ORTHODONTIC ALIGNMENT
- 1. The First Stage Of Comprehensive Treatment Dr Eithar Sharif supervision PROF DR MAHER FOUDA MANSOURA EGYPT
- 2. Arch wire . Alignment of symmetric crowding . Alignment of Asymmetric crowding . Crossbite correction . Impacted or unerupted teeth . Diastema closure .
- 3. The goals of the first phase of treatment are to bring the teeth into alignment and correct vertical discrepancies by leveling out the arches.
- 4. Arch wire Principles in the Choice of Alignment Arches 1. Initial archwires should provide light,continous force of approximately 50 grams . 2. The archwires should be able to move freely within the brackets, at least 2mil clearance is needed . 3. Rectangular archwires should be avoided because it can create unnecessary and undesirable root movement . 4. The springier archwire is preferable in the alignment.
- 5. Archwire Material The titanium-based arch wires, both nickel-titanium and betatitanium offer a better combination of strength and springiness than do steel wires . The NiTi wires, however, are both springier and stronger in small cross- section than beta-Ti. For this reason, NiTi wires are particularly useful in the first stage of treatment, and the remarkably flat load-deflection curve for superelastic NiTi makes it the preferred material. If steel is used at this stage, either multistranded wires or loops to increase springiness are needed. Beta-Ti rarely is the best choice for an initial arch wire.
- 6. Size of Wire As wire size increases, strength increases rapidly, while springiness decreases even more rapidly. For alignment, the smallest diameter (and therefore the springiest) wire that has adequate strength would be preferred .
- 7. Distance Between Attachments. As the distance increases between the points of attachment of a beam, strength decreases rapidly while springiness increases even more rapidly. The width of brackets determines beam length when continuous arch wires are used (unless brackets are bypassed): the wider the individual brackets smaller the interbracket span.
- 8. Alignment of Symmetric Crowding Archwire choices The flat load-deflection curve of superelastic NiTi makes it ideal for initial alignment when the degree of crowding is similar on the two sides of the arch. The superelastic wire provides remarkable range over which a tooth can be moved without generating excessive force. Under most circumstances initial alignment can be accomplished simply by tying 14 or 16 mil A-NiTi that delivers about 50 gm into the brackets of all the teeth, being careful not to tie too tightly, and observing the patient without the necessity of other changes . The size of the superelastic wire is not a critical variable, except that 18 mil wires should not be used in the 18 slot appliance.
- 9. Alignment of symmetric Crowding
- 10. The major objection to superelastic NiTi is that it is expensive. If a large range is not necessary, a triple-stranded 17.5 mil multistranded steel wire (3 X 8 mil) offers good properties at a fraction of the cost . In theory, this size would be too large for effective use in 18-slot brackets. Clinical research has shown, however, that in both the 18 and 22 appliance, if these wires are recontoured monthly and retied with elastomeric ligatures, the time to alignment is equivalent to A-NiTi.
- 11. One problem with superelastic wires for initial alignment is their tendency to "travel" so that the wire slips around to one side, protruding distally from the molar tube on one side and slipping out of the tube on the other. The most effective way to prevent travel is to tightly crimp a split tube segment onto the wire between two adjacent brackets, as done here between the central incisors. The location of the stop is not critical. Some preformed wires now have a dimple in the midline to prevent the arch wire from sliding excessively.
- 12. This panoramic radiograph show archwire travel to the point that on one side it penetrated into the ramus .
- 13. Preliminary alignment of the teeth within each arch is usually the first step in a treatment sequence aimed at achieving full orthodontic correction. Depending on the individual treatment goals and the treatment philosophy used, this step may involve leveling and aligning all of the teeth or may initially exclude some teeth to avoid round-tripping. Early alignment of rotated or malposed teeth allows these corrections to be maintained through the treatment period, permitting biologic adaptation and enhanced stability during the retention phase.
- 14. . Lining up the brackets is also important in any technique in which sliding mechanics will be used to close spaces between teeth. Initial leveling and alignment of the teeth has been simplified by the increased availabil- ity and use of wires with low load-deflection rates and shape memory characteristics. An arch form of nickel titanium wire inserted into malaligned brackets can accomplish what once required intricate bending of loops into stainless steel wires.
