Disappearing teeth. The story of infra-occlusion of primary molars: diagnosis, aetiology and management

A submerging tooth was classically defined by Dixon¹ as one that fails to maintain its position in the developing occlusion. More recently, the term infra-occlusion is defined as being when teeth are present with their occlusal surface below that of their neighbouring teeth, long after they should have reached occlusion (Figure 1).^2,3,4 In this regard, the primary teeth affected appear to remain stationary, while the adjacent teeth continue to erupt occlusally, with the normal vertical development of the face. It is common in the literature for the terms ‘submerged’ and ‘ankylosed’ to be used synonymously with infra-occlusion, which can create an element of confusion. The term we prefer is that of infra-occlusion.⁵

The prevalence of a condition is the proportion of a population who have a specific characteristic in a given time period. As individuals with infra-occluded teeth get older, some of these teeth may be shed naturally and therefore, the prevalence of infra-occluded teeth is lower within older age groups. Incidence is the rate of occurrences of new cases in a population and the differing use of these terms hampers the ability to undertake direct comparisons between studies. The reported prevalence of infra-occluded teeth is between 1.3%³ and 8.9%.⁵ The prevalence of infra-occlusion is higher in patients affected by hypodontia,⁶ occurs bilaterally⁴ and is more common in females than males.⁷ The primary mandibular molars are affected more than 10 times as often as the primary maxillary molars.^7,8

A strong familial association was also highlighted by Kurol⁵ who described infra-occlusion as a potentially multifactorial hereditary condition involving several genes or a single environmentally sensitive gene.⁹ A study by Odeh et al¹⁰ looking at the association between infra-occlusion and other associated dental variations in singletons and twins suggests the presence of a pleiotropic effect with delayed dental development and reduced tooth size. The authors also suggest that the underlying aetiological factors may be genetic and/or epigenetic.

Aetiology

A number of theories¹¹ for the aetiology of infra-occlusion have been suggested. These include:

• Ankylosis;
• Local trauma to Hertwig's epithelial root sheath;
• Disturbed local metabolism;
• Localized infection;
• Chemical or thermal irritation;
• Deficiency in bone growth;
• Abnormal tongue pressure.

Ankylosis is the most frequent cause of infra-occlusion.^4,5,12,13 This is anatomical fusion of the cementum to the alveolar bone thus preventing normal adaptive changes with facial and dentoalveolar growth.¹⁴ Ankylosis of the partially erupted primary molar prevents the vertical movement of this tooth relative to the others along the occlusal plane.¹⁵ This could potentially be attributed to changes in the local metabolism causing disruption of the periodontal ligament during the cyclical nature of primary root resorption (resorption-healing-resorption)¹⁶. This would facilitate close approximation of the bone and tooth structure providing an opportunity for ankylosis to occur.¹⁷ However, this theory does not explain why infra-occlusion is commonly seen affecting primary molars where the permanent successor is absent. Teague¹⁸ strongly supports the role that genetics plays in the aetiology of infra-occlusion, in particular with respect to those genes that affect size, shape, texture, number and eruption of teeth.

Classification of infra-occlusion

Many classifications of infra-occlusion have been described.^19,20,21 One simple classification was described by Brearley in 1973 (Table 1).²⁰ With increasing age, there is a decreased incidence in the frequency of ‘slight infra-occlusion’, but an increase in the presentation of the ‘moderate’ and ‘severe’ forms.²

How is this condition diagnosed?

Dental infra-occlusion is mainly diagnosed by clinical presentation, which reveals a tooth below the level of the occlusal plane. Other clinical findings may include tipping of adjacent teeth due to the tension on transseptal fibres,²² lateral open bites, overeruption of opposing teeth,²³ reduced mobility and/or a high-pitched tone on percussion. This abnormal sound has been described in the literature as ‘cracked tea cup’ and is indicative of ankylosis. Histological studies in animals have determined that at least 20% of the root surface must be ankylosed before a lack of mobility and the characteristic percussion sound can be detected.⁷

Commonly, the infra-occluded tooth can become tipped, suggesting that one area of its root has become ankylosed rather than the whole root surface (Figure 3). Peri-apical radiographic examination, other than confirming/dismissing whether a successor is present, is considered to be of limited value owing to the mechanism of the ankylosis process.²⁴ Obliteration of the periodontal ligament (PDL) space initially favours the labial and lingual root surfaces,²⁵ which is difficult to detect with conventional radiographs. Cone beam computed tomography can view the PDL space in detail, but there needs to be a clear justification for the use of such a higher-dose imaging technique.

Why is it important to diagnose an infra-occluded primary tooth?

The majority of infra-occluded teeth are shed naturally providing the permanent successor is present. However, complications can occur as a result of these infra-occluded teeth (Table 2). The cases shown in Figures 4 and 5 highlight the potential for severe infra-occlusion to result in significant space loss, and the requirement for more complex orthodontic and surgical treatment.

