GENERAL BRACKET DESIGN

Dr. Charles Burstone has clearly explained the adverse effects that can occur on adjacent teeth as a result of placing a continuous archwire in the edgewise slot of brackets on tipped teeth. There are three major factors that control the magnitude of force exerted on the adjacent teeth. The three factors are as follows; the dimension of the vertical opening of the archwire slot, the mesial-distal width of the slot and the degree of tipping of the tooth.

A  . 016 archwire in  a typical .018 edgewise slot
Width .135 - There can only be approximately 1 degree of tip of the tooth before adverse forces effect adjacent teeth when a continuous archwire is placed in the arch. 

B  . 016 archwire in a typical .022 edgewise slot
Width .135 - There can be approximately 7 degrees of tip of the tooth before adverse forces effect adjacent teeth when a continuous archwire is placed in the arch.

C  . 016 archwire in an archwire slot with a .035 vertical opening.

Width .050 - There can be approximately 20 degree of tip of the tooth before adverse forces effect adjacent teeth when a continuous archwire is placed in the arch 

The vertical dimension of the edgewise slot, the width of the edgewise slot, and the size of the archwire determine how far the tooth can be tipped before adverse forces are exerted on the adjacent teeth when a continuous archwire is inserted in the arch. 

Illustration 2:  Bite closure due to extrusive force on anterior teeth due to tipped cuspid

Increasing the vertical dimension of the edgewise slot would make it necessary to increase the vertical dimension of the archwire to take advantage of the pre-adjusted quality of the edgewise slot. Also, reducing the mesial distal dimension of the bracket would make it more difficult to correct and hold rotations. Therefore, there would be no net gain in solving bracket related treatment problems.

Illustration 2 demonstrates what happens when a continuous archwire is inserted into an arch where the cuspids and bicuspids are already tipped.  

Illustration 3: Angulation angles as relates to Apical base class I, II and III

               Apical Base Class I                 Apical Base Class II                 Apical Base Class III

Before the maxillary bicuspid and cuspid axial inclinations can altered when utilizing the standard angulation in the cuspid and bicuspid brackets, the incisors must be intruded, and the first molar must be extruded.  In the mandibular arch the opposite would need to occur.  That is, before the cuspid and bicuspid axial inclination can be altered the incisors would need to be extruded and the first molar intruded.  The same concept applies when treating apical base class II cases.

Optimum interdigitation of the teeth can be establish in apical base class II and III cases through dental compensation.   Dental compensation is easily accomplished by utilizing the interactive slot.  Modifying the axial inclination of the maxillary and mandibular incisors alone is not adequate when treating moderate apical base class II and III cases.

Cannon Ultra Bracket System

Illustration 4:  Mid-line cross-sectional view of a bracket

Notice that without the steel ligature tie in place the archwire is easily inserted into the interactive slot. However, once the archwire is inserted into the interactive slot and the steel ligature is placed, the archwire cannot become disengaged. The steel ligature tie prevents the archwire from coming out of the interactive slot. When used in this manner, the steel ligature tie functions quite differently than the steel ligature tie when used with the pre-adjusted edgewise bracket. With the pre-adjusted slot it is the tensile strength of the steel ligature tie (resistance to stretching) that holds the archwire in the pre-adjusted edgewise slot. The steel wire used as an orthodontic ligature tie is prone to stretching, since it is a small gauge wire that is very ductile and with little temper. When the steel ligature tie is used to hold the archwire in the vertical slot there are no longitudinal (stretching) forces on the steel ligature tie. The force exerted on the steel tie is one of compression. It takes far greater force to compress the steel ligature tie than to stretch it. The exact same principles apply to the use of elastomeric "O" rings and power chains.

The interactive slot of the Cannon Ultra bracket is unique in that the archwire is held into the interactive slot utilizing the same elements employed when securing an archwire into the pre-adjusted slot, but with considerable less friction between the archwire and/or the bracket and ligature tie.

Illustration 5:  Archwire ligation in the interactive slot for reduced friction

                                                                   A                                                          B

(A) Demonstrates the manner in which an archwire is ligated into the interactive slot with a steel ligature tie.  It is a figure eight tie with the ligature tie crossing as near the junction of the "T" hook and the stem on the bracket as possible.  The stem gets wider as it goes toward the edgewise slot, therefore the ligature tie can not move in an occlusal direction and put pressure on the archwire thereby increasing friction.

