Removable orthodontic appliance design: a scientific review

Removable orthodontic appliances are defined as simply as devices that can be inserted and removed from the mouth by the patient. (Proffit, 2018). Orthodontic force used in removable appliances depends on three factors, the rate of application, the intensity of the force and the resistance of the tissue to which it is applied. (Romanec et al., 2018). These devices are classified as active removable appliances and passive removable appliances. (Romanec et al., 2018).

Active appliances contain mechanical components that produce mechanical force, this is necessary to bring in dentoalveolar changes. Example: springs, screws. (Proffit, 2018). On the other hand, passive removable appliances mode of transmission of force is passive, which means mechanical force producing elements are unnecessary. (Proffit, 2018). These appliances also just hold and retain the achieved alignment of teeth after fixed orthodontic treatment. (Romanec et al., 2018). Example include habit breakers, space maintainers & Hawley’s retainer appliance. (Proffit, 2018).

Indications

One of the main indications of using removable appliances is “dentoalveolar malocclusion where simple tipping movements can be used to correct malocclusion”. (Cain, 1986). In addition, it can be used for overbite reduction with intrusion of the anterior teeth and extrusion of the posterior teeth. (Cain, 1986). Narrow arches with mild crowding can be treated with simple expansion appliances. (Cain, 1986). Unilateral crossbite with single malposition tooth can be treated with tipping movement. (Proffit, 2018). Removable habit breakers can be used to prevent and intercept the effects of abnormal habits. (Gawlik et al., 1995). Finally, for retention removable appliances can be used to maintain the corrected positions of the teeth after a fixed orthodontic treatment. (Proffit, 2018).

Contraindications

Clinical scenarios where use of removable appliances is insufficient include noticeable skeletal discrepancies, severe crowding and when bodily movements required. (Proffit, 2018). Presence of severe vertical discrepancies such as deep overbite, apical malposition and severe rotations are also contraindications to use removable appliances. (Proffit, 2018).

Advantages vs. Disadvantages

Figure 1 compares the major advantages and disadvantages of using removable appliances in orthodontics. (Proffit, 2018).

Removable appliance components

A variety of designs and shapes of removable appliances exist. However, the main components include active, retentive and an acrylic base plate. (Proffit, 2018). The active component exerts forces to allow tooth movement which include for example bows, springs, screws and elastics. (Cain, 1986). The retentive component is responsible for retaining the appliance in place and resist displacement, examples are clasps and bows. (Cain, 1986). Lastly, the acrylic base plate, this is composed of the bulk of the appliance and incorporates active and retentive components together into a single functioning unit. (Proffit, 2018).

The mechanics of removable appliances depend on the type of movement produced by removable appliances these include tipping, extrusion, intrusion, expansion and distalization. (Proffit, 2018).

When designing the removable appliance wire dimension must be calculated on the basis of the amount of force needed for the corresponding tooth movement. (Cain, 1986). Flexibility of wire depends on the length of wire and its diameter. (Proffit, 2018).

The formula below describes the relationship between diameter of wire and force;

D α PL^3/T^4

D – amount of deflection

P – amount of pressure

L – length of spring

T – thickness / diameter of spring. (Proffit, 2018).

Force is directly proportional to the fourth power of its diameter and inversely to the cube of its length. (Proffit, 2018). When diameter is doubled, deflection decreases 16 times and force is increased 8 times. When length is doubled however, flexibility increases 8 times, force reduced and range of action increases. (Proffit, 2018).

Force for a single rooted tooth should be in the range of 25-40 gm. (Proffit, 2018). Excessive force delay tooth movement, overload anchorage and cause discomfort to patient. (Cain, 1986). Direction of tooth movement is determined by the point of contact between spring and tooth. (Lohakare, 2008).

In this section clinical problems with possible appliance designs are described below.

Retroclined incisors

Z Spring (double cantilever spring), this facilitates labial movement of one or two incisors using 0.5mm hard round stainless-steel wire. (Isaacson et al., 2002). When designing the Z spring a line is drawn joining the present position and desired position of the tooth and a perpendicular bisector is drawn to this line. (Isaacson et al., 2002). Spring positioned perpendicular to the palatal surface of the tooth. (Isaacson et al., 2002). Activation of this appliance is done by opening both helices by 2-3mm at a time. To achieve minor rotations, activate by opening one of the helices only. (Isaacson et al., 2002). Treatment with Z-spring appliance is successful when provided there is enough space to align tooth along with patient compliance. (Proffit, 2018).

