The pre-adjusted edgewise appliance

Pre-adjusted appliance

The ‘edgewise’ appliance introduced by Edward H. Angle, which he named the ‘latest and best’ in orthodontics, provided excellent three-dimensional (3D) control of tooth movements. The rectangular bracket slot was greater in depth (0.028 in.) than the height (0.022 in.) and he used rectangular wires in edgewise mode (reverse of ribbon mode) to accomplish the full 3D control.

Charles H. Tweed later taught us how to use the edgewise appliance effectively and brought about the discipline of ‘order of bends’ to be built in the archwires. The ‘first-order bends’ compensate for labiolingual inadequacies of the bracket base to match the anatomical variations in the thickness of clinical crowns and arch form. The ‘second-order bends’ are built in the archwire to enhance anchorage and place tooth crowns and roots in a correct mesiodistal tip. Most importantly, the ‘third-order bends’, or torque bends, were devised to place the roots of the teeth in harmony with their skeletal bases and create an anatomically and functionally optimum occlusion. The torque bends are different in the anterior and buccal segments.

The process of complicated wire bending to obtain an optimal position of teeth when brackets are seated on a full complement of teeth would necessitate 76 primary bends (46 bends for angulation, inclination, and offset and 30 bends for prominence and occlusal gingival slot position).

Evolution of building the treatment into brackets

Several clinicians, therefore, incorporated modification of brackets to avoid or minimise wire bending.

  • •

    Holdaway suggested bracket angulation near the extraction site to avoid second-order bends and aesthetic positioning bends ( Fig. 53.1.i ).

    Figure 53.1.i

    Building the treatment in the brackets.

    (A) Holdaway suggested angulation of the brackets so that the mesial tipping of the anchor molar and second premolar can be prevented, and anchorage is enhanced. The second-order bends can be eliminated which offer great friction and are cumbersome to be accurately duplicated in each subsequent wire change. The angulation of the bracket on canines is so placed that during retraction, the root tip moves along the crown, so that roots of the teeth at extraction site are parallel. (B) Standard edgewise brackets on incisors lead to loss of natural mesiodistal root tip, which is later accomplished with artistic positioning bends. (C) Jarabak suggested elimination of the artistic positioning bends required at the end of the treatment and during incisor retraction by angulating the brackets.

  • •

    Lewis developed winged brackets for effective derotations and added bars for uprighting of the teeth ( Fig. 53.1.ii ).

    Figure 53.1.ii

    (A–C) Peter Lewis suggested a new approach to prevent rotations and tipping of the teeth. The mesial and distal bars are effectively used to prevent undesired tipping.

  • •

    Jarabak was the first to cut a slot in the bracket at an angle and Fizzell to mill the bracket slot at an angle to create the torque. Jarabak and Fizzell were the first to introduce a bracket with built-in torque and angulation ( Table 53.1 ).

    TABLE 53.1

    Evolution of building the treatment into brackets pre-SWA era

    Clinician Year Contribution
    Glendon Terwilliger 1941 Attempted soldering brackets into tip and torque positions
    Holdaway 1952 Angulating brackets in the mandibular buccal segment proportional to the severity of malocclusion
    Ivan F. Lee 1959 Torque bracket for anterior teeth (LEE TORQUE, Unitek, United States)
    Jarabak and Fizzell 1960 Demonstrated bracket having combined torque and angulation

  • •

    Lee is credited with developing the first commercially viable pre-torqued brackets.

  • •

    Ricketts used brackets with 7, 14 and 22 degrees torque in his bio-progressive technique ( Table 53.2 ).

