The transparent aligners or clear aligners are removable type appliances. Clear aligners for orthodontic therapy were first introduced in the United States in 1999 and were aggressively promoted by ‘Invisalign’. The ‘invisible braces’ or clear aligners provide an aesthetic edge over visible fixed appliances due to their transparent colour and lack of metal components. Clear aligners are now made available by many orthodontic traders of different names throughout the world. The clear aligners are discussed in detail in Chapters 41 and 42 .

The body of the appliance is made of PMMA and usually covers the palate up to the second molar region. The acrylic plate is made of self-cure or heat-cure acrylic material on a dental cast which was prepared from an accurate alginate impression.

The appliance’s wire components, such as a labial bow, clasps and springs, are retained in the acrylic body. The acrylic body should be of sufficient thickness to ensure optimum strength and rigidity to hold wire components. Fibre-reinforced composite (FRC) fibres are incorporated during acrylisation to reinforce polymeric parts of a removable orthodontic appliance.

Following the fabrication of wire components acrylisation is completed. The appliance should be embedded in water overnight before delivery to ensure that any residual monomer reacts with PMMA or is leached out in the water.

The acrylic body provides anchorage to the appliance by intimate contact with the oral mucosa and contour of the palate. Well-adapted acrylic extensions into the inter-dental embrasures and retentive wire components, which anchor onto the teeth, ensure the stability of the appliance. The appliance should be well fitting, not cause any undue pressure on oral soft and hard tissues and remain stable.

Retentive elements of the appliance are made of stainless steel (SS) spring hard round wire, which retain the appliance in the oral cavity primarily through clasp-like structures wrapped around the anchor teeth or between the teeth embrasures. Although several types of retentive clasps have been designed, the simpler ones are better for the ease of fabrication and adjustment. Commonly used retentive clasps are:

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  Ball-end hooks/pinhead

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  C-clasp

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  Arrowhead clasp: Single or double

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  Adams’ clasp

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  Delta clasp

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  Southend clasp

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  Passive labial bow

Wire dimensions used for making components of RAs are tabulated in Table 40.2 .

Ball-end hooks, pin-head hooks and single arrowhead clasps provide retention by engaging the clasp head into the buccal interdental embrasures gingival to the contact point. Retention clasps are mostly used in the posterior segment, commonly between the premolars. The clasps should be well adapted to the embrasure to minimise any interference with teeth in the opposite jaw during occlusion. A modification of pinhead clasp is a triangular clasp.

C-clasp: C-clasp wraps around to provide retention due to the placement of its free end gingival to the maximum contour of the buccal surface of the anchor tooth. The premolars and molars are the main source of tooth-borne anchorage.

Adams’ clasp or double arrowhead clasp : Devised by C.P. Adams, it is the most used retention clasp, which engages the first molar and is often a primary retaining device. It is a modification of Jackson’s clasp ( Fig. 40.2 C). The Adams clasp can be modified to serve an auxiliary function, such as holding rubber elastic for traction. The most useful modification is a traction hook or a loop. The clasp can also be modified by soldering a hook on its buccal bridge or by making a helix in the horizontal arm to engage elastics ( Fig. 40.2 C and D).

Retaining clasps.

(A) C clasp (red arrow) and pin head clasp (blue arrow). (B) Triangular head clasp. (C) Jackson’s clasp. (D) Modified Adams’ clasp with built-in helix for engagement of elastic (red arrow). (E) Modified Adams’ clasp with a soldered hook (blue arrow).

Delta clasp : This is a modification of Adams’ clasp, which is an excellent retentive clasp for premolars. William Clark recommends the delta clasp as a retaining component of the lower twin block on the premolars.

Southend clasp : It is a short labial bow wrapped around two central incisors designed to hold the appliance in the anterior end.

Labial bow: Conventionally, the labial bow is made as a wraparound wire around six anterior teeth up to the distal contact point of the canines. The loops usually are made about two-thirds the width of the canine’s mesiodistal width. The height of loops is extended 2–3 mm beyond the free gingival margin of the canine ( Fig. 106.7 B). The long labial bow can extend distal to the first and second premolars and even wrap around the distal to the last erupted molar, as the case may necessitate.

A passive labial bow stabiles the appliance in situ and maintains the position of the teeth it is wrapped around. When activated with sufficient relief on the palatal surfaces of the anterior teeth, it slowly retracts the anterior segment. A labial bow is an effective device for retroclining the labially proclined incisors. Retroclination of the incisors makes them look longer due to the extrusion associated with lingual tipping. Extrusion of maxillary incisors is rather tricky to control, which results in a further increase in deep bite and increased incisor visibility ( Fig. 40.3 ). A conventional labial bow has a limited range of action on tooth movement which is limited to lingual tipping.

Principles of bite opening and maxillary incisor retraction using a removable appliance. (A and B) The bite opening should be achieved before initiation of incisor retraction.

(C) The labial bow activation is initiated after bite opening and due to the trimming of acrylic on palatal surface of the plate to provide relief for the anterior teeth. (D) After incisor retraction. The lingual crown tipping is associated with some amount of extrusion and labial root tipping (black arrow). Blue allow indicates extrusive component.

A modification of a RA, where elastic traction force can be used to enhance lingual tooth movement of the maxillary anterior teeth and prevent extrusion or facilitate their effective intrusion, involves a conversion of labial bow U loops into hooks at a level higher than the gingival contour of the canines. A light elastic (2.5 ounces Begg elastic) substitutes the labial bow, which runs from either side of the hooks and under the small tags of light cure composite attachments temporarily bonded on incisors to engage the rubber band. The modified labial bow is a compromised substitute for an intrusion arch used in fully fixed appliances.

Several modifications of the labial bow have been suggested. These include a reverse loop labial bow ( Fig. 40.4 A and B), a Mills’ labial bow ( Fig. 40.4 C) and Roberts’ retractor ( Fig. 40.4 D).

Modified labial bows.

(A and B) Reverse loop labial bow. (C) Mills’ labial bow. (D) Roberts’ retractor.

Roberts’ retractor : It has an efficient spring design made from 0.5 mm diameter SS wire. The distal arm is supported with a tube. The retractor is adjusted just below the helix on the mesial arm of the loop by compressing the loop. The appliance produces light forces, which are efficient for the correction of the proclination of teeth. Activation up to 3 mm/visit is recommended ( Fig. 40.4 D).

Hawley’s retainer is a passive Hawley’s appliance. The appliance is used as a retainer device after extraction or non-extraction orthodontic treatment completion. The final labiolingual position of the anterior teeth is maintained by incorporating the first-order bends in the labial bow, similar to the intensity of the prescription of the fixed appliance used in treating the malocclusion ( Fig. 106.9 A).