(B) Shows the side view of the interactive slot with an archwire being held into place with a steel ligature tie.  The distance from the steel ligature tie to the tooth side of the interactive slot is of such a dimension that the archwire cannot become disengaged.  The archwire is .016.  There is considerable distance from the archwire to the steel ligature tie.  The steel ligature tie becomes the fourth side to form a .018 x .035 tube.  Compare to illustration 4.

  Illustration 6:  Ligature configurations

The maxillary second bicuspid demonstrates a steel ligature tie securing an archwire in both the edgewise slot and the interactive slot.  The maxillary first bicuspid shows an "0" ring securing both archwires in place.  The mandibular bicuspids shows an "0" ring securing an archwire in the interactive slot.  A power chain will secure both archwires in separate slots or a single archwire in either slot.

Illustration 7:  Low Friction en masse retraction ligature tie configuration

The configuration is that of a continuous figure eight tie.  Each time the ligature tie passes the "T" hook wrap the tie around the junction between the "T" hook and the stem.   Beginning with the left cuspid wrap the tie around the "T" hook then go under the occlusal tie wings of the left lateral and then go up and wrap the tie around the "T" hook on the left central. Continue with this procedure until the tie is wrapped around the "T" hook of all anterior brackets.  The ligature does not need to be excessively tight.  This will prevent space from opening between the anterior teeth and reduce friction.  The bicuspids have the usually low friction steel ligature tie previously described. 

Illustration 8:  En masse retraction utilizing power chain 

Since the initial retraction is primarily tipping of the anterior teeth, friction between the round archwire and the interactive slot is not of primary concern unless it is a maximum anchorage case.  Therefore, in most retraction cases a wide power chain from cuspid to cuspid is usually adequate.  Always use the low friction steel ligature for the bicuspids.

Illustration 9A:   Cannon Ultra Bracket System -- Maxillary Arch -- Left Quadrant                         

 Illustration 9B:  Cannon Ultra Bracket System -- Mandibular Arch -- Left Quadrant

Illustration 9C: I. D. Color chart for Cannon Ultra Bracket System

 Brackets and Buccal Tubes

As with the "torque" and "angulation" angles of the pre-adjusted edgewise bracket, there is no single set of measurements that will establish the proper occlusal gingival position of the brackets and buccal tubes for all cases.  The occlusal gingival length of the clinical crowns vary for different individuals, therefore bracket and buccal tube placement must be individualized for each case. 

It is true that there is no single set of measurements that will fit all cases, however there are some guidelines that seem to apply to all cases. The mesial distal center of the bracket should be placed on the mesial distal mid-line of the clinical crown of the teeth. The mesial distal center of the buccal tube, however, is placed over the facial buccal grove.  The occlusal-gingival center of the edgewise slot of the brackets and buccal tubes should be placed, as near as possible, to the facial axis point.  The facial axis point is that point on the facial axis which separates the gingival half of the clinical crown from the occlusal half.  If the brackets are properly placed in this manner, anterior guidance with cuspid rise will be established.

Sequence for Placement of Brackets and Buccal Tubes

The buccal tubes for the mandibular first molars are placed first.  The design of the buccal tubes is such that the edgewise slot is placed into the most occlusal portion of the buccal tube.  This design makes it easy to place the buccal tube in its proper occlusal-gingival position without creating occlusal interference with the maxillary first molar.  The buccal tubes are then placed on the maxillary first molars.  Maxillary and mandibular bicuspid brackets are placed next.  Great care should be taken to make sure that the edgewise slot of the brackets align with the edgewise slot of the buccal tubes.  Unlike the brackets, there are no tie wings on the buccal tubes, therefore, visually it might appear that the brackets are placed more occlusally than the buccal tubes.  The remaining brackets can be placed in any sequence desired, making sure that the brackets are properly placed as described above.  The second molar buccal tubes are placed in the same manner as was the first molar tubes.  The key to a successful strap-up is the proper positioning and alignment of the first molar buccal tubes and the bicuspid brackets. 