Interlocked incisors

This can be also called single tooth anterior crossbite, it can be treated by utilizing a posterior bite block with a ‘Z’ spring to relieve the bite before attempting to move the misplaced tooth. (Gill, 2013). Labial arch is sometimes added as a precaution to prevent labial proclination of the teeth involved. (Gill, 2013). However, in case of segmental anterior crossbite, inclined bite plate is utilized by unlocking the lingual position of the maxillary incisors and correcting the bite in the front region. (Gill, 2013).

Single tooth buccal movement

T spring appliance is used for buccal movement of premolars & sometimes canines. (Gill, 2013). It is composed of T shaped arm 0.5mm round stainless-steel wire with the end embedded in acrylic. (Gill, 2013). This appliance is activated by extending the free end of the arm in the direction of the desired movement. (Gill, 2013).

Proclined upper incisors

In this case a labial bow made of 0.7mm hard round stainless-steel wire is used. (Proffit, 2018). Labial bow contacts the most labial part of the tooth. (Proffit, 2018). In addition, it is also utilized in cases of minor overjet reduction, anterior space reduction closures and for providing anterior retention. (Gill, 2013). Roberts retractor is a variation of a labial bow and is activated by coil closing. (Proffit, 2018). Made of thin gauge stainless steel wire 0.5mm which is highly flexible. (Gill, 2013). Indicated in severe anterior proclination, overjet of over 4mm. (Proffit, 2018). However, it lacks adequate stability on a vertical plane and require 0.7mm diameter distal tube to be added. (Proffit, 2018).

Closure of space distal to canine

Long labial bow, this appliance extends from the first maxillary premolar to the opposite first premolar. (Proffit, 2018). Activation of the appliance is done by compressing two loops. (Lohakare, 2008). This activation of 1.5 to 2 mm is done to achieve about 1 mm of retraction of maxillary incisors per month. (Gill, 2013). Labial bow in general should uniformly touch the labial surfaces of all teeth being retracted. (Proffit, 2018). At every follow up appointment with labial bow appliance the clinician should adjust the bow and trim the acrylic on the palatal side to create space for tooth movement. (Carano et al., 2003).

Rotation

Labial bow can also be used to resolve minor rotations in the incisor areas. (Rohaya et al., 2006). For Retention clasps are placed on first molars and anchorage is provided by the baseplate on the palate. (Rohaya et al., 2006). This baseplate should be kept in contact with the palatally rotated surface and clear of contact on the labially rotated surface. (Proffit, 2018).

Mesio-distal tipping
Mesio-distal tipping is also facilitated in midline diastema closure. (Proffit, 2018). Finger Spring (single cantilever) is indicated in this type of movement. (Waters, 1970). The coil of the finger spring rests on the long axis of the tooth in question, halfway between the initial and final position of the tooth and perpendicular to the direction of movement. (Waters, 1970). Prior to using this appliance, the teeth must be located correctly buccolingually. (Proffit, 2018). Finger spring is made of 0.5mm-0.6mm hard round stainless-steel wire (Waters, 1970), and it is activated by moving the arm of the spring toward the tooth intended to be moved. (Waters, 1970). Another appliance used for diastema closure is modified split labial bow it is composed of a free end of buccal arm hooks onto distal surface of each central incisor. (Proffit, 2018).

Canine retraction

Buccal canine retractor is used for the highly placed canine in buccal sulcus, it has standardized dimensions for the stability of stiffness ratios. (Littlewood & Mitchell, 2019). Active arm should engage the mesial surface of canine at gingival level and posterior arm should cross the mesial surface of premolar i.e. transverse arm. (Littlewood & Mitchell, 2019). The placement of the coil of the appliance is at the centre of original and required position of tooth. (Proffit, 2018). It is activated by coil closing, however, because it is stiff, it cannot be activated by more than 1mm. (Lohakare, 2008). It is important to note that canine retraction is only favourable when buccally displaced canine crown is mesially tipped while the root is distally positioned. (Proffit, 2018).

