Discussion
This case report has detailed a pleasing overall treatment outcome which addressed the presenting malocclusion and eliminated the future need for prosthodontic treatment for the congenitally missing 35 and 45. The spontaneous eruption of the 38 and 48 into satisfactory clinical positions further enhanced the treatment outcome from a functional perspective. The eruption of the 38 and 48 was made possible through effective mesialisation of the mandibular first and second permanent molars during the orthodontic treatment. Such mesial movement created space distal to the mandibular second molars which subsequently created an unobstructed eruption path for the 38 and 48. Although an argument could be made for improving the alignment of the 38 and 48 with a second phase of fixed appliances, this was not desired by the patient. Should the 38 and/or 48 not erupt further and require removal for any dental health reason, such removal would be undoubtedly more straightforward compared to their original pre-treatment position and angulation (Figure 2b). This improvement in angulation and vertical position can be attributed to the effective orthodontic mesialisation of the mandibular first and second permanent molars.
The mandibular right second primary molar tooth was not visible clinically at the initial orthodontic consultation. The pre-treatment panoramic radiograph was rather confronting as it revealed a severe submergence of the 85 and its resultant effect upon the succedaneous 45. This problematic outcome emphasizes the need for routine and periodic assessment of growing patients. The dentition of a growing patient is as an individual and dynamic entity which requires all dental practitioners to be familiar with the eruption sequence of teeth. Any deviation from the norm should be diagnosed and managed in a timely manner, in order to preserve dental arch integrity and to avoid negative outcomes.20 In retrospect, earlier diagnosis of the infraoccluded 85 (i.e. when this tooth was still in a supragingival position) would have been ideal and may have completely avoided the surgical and orthodontic management challenges encountered in the fourth quadrant.
Congenitally missing premolar teeth are relatively common and are often associated with both short and long-term clinical dilemmas. Numerous factors must be carefully considered in the diagnostic and treatment planning process. It is imperative that every relevant treatment option be evaluated from the cost-benefit perspective and thoroughly discussed with the patient as part of the informed consent process.
Maintaining an over-retained primary molar when the succedanenous premolar is congenitally missing may be a reasonable option for some individual patients, particularly if the primary molar demonstrates no sign of infraocclusion, is unrestored, non-mobile and has significant remaining root structure. Such a tooth may be expected to have a good long-term prognosis and decent clinical longevity. It has been reported that after the age of 20 years, retained primary molars display minimal change with respect to infraocclusion, tipping of adjacent teeth and root resorption. Therefore, should a primary molar still be present in the dental arch at 20 years of age, it may have a good prognosis for long-term survival.21
However, it is difficult to argue that an intact primary tooth would have superior clinical longevity in comparison to a healthy permanent tooth. For some patients, it may be advantageous to remove primary molars and appropriately close the extraction spaces in conjunction with orthodontic treatment. Such space redistribution may create sufficient space for third permanent molars to successfully erupt, as demonstrated in this case report.
Anchorage is defined as a resistance to unwanted tooth movement22 and anchorage control is a fundamental principle of contemporary orthodontic treatment.7,23Anchorage can be controlled through the use of intra-oral or extra-oral devices. The development of non-osseointegrating titanium bone screws has revolutionised orthodontic anchorage control as they offer a simple, safe and effective option with reduced patient compliance.24 These mechanically retained bone screws are generically referred to as temporary anchorage devices (TADs) and due to their placement into inter-radicular, mandibular or palatal bone, their clinical application is also known as skeletal anchorage.7,22-24
The use of TADs can permit safe, predictable and effective orthodontic mesialisation of the mandibular first and second permanent molars. Although such movement can also be achieved through the implementation of intermaxillary elastics or bite correcting springs, TADs can significantly reduce the amount of patient compliance required for challenging tooth movements. With the benefit of hindsight, the authors concede that the TADs used for this patient may have provided better indirect anchorage control if rigid wire splinting was used between the TADs and the mandibular canines or premolar teeth.25,26 In addition, mesial force applied directly to the TADs could also have been implemented following uprighting of the 46.27
The mesialisation of first and second permanent molars to provide space for third molars has been widely discussed in the literature. When a premolar or a second primary molar is extracted and the remaining posterior teeth have been moved mesially, the probability of spontaneous eruption of the third molars does increase.28-33 In general, the greater the distance of first and second molar mesialisation, the greater the probability of spontaneous normal eruption of the third molar teeth.28
Removal of significant alveolar bone was required to extract the ankylosed 85 and impacted 45. Fortunately, subsequent spontaneous bone regeneration generally allows bony defects to fill with bone without the presence of a stimulus or grafting material.34 This phenomenon also occurs in patients who have had large cysts removed from the ramus of the mandible. In addition, orthodontic tooth movement through the area of an alveolar bony defect can further facilitate regeneration of bone.34 Orthodontic tooth movement can both stimulate and help maintain bone formation, however, how this process occurs is not completely understood.35 Two hypotheses exist with respect to the process of osteogenesis. The first hypothesis is a bone bending theory as described by Epker and Frost36,37, which postulates that the flexion of the alveolar bone creates convexities and concavities which in turn stimulates osteogenesis and resorption. The second hypothesis is known as the pressure tension hypothesis.38,39 This hypothesis relates to the concept of creates pressure and tension areas that occur within the periodontal ligament as the tooth moves. The pressure and tension stimulates simultaneous bone resorption and osteogenesis which then allows for tooth movement. For the patient described in this case report, the large bony defect which resulted from surgical removal of the 85 and 45 was completely resolved, and likely occurred through a combination of normal bone healing and by the osteogenic effects of orthodontic tooth movement.
In recent times, there has been a resurgence in the technique of tooth autotransplantation, which refers to the technique of transplanting embedded, impacted or erupted teeth from one site into another in the same individual.40 Autotransplantation can provide a cost-effective and biocompatible solution for adolescent patients with congenitally missing teeth or significantly compromised teeth when a suitable donor tooth is available.41 Although carefully considered individual case selection and surgical skill are the critical determinants for success, advances in three-dimensional computed tomography and rapid prototyping have the potential to significantly reduce the technique sensitivity of the autotransplantation procedure.41
The patient in this case report was also considered to be a potential candidate for tooth autotransplantation. The left mandibular third molar (38) may have eventually become a reasonable donor tooth to replace the congenitally missing 35. However, the 38 did not demonstrate sufficient tooth development at the time of the planned removal of the ankylosed 85 and impacted 45 to permit safe autotransplantation under a single general anaesthetic. The clinicians involved in the management of this case determined that effective mesialisation of the mandibular molars would result in a very high likelihood of successful eruption of the 38. The favourable third molar eruption outcome which was attained with the orthodontic treatment ultimately justified the clinical decisions made at the treatment planning stage.
Although restorative implants are increasing in acceptance and popularity amongst both patients and clinicians, such treatment may not always represent the most ideal option for compromised or congenitally missing teeth. Actively growing adolescent patients may also pose several management issues for future implant placement.42 Given this patient’s relatively young age, presence of third molars of suitable morphology and the need for comprehensive orthodontic treatment, maintaining the over-retained 75 with a view to eventual implant replacement was not considered to be the ideal option for this individual patient.