- 15. . Preadjusted brackets of various prescriptions allow a greater degree of tooth alignment to be attained in this early stage of treatment. The convenience of flexible wires and preadjusted brackets, however, should not serve as a substitute for detailed diagnosis of the patient and careful planning of mechanical intervention. Indeed, the use of wires with low loaddeflection characteristics may actually increase the possibility that the undesirable side effects of poorly planned orthodontic mechanics will be expressed to a greater extent before being detected.
- 17. Analysis of the force systems that will be generated by insertion of straight wires into malaligned brackets will show the unanticipated side effects before they occur. Depending on the individual situation, these side effects may be desirable or not. If unfavorable side effects can be anticipated, steps can be taken to control them, or an alternate mechanics plan can be developed to achieve the desired outcome in a different way. 16 Seminars in Orthodontics, Vol 7, No 1 (March), 2001:
- 18. Quantifying Extraction Decisions While the frequency of premolar extraction cases varies among orthodontists, it is estimated that one-third of all orthodontic patients have such a severe malocclusion that some pattern of premolar extraction is deemed necessary in order to resolve the problems and align the teeth (Proffit ). TSALD is the most important factor necessitating the decision to extract premolars (Baumrind et al.). Differential diagnosis involves determining whether first or second premolars should be extracted in the maxilla and/or in the mandible. Basic guidelines towards choosing premolar extraction patterns are available in textbooks (e.g., Moyers ; Proffit ), but more detailed factors for consideration are available in the primary orthodontic literature.
- 19. First Premolar Extractions General guidelines suggest extracting first premolars when the TSALD source area is primarily in the anterior portion of the arch. Removing the first premolars is a straightforward way to correct anterior crowding, excessive overjet and protrusion. This correction works by making space for the alignment and retraction of incisors and canines. Extracting premolars close to the area of crowding is beneficial because at the point when crowding or protrusion is corrected, little extraction space remains to be closed (Schoppe , Graber , Dewel , Moyers , Proffit ). Other indications for a first premolar extraction pattern instead of a second premolar extraction pattern include excessive overbite, Class II, division 2 malocclusion type, and serial extraction therapy
- 20. Brandt and Safirstein stated that placing the extraction site closer to the anterior gives a mechanical advantage in leveling the arch as space is closed. This advantage is helpful when treating patients with a deep bite. Creekmore said that, as a rule of thumb, he preferred extracting maxillary first premolars for non-surgical treatments of non-growing Class II, division 2 malocclusioans. He specified that in these cases he treated the mandible without premolar extraction. Dewel specified another reason for first premolar extractions over second premolar extractions. He stated that when treating a young patient with serial extraction, who will require the removal of permanent teeth, the first premolars should be removed so that erupting canines can more easily drift distally into the extraction space.
- 21. Second Premolar Extractions The basic indication for second premolar extraction is that there is moderate anterior crowding with no protrusion and the patient has good facial balance (Dewel , Graber , Moyers , Brandt and Safirstein , Creekmore , Proffit ). The crowding guideline is subjective as de Castro describes a second premolar extraction instance as being when there is a TSALD of 5 mm or more, while Schoppe describes it as being a TSALD of 7.5 mm or less. Either way, removing the second premolars will give enough space to resolve minor crowding while not changing the profile. It also leaves the incisors in their original position over basal bone without inclining them lingually which is undesirable (Dewel , Schoppe ).
- 22. Other considerations for removing second premolars instead of first premolars include posterior crowding, anterior open bite, Class III correction, and facilitation of intentional anchorage slippage. When second or third molars are crowded, ectopic, or impacted, they can be helped by creating space in the buccal segments of the arch. This space is created by extracting second premolars so that the first molar can move mesially (Logan , de Castro). This extraction pattern is also advantageous for correcting anterior open bites,
- 23. because it is easier to accentuate the curve of Spee and lessen relapse of the open bite after treatment (Brandt and Safirstein ). Also, by reducing the posterior vertical dimension through the removal of posterior occlusal surface area, an immediate increase in anterior overbite is accomplished, facilitating closure of an open bite (Logan ). Second premolar extraction in the maxilla can also be helpful in camouflaging Class III malocclusions when combined with a first premolar extraction in the mandible. This pattern allows more retraction of the lower incisors while allowing more mesial maxillary molar movement to correct the malocclusion (Schoppe ). Intentional anchorage slippage can be facilitated by the extraction of second premolars. This is desirable when there will be excess extraction space remaining after TSALD resolution and the patient has good facial harmony.
