This chapter concerns those appliances that are fabricated mainly in acrylic and wire, and (as the name suggests) can be removed from the mouth. Most removable appliances are made for the upper arch. Functional appliances are made of the same materials, but work primarily by exerting intermaxillary traction and so are considered separately in Chapter 18.
16.1. INDICATIONS FOR THE USE OF REMOVABLE APPLIANCES
Although widely utilized in the past as the sole appliance to treat a malocclusion, with the increasing availability and acceptance of fixed appliances the limitations of the removable appliance have become more apparent. The removable appliance is only capable of producing tilting movements of individual teeth, which can be used to advantage where simple movements are required to correct a mild malocclusion but can lead to a compromise result if employed where more complex tooth movements are indicated. As a result the role of the removable appliance is changing, and it is becoming more widely used to transmit forces to blocks of teeth and as an adjunct to fixed appliance treatment. Removable appliances provide a useful means of applying extra-oral traction to segments of teeth, or an entire arch, to help achieve intrusive and/or distal movement. Examples of these types of appliance include the en masse appliance, which is described in Section 16.4.6 or the maxillary and buccal segment intrusion splints discussed in Chapter 12. Removable appliances are also employed for arch expansion, which is another example of their usefulness in moving blocks of teeth.
Removable appliances are particularly helpful where a flat anterior bite-plane or buccal capping is required to influence development of the buccal segment teeth and/or to free the occlusion with the lower arch. They are used passively as space maintainers following permanent tooth extractions and also as retaining appliances following fixed appliance treatment, as wear can gradually be reduced, allowing the occlusion to ‘settle in’. The advantages and disadvantages of removable appliances are summarized in Table 16.1.
On occasion it may be helpful to ‘test out’ the cooperation of patients whose motivation for more complex treatment is uncertain by fitting a removable appliance and reviewing progress before deciding whether to proceed.
Lower removable appliances are generally poorly tolerated by patients. This is due in part to their encroachment upon tongue space, but also the lingual tilt of the lower molars makes retentive clasping difficult.
16.2. DESIGNING REMOVABLE APPLIANCES
16.2.1. General principles
The design of an appliance should never be delegated to a laboratory (to do this is equivalent to driving a car blindfolded with the passenger giving directions) as they are only able to utilize the information provided by the plaster casts. Success depends upon designing an appliance that is easy for the patient to insert and wear, and is relevant to the occlusal aims of treatment.
16.2.2. Steps in designing a removable appliance
Four components need to be considered for every removable appliance:
Retaining the appliance
This gives the acronym ARAB, which may help to jog the memory. A detailed consideration of each of these components is given in the sections below.
Generally, extractions should be deferred until after an appliance is fitted. The rationale for this is twofold:
If the extractions are carried out first, there is a real risk that the teeth posterior to the extraction site will drift forward, resulting in an appliance that does not fit well or even does not fit at all. This is most noticeable when upper first permanent molars have been extracted or there is a conspicuous delay before the appliance is fitted.
Occasionally a patient decides after an appliance is fitted that they do not wish to continue with treatment. It is obviously preferable if this change of mind occurs before any extractions have been undertaken.
Rarely, it is necessary to carry out extractions first, for example when a displaced tooth will interfere with the design of the appliance. However, even in these cases it is preferable to take impressions for the fabrication of the appliance before the extractions and to instruct the technician to remove the tooth concerned from the model. The appliance should then be fitted as soon as practicable after the teeth are extracted.
16.3. ACTIVE COMPONENTS
Springs are the most commonly used active component. Their design can readily be adapted to the needs of a particular clinical situation and they are inexpensive. However, a skilled technician will make the difference between a spring that works efficiently with the minimum of adjustment on fitting and one that requires the clinician to try to compensate for its inadequacies at every visit.
The expression for the force F exerted by an orthodontic spring is the only formula remembered by the author and on this basis is recommended to the reader as being worthwhile:
where d is the deflection of the spring on activation, r is the radius of the wire, and l is the length of the spring. Thus even small changes in the diameter or length of wire used in the construction of a spring will have a profound impact upon the force delivered. It is obviously desirable to deliver a light (physiological) force (Chapter 15) over a long activation range, but there are practical restrictions upon the length and diameter of wire used to construct a spring. The span of a spring is usually constrained by the size of the arch or the depth of the sulcus. However, incorporating a coil into the design of a spring increases the length of wire and therefore results in the application of a smaller force for a given deflection. A spring with a coil will work more efficiently if it is activated in the direction that the wire has been wound so that the coil unwinds as the tooth moves.