    TABLE 53.2

    Evolution of building the treatment into brackets: Pre-adjusted appliance (1970) and thereafter

    S.no. Appliance Clinician Year Salient features
    1. SWA Lawrence F. Andrews 1972 Based on six keys
    2. Bioprogressive therapy Robert M. Ricketts 1976 0.018 × 0.030 bracket slot size
    3. Roth Ronald Roth 1979 Overcorrection, gnathological concept
    4. Vari-simplex Alexander 1983 Three different designs of brackets
    5. Tip-edge P. C. Kesling 1986 Torque in base and tip in face. Tip Edge appliance system. Controlled tipping with torque features.
    6. MBT McLaughlin, Bennett and Trevisi 1997–2001 Versatile appliance, modified values, clinical progression in design for excellence.
    7. Bi-dimensional system Gianelly 2000 Variable slot for anterior/buccal brackets

The original edgewise brackets have a uniform thickness of the base; that is, the distance between the base of the bracket and the centre of the slot is same. When edgewise brackets are assigned on teeth, the anatomical arrangement of the arch is lost, or in other words, they become regular in the labial facial side at the labial/buccal surfaces of the crowns ( Fig. 53.2.i ).

Figure 53.2.i

(A) Standard edgewise bracket distance between the bracket base and the centre of the slot is same in each bracket. Therefore, when the brackets are placed, they become as regular in the facial prominence as the crown. (B) With the unbent archwire ligated, the facial surface of each crown becomes equidistant from the embrasure line, which is undesirable.

An attempt to place the bracket at an angle to achieve the desired tip requires the bracket base to be bent over the contours of the tooth crown. A mismatch of the bracket base and tooth contour leads to the rocking of the bracket ( Fig. 53.2.ii ). The bracket does not fit properly on the labial contour of the tooth and tends to rock occlusally or gingivally. The slot is 90 degrees to the base in edgewise bracket. The effect of the 90-degree angle of the bracket slot to the base will lead to undue torque of the tooth ( Fig. 53.2.iii ).

Figure 53.2.ii

Rotational effect.

(A) The bracket base of standard edgewise bracket is bent to follow the contour of the tooth surface. (B) When the bracket is angulated on the tooth to accomplish tip, a rocking potential is created.

Figure 53.2.iii

(A and B) Untorqued edgewise brackets located at LA point.

The undesirable effects include occlusal gingival displacement of the teeth. The effects are more pronounced when full-size wires are in place. The occlusal-gingival errors are corrected with compensatory second-order bends in the archwire. The unwanted torque effects, that is, labiolingual displacement of roots–crown are also the consequence of a standard slot of the edgewise bracket. Therefore, it is pertinent to incorporate compensatory torque in the wire. A trained edgewise orthodontist spends a considerable time in the process of archwire bending. Each subsequent archwire requires the bends and torques to be replicated, which requires precise and meticulous wire bending by skilled hands ( Fig. 53.3 ).

Figure 53.3

Effect of standard edgewise brackets.

(A) The standard edgewise bracket can unintentionally cause rocking of bracket occlusally or gingivally. (B) When the vertical components of the brackets are cited parallel to FACC and base point cited at FA point, the angle of the slot varies to many different angulations. (C) When standard edgewise brackets are placed, the teeth tend to lose their mesiodistal angulations, and all roots tend to be parallel which is not desirable. The blue lines show their original tip.

As edgewise brackets are placed with their slots parallel to the occlusal plane or the incisal edge of the teeth, the teeth tend to lose their natural mesiodistal inclination and become more or less upright. Therefore, additional second-order or artistic bends are required to finish the occlusion.

Tweeds philosophy involved a greater emphasis on anchorage conservation with tip back bends and heavy forces for retraction with loops made in edgewise wires with the incorporation of torque and V bends. His challenges were resolved with the development of a pre-adjusted (straight-wire) appliance.

Major shortcomings of the edgewise bracket are listed in Table 53.3 .

TABLE 53.3

The major shortcomings of standard edgewise appliance

  • 1.

    Brackets bases are perpendicular to bracket stem

  • 2.

    Brackets bases are not contoured occlusogingivally

  • 3.

    Slots are not angulated

  • 4.

    Bracket stems are of an equal faciolingual thickness

  • 5.