- 24. De Castro specifies that when needing to move the molars forward more than 2.5 mm on each side, a second premolar extraction pattern is indicated. By removing second premolars instead of first premolars, first molars are easily slipped forward instead of necessitating unwanted retraction of the anterior teeth to close the remaining space (Dewel , Schoppe , Dewel , Logan , de Castro ). This occurs not only because the teeth mesial to the first molars are removed, giving a clear path of movement, but also because the weaker anterior anchorage is increased from six to eight teeth producing more resistance (Dewel ).
- 25. Alignment in Premolar Extraction Situations In patients with severe crowding of anterior teeth, it is necessar to retract the canines into premolar extraction sites to gain enough space to align the incisors. In extremely severe crowding, it is better to retract the canines independently before placing attachments on the incisors. This can be done either with segmental retraction loops, or by sliding the canines along a relatively rigid wire (16 steel, for instance) that does not contact the incisors. Sliding the canines produces more stress on the posterior anchorage, so critical anchorage is an indication for the retraction loops.
- 26. In more typical and less extreme crowding, it is possible to simultaneously tip the canines distally and align the incisors. Until recently, the best way to do this was to use a loop arch wire . The principle is that described by Stoner 3 as a "drag loop." The loop in the extraction site is gabled sharply and activated slightly, producing a gentle space-closing force with an anti-tip moment on the posterior teeth. As the activated distal loop closes, the loop mesial to the canine opens, allowing the canine to tip back independently while the incisors are being aligned.
- 27. The same independent distal movement of the canines now can be obtained with an A-NiTi arch wire without loops, and A-NiTi coil springs from the first molars to tip the canines distally . When this is done, the spring should be chosen to deliver only 50 gm, and an arch wire preformed by the manufacturer to have an exaggerated reverse curve of Spee should be chosen, to limit forward tipping of the molars. As with the drag loop, the idea is to pit distal tipping of the canines against forward bodily movement of the molars.
- 28. For this patient arolled or drag loop in 14 mill steel wire is being used to tip the canine distally into the extraction site while the incisor are being aligned independently.
- 29. When additional arch length needed, advanced stop in the flexible initial archwire are useful.
- 30. Alignment in Nonextraction Situations Alignment in nonextraction cases requires increasing arch length, moving the incisors further from the molars. In this circumstance, just tying a superelastic wire into the bracket slots is ineffective. Two objects cannot occupy the same space at the same time, so alignment cannot occur until space to allow it is created. With multistrand wires, the easiest way to increase arch length during alignment is to bend a loop mesial to the molars so that the wire is held just anterior to the incisors before it is tied in. At subsequent appointments the adjustment loop is opened, again advancing the wire slightly, until the teeth come into alignment.
- 31. The superelastic equivalent of this procedure is to crimp a stop on the wire at the molar tube, so that it holds the wire just in front of the incisors . The greater range of the superelastic wire means that the activation can be somewhat greater. At subsequent appointments, if more arch length is needed, an additional stop or stops can be quickly slipped into position, without even removing the wire or religating it. Obviously, this type of arch expansion will carry the incisors facially, and so it is not indicated in the presence of severe crowding unless incisor protrusion is desired.
- 32. Alignment of Asymmetric Crowding When all or nearly all the crowding is in one place, what is needed is an arch wire that is rigid where the teeth are already aligned, and quite springy where they are not. It is easy to add a small diameter superelastic wire as an auxiliary spring, so that a stiff main arch (16 or 18 steel) can be tied into all the teeth except the displaced one (or two the same system works with small segments of two teeth).
- 33. A segment of superelastic NiTi can be laid in the brackets on top of the main arch wire, or tied below the brackets of the anchor teeth, and tied to the bracket on the displaced tooth . With this arrangement, the correct light force to bring the displaced tooth into alignment is provided by the NiTi wire, and the reciprocal force is distributed over all the rest of the teeth. The result is efficient movement of the displaced tooth, with excellent preservation of arch form.
- 34. Alignment of severely crowded lower incisors with superelastic equivalent of the original “ drag loop.’’
