begg mechanics1

BEGG MECHANICS INTRODUCTION: Begg technique has been in practice for the last 5 decades serving the orthodontic faculty as the universal appliance for all the types of malocclusion for more than 20 years, all round the world including America, the birthplace of edge-wise appliance. However, there was a sidden negative growth of the begg technique though ata the same time many modifications and refinement were introduced. During these many years there has been many research, publications, seminars and hands-on courses conducted almost all around the world on this indigenous technique introduced by Raymond Begg of Australia. But sadly very little was and is being published on the biomechanics of Begg technique.this inspite of the biomechanics of bite-opening of begg technique has got the maximum interest, since it was believed that begg is very efficient in management of deep bite cases The notable exceptions who enunciated some aspects of the biomechanical basis of begg treatment were: Sims-AJO 1977; Nikolai-AJO-1981 ; Kesling 1985 ; Swain-1975 ; Hocevar-AJO1982 ; Mulie &Ten HOeve-JCO1976 More authors ventured in introducing their own modifications to the appliance rather than to explain firther the biomechanics of the begg appliance. Should have Burstone or one of his students been a Begg practioner, then Raymond Begg would have been the happiest soul. Special features of Begg mechanics The difference between the design of the Begg bracket and the edgewise bracket, as also the difference in the philosophies of the two treatment modalities must be taken into account for enunciating the Begg biomechanics. We are likely to be misled if we blindly apply all the tenets of edgewise mechanics to the Begg mechanics. Behavior of the Begg bracket in the first, second and third order mechanics:

Begg bracket (in reality the ribbon arch bracket) is often called a ‘unipoint’ bracket to indicate the minimum contact it has with the archwire, thus permitting the freedom for free tipping that is an integral part of the Begg treatment. However, this is not a correct usage (as was rightly pointed out by Swain,) nor is the freedom for tipping uniform in all the three planes. The bracket slot behaves differently in the three planes as under: 1. First order mechanics: If the slot is filled by the archwire, i.e. when 0.020” archwire is employed in the 0.020”slot, there is hardly any play between the bracket and the archwire. Here, the archwire experiences the horizontal binding that is inherent in the ribbon arch bracket. Undersize wires, obviously, will allow more play (that progressively increases as the wire size becomes smaller and smaller). Still, with a slight change in the archwire bracket relationship, the wire starts contacting the bracket at two points, one on a phalange and the other in the pillar. 2. Second order mechanics: Even using the full size archwire and a hook pin or a third stage pin, the freedom for mesio-distal tipping is quite large. When the full slot height 0.040” is available with the use of stage I pin, the freedom is still more. 3. Third order mechanics: Whatever size round archwire is used, the freedom for labio / bucco lingual crown or root torque is maximum. (With the use of ribbon rectangular archwire, this freedom will obviously get reduced or eliminated.)

Following are some of the examples to highlight the differences between the mechanics of the two therapies. 1) When groups of teeth are rigidly held together, it is generally recommended that the center of resistance of teeth segments

be taken into account. This

consideration may not apply to Begg treatment in most instances in the stage I and II because of the relatively loose fit of the undersize round wires in the bracket slots. Even in the third stage, when the 0.020” round wire almost fills the slot, there

is no freedom for the teeth for the first order movement but there is enough freedom for the second and third order movements, with the result that any tooth can move independent of other teeth in the mesio-distal and bucco-lingual directions. 2) Conventionally, in edgewise practice the wire bracket interface is the site where all the forces or moments are applied. While this is true in some situations in Begg practice, under other situations the forces are applied on the tooth surface away from the bracket as for example, through the spurs of the torquing auxiliary or through the bent stem of a rotation spring. 3) Forces and moments employed in Begg treatment are relatively low. Thus, the force applied for crown tipping movement in Begg practice is 5 to 15 gms per tooth as against the force value of 50 to 75 gms mentioned by Proffit for the same movement. The difference, obviously, is because of the higher frictional resistance in the edgewise appliance. 4) The Begg slot is closer to the tooth than the edgewise slot (or even the Tip edge bracket slot which also employs Begg mechanics). Hence the moments generated by the same force on a Begg bracket are smaller than on the edgewise bracket under identical application. 5) In a Begg bracket, a vertical deflection in the arch wire will generate a force but no moment unlike in an edgewise bracket (ignoring the moment which this force will cause because it is acting away from the CRes of the tooth). Mulligan’s ‘commonsense mechanics’ rules do not hold good in the vertical direction for Begg brackets. For example, a centre bend between the molar tube (or bracket) and the canine bracket will create only moments and no vertical forces in the edgewise appliance. On the other hand, such a bend in the molar tube and the canine bracket will create a moment and a vertical force on the molar but only a vertical force on the canine in the Begg appliance.

