a total curve, enlarging about its geometric centre. This internally centered curve orientation provides a new method for reliable comparison of arch forms in both serial and cross sectional investigations.

Effect of muscle forces can be noted in cases with patients with scars, hemifacial hypertrophy, atrophy and macroglossia.

2) Lower Incisor Crowding

PR=C, explains why mandibular incisor teeth exhibit many crowded positional variations and of all the teeth in the mouth, the least stability following positional changes due to treatment. In this anterior segment, the radius is smallest and the pressure is greatest, thus having a critical influence on this segment.www.indiandentalacademy.com

LIMITATIONS:1) It explains typical normal arch forms. It may not explain different

malocclusions.

2) It fits majority of cases but may no fit in arches which are tapered or square. In such cases the arch form must be altered accordingly

3) PR=C is a hypothesis and not a theory. Long term studies required for its verification.


ROCKY MOUNTAIN DATA SYSTEM

ROCKY MOUNTAIN DATA SYSTEM computer derived formula relies upon

measurements taken from inter molar width, inter cuspid width and arch

depth as measured from the facial surface of the incisors to the distal

surface of the terminal molar.

This allows computer to be programmed with Cartesian X & Y co-ordinates

that are necessary for arch computation.

Facial type is also considered

Arch design applicable only to the lower arch


INDIVIDUALIZED IDEAL ARCHESProposed by Larry White in 1978.

Undertook a study to see how a collection of ideal, untreated arches conformed to the predetermined arch forms of the most popular formulae.

Models of 24 orthodontically untreated superior, adult occlusions were collected and tracings made on acetate paper & overlays were superimposed.

The closeness of fit was evaluated and graded as ‘good fit’ , ‘moderately good fit’ and ‘poor fit’.

TYPE GOOD FIT MODERATE POOR FIT

BONWILL-HAWLEY

4 (8.33%) 19 (39.5%) 25 (52.8%)

BRADER 6 (12.5%) 21 (43.7%) 21 (43.7%)

CATERNARY 3 (27.08%) 22 (45.8%) 13 (27.08%)

RMDS 2 (8.33%) 22 (91.6%) -www.indiandentalacademy.com

Absence of Arch Symmetry

White also evaluated the symmetry of arches and the most conspicuous finding was the total absence of arch symmetry.

Thus reached to 2 conclusions:

1. No generalised, universal arch forms seems to be applicable.

2. Majority of dental arches are assymmetrical

Thus he advocated individualising arches by simple technique called “OCCLUSAL MAPPING”.

Draw occlusal surfaces of teeth from xray or photos. Proximal contacts are marked and a line is drawn through the mesio-distal dimensions of each tooth & connecting the lines across the proximal contacts.

This line represents the centre of the basic arch perimeter.


(1) (2)

(3)www.indiandentalacademy.com

RICKETTS PENTAMORPHIC ARCH FORMSConsidered the following factors in the determination of the arch form:

Arch correlation, size, arch length, where the arch was measured, contact details and form at the bracket location.

Originally 12 arch forms were identified from different studies. These were narrowed to 9 by computer analysis. Studies of other normal and stable treated patients resulted in elimination of all but 5 forms.

These Pentamorphic arch forms were such that they would fit most facial forms


ROTH TRU ARCH FORMDeveloped from biologically and clinically derived broad curves observed in

patients treated with Cetlin mechanics of functional appliances such as FR which

are referred to as “ Natural or Non-Orthodontic”.

The Roth Tru Arch was derived from his extensive clinical testing & recording of

jaw movement patterns in treated patients who were out of retention and had

remained stable.

This arch form mainly was wider by a few millimeters,

primarily in bicuspid area when compared o Andrews

normals and coincided exactly when superimposed on

Ricketts pentamorphic arch forms.

This arch form over corrects arch width slightly: over correction in all 3 planes of space is a part of Roth’s end of fixed appliance therapy goal.


MATHEMATIC & GEOMETRIC MODELS FOR ARCH FORMSMathematic models have been used for describing arch forms.

Lu(1964) claimed that the dental arch could be satisfactorily described by a polynomial equation of the 4th degree.

Sanin(1970) investigated the size and shape of ideal arches and confirmed the views of Lu.

Pepe(1975) analysed a sample of 7 models of normal occlusion by digitization and curve fits. The results showed that 4th order polYnomial equations were better than caternary curve fits and also suggested that 6th degree polynomial equations appear to have potential as clinical indicators of arch form.