- 35. Crossbite Correction It is important to correct posterior crossbites and mild anterior crossbites (one or two displaced teeth) in the first stage of treatment. Severe anterior crossbites (all the teeth), in contrast, are usually not corrected until the second stage of conventional treatment, or might remain pending surgical correction. For both posterior and anterior crossbites, it is obviously important to make the appropriate distinctions between skeletal and dental problems, and to quantitate the severity of the problem.
- 37. Individual Teeth Displaced Into Anterior Anterior crossbite of one or two teeth almost always is an expression of severe crowding, and is most likely to occur when maxillary lateral incisors that were somewhat lingually positioned to begin with, are forced even more lingually by lack of space. Correction of the crossbite requires first opening enough space for the displaced teeth, then bringing them into proper position in the arch . Crossbite
- 38. Occlusal interferences may make this difficult. The patient may tend to bite brackets off the displaced teeth, and as the teeth are moved "through the bite", occlusal force pushes them one way while the orthodontic appliance pulls them the other. It may be necessary to use a bite plate temporarily to separate the posterior teeth and create the vertical space needed to allow the teeth to move. The older the patient, the more likely it is that a bite plate will be needed. During rapid growth in early adolescence, often incisors that were locked in anterior crossbite can be corrected without a bite plate. After that, one probably will be required.
- 39. Transvere Maxillary Expansion by Opening the Midpalatal Suture It is relatively easy to widen the maxilla by opening the midpalatal suture before and during adolescence, but this becomes progressively more difficult as patients become older. The chances of successful opening of the suture are nearly 100% before age 15, but begin to decline thereafter because of the increased interdigitation of the sutures.'
- 41. Patients who are candidates for opening the midpalata suture may have such severe crowding that even with this arch expansion, premolar extraction will be required. In these patients, however, separation of the suture should be the first step in treatment, before either extraction or alignment. The first premolar teeth are useful as anchorage for the lateral expansion and can serve for that purpose even if they are to be extracted later, and the additional space provided by the lateral expansion facilitates alignment.
- 42. Sometimes, transverse maxillary expansion can provide enough additional space to make extraction unnecessary, but rarely is it wise to use sutural expansion as a means of dealing with a crowding in an individual who already has normal maxillary width . Opening the midpalatal suture should be used primarily as a means of correcting a skeletal crossbite, making a narrow maxilla normal, not a normal maxilla abnormally wide.
- 43. In the early permanent dentition, the basic mechanism for separation of the midpalatal suture is a jackscrew built into a fixed appliance that is rigidly attached to as many posterior teeth as possible. The appliance can be made so that it has plastic palate-covering shelves or can consist solely of a metal or plastic framework against the teeth, not contacting the palatal tissue.
- 44. When the maxillary teeth move transversely, some extrusion may occur, and even if it does not, the tooth movement creates cuspal interferences that cause the mandible to rotate down and back. In deep bite patients who are still growing or in patients with a mild Class III tendency, this can be advantageous, but it is a problem in long face patients with a narrow maxilla. Adding bite blocks to a bonded expander is the best way to overcome this problem
- 45. Separation of the midpalatal suture can be produced by either rapid or slow expansion with the fixed appliance remains in place for approximately the same length of time. With rapid expansion, the expansion itself is carried out in approximately 2 weeks, but the screw should then be stabilized and the appliances maintained in place for 3 to 4 months of retention. With slow expansion, approximately 2 1/2 months are needed to obtain the expansion, and the appliance can be removed in another 2 months.
- 46. Some degree of relapse can be expected after palatal expansion because of the elasticity of the palatal soft tis- sue. Therefore it is wise to overcorrect the crossbite initially . Even if 3 to 4 months of stabilization with the palate-separating device have been provided, additional retention of the crossbite correction is needed when the fixed appliance is removed. A palate-covering removable retainer is satisfactory but may be somewhat awkward in combination with fixed appliances to align the teeth as the first stage of treatment proceeds. An alternative is a heavy labial arch wire placed in the headgear tubes, which will maintain the lateral expansion while light resilient arch wires are being used to align the teeth , or a lingual arch.
- 47. Aheavy labial archwire placed in headgear tubes .
- 48. Correction of Dental Posterior Crossbites. Three approaches to correction of less severe dental : crossbites are Aheavy labial expansion arch. 1. An expansion lingual arch. 2. Cross -elastics. 3.