    The maxillary molar offset is not built-in

The first integrated pre-adjusted appliance system and philosophy by L. F. Andrews

The previous attempts to incorporate torque and slot inclination in the bracket were based on empirical observations and the experience of the clinicians. The need for a scientific rationale and detailed study of occlusion paved the way for the creation of programmed pre-adjusted appliances. The study of non-orthodontic occlusion models led to the synthesis of the ‘six keys’ to normal occlusion by L. F. Andrews.

Andrews’ constant clinical experience and observations of variation in treatment outcome and apparent inadequacies in treatment records displayed at meetings, though they were treated to Angle’s concept of class I cusp to fossa relations, led him to redefine the goals of occlusion and features at the end of treatment. He proposed that treated malocclusion should have a resemblance to the natural dentition and occlusion of normal occlusion.

The measurements of non-orthodontic normal models

L. F. Andrews began to systematically quantify features of dental casts of individuals who (1) had a negative history of orthodontic treatment, (2) were straight and pleasing in appearance, (3) had a bite which looked correct and (4) in their judgement, would not benefit from orthodontic treatment.

120 non-orthodontic set of study models of normal occlusion sourced between 1960 and 1964, were subjected to analysis of the features of occlusion, resulting in the identification of six consistent characteristics common to all and possibly the hallmark of the normal occlusion. Furthermore, a study involving 1150 orthodontically treated cases was conducted to ascertain the potential interrelation of these features and the predictive nature of these characteristics in influencing treatment outcomes. These findings facilitated the derivation of ‘six keys of normal occlusion’, which were considered prerequisites to describing the normal occlusion. The six keys of normal occlusion and goals of orthodontic treatment are as follows :

  • Key 1 . Molar relationship: In addition to Angle’s concept of the cusp-to-fossa relationship of the mesiobuccal cusp of the maxillary first molar and buccal groove of the mandibular first molar, Andrews found that the distal surface of the distobuccal cusp of the upper first permanent molar made contact and occluded with the mesial surface of the mesiobuccal cusp of the lower second molar. The greater the intimacy of the distal surface of the distobuccal cusp of the upper first permanent molar to the mesial surfaces of the mesiobuccal cusp of the lower second molar, the better the prospects for normal occlusion. The maxillary premolars and canines exhibit a cusp-embrasure relationship on the buccal and a cusp-fossa relationship on their lingual occlusion. The orthodontic treatment, therefore, should aim to attain above mentioned molar relations ( Fig. 53.4 A–E).

    Figure 53.4

    Andrews concept of occlusion.

    (A) Molar relationship as the key I as observed by L. F. Andrews in non-orthodontic normal. (B) Improper molar relationship. (C) Improved molar relationship. (D) More improved molar relationship. (E) Proper molar relationship. (F–I) Crown tip or crown angulation. The long axis of crown measured from line 90 degrees to occlusal plane. (F) Positive crown tip. (G) Negative crown tip. (H) Ordinarily occluded teeth demonstrate the gingival portion of crown more distal than the occlusal portion of the crown. (I) A rectangle which when angulated occupies more mesiodistal space than a non-angulated rectangle (i.e. upper central and lateral incisors if placed upright on losing their mesiodistal tip, would show spaces).

  • Key 2 . Crown angulation, or the mesiodistal ‘tip’: The tooth angulation in mesiodistal direction by Andrews confines to the crown only and not the entire tooth (crown and root). Crown angulation is judged at the ‘mid-development ridge’ of the crowns of incisors, canines and premolars and the ‘dominant vertical groove’ on the buccal surface of the molars. Each tooth has its unique angulation or crown tip in the mouth while in normal occlusion. The tip is viewed as a gingival portion of the clinical crown being distal to the incisal portion.

    • The quantum of the crown tip is depicted in degrees. The crown tip is an angle between the long axis (LA) of the crown and an imaginary line vertical to the occlusal plane. In normal occlusion, a ‘plus reading’ (+) is denoted whereby the gingival portion of the LA of the crown is distal to the incisal portion. In a reverse situation, when the gingival portion of the LA of the crown is mesial to the incisal portion, a ‘minus reading’ (–) is assigned. The distal tip of the crowns is responsible for a normal arch form, and lack of which will be seen as spacing and poor proximal contact points. Presuming the tooth crown as a piece of a rectangular structure, when placed tipped, will occupy more space ( Fig. 53.4 F–I).