In practice the smallest diameter of wire that can be used for spring construction is 0.5 mm. However, wire of this diameter is liable to distortion or breakage and therefore the spring has to be strengthened by being sleeved in tubing (e.g. the Roberts retractor) or protected with acrylic (e.g. the palatal finger spring).
The effect of wire diameter upon the force delivered by a spring can be appreciated by considering the amount of activation required to deliver a force in the region of 30–50 g for the same design of buccal canine retraction spring (Fig. 16.1) fabricated using wires of two different diameters. For a spring composed of 0.5 mm wire an activation of about 3 mm will be required. For the same spring composed of 0.7 mm wire an activation of 1 mm is required. It can readily be appreciated that the 0.7 mm spring gives little margin for error — an activation of 1.5 mm would give an excessive force, but an activation of 0.5 mm would deliver insufficient force.
The stability ratio of a spring is readily appreciated when trying to adjust a buccal canine retraction spring. In mechanical terms it is:
In practice, springs which have a high stability ratio, for example, the palatal finger spring are straightforward to adjust, whereas those with a low stability ratio are difficult to position precisely on the tooth to be moved.
Screws are less versatile than springs, as the direction of tooth movement is determined by the position of the screw in the appliance. They are also bulky and expensive. However, a screw appliance may be useful when it is desirable to utilize the teeth to be moved for additional clasping to retain the appliance. This is helpful when a number of teeth are to be moved together (for example in an appliance to expand the upper arch (Fig. 16.2)) or in the mixed dentition where retaining an appliance is always difficult.
There are basically two types of screw. The most commonly used type consists of two halves on a threaded central cylinder (Fig. 16.3) turned by means of a key which separates the two halves by a predetermined distance, usually about 0.2 mm for each quarter turn. The other variety is the spring-loaded piston screw (Fig. 16.4) which is activated by moving the whole screw assembly forwards by means of a screwdriver.
Activation of a screw is limited by the width of the periodontal ligament, as to exceed this would result in crushing of the ligament cells and cessation of tooth movement (see Chapter 15).
Special intra-oral elastics are manufactured for orthodontic use (see Chapter 17, Fig. 17.20). These elastics are usually classified by their size, ranging from 1/8 inch to 3/4 inch, and the force that they are designed to deliver, usually 2 oz, 3.5 oz or 4.5 oz. Selection of the appropriate size and force is based upon the root surface area of the teeth to be moved and the distance over which the elastic is to be stretched. The elastics should be changed every day.
16.4 COMMONLY USED COMPONENTS
16.4.1. Labial movement of the incisors
Z-spring (Fig. 16.5)
This is actually a small double-cantilever spring fabricated in 0.5 mm wire. It has the advantage that the direction of movement can be altered. Good anterior retention is required to resist the displacing effect of this spring.
Activation is by pulling the spring about 1–2 mm away from the baseplate at an angle of approximately 45° in the direction of desired movement (so that the spring is not caught on the incisal edge as the appliance is inserted).
Double-cantilever spring (Fig. 16.6)
This spring is designed for moving more than one tooth labially and is made in 0.7 or 0.8 mm wire depending upon its length.
Activation is in the same way as for the Z-spring.
This design is used for proclining more than one tooth and also allows variation in the direction of individual tooth movement.
Activation is the same as for the Z-spring.
Screw appliance (Fig. 16.7)
This design is helpful where retention is limited, as the incisors to be moved can also be clasped for additional retention. However, a screw appliance tends to be bulky, which limits its application to the movement of at least three incisors.
Activation is by giving the screw one-quarter turn every three to four days.
Landin screw (Fig. 16.4)
The piston or Landin screw is used for proclining one incisor, but the direction of tooth movement is determined by the technician’s placement of the screw in the acrylic. Now rarely used.
Activation is by turning the screw with a watchmaker’s screwdriver (or flat plastic instrument) in the direction of movement (about 1 mm).
16.4.2. Palatal movement (retraction) of the incisors
Removable appliances are only indicated for overjet reduction if the upper incisors are proclined and the overjet not significantly increased.
Labial bow (0.7 mm wire)
A conventional labial bow is really too stiff for active overjet reduction. Although splitting the bow increases its flexibility, it also increases its liability to distortion and to causing tissue trauma. Other designs are preferable.