- 49. 1.Aheavy labial expansion The inner bow of a facebow is also a heavy labial arch, and expansion of this inner bow is a convenient way to expand the upper molars. This expansion is nearly always needed for patients with a Class II molar relationship, whose upper arch usually is too narrow to accommodate the mandibular arch when it comes forward into the correct relationship because the upper molars are tipped lingually. The inner bow is simply adjusted at each appointment to be sure that it is slightly wider than the headgear tubes and must be compressed by the patient when inserting the facebow. If the distal force of a headgear is not desired, a heavy labial auxiliary can provide the expansion effect alone. The effect of the round wire in the headgear tubes, however, is to tip the crowns outward, and so this method should be reserved for patients whose molars are tipped lingually.
- 51. 2.An expansion lingual arch Ahighly flexible lingual arch, like the quad helix design is an excellent choice for correction of a dental crossbite.
- 52. 3.Cross-elastics It is typically running from the lingual of the upper molar to the buccal of the lower molar . These elastics are effective, but their strong extrusive component must be kept in mind. As a general rule, adolescent patients can tolerate a short period of cross-elastic wear to correct a simple crossbite ,
- 53. , because any extrusion is compensated by vertical growth of the ramus, but cross-elastics should be used with great caution in adults. As any posterior crossbite is corrected, interference of the cusps increases posterior vertical dimension and thereby tends to rotate the mandible downward and backward, even if crosselastics are avoided. The elastics accentuate this tendency.
- 54. Cross-elastic from the lingual of the upper molars to the buccl of the lower molars .
- 55. I mpacted or Unerupted Teeth Bringing an impacted or unerupted tooth into the arch creates a set of special problems during alignment. The most frequent impaction is a maxillary canine or canines, but it is occasionally necessary to bring other unerupted teeth into the arch, and the same techniques apply for incisors, canines and premolars. Impacted lower second molars pose a different problem and are discussed separately.
- 56. The problems in dealing with an unerupted tooth fall into three categories: (1) surgical exposure, (2) attachment to the tooth, and (3) orthodontic mechanics to bring the tooth into the arch.
- 57. 1. Surgical Exposure. It is important for a tooth to erupt through the attached gingiva, not through alveolar mucosa, and this must be considered when flaps to expose an unerupted tooth are planned. If the unerupted tooth is in the mandibular arch or on the labial side of the maxillary alveolar process, a flap should be reflected from the crest of the alveolus and sutured so that attached gingiva has been transferred to the region where the crown is exposed
- 58. If this is not done, and the tooth is brought through alveolar mucosa, it is quite likely that tissue will strip away from the crown, leaving an unsightly and periodontally compromised gingival margin. If the unerupted tooth is on the palatal side, similar problems with the heavy palatal mucosa are unlikely, and flap design is less critical. Occasionally, a tooth will obligingly erupt into its correct position after obstacles to eruption have been removed by surgical exposure, but this is rarely the case after root formation is complete. Even a tooth that is aimed in the right direction usually requires orthodontic force to bring it into position.
- 59. Method of Attachment. The least desirable way to obtain attachment is for the surgeon to place a wire ligature around the crown of the impacted tooth. This inevitably results in loss of periodontal attachment, because bone destroyed when the wire is passed around the tooth does not regenerate when it is removed. Occasionally no alternative is practical, but wire ligatures should be avoided whenever possible.
- 60. Before the availability of direct bonding, a pin was sometimes placed in a hole prepared in the crown of an unerupted tooth, and in special circumstances, this remains a possible alternative. The best approach, however, is simply to expose an area on the crown of the tooth and directly bond an attachment of some type to that surface . In many instances, a button or hook is better than a standard bracket because it is smaller. Then, a piece of fine gold chain is tied to the attachment, and before the flap is repositioned and sutured into place, this is positioned so that it extends into the mouth. The chain is much easier to tie to than a wire ligature.
- 62. Mechanical Approaches for Aligning Unerupted Teeth. orthodontic treatment to open space for the unerupted tooth and allow stabilization of the rest of the dental arch must begin well before the surgical ex- posure. To align the other teeth a heavy stabilizing arch wire can be in position at the time of surgery. This allows postsurgical orthodontic treatment to start immediately.