Cubic Spline Function : used for modelling of various assymmetrical objects.Is an adaptation of the draftsmans spline. The mathematical adaptation of physical spline consists of a set of individual cubic polynomials between successive knot points and has been developed for use in describing normal dental arches


RESEARCH ARCH FORM/ CLINICAL ARCH FORM

Acc. To McLaughlin & Bennet, there is a difference between the clinical

and research arch form.

Braun etal (1966) represented arch form by a complex mathematical

formula known as “ Beta Function”. They measured the center of each

incisor incisal edge, cusp tips of canines and premolars and the M-D and

D-B cusp tips of molars. This research arch form can be surprisingly

tapered.

In contrast clinicians arch wire shape must be based on the points where

the wire will lie in the bracket slots of correctly positioned brackets.

This arch form relates to the mid point on the labial surface of the clinical

crowns of the teeth, and should include estimation for the in out which is

built into the bracket system. www.indiandentalacademy.com

Felton(1987) evaluated a wide range of manufactured arch wires from orthodontic companies and found that the arch forms fell into tapered, ovoid or square groups( first classified by Chuck in 1932). When superimposed they differed only in ICW (approx 6mm).


TAPERED ARCH FORM

-narrow intercanine width, used in patients with narrow arches and gingival recession in premolar canine region.

SQUARE ARCH FORM

-Used in cases with broad arches and those who require buccal uprighting

OVOID ARCH FORM

Good reliable arch form for a majority of the cases. Advisable to stock wires in ovoid shape, which then can be altered depending on the case


RELAPSE TENDENCY AFTER ORTHODONTIC TREATMENT

1. In 1969, Reidel reviewed literature concerning stability of arch form;

- When intercanine and inter molar width had been changed during orthodontic treament, there was a strong tendency for these teeth to return to their pre treatment positions

- Thus he postulated that arch form particularly in the mandibular arch cannot be permanently altered during appliance therapy (primarily in nonextraction cases)

2. In extraction cases, Strang(1946) and Howe(1960) , suggested that intermolar width was normally decreased during extraction treatment. However, if canines were moved distally into extraction sites, they could be expanded buccally to limits offered by their new distal location. But Lee (1999) proposed that retraction of canines into extraction sites was not generally associated with stable expansion. www.indiandentalacademy.com

3. Shapiro(1974) studied arch lengths, intercanine and intermolar widths in

22 extraction and 58 nonextraction cases and concluded that:

- Mandibular intercanine widths (ICW) showed strong tendency to return to

its pre treatment dimensions except in Class II div 2.

- Expansion in ICW in treated Class II div 2 cases showed significantly

greater stability than Class I or Class II div 1. Coz Class II div 2 shows deep

bite with lower canines inclined lingually. When bite is opened incisal edges

of lower and canines move labially with the body of the tooth remaining in

the same position.

- Mandibular arch length decreased substantially in every group during pot-

retention period.


4. Lee(1999) noted that in cases where the canine is lingually displaced in assoc. with

crowding, relief of crowding with expansion of the ICW may reasonably be expected

with moderate amount of stability.

5. Setwyn & Barnett(1991) noted that in correction of cross bites, the balance of

forces on the dentition can be maintained while achieving significant movements

without upsetting the muscular equilibrium. Lower dentition can be induced to occupy

space previously occupied by the upper incisors in class II div 2 cases without

encroaching the lips.

6. De La Cruz etal(1995) also suggested that patients pretreatment arch form

appeared to the best guide for future arch form stability, but also said that minimizing

treatment change was no guarantee of stability

7. Burke etal (1998) conducted a meta analysis to review 26 papers on mandibular

ICW changes, and concluded that regardless of patient diagnostic and treatment

modalities, mandibular ICW tends to expand during treatment by 1-2mm and

contracts post retention by the same amountwww.indiandentalacademy.com

PRACTICAL SOLUTIONS TO MAINTAIN ARCH FORM:

4 components of arch form:

i. Anterior Curvature: based on Inter canine

width

ii. Intercanine width: is the most critical aspect

of the arch form.

iii. Posterior Curvature: consensus seems to

favour a gradual curvature between canines

and premolars

iv. Inter-Molar Width: here the treatment

changes are more stable.


Arch Form Early in Treatment:

A 3 step procedure is followed to choose the arch form;

a. Template placed on model to decide appropriate arch form

b. If buccal uprighting required a wider arch form selected

c. Decide the contour and width of the lower posterior segment.