Canine teeth have large surface area in nature and require more force, anchorage must be monitored carefully to ensure that it is not lost. (Sharma et al., 2015). If the arch is crowded, then space will need to be created prior to alignment of the canines and fixed appliances may be needed. (Sharma et al., 2015). That is why a base plate in the upper jaw is included in the design for anchorage. (Sharma et al., 2015). On the other hand, reverse loop canine retractor is particularly indicated for use in the lower arch where sulcus depth is limited. (Sharma et al., 2015). It retains increased flexibility and lighter forces, this directly related to the length of wire used. (Proffit, 2018). This appliance is activated by trimming 1 mm of the wire at the free end and reform it to contact the mesial of the canine. (Lohakare, 2008). 3 mm activation per month is sufficient, thus tooth moves by 1 mm per month. (Lohakare, 2008).

Distalization

Distalization is done by utilizing an appliance design to move the molars distally and creating space in the dental arch. (Proffit, 2018). Some designs incorporate screws with the axis parallel to the line of the arch with retention clasps on permanent molars and first premolars. (Lohakare, 2008). Minimum of 3 retention points or clasps should be acquired in the design. (Lohakare, 2008). Anchorage can be reinforced with extraoral traction. (Proffit, 2018). The baseplate should be split as indicated by the position of the tooth to be moved. (Lohakare, 2008).

A removable appliance retained with Adam’s clasps and incorporating a helical finger spring adjacent to the tooth to be moved is the very effective, this appliance is reported to be the ideal design for distally tipping one molar. (Proffit, 2018). This removable appliance is mainly indicated after premature loss of primary molar, it is activated by 1 to 2 mm per month to regain up to 3 mm of space. (Proffit, 2018). After regaining the space, it must be retained by a space maintainer appliance until the eruption of the successor tooth in space. (Lohakare, 2008).

Expansion

Expansion can be done by a removable palatal expander and Jack screw appliance; it is available in a variety of sizes and ranges of activation. (Lohakare, 2008). It is designed as an acrylic base plate that is split on the midline with screw and retained with 4 Adams clasps on posterior teeth. (Proffit, 2018). Screw lies on an imaginary line passing between first and second premolar. (Mah et al., 2004). It is mainly utilized for correction of anterior and posterior cross bite. (Proffit, 2018). Activation is done by patient at regular intervals using a key, quarter turn opening of screw twice per week which will provide 1mm movement per month. (Mah et al., 2004).

Three-way appliance is an appliance used in expansion that incorporates a screw design that allow expansion of the anterior and lateral segment. (Mah et al., 2004). This is done by tipping and tilting of the premolars or molars in the buccal direction and anterior teeth in the labial direction. (Lohakare, 2008). It is mainly indicated in mild class III cases with slight amount of upper arch crowding. (Lohakare, 2008). One of the main advantages of this design is that the patient can activate it at home, but its success also depends on the patient’s cooperation. (Proffit, 2018).

Coffin spring is an appliance with a spring that resembles a Greek letter Omega. (Agarwal & Mathur, 2010). Indicated in patients with narrow arch, posterior cross bite and crowding to teeth up to 3mm. (Agarwal & Mathur, 2010). Its spring is 1.25 mm in diameter and its free ends are incorporated in the acrylic plate which provide slow expansion. (Agarwal & Mathur, 2010). When compared to expansion screw appliance, the coffin spring is less expensive, easier to keep clean and patient does not have to adjust the appliance. (Agarwal & Mathur, 2010). However, it requires high degree of retention with four clasps and its activation is not controlled. (Proffit, 2018). The activation is usually done by pulling the two ends apart by 1-2mm digitally. (Agarwal & Mathur, 2010).

Intrusion & Extrusion

Anterior bite plane is made of a thickened acrylic palatal plate that extends to the upper incisors, to which the lower incisors occlude, relieving the occlusion on the posterior teeth. (Proffit, 2018). It is indicated in class I deep bite with low facial height and contraindicated in high facial height and skeletal deep bite. (Forsberg et al., 1984). Surface of bite plane is horizontal and parallel to occlusal plane. (Forsberg et al., 1984). The appliance is recommended to be worn full time because overbite reduction takes longer time to achieve and requires retention. (Proffit, 2018).

In the literature it was shown that in case of extrusion, most commonly used are functional appliances in the late mixed dentition. (Proffit, 2018).