Roberts retractor (Fig. 16.8)
This spring is made of 0.5 mm wire which is sheathed with tubing distal to the coils. While useful for retracting proclined incisors, it is difficult to repair and requires an adequate depth of sulcus. A separate retaining appliance is advisable after overjet reduction.
Activation is by bending the arms of the spring towards the incisors.
Strap spring (Fig. 16.9)
The spring is also known as a self-straightening arch. It is fabricated by winding 0.5 mm wire onto a heavier labial bow with one end attached to the U-loop of the bow and the other free-sliding. It has the advantage that the same appliance can be used for canine retraction, overjet reduction, and retention, as the strap spring can easily be added and removed. Some orthodontists recommend using two strap springs to prevent flattening of the arch, whilst others feel that this is unnecessary. Certainly, it is easier to adjust and insert one strap spring than two.
Activation is by tightening the base labial bow, and not the strap spring itself. It is important to ensure that the free-sliding end is indeed free-sliding. If it is not, then it is usually necessary to replace the strap spring.
Using elastics to reduce an overjet is popular with patients as they are less visible than a metal spring. An appliance used for canine retraction can be converted for overjet reduction with elastics by dividing the labial bow and fashioning hooks adjacent to the upper canines. Alternatively, a purpose-designed appliance can be made. For most patients a 3.5 oz, 5/8 inch elastic is required. This method of overjet reduction should be avoided in cases with very proclined incisors as the elastics tend to slide up the teeth and retract the gingivae instead.
Whatever the means of overjet reduction, if a bite-plane has been used for overbite reduction this must be trimmed away behind the incisors as they are retracted. Adjustment should be made from the fitting surface, as well as anteroposteriorally (Fig. 16.10). Contact needs to be maintained with the lower incisors to prevent their re-eruption, and the bite-plane should only be trimmed away completely during the final phases of overjet reduction.
16.4.3. Mesial/distal movement of incisors
Palatal finger spring
See the section below on the canine palatal finger spring.
Activation (half a tooth width) as for the canine palatal finger spring.
16.4.4. Retraction of canines
It is sometimes tempting to start retraction of a canine before it has erupted sufficiently. Placement of an active spring on the inclined mesial cusp of a canine tooth will at best delay further eruption and may intrude the tooth.
Palatal finger spring (Fig. 16.11)
This design of spring has better vertical stability than a buccal retractor — this is readily appreciated when trying to adjust a buccal spring. It is wise to ask the technician to box out the acrylic overlying the spring and to place a guard wire to prevent distortion. To make adjustment of a spring as straightforward as possible the coil should be positioned midway between the starting position of the tooth and the intended finishing location. Where possible, a palatal finger spring should be used in conjunction with a labial bow, as this will help to guide the tooth around the arch and prevent flaring of the tooth buccally.
Before activation is attempted the spring should be adjusted so that it is lying at the level of the gingival margin with a point of application at 90° to the intended direction of movement. The spring can be activated at any point between the coil and where it emerges from underneath the guard wire, but placing the bend nearer to the tip of the spring moves the point of application more buccally. As a rule of thumb an optimal force for canine retraction is delivered by an activation of just under half a tooth width.
Buccal canine retractor — 0.5 mm tubed
Where a canine needs to be moved palatally a buccally approaching spring is required. The author’s preferred design is shown in Fig. 16.1.
Activation is by winding up the coil or by adjusting the anterior leg. However, this has the effect of lowering the point of application of the spring, and a compensatory adjustment of the more posterior leg is needed to correct this.
Buccal canine retractor — 0.7 mm
There are several permutations of the 0.7 mm buccal canine retractor. A similar design to the 0.5 mm retractor in Fig. 16.1 or the type shown in Fig. 16.12 can be used. This type of spring is known colloquially as a ‘cut and bend’ spring, as this is the manner by which it is activated.
Activation will depend upon the design of spring used, but to be effective it must curve around and engage the mesial aspect of the tooth. The disadvantage of a retractor formed in 0.7 mm wire is that an activation of about 1 mm is required to deliver an optimal force for canine retraction, and this is difficult to achieve precisely in practice.
16.4.5. Buccal movement of premolars and molars
This spring is used for the buccal movement of a single premolar or molar tooth (Fig. 16.13). Good retention is required to resist the displacing effect of the spring.
Activation is by pulling the spring away from the acrylic at an angle of 45°.
