Removable appliances can serve effectively for retention against intraarch instability and are also useful as retainers in patients with growth problemsi9 (in the form of modified functional appliances or part-time headgear). If permanent retention is needed, a fixed retainer should be used in most instances, and fixed retainers are also indicated for intra-arch retention when irregularity in a specific area is likely to be a problem.
Hawley Retainers :-
By far, the most common removable retainer is the Hawley retainer, designed in the 1920s as an active removable appliance. It incorporates clasps on molar teeth and a characteristic outer bow with adjustment loops, usually spanning from canine to canine. Because it covers the palate, it automatically provides a potential bite plane to control overbite.
The ability of this retainer to provide some tooth move- ment was a particular asset with fully banded fixed appli- ances, since one function of the retainer was to close band spaces between the incisors. With bonded appliances on the anterior teeth or after using a tooth positioner for finishing, there is no longer any need to close spaces with a retainer.
However, the outer bow provides excellent control of the incisors even if it is not adjusted to retract them, especially if the anterior section has acrylic added to fit more tightly, or perhaps even better, if the anterior segment is formed from a clear polymer .
When first premolars have been extracted, one function of a retainer is to keep the extraction space closed, which the standard design of the Hawley retainer cannot do. Even worse, the standard Hawley labial bow extends across a first premolar extraction space, tending to wedge it open. A common modification of the Hawley retainer for use in extraction cases is a bow soldered to the buccal section of Adams clasps on the first molars, so that the action of the bow helps hold the extraction site closed. Alternative designs for extraction cases are to wrap the labial bow around the entire arch, using circumferential clasps on second molars for retention, or to bring the labial wire from the baseplate between the lateral incisor and canine and to bend or solder a wire extension distally to control the canines. The latter alternative does not provide an active force to keep an extraction space closed but avoids having the wire cross through the extraction site and gives positive control of canines that were labially positioned initially .
The clasp locations for a Hawley retainer must be selected carefully, since clasp wires crossing the occlusal table can disrupt rather than retain the tooth relationships established during treatment. Circumferential clasps on the terminal molar may be preferred over the more effective Adams clasp if the occlusion is tight.
The palatal coverage of a removable plate like the maxillary Hawley retainer makes it possible to incorporate a bite plane lingual to the upper incisors to control bite depth. For any patient who once had an excessive overbite, light contact of the lower incisors against the baseplate of the retainer is desired.
Removable Wraparound (Clip) Retainers :-
A second major type of removable orthodontic retainer is the wraparound or clip-on retainer, which consists of a plastic bar (usually wire-reinforced) along the labial and lingual surfaces of the teeth. A full-arch wrap- around retainer firmly holds each tooth in position. This is not necessarily an advantage, since one object of a retainer should be to allow each tooth to move individually, stimulaing reorganization of the PDL. In addition, a wraparound retainer, though quite esthetic, is often less comfortable than a Hawley retainer and may not be effective in maintaining overbite correction. A full-arch wraparound retainer is indicated primarily when periodontal breakdown requires splinting the teeth together.
A variant of the wraparound retainer, the canine-to- canine clip-on retainer, is widely used in the lower anterior region. This appliance has the great advantage that it can be used to realign irregular incisors if mild crowding has devel- oped after treatment, but it is well tolerated as a retainer alone. An upper canine-to-canine clip-on retainer occasionally is useful in adults with long clinical crowns but rarely is indicated and usually would not be tolerated in younger patients because of occlusal interferences.
In a lower extraction case, usually it is a good idea to extend a canine-to-canine wraparound distally on the lingual only to the central groove of the first molar . This is called a Moore retainer. It provides control of the second premolar and the extraction site but must be made carefully to avoid lingual undercuts in the premolar and molar region. Posterior extension of the lower retainer, of course, also is indicated when the posterior teeth were irre- gular before treatment.
Clear (Vacuum-Formed) Retainers :-
A retainer made with a clear heat-softened plastic that is sucked down tightly over the teeth with a device that creates a vacuum to do that is another form of the older wrap- around retainer made with acrylic and wire. Because the material is transparent and thin, a vacuum-formed retainer is all but invisible, and most patients prefer them. At present this is the most widely used retainer for the maxillary arch, and patients using a clear retainer report greater satisfaction with their treatment than those with other types of retain- ers. In terms of effectiveness of maintaining incisor alignment, a Swedish study reported no difference between these retainers and a bonded wire retainer. This implies excellent compliance with the removable suck-down retainer, and it does seem that patients are more likely to wear a clear retainer full time.
As with anything else, there are limitations to vacuum- formed retainers:
(1) the thickness of the material over the occlusal surface of the teeth can become a problem, espe- cially if both the upper and lower arches are retained in this way. It does no harm to open small holes in the occlusal surface of the retainer at points of occlusal contact (as seen with equilibrating paper) to keep from separating the teeth so much, but the combination of a vacuum-formed upper and a fixed lower retainer greatly reduces this problem;
(2) the retainer maintains alignment but does not control deep- ening of the bite as well as a palate-covering Hawley retainer; and
(3) after a few months, the retainer tends to crack and discolor to the point that it has to be replaced. One study reported that using the final aligner in an Invisalign sequence as a retainer was not as effective as other retainer types, perhaps because a thinner material is used in clear aligner therapy,
Positioners as Retainers :-
A tooth positioner also can be used as a removable retainer, either fabricated for this purpose alone, or more commonly, continued as a retainer after serving initially as a finishing device. Positioners are excellent finishing devices and under special circumstances can be used to an advantage as retain- ers. For routine use, however, a positioner does not make a good retainer. The major problems are:
1. The pattern of wear of a positioner does not match the pattern usually desired for retainers. Because of its bulk, patients often have difficulty wearing a posi- tioner full time or nearly so. In fact, positioners tend to be worn less than the recommended 4 hours per day after the first few weeks, although they are reasonably well tolerated by most patients during sleep.
2. Positioners do not retain incisor irregularities and rotations as well as standard retainers. The problem with alignment follows directly from the first one: a retainer is needed nearly full time initially to control it. The flexible material of a positioner does hold a tooth tightly enough to control rotations.
3. Overbite tends to increase while a positioner is being worn during finishing. This effect extends into retention and perhaps is greater when the positioner is worn only a small percentage of the time.
A positioner does have one major advantage over a stan- dard removable or wraparound retainer, however-it main- tains the occlusal relationships as well as intra-arch tooth positions. For a patient with a tendency toward Class III relapse, a positioner made with the jaws rotated somewhat downward and backward may be useful. Although a posi tioner with the teeth set in a slightly exaggerated “supernormal” from the original malocclusion can be used for patients with a skeletal Class II or open bite growth pattern, it is less effective in controlling growth than a functional appliance or nighttime headgear.
In fabricating a positioner, it is necessary to separate the teeth by 2 to 4 mm. This means that an articulator mounting that records the patient’s hinge axis allows more accurate fabrication. As a general guideline, the more the patient devi- ates from the average normal, and the longer the positioner will be worn, the more important it is to make it on articulator-mounted casts. If a positioner is to be used for only 2 to 4 weeks as a finishing device in a patient who will have some vertical growth during later retention, and if the patient has an approximately normal hinge axis, an individu- alized articulator mounting makes little or no practical difference.
The usual sign of a positioner made to an incorrect hinge axis is some separation of the posterior teeth when the incisors are in contact. Patients wearing a positioner as a retainer should be checked carefully to see that this effect is not occurring.