However, a horizontal wire deflection in the archwire will cause either moments, forces or both at the Begg brackets similar to the edgewise mechanics.

Basic Begg Movements With the above background in mind, we will now discuss the three basic movements in Begg treatment viz. incisor intrusion, (crown) tipping and root movements. In the following sections, the mechanics of controlled tipping and root movements is explained on the basis of M/F ratio as is customarily done. The mechanism of intrusion could likewise be described, but it is more easily understood by considering the net intrusive force magnitude and direction instead in terms of M/F ratio. Rotational Control in Refined Begg The difference in the behavior of Begg bracket in the first, second, and third order displacements must be properly understood. The freedom offered by the Begg bracket: a.

In the horizontal plane (First order) is the least (especially with full size wires).

b.

For second order (mesio-distal) is relatively greater (more so with Stage I pins).

c.

The freedom for third order displacement on a round wire is maximum.

Therefore, Begg bracket is a “unipoint” bracket only for mesio-distal or labio (bucco)lingual displacements. It causes a “two-point contact” with the archwire in the first order – rotational displacement -, which creates a force couple. “The ribbon arch was actually superior to the edgewise appliance for rotation of individual teeth.” – Dewel.

The efficacy of rotational control will diminish as the difference between the slot size and the wire size increases, because of: a. Undersized wires b. Oversized slot (bad manufacturing or when slot opens during the treatment) Hence, good quality brackets must be used for obtaining rotational control. Also, higher size wires (0.018” and 0.020”) should be employed as early as possible.

Full engagement of the arch wire in the slot is imperative. This is possible only with adequate strength bonding. Anti-rotation brackets and rotation modules can achieve over-correction of rotation. ----------------------------------------------------------------------------------------------------------Mechanics of intrusion Lack of true intrusion of the maxillary incisors was one of the major weaknesses of traditional Begg (bite opening was mainly on account of molar extrusion and some intrusion of the lower incisors), which is overcome to a significant extent in refined Begg. Whether the upper incisors intruded, remained at the same level or actually extruded was a bebated issue. The mechanism was satisfactory in growing patients with ana average exposure of upper incisors, but it failes to improve the excess exposure of the so-called “gummy smiles”.this is overcomew significantly in the refined begg. It must be emphasised that anterior teeth can be intruded by orthodontic means only to the extent of 3-4 mm. An adult case of gummy smile requiring surgery cannot be converted to a non surgical orthodontic treatment by any presently available mechanotherapy. However, cases of gummy smile during childhood can be successfully treated by partly intruding the incisors and partly by preventing the downward displacements of the alveolus / maxilla. All the six anterior teeth are intruded together in Begg practice. The round archwire derives its bite opening force from the anchor bends. This force acts on the

