The iatrogenic anterior open bite; a potential side-effect of thermoplastic orthodontic retainers
From Volume 45, Issue 4, April 2018 | Pages 342-344
Article
Orthodontic treatment is provided for patients of all ages and is delivered by a variety of practitioners, including specialist orthodontists in hospital and practice, general dental practitioners, and an increasing number of orthodontic therapists. Additionally, and most alarmingly, there is a cohort of self-treatments using Internet-provided appliances. It is essential that the treatment performed is appropriate, with a formal treatment plan agreed by both the clinician and patient as part of the informed consent process. All orthodontic treatment plans must include a discussion about retention, as this forms part of the consent process. Un-informed patients may not have proceeded with treatment had they known that they were required to wear removable retainers indefinitely. Long-term retention can occasionally lead to unexpected problems.
Moyers defined retention as ‘the holding of teeth following orthodontic treatment in the treated position for the period of time necessary for the maintenance of the result’.1 In contemporary orthodontics, this is frequently translated to life-long indefinite retention, with the long-term frequency of wear monitored by the patient. Traditional retainers are constructed from heat-cured acrylic that is colleted around the teeth, covering the palate and extending lingually in the mandible, with custom-formed stainless steel wire clasps and bows used to retain the appliances. Examples of these appliances include the ‘Hawley’ retainer and ‘Begg’ retainer (Figure 1). This retainer design is still widely prescribed, however, the introduction of vacuum- or pressure-formed thermoplastic retainers (VFR/PFR) is gathering popularity. Thermoplastic occlusal coverage appliances date back to Nahoum's Dental Contour Appliance of 1964.2 Later, in 1993, Sheridan described the original design for the Essix retainer with coverage from canine to canine only for maintenance of labial segment orthodontic corrections.3 Risks of demineralization and erosion, or occlusal and incisal disruptions were acknowledged. The design was modified to provide complete occlusal coverage with occlusal equilibration of both upper and lower retainers.4 These modifications also help to reduce the risk of accidental inhalation and maintain buccal segment corrections. Interestingly, thermoplastic Essix retainers were never intended to be used in patients with pre-treatment open bite tendencies and, after a very short 2–4 week period of full-time wear, patients were instructed to wear at night-time only.
Construction of a thermoplastic retainer uses acrylic blanks, commonly 1.0 mm, but are also available in various compositions and thicknesses dependent on brand. The process to construct thermoformed retainers is to heat the plastic blank until pliable and flexible, and then it is thermoformed against a working model by means of vacuum or pressure. The original thickness of the plastic sheet thins by 30–50% as it forms around the cast, depending on the thermoforming unit used. The original Essix retainer described was constructed from a 0.75 mm copolyester blank that reduced in thickness to 0.4 mm, as it was thought thicker varieties lacked the necessary flexibility.3 Studies have shown that thermoplastic retainers are more cost-effective, preferred by patients,5 and better at maintaining alignment of the lower labial segment6 in comparison to more traditional retainers.
Case example
The patient was a 14-year-old female, referred for a second opinion by a specialist orthodontist who had not performed the orthodontic treatment. The patient's complaint was her inability to close her front teeth together. It was understood that she had undergone functional appliance treatment, followed by provision of upper and lower fixed appliances on a non-extraction basis. She presented with a Class II division 1 incisor relationship with increased vertical proportions, a half unit II left buccal segment relationship and crossbite tendency on the right side, further complicated by bilateral lateral open bites with contact solely on the second molars (Figure 2). Following removal of her fixed appliances, she had been provided with vacuum-formed retainers that she reported she had worn full-time for just over 12 months. Examination of these retainers showed their extension to be to the first permanent molars only. At the time of fixed appliance removal, the patient stated that she had good contact on all of her teeth. The post-debond study models taken by the initial practitioner (Figure 3) demonstrated this, in addition to fully erupted mandibular and partially erupted maxillary second permanent molars. It was not clear if the original design had not extended to the second molars, or whether the patient had modified the retainers herself.
It would appear that the bilateral buccal segment open bites were iatrogenic in aetiology due to inadequate coverage of the erupting terminal molar teeth. The retainers had acted as a bite plane selectively discluding the second molars and allowing overeruption.
The following options were considered and discussed:
The patient requested no further fixed appliance treatment. Therefore upper and lower Hawley retainers were provided for night-time only wear to allow for vertical settling whilst maintaining alignment of the teeth. The authors preferred a Hawley type appliance rather than a Begg appliance due to improved retention. Over a period of six months, the vertical occlusion re-established, providing the patient with contact throughout the arch, except for the lower left first premolar. It is likely that further improvement will occur. No attempt was made to correct the other features of the patient's post-treatment malocclusion as she did not wish to have any further treatment (Figure 4).
This case report illustrates how a deceptively simple appliance can disrupt the occlusion, and how a simple technique can be used to improve iatrogenic lateral open bites similar in nature.
The exact reason for the retainer with inadequate coverage is not known but it is useful to consider what may have happened:
Other authors have described alternative removable appliances to achieve similar effects, for example Chate and Falconer7 and Dellinger.8
Conclusion
Thermoplastic retainers have become increasingly popular. There is no doubt that they are relatively cheap and easy to manufacture when compared with Hawley retainers. The popularity of thermoplastic aligners as active appliances is well documented. Clear thermoplastic retainers and aligners are deceptively simple, yet also deceptively powerful, as poor design or patient modification can create a situation where occlusal disruption is a very real risk. It is important to note that the advice discussed here for avoiding adverse occlusal effects following overeruption of terminal molar teeth is applicable to any type of long-term appliance that has occlusal coverage, such as soft bite raising appliances or mandibular advancement splints. The recent development of self-treatment with aligners may lead to adverse bite opening if a sound understanding of occlusion is not appreciated. As with any type of fixed or removable appliance prescribed to a patient, it is essential that patients are advised not to self-modify the appliances in anyway.