Bite Opening:  Pre-adjusted edgewise Bracket vs. Cannon Ultra Bracket

One of the objectives of the design of the Cannon Ultra bracket system is to have a narrow interactive slot (.050 M/D x .035, O/G) in conjunction with the pre-adjusted edgewise slot, which can be utilized to align and correct rotations of teeth with minimal adverse forces on the adjacent teeth. When correcting rotations the interactive slot gives metal to metal contact between the archwire and the bracket to hold the archwire firmly in the slot. Whereas the pre-adjusted edgewise slot must rely on some type of tie, either steel or elastomeric, to hold the archwire in the slot. The interactive slot is also much more efficient in bite opening procedures than the pre-adjusted edgewise slot. When the reverse curve bite opening archwire is inserted into the narrow deep interactive slot the action of the archwire is exerted on the teeth. When the reverse curve bite opening archwire is inserted into the wide pre-adjusted edgewise slot the action of the archwire is exerted primarily on the brackets.

Illustration 10: . 018 reverse curve nickel titanium archwire

A.  A .018 reverse curve archwire is engaged into the .018 pre-adjusted edgewise slot of the bicuspid and cuspid brackets. The archwire is not engaged into the pre-adjusted edgewise slot of the central and lateral brackets. Note that there will not be any intrusive force exerted on the centrals and laterals when the archwire is engaged. The wide (.135 M/D x .018 O/G) archwire slots of the bicuspids and cuspids flatten he archwire. The force is exerted on the brackets instead of the teeth.

B: A .018 reverse curve archwire is engaged into the interactive slot (.050 M/D x .035, O/G) of the bicuspids. The archwire is not flattened and when engaged into the interactive slots of the cuspids, centrals and laterals the intrusive force will increase in magnitude from the cuspids to the centrals.

En Masse Retraction with the Pre-adjusted Bracket

For many years looped archwires have been used extensively with the edgewise appliance when retracting anterior teeth.  The advent by Dr. Andrews of the "Fully Programmed Translation Brackets" with anti-tip and anti-rotation built into the placement of the pre-adjusted slot in the brackets was an attempt to replace the looped archwires utilized for anterior retraction.  The "Fully Programmed Translation Brackets" were not an overwhelming success.  The anti-tip and anti-rotation functions were an attempt to overcome problems associated with retracting anterior teeth utilizing a single straight archwire in the edgewise slots.  The anti-tip and anti-rotation functions actually make finishing the case more difficult.  They are counterproductive in the final alignment of the teeth.

Looped archwires continue in wide use today for the retraction of the anterior teeth.  They are designed so that a force system can be established to retract the anterior teeth bodily.  A balance must be accomplished between the "torque" forces and the retraction forces to prevent the anterior teeth from tipping instead of the desired bodily movement.  If the forces are not balanced, the anterior teeth will tip distally, the bite will close, and  the posterior teeth will move forward and close the extraction site before a class I relationship is achieved.  The looped archwires are numerous and vary as to size, shape and complexity.  They can be extremely difficult to properly adjust and maintain. 

A recent article in an orthodontic trade publication advertising looped archwires showed a sampling of looped archwire designs utilized in orthodontics.

Illustration 11:  Samples of looped archwires

En Masse Retraction with the Cannon Ultra Bracket

The Cannon Ultra Bracket System utilizes two archwires simultaneously so that bodily en masse retraction of the six anterior teeth can be accomplished without complicated archwire configurations and/or external anchorage.

Once the teeth are aligned so that a .018 steel archwire can be inserted into the interactive slots of the maxillary and mandibular teeth along with a .018 x .025 nickel titanium in the pre-adjusted edgewise slots, retraction of the anterior teeth can begin.  A bi-lateral gable bend of approximately 25 degrees is placed into the .018 steel archwire in the middle of each extraction site.  Power chain along with elastics, if needed, are used to retract the anterior teeth.  The rigidity of the steel archwire with the gable bends, will not allow the nickel titanium archwire to bow in a gingival direction in the extraction regions.  In fact, the dental arch and the nickel titanium archwire will take on the shape of the steel archwire.  Therefore, the cuspids cannot tip distally, nor can the bicuspids tip mesially beyond the "angulation" angle buit into the pre-adjusted slot of the brackets.  The centrals and laterals cannot tip lingually beyond the "torque" angle built into the pre-adjusted slot of the brackets.

View step by step treatment procedures in the Case Review heading at Home Page.

Illustration 12:  Bodily en masse retraction

Customizing "Torques" and "Angulations"

The interactive slot of the Cannon Ultra Bracket System is utilized to establish the dental compensation needed to establish optimum interdigitation of the teeth in apical base discrepancy cases.

View step by step treatment procedures in Case Review at Home Page.