Retainer (orthodontics)

The underneath surface of an upper Wrap Around Hawley retainer resting on top of a retainer case

The best-known removable retainers are the Hawley retainers, which consist of a metal wires that typically surround the six anterior teeth and keep them in place. Hawley retainers are one of the oldest types of removable retainers.[1] Named for its inventor, Dr. Charles A. Hawley, the labial wire, or Hawley bow, incorporates 2 omega loops for adjustment. It is anchored in an acrylic baseplate that sits in the palate (roof of the mouth). They are made from metal wire running along the outside of the teeth. There are many adaptations possible with Hawley retainers.[1] The advantage of this type of retainer is that the metal wires can be adjusted to finish treatment and continue minor movement of the anterior teeth as needed.[3] It also benefits from being robust and rigid, easy to construct and allows prosthetic tooth/teeth to be added onto with metal stops placed mesial and distal to the prosthetic teeth to prevent any relapse. To help fix rotations; acrylic facing can be added to the labial bow and a bite plane added to maintain the result of deep overbite correction. Also, to control the position of the canine, reverse U-loop can be employed. Additionally, to avoid occlusal interferences, the labial bow can be soldered to the cribs.[1]

The main disadvantages of this type of retainer is its inferior aesthetics, interference with speech, risk of fracture and inferior retention of lower incisors in comparison to vacuum-formed retainers.[1]

Recently, a more aesthetic version of the Hawley retainer has been developed. For this alternative, the front metal wire is replaced with a clear wire called the ASTICS. This retainer is intended to be adjustable similarly to the traditional Hawley retainer, which is not practical with vacuum-formed retainers.

Research shows that Hawley retainers are not effective for preventing incisor irregularity relapse.[4] Hawley retainers also affect speech, especially the d, s, t, and i sounds, however as they are often only worn at night time, this concern may not be so prevalent.[5] Research shows that participants that wear Hawley retainers report being more embarrassed about the appliance than wearing vacuum formed retainers and they found Hawley retainers more difficult to wear.[6] However, if the patient has concerns with regards to the visible metal wire, a clear polyethylene bow can be used to enhance aesthetics.[1]

The process of making an acrylic retainer was featured on the How It's Made TV show on the Science Channel on Season 23, Episode 16.

Another common type of removable retainer is the vacuum formed retainer (VFR). This is a polypropylene or polyvinylchloride (PVC) material. VFRs are made using a thermoforming process, using vacuum- or pressure-thermoforming.[7] This clear or transparent retainer normally fits over the entire arch of teeth, however some designs fit only from canine to canine (clip-on retainer). The retainer is clear and so virtually invisible when worn. Hence, it can offer an aesthetic advantage relative to other retainers. VFRs, if worn 24 hours per day, do not allow the upper and lower teeth to touch, as the retainers cover the occlusal (biting) surfaces of the teeth.[8] Some orthodontists feel that it is important for the top and bottom chewing surfaces to meet to allow for "favourable settling" to occur, thus the need for wearing the retainer only intermittently.[8] VFRs are most commonly worn overnight and removed while eating.[1] If worn while eating, they can behave as a reservoir enclosing the teeth with cariogenic substances and lead to decalcification of teeth over time (formation of cavities). The same can result if the retainer is inserted straight after a meal or drink. VFRs tend to be favoured as an orthodontic retainer as they are more aesthetic, interfere less with speech and are more economical than other removable retainers.[8] However, for patients with disorders such as bruxism, VFRs can be prone to rapid breakage and deterioration due to grinding of the VFR against the opposing teeth. There is, however, an increased ease of fabrication over other retainers if they do break.[8] Another advantage of VFRs is that there is evidence to suggest superior retention of lower incisors compared to other removable retainer types, but the best[9] retention was achieved with fixed retainers.[7]

The Begg retainers have the labial bow extending around the distal aspect of the terminal molar.[10] They allow occlusal settling, as no wire work crosses the occlusion. Begg retainers are robust enough to be worn during eating,[11] however they are less retentive than Hawley retainers and the labial bow is more prone to distortion.[12]

The Barrer retainers (aka Spring retainers) carry acrylic bows both labially and lingually.[10] The original appliance extended only to the canines, however due to the risk of swallowing or aspiration, a modification which includes cribs on the first molars has been described. These retainers can be used to realign minor lower incisor relapse.[12]

Dental practitioners and orthodontists are sometimes resistant to providing younger patients with removable retainers due to the potential issues of the patient not wearing their retainer consistently after completing their orthodontic treatment. However, the recent innovation of SMART Retainers and related mobile applications that track, remind, and reward the patients for their continued use of their retainer may lead to a significant decrease in the number of adolescents that obtain additional orthodontic treatment in the future due to them not complying with their retainer usage. The mobile application has a unique feature that allows for parental notifications. If the child does not open their Smart Retainer Case, the parent can receive mobile notifications to help reduce retainer non-compliance.[13]