- 63. An auxiliary NiTi wire, overlaid on the stabilizing arch in the same way of asymmetric alignment Situations is the most efficient way to bring an impacted tooth into position. The alternatives are to use a special alignment spring, either soldered to a heavy base arch wire or bent into a light arch wire (
- 64. Another possibility, magnetic force to initiate movement of an unerupted tooth. Magnetic attraction between an attachment bonded to the tooth and an intraoral magnet would produce the tooth movement. The technique involves bonding a small magnet to an unerupted maxillary canine, and using magnetic attraction to a larger magnet contained within a palate-covering removable appliance.
- 65. Case Report A 10 year old boy was brought by his parents to the orthodontic department . Their chief complaint was the non-eruption of the upper left permanent central incisor. The child was physically healthy and had no history of medical and dental trauma. The patient had a skeletal class I malocclusion and balanced facial pattern. Intraoral examination revealed an early mixed dentition and an Angle’s class I molar relationship. Clinical examination showed a missing maxillary permanent left central incisor and no apparent arch length discrepancy in both maxillary and mandibular arches. Inadequate space distribution of the maxillary incisors causing midline deviation was due to drifting of the adjacent teeth into unoccupied space.
- 66. An intraoral periapical radiograph of upper anterior region demonstrated a supernumerary tooth and an impacted permanent left central incisor . To confirm the position of supernumerary tooth, upper anterior occlusal radiograph was taken which showed the presence of supernumerary tooth on the palatal side Several treatment alternatives were explained to the patient and his parents. They agreed for the extraction of supernumerary tooth surgically followed by surgical exposure of impacted central incisor and alignment of the impacted incisor into the arch by orthodontic treatment .
- 67. Figure 1: Intraoral photograph showing the unerupted 21
- 68. Figure 2: Intraoral periapical radiograph showing supernumerary tooth and an impacted permanent left central incisor
- 69. Figure 3: Anterior occlusal radiography showing palatally placed supernumerary tooth
- 70. Figure 4: Intraoral periapical radiograph showing absence of supernumerary tooth, after surgical extraction
- 71. Figure 5: Showing use of elastics for orthodontic traction
- 72. Figure 6: Showing insufficient space in the maxillary arch
- 73. Figure 7: Showing the open coil spring used for space regaining
- 74. Figure 8: Showing 0.012’’ Australian S.S. archwire
- 75. Figure 9: Showing the proper alignment of 21 in the maxillary arch
- 76. Figure 10: Showing the proper alignment of 21 in the maxillary arch
- 77. Unerupted/Impacted Lower Second Molars. Correction of an impacted second molar requires that the tooth be moved posteriorly and uprighted. In most cases, if the mesial marginal ridge can be unlocked, the tooth will erupt on its own. When the second molar is not severely tipped, the simplest solution is to place a separator between the two teeth. For a more severe problem, one possibility is to solder an auxiliary spring to the arch wire mesial to the first molar and extend it posteriorly into the embrasure between the first and second molars. The long arm of such a spring gives excellent flexibility and range of action, but also makes it difficult to keep the spring in place and may lead to substantial soft tissue irritation. A better alternative is to surgically expose the second molar, bond a tube to the buccal surface, and then use an auxiliary spring inserted into this tube to upright the second molar
- 78. Diastema Closure A maxillary midline diastema is often complicated by the insertion of the labial frenum into a notch in the alveolar bone, so that a band of heavy fibrous tissue lies between the central incisors.In this case almost always requires surgery to remove the interdental fibrous tissue and reposition the frenum.
- 79. If the frenum is removed while there is still a space between the central incisors, scar tissue forms between the teeth as healing progresses, and a long delay may result in a space that is more difficult to close than it was previously.
- 80. It is better to align the teeth before frenectomy. Sliding them together along an arch wire is usually better than using a closing loop, because a loop with any vertical height will touch and irritate the frenum. If the diastema is relatively small, it is usually possible to bring the central incisors completely together before surgery . If the space is large and the frenal attachment is thick, it may not be possible to completely close the space before surgical intervention. The space should be closed at least partially, and the orthodontic movement to bring the teeth together should be resumed immediately after the frenectomy, so that the teeth are brought together quickly after
- 81. the procedure. When this is done, healing occurs with the teeth together, and the inevitable postsurgical scar tissue stabilizes the teeth in their correct position instead of creating obstacles to final closure of the space A maxillary midline diastema tends to recur, A bonded fixed retainer is recommended