teeth through the brackets which are placed on the labial surfaces of the incisors, i.e. away from the long axis of the teeth on which the CRes. of the individual teeth are located. Depending on the direction of the intrusive force in relation to the long axis of the tooth, the tooth would undergo varying amounts of intrusion (translation) and labial crownlingual root tipping (rotation). Such rotational displacement is generally undesirable (the exception being lingually inclined incisors as in Cl. II div. 2 cases), and is resisted in the case of upper incisors by using Cl. II elastics during stage I. However, the Cl II force not only has a horizontal component for providing this resistance, it also has a vertical component which reduces the magnitude of the intrusive force of the wire. Further, the horizontal component of the elastic force affects the direction of the net resultant force. Thus, the interplay between the wire generated intrusive force and the elastic force determines both the magnitude and direction of the net resultant force acting on the teeth. Both these deserve a detailed discussion to appreciate how Refined Begg is able to achieve true intrusion unlike in the traditional practice.. I. Consideration of the magnitude of intrusive force. A. Optimal intrusive force value. Many authors13,14,15 have suggested optimum intrusive force values ranging from 15-30 g per upper incisor and slightly higher values for upper canines. The earlier writings6,16 on Begg Treatment did not specify the precise force values in the Begg bite opening mechanics. Later, Kesling17 (1985) stated that the upper and lower bite opening bends generate intrusive forces of approximately 1.5 oz and 1.2 oz magnitude respectively at the upper and lower midlines. The extrusive component of the light Cl. II elastics on the upper incisors is approximately 1 oz. Hence the net intrusive force on the upper incisors is approximately 0.5 oz. (Fig. 3). This value (0.5 oz at the midline, i.e., 14 gm for three teeth or approximately 5gm per tooth) is far below the optimal force suggested by others. It may be one of the reasons for the lack of intrusion of upper anteriors in conventional Begg treatment. Probably the force is just enough to prevent their normal eruption and to maintain their height, but insufficient to intrude them. I suggest that for active intrusion the upper anteriors should receive approximately 60 gm net force in the midline after negating the extrusive component of Cl II elastics . B. Role of light Class II elastics . A net intrusive force of 60 gm can be obtained by a combination of 75 gm intrusive force and appropriate modification of elastics, as follows : a. By using light elastic force for longer periods (from 2 to 5 days), a very light Cl. II force is provided most of the time, since the elastic force diminishes rapidly in the oral environment. Such very low force values do not adversely affect concomitant retraction, because forces in the vicinity of 5 gm are known to be capable of achieving tipping movements. b. Sims21 has suggested the use of 3/8″ ultra light elastics (e.g., “road-runner elastics” of M/s. Ormco) instead of the routinely used 5/16″ light elastics (e.g.

T.P. yellow elastics). He goes to the extent of continuing the same elastics for 4-5 days, till they break. i.

It is realised that the above suggested force values are only a rough estimate. It is impossible to precisely calculate the required intrusive force every time, for every patient, because there are many variables.The discussion, however, does point to the need to use higher intrusive forces (than those employed in conventional Begg treatment), in combination with very light Cl. II elastic forces for active upper incisor intrusion.

I however, wish to emphasize that I do not recommend the use of high intrusive forces all the time in a stereotype fashion. This is the maximum force which is required at a particular stage. How to vary the magnitude of the force and where to use the maximum permissible force is discussed later. II. Consideration of the direction of the resultant force. As pointed out by Hocevar20, teeth respond only to the resultant of the forces, and not to the individual components of the force system. The anterior teeth would respond to a resultant of the wire generated intrusive force and the elastic generated retractive force. This resultant force should ideally pass through the center of resistance of the upper incisors (which is very difficult to achieve), or at least should lie very close to and directed as much parallel to the long axis of the teeth as possible. The direction and the magnitude of the resultant force both depend upon the interplay between 1. The magnitude of the intrusive force – its direction being almost constant i.e. tangential to the arc which the anterior segment of the archwire would subscribe if released from the brackets. 1. The magnitude and direction both of the elastic force. a. Since extremely high intrusive force will cause reciprocal undesirable reactions on the anchor molars, it is better to adopt different strategies which will keep the intrusive force within limits and still give a gradually increasing vertical orientation of the resultant. The strategies are in the form of first changing the magnitude and then the orientation of the elastic force instead of always relying on a constant Cl. II elastic force. Starting from light intrusive force and light Class II forces in the case of severely proclined incisors, the intrusive forces are gradually increased as the incisors become less inclined. Simultaneously the elastic force is reduced and their application is changed from Class II to Class I mode, and further changed by applying them from a transpalatal arch. Conversely, in Class II div. 2 cases different combination of intrusive forces and elastic wear will be employed starting from no elastics, depending on the changing incisor inclination. III Arch wire design (placement of bite opening bends at different sites) and its effect on the individual teeth