  • Key 3 . Crown inclination (labiolingual or buccolingual inclination): Crown inclination refers to the labiolingual or buccolingual inclination of the LA of the tooth crown and not the inclination of the LA of the entire tooth. The crown inclination of each tooth in the arch has a unique scheme which constitutes a normal functioning occlusion and the health of the dentition. In general, the anterior teeth have their incisal edges placed labially to the gingival portions, which have sometimes been referred to as torque.

    • The crown inclination or torque is measured about the occlusal plane. An angle formed between a vertical line from the occlusal plane and a line that is tangent to the middle of the labial or long buccal axis of the clinical crown, as viewed from the mesial or distal, represents crown inclination. A plus reading is given if the gingival portion of the tangent line (or of the crown) is lingual to the incisal portion. A minus reading is assigned when the gingival portion of the tangent line (or of the crown) is labial to the incisal portion. In normal occlusion, anterior teeth have a positive crown inclination and buccal segments have a negative crown inclination. The buccal crown inclination is progressive from canine to second molar ( Fig. 53.5 ).

      Figure 53.5

      Andrews concept of occlusion.

      (A) Crown inclination is determined by the resulting angle between a line 90 degrees to the occlusal plane and a line tangent to the middle of the labial or buccal clinical crown. (B) Lingual crown inclination generally occurs in normally occluded upper posterior crowns. The inclination is constant and similar from the canines through the second premolars and slightly more pronounced in the molars. (C) The lingual crown inclination of normally occluded lower posterior teeth progressively increases from the canines through the second molars. (D) Improperly inclined anterior crowns result in all upper contact points being mesial, leading to improper occlusion. (E) A demonstration, on an overlay, that when the anterior crowns are properly inclined, the contact points move distally, allowing for normal occlusion. (F) Spaces resulting from normally occluded posterior teeth and insufficiently inclined anterior teeth are often falsely blamed on tooth size discrepancy.

    • A.

      Anterior teeth (central and lateral incisors): Normal inclinations of anterior teeth are essential for normal distal positioning of the contact points of the upper teeth in their relationship to the lower teeth, permitting proper occlusion of the posterior crowns. The crown inclinations are sufficient to prevent overeruption of anterior teeth and to deepen the bite. The appropriate inclination of the anterior crowns leads to the seating of the buccal cusp to fossa relations and contact points. Even in situations when buccal teeth are seated well, the improper inclination will result in spaces somewhere between the anterior and posterior teeth ( Fig. 53.5 D–F).

    • B.

      Upper and lower posterior teeth (canines through molars): A lingual crown inclination in the buccal teeth progressively increases from the canines through the second molars.

  • Key 4 . Rotations: Teeth should be free from unusual rotations. When the teeth are rotated, say molars, they encroach upon the arch length ( Fig. 53.6 ).

    Figure 53.6

    A rotated molar occupies more mesiodistal space, creating a situation shown in dotted out line unreceptive to normal occlusion.

  • Key 5 . Tight contacts: The arch should be free of spaces. The teeth should meet tightly at proximal contact points. Lack of contact points is suggestive of improper occlusion except in conditions of tooth size problems, such as microdontic lateral incisors. Such situations will require restorative work to create contact points.

  • Key 6 . Occlusal plane: The plane of occlusion in non-orthodontic normal is flat to slightly curved. A deep curve of Spee represents less than excellent orthodontic treatment ( Fig. 53.7 ).

    Figure 53.7

    Andrews concept of occlusion.

    (A) Flat to a slightly curved curve of Spee is the sixth key of Andrews’ normal occlusion. (B) Deep curve of Spee results in a more confined area for the upper teeth, creating spillage of the upper teeth progressively mesially and distally. (C) A reverse curve of Spee results in excessive room for the upper teeth.

