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Arch forms 1 /orthodontics course training by Indian Dental Academy /certified fixed orthodontic courses by Indian dental academy


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Arch forms 1 /orthodontics course training by Indian Dental Academy /certified fixed orthodontic courses by Indian dental academy

  1. 1.
  2. 2. INDIAN DENTAL ACADEMY Leader in continuing dental education
  3. 3. 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
  4. 4. INTRODUCTION: 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. 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.
  5. 5. 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
  6. 6. 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).
  7. 7. 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.
  8. 8. A review of the change in arch form show the following findings: male arches grow wider that female arches Lower inter- canine 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.
  10. 10. 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”
  11. 11. 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
  12. 12. 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”
  13. 13. 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”
  14. 14. 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. Caternary Arch Form
  15. 15. 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
  16. 16. 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.
  17. 17. 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.
  18. 18. 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 Constant He 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 LINGUAL P R C MOLAR 9.2 12.2 112.2 INCISORS 15.2 7.5 112.5 113 PR=C, was applied and was noted that the product of P and R was the same
  19. 19. 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).
  20. 20. 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.
  21. 21. 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.
  22. 22. 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
  23. 23. INDIVIDUALIZED IDEAL ARCHES Proposed 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 BONWILLHAWLEY 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%) -
  24. 24. 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.
  25. 25. (2) (1) (3)
  26. 26. RICKETTS PENTAMORPHIC ARCH FORMS Considered 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
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  28. 28. ROTH TRU ARCH FORM Developed 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.
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  30. 30. MATHEMATIC & GEOMETRIC MODELS FOR ARCH FORMS Mathematic 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 6 th 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
  31. 31. 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.
  32. 32. 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).
  33. 33. 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
  34. 34. 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 stable expansion.
  35. 35. 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 potretention period.
  36. 36. 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 amount
  37. 37. 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.
  38. 38. 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.
  39. 39. 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.
  40. 40. 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
  41. 41. 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
  42. 42. 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
  43. 43. 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.
  44. 44. 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.
  45. 45. 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.
  46. 46. 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.
  47. 47. 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- Tweed
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