Removable Appliance Design: Clinical scenarios and corresponding active appliance design

Removable orthodontic appliances can be designed and modified to address a wide range of dental problems during the developing dentition. The appliance selected depends on the type and direction of tooth movement required, the available space, anchorage, and the patient’s cooperation. The following clinical scenarios demonstrate common orthodontic problems and the corresponding active removable appliance designs used to manage them.

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. (Profitt, 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 relief 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).

Proclained upper incisors

In this case a labial bow made of 0.7mm hard round stainless-steel wire is used. (Profitt, 2018). Labial bow contacts the most labial part of the tooth. (Profitt, 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. (Profitt, 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. (Profitt, 2018). However, it lacks adequate stability on a vertical plane and require 0.7mm diameter distal tube to be added. (Profitt, 2018).

Closure of space distal to canine

Long labial bow, this appliance extends from the first maxillary premolar to the opposite first premolar. (Profitt, 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. (Profitt, 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. (Profitt, 2018).

Mesio-distal tipping

Mesio-distal tipping is also facilitated in midline diastema closure. (Profitt, 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. (Profitt, 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. (Profitt, 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. (Profitt, 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. (Profitt, 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. (Profitt, 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. (Profitt, 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. (Profitt, 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. (Profitt, 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. (Profitt, 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. (Profitt, 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. (Profitt, 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. (Profitt, 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. (Profitt, 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. (Profitt, 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. (Profitt, 2018).

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

References

• Agarwal, A. and Mathur, R., 2010. Maxillary expansion. International journal of clinical pediatric dentistry, 3(3), p.139.

• Carano, A., Bowman, S.J. and Valle, M., 2003. A fixed reverse labial bow for moderate Class III interceptive treatment. Journal of clinical orthodontics: JCO, 37(1), pp.42-46.

• Forsberg, C.M. and Hellsing, E., 1984. The effect of a lingual arch appliance with anterior bite plane in deep overbite correction. The European Journal of Orthodontics, 6(1), pp.107-115.

• Gill, D.S., 2013. Orthodontics at a Glance. John Wiley & Sons.

• Isaacson, K.G., Muir, J.D. and Reed, R.T., 2002. Removable orthodontic appliances. Wright.

• Littlewood, S.J. and Mitchell, L., 2019. An introduction to orthodontics. Oxford university press.

• Lohakare, S. S. (2008). Orthodontic removable appliances. JAYPEE BROTHERS PUBLISHERS.

• Mah, J. and Lotte, B., Lotte Brian Walter, 2004. Removable expansion appliance. U.S. Patent Application 10/192,305.

• Rohaya, M.A.W., Shahrul Hisham, Z.A. and Doubleday, B., 2006. Randomised Clinical Trial: Comparing the Efficacy of Vacuum-formed and Hawley Retainers in Retaining corrected tooth rotations. Malaysian Dental Journal, 27(1).

• Sharma, R., Mittal, A.K., Sidana, A. and Agarwal, V., 2015. Canine retraction in orthodontics: a review of various methods. Medico Research Chronicles, 2(1), pp.85-93.

• Waters, N.E., 1970. The mechanics of finger and retraction springs of removable orthodontic appliances. Archives of oral biology, 15(4), pp.349-363.