This design is applicable if it is required to move more than one tooth buccally, for example correction of a crossbite by upper arch expansion (see Fig. 16.2).
Activation: the patient should give the screw a one-quarter turn twice a week (for example on a Wednesday and a Saturday). If opened too far, the screw will come apart; therefore patients should be warned that if the screw portion becomes loose they should turn it back one turn and not advance the screw again.
16.4.6. Mesial/distal movement of premolars and molars
Palatal finger spring
See the section on the canine palatal finger spring.
Nudger appliance (Fig. 16.14)
This appliance is used in conjunction with headgear to bands on the first molar teeth. It is usually used to achieve distal movement of the molar teeth when it is intended to go onto fixed appliances to complete alignment. The appliance incorporates palatal finger springs to retract the first permanent molars. The appliance is worn full-time and the patient asked to wear the headgear for 12 to 16 hours per day. The palatal finger springs are only lightly activated with the aim of minimiszing forward movement of the molars when the headgear is not worn. This appliance is also very useful if unilateral distal movement is required. In this case the contralateral molar can be clasped to aid retention. If overbite reduction is required then a bite-plane can be included in the appliance. It is advisable to fit the bands on the molar teeth and then take an impression to fabricate the appliance.
Activation: the palatal finger springs are activated 1–2 mm.
This is similar to the design used for buccal movement of one or two molars or premolars, except that the screw is positioned to open anteroposteriorly.
En masse appliance
The en masse appliance is used for distal movement of the upper buccal segments with headgear. There are several variants, but essentially the appliance comprises extra-oral traction, either as an integral part of the appliance or by tubes soldered to the bridge of the cribs, which allow the insertion of a face-bow and a means of expansion to maintain arch coordination.
Activation: the active force is provided by the headgear and at least 14–16 hours wear per day is required. If the appliance incorporates a midline screw this should be given a one-quarter turn per week. A coffin spring is activated by pulling apart the two halves of the appliance. (Sometimes appliances with a coffin spring arrive from the laboratory without the acrylic being divided — it is advisable to cut the acrylic down the middle before attempting to activate the coffin spring!)
16.4.7. Palatal movement of an individual tooth
Self-supporting buccal spring
As for the buccal canine retractor.
Activation is the same as for the buccal canine retractor, bearing in mind the root surface area of the tooth to be moved (i.e. less activation for a lateral incisor).
16.5. RETAINING THE APPLIANCE
16.5.1. Adams clasp
This crib was designed to engage the undercuts present on a fully erupted first permanent molar at the junctions of the mesial and distal surfaces with the buccal aspect of the tooth (Fig. 16.15). The crib is usually fabricated in hard 0.7 mm stainless steel wire and should engage about 1 mm of undercut. In practice this means that in children the arrowheads will lie at or just below the gingival margin. However, in adults with some gingival recession the arrowheads should lie part way down the crown of the tooth (Fig. 16.16).
This crib can also be used for retention on premolars, canines, central incisors, and deciduous molars. However, it is advisable to use 0.6 mm wire for these teeth. When second permanent molars have to be utilized for retention soon after their eruption it is wise to omit the distobuccal arrowhead, as little undercut exists and if included it may irritate the cheek.
The reason for the popularity of the Adams crib is its versatility as it can be easily adapted:
Extra-oral traction tubes, labial bows, or buccal springs can be soldered onto the bridge of the clasp (Fig. 16.17; see also Fig. 16.9).
Hooks or coils can be fabricated in the bridge of the clasp during construction (Fig. 16.18).
Double cribs can be constructed (see Fig. 16.12).
Adjustment: the crib can be adjusted in two places. Bends in the middle of the flyover will move the arrowhead down and in towards the tooth. Adjustments near the arrowhead will result in more movement towards the tooth and will have less effect in the vertical plane (Fig. 16.19).
16.5.2. Other methods of retention
Southend clasp (Fig. 16.20)
This clasp is designed to utilize the undercut beneath the contact point between two incisors. It is usually fabricated in 0.7 mm hard stainless steel wire.
Adjustment: retention is increased by bending the arrowhead in towards the teeth.
Ball-ended clasps (see Fig. 18.12)
These clasps are designed to engage the undercut interproximally. This design affords minimal retention and can have the effect of prising the teeth apart.
Adjustment: the ball is bent in towards the contact point between the teeth.
Plint clasp (Fig. 16.21)
This clasp is used to engage under the tube assembly on a molar band.