I recommend the following modification for a uniform intrusion of the six upper anteriors : A mild gingival curve is incorporated in the anterior section, starting from the mesial of the cuspid circle on one side to the corresponding point on the other side. This is based on the recommendation of Swain. At the midpoint, this should lift the archwire over the brackets by about 3 mm. This intrusive action is further augmented by incorporating a vertical step up bend, mesial to the molar tube, 3-5 mm in height. This causes the intrusive force to be applied from a point higher up in relation to the occlusal plane, thus offsetting the relative extrusive action on the canine that the gingival curve in the anterior section will have. Even such a design may extrude the canine relative to the lateral incisors. This is prevented by making the cuspid circle in such a way that the posterior segments of the archwire are kept gingival and the anterior segment is kept occlusal. The vertical step-up bend, though very effective in anterior intrusion, is also likely to tip the anchor molars distally. Therefore, it should be discontinued as soon as the intrusion requirements are achieved. It might be prudent to take vertical anchorage from both the first and second molars in such cases, and also to support the anchorage with distal vertical elastics. Summery of Intrusion Mechanics It is easier to understand the mechanics of intrusion by considering the application of a single force or the resultant of more than one force in relation to the centre of resistance of individual incisors or the entire segments of two incisors, four incisors or the six anteriors. True intrusion is possible when the application of force is through the concerned centre of resistance (or very close to it). During stage I, the upper anteriors are subjected to two forces viz. The retractive force of the Cl. II elastics and the intrusive force generated by the anchor bend in the archwire. The resultant of these two will determine how the upper anterior teeth respond to the intrusion needs. The magnitude and direction of the resultant force can be altered by changing a. Magnitude and / or direction of the retractive force. b. Magnitude of intrusive force. By a careful manipulation of these two forces the magnitude and the direction of the resultant force can be so adjusted as to obtain a predominantly intrusive action, a predominantly retrusive action or a combination of the two, as below :

1. In extreme cases of proclination or retroclination, the resultant force is made to pass palatal or labial respectively in relation to the C

Res

to correct the inclination before

attempting intrusion. In cases of severe proclination the intrusive force is kept low while the retrusive force is average. The combination gives a resultant force which passes palatal to the incisors, thus correcting the excess proclination. While treating the other extreme of retroclined teeth, only an intrusive force is used (omitting the Cl.II elastics). This force acts labial to the C Res and corrects the retroclination. 2. After the inclination corrects, a varied strategy is employed which helps in keeping the resultant force close to the C

Res

This is done by initially increasing the intrusive

force (by increasing the anchor bend in 0.016 wire for one or two visits followed by the use of 0.018 archwire). Thereafter the force of Cl.II elastics is reduced and then the direction of the elastics is changed to Cl.I and subsequently to an oblique direction from T.P.A. or from the power arm. This helps in keeping the resultant force close to the C Res and the direction parallel to the long axis of the teeth. Mechanics of Begg tipping In general, uncontrolled tipping is undesirable because it can hasten root resorption, as emphasized by Reitan. This is especially true of uncontrolled tipping in the labio-lingual direction. (The few exceptions wherein uncontrolled tipping is beneficial for one or two visits are: lingual tipping of excessively proclined

incisors, and labial

uprighting of retroclined upper incisors in Cl. II Div. 2 or the lower incisors in Cl. III.) Controlled Lingual tipping of upper anteriors Intrusion and tipping of upper incisors are intimately related not only because they are simultaneously carried out but also because when they are judiciously balanced, they help in overcoming the other significant drawback of conventional Begg namely uncontrolled tipping of incisors. This is achieved by manipulating the wire generated intrusive force and the retractive component of the force from the elastics. Each of them, if applied independently (since they are acting at the bracket level i.e. away from the C Res of the teeth), cause part translation and part rotation (in other words, uncontrolled

tipping). The translatory movements are in different directions – in the vertical direction from the intrusive force and in the posterior direction from the retractive force. However, what is important to note is that the moments generated by both of them are in the same labio-lingual plane but act in opposite directions. The moment caused by the intrusive force is crown labial - root lingual, while that produced by the retractive force is crown lingual - root labial. Therefore, the moment from the intrusive force can act as the counter moment against the moment produced by the elastic force. The ratio of the former to the retractive component of the elastic force is the M/F ratio which governs the type of tipping while retracing the anterior teeth. If the intrusive force is inadequate (producing a smaller moment than required), or if the elastic force is too large, the M/f ratio will be insufficient for controlled tipping. Hence, the important consideration is to keep the Cl. II elastic force very light and use adequate amount of intrusive force so that a correct M / F ratio is obtained. Many years ago, Swain pointed out the possibility of tipping the crowns lingually while maintaining the root apices at the same position, based on the study of Adler. Another very useful addition to Refined Begg mechanics is the MAA which can also provide a moment in the labio – lingual plane by creating a force couple. This is akin to the third order couple provided by a rectangular wire in the edgewise slot. but of a comparatively very small magnitude. After the bite has opened in the first stage, the intrusive force level needs to be reduced, which also reduces the M / F ratio. When this happens, there is a greater likelihood of uncontrolled tipping (during the latter part of first stage and the whole of second stage). Hence, after the intrusive force is reduced, the M / F ratio must be supplemented with the moment from a MAA for lingual root torque. The light forces generated by MAA against the crown surfaces are given in the following table. ( The moments created on the individual teeth will be equal to the forces multiplied by the height of the MAA box.) These may be compared with the forces produced by the torquing auxiliary in (table No.1) Preventing Uncontrolled Tipping of Lower Incisors