  • Key 7 is a new addition to Andrews’s six keys. A normal Bolton ratio is needed for proper occlusion, and a lack of which will not allow adequate coordination of upper and lower arches in the position of tight contacts. These situations can exist when there is a mandibular anterior excess or microdontic incisors; the maxillary laterals are the most affected teeth.

He also decided the location of the bracket on the crown of each tooth based on their contours in occlusogingival direction and mesiodistal direction ( Fig. 53.8 ).

Figure 53.8

Location of bracket area on the tooth surface and determining the contours of the tooth crown surface in mesiodistal and occlusogingival directions.

(A) Bracket area. (B) Vertical contour. (C) Horizontal contour.

The values of crown prominence, maxillary molar offset, crown angulation and crown inclination of the maxillary arch ( Fig. 53.9 ) and crown prominence, crown angulation and crown inclination of the mandibular arch ( Fig. 53.10 ) were collected. This data constituted the ‘norms’ which were used as reference values in designing the pre-adjusted bracket system or the straight wire appliance.

Figure 53.9

Features of Andrews appliance.

Values of maxillary arch measurements. (A) Crown prominence. (B) Maxillary molar offset. (C) Angulation. (D) Inclination.

Figure 53.10

Features of Andrews appliance.

Values of mandibular arch measurements. (A) Crown prominence. (B) Inclination. (C) Angulation.

The information he gathered was crystallised in evolving the first pre-adjusted appliance that is called ‘Straight Wire Appliance’ more popularly abbreviated as SWA.

The features of straight wire appliance

Andrews’ appliance was undoubtedly much superior appliance to the existing standard edgewise appliance system. The new terminology was given by Andrews pertaining to SWAs ( Table 53.4 and Fig. 53.11 ).

TABLE 53.4

Common terminologies used in straight wire appliance

S. no. Terminology Description
1. LACC Long axis of clinical crown.
2. LA point Centre of the clinical crown or LACC.
3. FACC Facial axis of clinical crown.
4. Andrews plane An imaginary plane that would intersect the crowns of properly positioned teeth at the LA point, assuming flat or normal curve of Spee ( Fig. 53.11 ).
5. Bracket base The most lingual portion of the bracket stem.
6. Bracket stem The portion of a bracket between the bracket base and the most lingual portion of the slot (the slot base) excluding tie-wings.
7. Slot base The lingual wall of the slot.
8. Slot point The centre point of the slot axis.
9. Base point On the bracket base, the point that would fall on a lingual extension of the slot axis.
10. Slot axis The buccolingual (or labiolingual) centreline of the slot. It is equidistant from the gingival and occlusal slot walls and is centered mesiodistally. When the bracket is properly placed, the slot axis, if extended lingually, would include the base point and the LA-point, and it would be included by a labial or buccal extension of the Andrews plane.
11. Crown angulation Crown angulation is described in mesiodistal direction, also known as crown tip. Andrews quantified the tip and designated with signs − or +. The degree of the crown tip is the angle formed by the long axis of the clinical crown (as viewed from labial or buccal perspective) and a line perpendicular to the occlusal plane. A ‘plus reading’ is awarded when the gingival portion of the LACC is distal to the incisal portion. A ‘minus reading’ is given when the gingival portion of the LACC is mesial to the incisal portion.
12. Crown inclination Andrews described crown inclination in labiolingual direction. Each tooth in the arch has its unique crown inclination in labiolingual direction. Tweed called it torque. Andrews denoted − and + signs to the readings, based on the prominence of the gingival portion of the tooth crown. Crown ‘plus’ or ‘minus’. A plus reading is given if the gingival portion of the crown is lingual to the incisal portion ( Fig. 53. 5 ). A minus reading is earned when the gingival portion is labial or buccal to the incisal portion.
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May 10, 2026 | Posted by in Orthodontics | 0 comments

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