Adjustment: by moving the clasp under the molar tube.
Labial bows (Fig. 16.22)
A labial bow is useful for anterior retention, particularly if mesial or distal tooth movement is planned, as it will help to guide tooth movement along the arch and prevent buccal flaring. Fitted labial bows provide particularly good retention and are often employed in retaining appliances following fixed appliance treatment.
Adjustment: this will depend upon the exact design of an individual bow. However, the most commonly used type with U-loops is adjusted by squeezing together the legs of the U-loop and then adjusting the height of the labial bow by a bend at the anterior leg to compensate (Fig. 16.23).
The other individual components of a removable appliance are connected by means of an acrylic baseplate, which can be a passive or active component of the appliance.
16.6.1. Self-cure or heat-cure acrylic
Heat-curing of polymethylmethacrylate increases the degree of polymerization of the material and optimizes its properties, but is technically more demanding to produce. It is common practice to make the majority of appliances in self-cure acrylic, retaining heat-cure acrylic for those situations where additional strength is desirable, for example some functional appliances.
16.6.2. Anterior bite-plane
Increasing the thickness of acrylic behind the upper incisors forms a bite-plane onto which the lower incisors occlude. A bite-plane is prescribed when either the overbite needs to be reduced by eruption of the lower buccal segment teeth or elimination of possible occlusal interferences is necessary to allow tooth movement to occur.
Anterior bite-planes are usually flat. Inclined bite-planes may lead to proclination or retroclination of the lower incisors, depending upon their angulation, and therefore should be avoided.
When prescribing a flat anterior bite-plane the following information needs to be given to the technician:
How far posteriorly the bite-plane should extend. This is most easily conveyed by noting the overjet.
The depth of the bite-plane. To increase the likelihood that the patient will wear the appliance, the bite-plane should result in a separation of only 1–2 mm between the upper and lower molars. The depth is prescribed in terms of the height of the bite-plane against the upper incisors, for example ‘half height of the upper incisor’.
In a proportion of cases more than 1–2 mm of overbite reduction is required, and therefore it will be necessary to add to the height of the bite-plane during treatment.
16.6.3. Buccal capping
Buccal capping is prescribed when occlusal interferences need to be eliminated to allow tooth movement to be completed and reduction of the overbite is undesirable. Buccal capping is produced by carrying the acrylic over the occlusal surface of the buccal segment teeth (Fig. 16.24) and has the effect of propping the incisors apart. The acrylic should be as thin as practicably possible to aid patient tolerance. To assist adjustment of posterior clasping, the buccal capping can be extended only halfway across the buccal segment teeth. During treatment it is not uncommon for the bite-plane to fracture away and it is wise to warn patients of this, advising them to return if a sharp edge results. However, if as a result a tooth is left free of the acrylic and is liable to over-erupt, a new appliance will be necessary (as additions to buccal capping are rarely successful).
16.7. FITTING A REMOVABLE APPLIANCE
It is always useful to explain again to the patient (and their parent/guardian) the overall treatment plan and the role of the appliance that is to be fitted. It is also prudent to delay any permanent extractions until after an appliance has been fitted and the patient’s ability to achieve full-time wear has been demonstrated.
Adams pliers (no. 64)
Spring-forming pliers (no. 65)
Maun’s wire cutters
Pair of autoclavable dividers
Steel rule (these are generally cheaper from ironmongers than from dental supply companies)
A straight handpiece and an acrylic bur (preferably tungsten carbide)
A pair of robust hollow-chop pliers is a useful addition, but not essential
Fitting an appliance can be approached in the following way (see also Table 16.2):
Check that you have the correct appliance for the patient in the chair (everyone will make this mistake at some stage) and that your prescription has been followed.
Show the appliance to the patient and explain how it works. It is advisable to stress to the patient that they should not remove the appliance by the springs.
Check the fitting surface for any roughness.
Try in the appliance. If it does not fit check the following:
Have any teeth erupted since the impression was taken? If necessary, adjust the acrylic.
Have any teeth moved since the impression was recorded? This usually occurs if any extractions have been recently carried out. Occasionally, to salvage the situation, it is necessary to bend the cribs forward to compensate for anterior movement of the molars.
Has there been a significant delay between taking the impression and fitting the appliance?
Adjust the retention until the appliance just clicks into place.
If the appliance has a bite-plane or buccal capping, this will need to be trimmed so that it is active but not too bulky.