The problem is of a different nature in the case of lower incisors. Intrusive force alone can flare the teeth, (except in the initial stages of deep bite cases where contact of the incisal edges with the palatal surface of upper incisors or with the palatal mucosa will prevent their labial movement). Flaring occurs because lower incisors are subjected to a crown labial- root lingual moment from the intrusive force in the archwire, while there is no restraining force on these teeth similar to the Cl II elastic force on the upper incisors. Such flaring can be avoided by the following means: Minimise the moment by reducing the intrusive force, or by placing the brackets as much gingivally as is permitted by hygiene considerations. A. Provide a holding force by bending the diatal ends of the arch wire as suggested by Hocevar. Provide a counter moment by using a MAA for labial root torque or a reverse torquing auxiliary. During the second stage, when Cl I elastic foce needs to be used, a MAA for lingual root torque may be required to prevent uncontrolled crown lingual-root labial tipping of lower incisors (similar to what happens in the upper arch) since intrusive force gets reduced at this stage. Mechanics of root movement The third stage of Begg treatment involves predominantly root movements in a labiolingual or mesiodistal direction. A doubt is expressed by some edgewise operators as to how it is possible to obtain a high M/F ratio required for the root movements using the Begg torquing auxiliary and uprighting springs. The even lighter auxiliaries and springs employed in Refined Begg may magnify this apprehension. However, a careful scrutiny of the forces generated by the torquing auxiliary and the uprighting springs in relation to the light Cl.II elastic force will help in dispelling this apprehension. The force values are given in the following tables. Forces generated (in grams) by the commonly used four spur and two spur torquing auxiliaries with 5 mm spur length. (Table 2) The auxiliary commonly used is the one made in 0.012 premium plus wire. Although the forces produced by this auxiliary are low, the moments generated by these

forces are sufficient because the moment arm is much greater in a torquing auxiliary than in a rectangular archwire twisted for torquing effect. It may be noted that the forces produced by a 4 spur torquing auxiliary made in 0.012” wire are approximately twice of those from a MAA made in 0.009” wire. In other words, the M/F ratio for root movements is about twice of that employed in controlled tipping during the initial stages. The forces generated by the uprighting springs made from different wires (Except in the case of minisprings which were the commercially available once, other springs had the following features: 2 1/2 turns, the inner diameter of helical 0.036”, length of the arm 4 mm and angulation 135o )(table 3) I recommend uprighting springs made of 0.010” wire for the incisors and of 0.012” wire for canines and premolars. As Nikolai has pointed out, greater moments are required for the mesio-distal root movements than for the bucco-lingual root movements, since holding force for the former is greater due to the mesio-distal crown contact. Thus the forces produced by the torquing auxiliary are smaller than the forces generated by the uprighting springs for the same individual teeth. One must remember that the action of uprighting springs and torquing auxiliaries is in all the three planes namely sagittal, vertical and transverse. The sagittal forces are easily appreciated. The uprighting springs on the anterior teeth for distal root movement and the torquing auxiliary for palatal root torque, both have an extrusive effect on the anteriors and an intrusive effect on the molars. The intrusive effect on the molars is responsible for a transverse buccal rolling action on the molars. Such undesired reactions should be carefully monitored and neutralized. Does Begg appliance satisfy the appliance design criteria ? Burstone has set the following criteria for an efficient appliance design : 1. Force system a. Precise M/F ratios for different types of tooth movement. b. Constancy and low magnitude of moments and forces. 2. Ease of use. 3. Patient comfort. 4. Minimal patient co-operation. The Refined Begg appliance admirably satisfies all the above requirements. It can provide the required M/F ratios for controlled tipping in the initial stages and for root

movements in the final stage. The forces used are light and fairly constant. In fact the ultra-light forces advocated by Refined Begg are physiologically more acceptable. Provided the appliance is handled carefully, it is easy to practice and manipulate. The light forces are comfortable for the patient. Finally, one has to depend on patient’s cooperation only to a limited extent. Thus the Refined Begg should be considered as an appliance on par with any other modern appliance.