Initially, 0.015” Multistranded/ 0.014” SS/ 0.016” HANT used.

Commonly ovoid arch preferred to minimize inventory.

These are followed by 0.016”, 0.018” SS which influence arch form.

Arch Form in Mid Treatment:

19x25 SS with hooks preferred after leveling & aligning with rectangular wires as they provide good control of arch form

Advisable to stock in all three shapes as have a “Rebound Tendency” towards original shape.


ARCHWIRE COORDINATION

Important step throughout treatment. Any arch form selected should be

coordinated to the patients arch and coordinated between maxillary and

mandibular arch forms.(more critical in heavier round and rectangular

wires)

When lower wire superimposed with upper,

it should lie 3mm inside the upper wire.


ARCH EXPANSION

Arch expansion can be done using 19x25” SS wire, or used to maintain

expansion achieved by quad helix or RME. The chosen wire is

expanded in the molar region or a square arch form is used for a limited

period. Expansion in canine region is usually not stable but literature

shows if expansion done in early permanent dentition, it is quite stable.

Correct technique for Expansion :

-During expansion of wire, avoid over expansion and distortion of arch

form.

- When the ends of expanded wire are

held and pressed back toward the arch form,

the wire should match that shape www.indiandentalacademy.com

If over expanded, it will not match the chosen arch form, and will lead to narrowing of Inter Canine Arch Width

If greater expansion required a Jockey Arch is made use of. It is a heavier

guage wire 19x25 or heavy round wire that is placed over the normal

archwire to achieve expansion


RECENT DEVELOPMENTS:

The introduction of 3D radiographic imaging with Cone Beam Computed

Tomography (CBCT) has lead to multitude of clinical application across all

dental disciplines.

Frontal views of dental arches aid in the assessment of vertical and

transverse dimensions to evaluate inter.arch discrepancies


Occlusal views of arches reveal relative tooth positions and shape of alveolar

bone support. Arch form tracings are made at the height of the alveolus. The

arch forms can then be superimposed to reveal compatibility. Also used for

selection and fabrication of arch wires.


Arch Form In Lingual Orthodontics:Due to the lingual morphology of the teeth, a straight wire cannot be engaged lingually. The arch wire form is changed accordingly.

The wires used here are “Mushroom Shaped”, with an offset present between canine and premolar.


During sliding mechanics, there is a transverse bowing of the arch leading to distortion of the arch form.

To prevent this the posterior legs of the archwire are bowed outward to compensate for the transverse bowing of the arch.


CONCLUSION

Current literature illustrates many divergent views on

the shape of arch form.

It is now generally believed that the arch shape is determined

by an interplay between genetic and many varied

environmental factors such as pressure from soft tissues;

shape and position of jaws; alteration in eruptive mechanism

and morphology of teeth.

Clinicians should therefore be cautious when treating

individuals to a mathematically derived ideal.

The common consensus though seems to be that

individualization and coordination of arch forms for

each patient is a must to obtain optimum long term

stability.www.indiandentalacademy.com

REFERENCES:1. SystemizedOrhtodontic Treatment Mechanics- McLaughlin,Bennet & Trevisi

2. Bioprogressive Therapy, Book 1- Ricketts, Bench, Gugino, Hilgers, Schulhof

3. Rocky Mountain Data System arch forms. JCO 1975,9:776

4. Dental Arch form related to intraoral force PR=C, Brader.AJO1972;61:541-561

5. Polynomial Caternary Curve fits, Pepe. J Dent Res,1975;54:1124

6. Dental Arch Analysis : A literaature review, Rudge. Eujo,1981;3:279

7. Computerized analysis of shape and stability of mandibular arch form. AJODO1987;92:478-83

8. Long term changes in arch form after orthodontic treatment and retention. AJODO,1995;107:518-30

9. Arch width and form: A Review, Lee. AJODO,1999;115:305-13

10. MBT archform and Archwire sequencing

11. Contemporary Orthodontics- William Proffit , Mosby, 3rd edition

12. The Essence of Orthodontics,-Graber Lee

13. Text book of Orthodontics- Graber Vanarsdall

14. Orthodontics- Tweedwww.indiandentalacademy.com

THANK YOUwww.indiandentalacademy.comLeader in continuing dental education


Arch Forms 1-Ortho / orthodontic courses by Indian dental academy

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Transcript of Arch Forms 1-Ortho / orthodontic courses by Indian dental academy