Habit breakers

Habit breaker such as tongue guard and palatal crib are used in case of thumb sucking or tongue thrust habits. (Proffit, 2018). The active components of this appliance are loops on labial bow which interfere with the habit of the patient. (Gawlik et al., 1995). Adam’s clasps or Southend clasps are sometimes incorporated in the design for retention. (Proffit, 2018). The crib or loops must be in line with the maxillary incisor tip to prevent interference with lower incisors in maximum intercuspation. (Abraham et al., 2013).

Oral screen

Oral screen is a simple passive myofunctional appliance that eliminates abnormal forces of lips and perioral musculature on dentoalveolar structures and allows forces of tongue to expand the lower arch (Arora et al., 2013). It is effective in treating acquired malocclusions due to abnormal habits like lip, thumb sucking and mouth breathing (Arora et al., 2013).

Retention

Hawley retainer

An appliance that extend from canine-to-canine with an anterior bow and clasps on molars. (Proffit, 2018). The adjustment loop of the Hawley anterior bow often keeps the wire from having full contact with the canines. (Proffit, 2018). Adam’s clasp or C clasp are incorporated for retention and sometimes bite planes can be added to maintain overbite reduction. (Rowlands et al., 2007). Labial bow can be activated to close residual spaces. (Rowlands et al., 2007).

Barrer retainer

“An appliance that is designed for minor adjustment of rotated incisors”. (Sandler & Reed, 1988). It is usually acrylate bows both labially and lingually and used in the mandible. (Rowlands et al., 2007). Barrer retainer in the mandible was shown to be more effective than wire Hawley bow in retaining the contacts in lower incisors. (Sandler & Reed, 1988).

Glass-reinforced clear polymer retainer

A removable maxillary retainer with a clear outer bow, which fits more tightly than a metal wire and is better aesthetically. (Proffit, 2018). However, it cannot be adjusted to modify tooth positions without starting over with a new retainer. (Proffit, 2018).

Vacuum Essix retainer

A retainer made with a clear thermoplastic plate created with vacuum on the patient’s cast that is fitted tightly on the patient’s teeth. (Jäderberg et al., 2012). Because the material is transparent and thin, most patients prefer them, therefore reported better compliance. (Jäderberg et al., 2012).

Adam’s Clasp

Is composed of two arrowheads connected together with a bridge, the bridge is 45 degree to long axis, the two arms engage mesial & distal proximal undercuts. (Inman, 2008). Adams clasps provide good retention and ease of fabrication. (Mansuri & Singh, 2014). Partially or fully erupted primary & permanent molars can be engaged with Adams clasps. (Inman, 2008). A lot of modifications of Adams clasps were reported in the literature such as Adams with J hook, incorporated helix, distal extension engaging elastics or Adams with a soldered buccal tube for extraoral anchorage. (Mansuri & Singh, 2014).

Southend clasp

Southend clasp is mainly used in the anterior teeth because it requires a minimal undercut. (Stephens, 1979). The wire should follow the curvature of gingival margin of the teeth (Mansuri & Singh, 2014). It can be used when the upper incisors are not proclined. (Stephens, 1979).

Triangular clasp

Small triangular clasp that engage the proximal surface undercut between two teeth. (Broussard, 1981). Triangular clasps are used in addition to existing clasps but when additional retention is required. (Mansuri & Singh, 2014). It was reported that its use doesn’t lead to gingival trauma. (Mansuri & Singh, 2014).

Ball end clasps

Stainless steel wires with a ball structure on one end, this ball end engages in proximal undercut between adjacent posterior teeth (Mansuri & Singh, 2014).

C clasp

Engages bucco-cervical undercut, the wire from proximal undercut, along cervical margin, carried over into lingual arm. It has a simple design and easy fabrication. (Mansuri & Singh, 2014). However, the tooth needs to be fully erupted. (Mansuri & Singh, 2014).

Patient instructions

• Patient must wear the appliance at all times except the time of oral hygiene practice. (NHS, 2019).
• Patient must be able to wear the appliance during eating and sleeping but must be removed during sports or swimming. (NHS, 2019).
• The appliance must be placed in a protective box to protect the appliance from accidental loss or damage. (NHS, 2019).
• Patient must clean the appliance daily with toothpaste and a small soft bristle toothbrush to prevent attrition of the acrylic plate. (NHS, 2019).