The active element(s) to be used in the first stage of treatment should be gently activated, provided that extractions are not required to make space available into which the teeth are to be moved.
Give the patient a mirror and demonstrate how to insert and remove the appliance. Then let them practice.
Go through the instructions with the patient (and parent or guardian), stressing the importance of full-time wear. A sheet outlining the important points and containing details of what do in the event of problems is advisable, but unfortunately is not always read (Table 16.3). Medicolegally it is prudent to note in the patient’s records if instructions have been given.
Arrange the next appointment.
If a working model is available, it is wise to store this with the patient’s study models as it may prove helpful if the appliance has to be repaired
Your appliance should be worn all the time, including meals and in bed at night
Your appliance should only be removed for tooth cleaning and during vigorous sports (when it should be stored in a strong container)
It is usual to experience some discomfort and a little difficulty with speech initially, but this should pass in a few days as you become accustomed to wearing the appliance
It is important to avoid hard or sticky foods and chewing gum
If you cannot wear your appliance as instructed or if it becomes damaged or causes pain, please contact (…) immediately.
16.8. MONITORING PROGRESS
Ideally, patients wearing active removable appliances should be seen every 3 to 4 weeks. Activation of an appliance more frequently than this will increase the risk of anchorage loss and root resorption (see Chapter 15). The exception to this guideline is the screw appliance where only a small amount of activation is possible at a time and therefore more frequent small activations are required. Passive appliances can be seen less frequently, but it is advisable to check, and if necessary adjust, the retention of the clasps every 3 months.
During active treatment it is important to establish that the patient is wearing the appliance as instructed. A more accurate answer may given in response to the question ‘How much are you managing to wear your brace?’ rather than ‘Are you wearing your brace full-time?’ Indications of a lack of compliance include the following:
the appliance shows little evidence of wear and tear;
the patient lisps (ask the patient to count from 65 to 70 with, and without, their appliance);
no marks in the patient’s mouth around the gingival margins palatally or across the palate;
16.8.1. At each visit
If wear is satisfactory the following should be checked at each visit:
The treatment plan: this may seem facetious, but it is all too easy to lose sight of the precise aims of treatment. Referring back to the original plan will ensure that each step is carried out methodically and will act as a reminder of how long treatment has been under way, so that progress can be monitored.
The patient’s oral hygiene.
Loss of anchorage by recording overjet and buccal segment relationship.
Tooth movement since the last visit: a good tip is to use dividers which can be imprinted into the records.
Retention of the appliance by asking the patient and adjusting the clasps or labial bow (see Section 16.5) as indicated.
Whether the active elements of the appliance need adjustment (see Section 16.4).
Whether the bite-plane or buccal capping need to be increased and/or adjusted.
Record what action needs to be undertaken at the next visit.
16.8.2. Common problems during treatment
Slow rate of tooth movement
Normally tooth movement should proceed at approximately 1 mm per month in children, and less in adults. If progress is slow, check the following.
Is the patient wearing the appliance full-time? If the appliance is not being worn as much as required, the implications of this need to be discussed with the patient and the parent. If poor cooperation continues, resulting in a lack of progress, consideration will have to be given to abandoning treatment.
Are the springs correctly positioned? If not, explain again to the patient the purpose of the spring and show them how to insert the appliance correctly.
Are the springs underactive, overactive, or distorted? If the springs were correctly adjusted at the patient’s last visit (see Section 16.4), check that the patient is not using them to remove the appliance or putting it in their pocket during meals.
Is tooth movement obstructed by the acrylic or wires of the appliance? If this is the case, these should be removed or adjusted.
Is tooth movement prevented by occlusion with the opposing arch? It may be necessary to increase the bite-plane or buccal capping to free the occlusion.
Frequent breakage of the appliance
The main reasons for this are as follows:
The appliance is not being worn full-time.
The patient has a habit of clicking the appliance in and out (see below).
The patient is eating inappropriate foods whilst wearing the appliance. Success lies in dissuading the patient from eating hard and/or sticky foods altogether. Partial success is a patient who removes their appliance to eat hard or sticky foods!
Appliance quickly becomes loose fitting
The most common cause of this is a patient who is clicking the appliance in and out. This habit can also lead to intrusion of the teeth, which are clasped by the appliance and to frequent breakages. The patient’s close family are often very grateful if the habit is stopped, as the clicking noise that it generates can be very irritating.
Excessive tilting of tooth being moved
Removable appliances are only capable of tilting movements. However, this is exaggerated by the following:
The further that the spring is from the centre of resistance of the tooth the greater is the degree of tilting. Therefore a spring should be adjusted so that it is as near the gingival margin as possible without causing gingival trauma.
Excessive force is being applied to the tooth, as this has the effect of moving the centre of resistance more apically.
This can be increased by the following:
Part-time appliance wear, thus allowing the anchor teeth to drift forwards.
The forces being applied by the active elements exceed the anchorage resistance of the appliance. Care is required to ensure that the springs, etc. are not being overactivated or that too much active tooth movement is being attempted at a time.
If anchorage loss is a problem see Chapter 15.
This can occur for two reasons:
Poor oral hygiene. In the majority of cases the extent of the inflammation exactly matches the coverage of the appliance and is caused by a mixed fungal and bacterial infection (Fig. 16.25). This may occur in conjunction with angular cheilitis. Management of this condition must address the underlying problem, which is usually poor oral hygiene. However, in marked cases it may be wise to supplement this with an antifungal agent (e.g. nystatin, amphotericin, or miconazole gel) which is applied to the fitting surface of the appliance four times daily. If associated with angular cheilitis, miconazole cream may be helpful.
Entrapment of the gingivae behind the upper incisors during overjet reduction between the incisors themselves and the acrylic of the bite-plane (Fig. 16.26). A mistake commonly made during overjet reduction is to trim away the fitting surface of the appliance to allow for palatal movement of the incisors only, forgetting that space should also be created for retraction of the palatal gingivae. To prevent this from occurring, it is necessary to achieve good over-bite reduction in the initial stages of appliance therapy and trim the acrylic as shown in Fig. 16.10 during overjet reduction.
Lack of overbite reduction
Lack of progress with overbite reduction can be a problem in patients who are not actively growing vertically, such as adults or those with a horizontal direction of mandibular growth. In these cases it may be necessary to proceed onto fixed appliances. In children, the most common reason for lack of progress with overbite reduction is that the appliance is not being worn during meals. Patients should be advised that their treatment will be quicker and more successful if they wear their appliance for eating, and that adaptation will be enhanced if they start with softer foods.
16.9. LOWER REMOVABLE APPLIANCES
Lower removable appliances are rarely used because they are poorly tolerated by patients. Not only do they encroach upon tongue space, but retention is a problem owing to the lingual tilt of the lower molars and the displacing action of the tongue. In addition, it is difficult to incorporate lingual springs and there is
limited depth of sulcus for buccal springs. Various designs have been suggested to overcome these shortcomings, but where tooth movement in the lower arch is required a fixed appliance is usually more efficient. Therefore the most commonly used design of lower removable appliance is the retainer.
16.10. APPLIANCE REPAIRS
Before arranging for a removable appliance to be repaired the following should be considered:
How was the appliance broken? If a breakage has been caused by the patient failing to follow instructions, it is important to be sure any cooperation problems have been overcome before proceeding with the repair.
Would it be more cost-effective to make a new appliance, perhaps incorporating the next stage of the treatment planned?
Occasionally it is possible to adapt what remains of the spring or another component of the appliance to continue the desired movement. For example, a long labial bow can be cut and adapted to form a buccal retractor.
Is the working model available, or is an up-to-date impression required to facilitate the repair?
How will the tooth movements which have been achieved be retained while the repair is being carried out? Often there is no alternative but to try and carry out the repair in the shortest possible time.
PRINCIPAL SOURCES AND FURTHER READING
Houston, W. J. B. and Isaacson, K. G. (1980). Orthodontic treatment with removable appliances (2nd edn). Wright, Bristol.
Houston, W. J. B. and Waters, N. E. (1977). The design of buccal canine retraction springs for removable orthodontic appliances. British Journal of Orthodontics, 4, 191–5.
Kerr, W. J. S., Buchanan, I. B., and McColl, J. H. (1993). Use of the PAR Index in assessing the effectiveness of removable orthodontic appliances. British Journal of Orthodontics, 20, 351–7.
This study found that when removable appliances were used in selected cases, 89 per cent showed an improved or a greatly improved result (as indicated by the PAR Index).
Lloyd, T. G. and Stephens, C. D. (1979). Spontaneous changes in molar occlusion after extraction of all first premolars: a study of Class II division 1 cases treated with removable appliances. British Journal of Orthodontics, 6, 91–4.