A study by Kurol and Thilander²⁶ found that the exfoliation of the infra-occluded primary tooth was on average 6 months later than with non-infra-occluded primary molars. In this study, of the 149 infra-occluded primary molars with permanent successors present, all except five (>96%) spontaneously exfoliated. Therefore, Kurol and Koch¹⁴ have suggested that a conservative management regimen in the first instance is advisable when the permanent successor is present.

The extent of possible complications depends on different factors, including patient age and stage of dental development at which the infra-occlusion begins, whether the permanent successor is present, and the number and type of teeth affected. The earlier an infra-occluded primary tooth is diagnosed, the more treatment options that are available to both the patient and clinician.

Management of infra-occluded teeth

Treatment largely depends on whether a permanent successor is present, and how close to exfoliation the primary molar is. In broad terms treatment options include:

• Monitoring and maintaining;
• Extraction +/- space maintenance.

Factors that influence choice of treatment

Clinicians must consider a number of factors before deciding which treatment option to recommend. These include:

• Presence of permanent successor;
• Condition of the infra-occluded primary molar;
• Severity of the infra-occlusion;
• Tipping/overeruption of adjacent teeth;
• Dental age;
• Malocclusion;
• Wishes of the patient/parent/guardian.

Ultimately the final clinical decision will be made by considering all of these factors.

Treatment of infra-occluded primary molars with permanent successors

Management of infra-occluded primary molars when a permanent successor is present is outlined in Figure 6. If a permanent successor is present, the infra-occluded tooth usually exfoliates normally.^4,27,28 It can, however, result in delayed eruption of the permanent successor compared to the contralateral unaffected tooth.²⁵ It is also possible for the permanent successor to be redirected from its normal path of eruption (Figure 7). A systematic review has recommended conservative monitoring for up to 6 months, and if the tooth does not exfoliate spontaneously within that time, they should be removed, as arch-length reduction, alveolar bone defects, impacted permanent successors and occlusal disturbances can all occur when the removal is delayed.²⁹

Slight or moderately infra-occluded primary molars should be monitored clinically at 3–6-month intervals (Figure 9). The use of a William's probe can aid clinical assessment. Intra-oral photographs in this instance can be a very useful adjunct to the contemporaneous clinical records. Study models and the increasing availability of intra-oral scans may also be useful as they provide a more helpful 3D representation. If there has been a significant change or progression in the position of the tooth over a short period, radiographic review should be undertaken to ensure that the permanent successor is developing normally. Occasionally, it may be beneficial to re-establish the occlusal height or mesio-distal length of the infra-occluded primary molar. Treatment in this instance can range from direct composite onlays to the Hall technique stainless steel crowns.³⁰

Extraction should also be considered if the primary molar is severely infra-occluded with the adjacent teeth tipping to prevent the successor from erupting. Orthodontic uprighting of the adjacent tipped teeth can facilitate the extraction, or allow the natural exfoliation, of the primary tooth. Subsequent to a decision to remove a primary tooth, consideration should be given to space maintenance to facilitate eruption of the permanent successor; however, this largely depends on the stage of development of the successor, and how close it is to eruption.

Treatment of infra-occluded primary molars without permanent successors

Individuals with one infra-occluded tooth often present later with additional primary teeth also infra-occluded, as the condition is often bilateral.^4,17,31 If the permanent successor is absent, the decision to extract or maintain the infra-occluded tooth depends on the condition of the infra-occluded tooth, degree of infra-occlusion, rest of the malocclusion and age of the patient.⁸ There are several potential treatment options, and these may range from maintaining the primary molar, to removal followed by space closure or provision of a prosthesis. These options need to be carefully considered as delayed intervention may result in a difficult extraction with significant alveolar bone loss, whereas loss of the primary unit earlier may prevent further vertical bone growth, thus limiting future treatment options (Figure 10).^27,32 It is important, therefore, that the patient is assessed jointly by an orthodontist and paediatric/restorative dentist to consider all of the potential treatment options.

There are a number of potential treatment options available, including:^27,33,34,35

• Monitoring the infra-occluded tooth;
• Restoration of occlusal height;
• Extraction of the infra-occluded tooth and space closure or maintenance.

Monitoring the infra-occluded tooth

Early detection and appropriate management can reduce complications. In a young growing patient, it is likely that the tooth will infra-occlude further with progressive skeletal growth. The progression of the infra-occlusion needs to be discussed with the parents/carers and the patient, and it is important to highlight the need for follow-up appointments to evaluate the rate of progression. Monitoring of a slight to moderately infra-occluded tooth can be undertaken with study models, by using a simple William's periodontal probe clinically and through intra-oral photographs/intra-oral scans. Initially the patient should be seen 3–6 monthly intervals to determine the rate of progression of the infra-occlusion. Once the rate of progression has been established, an individualized follow-up period with appropriate interventions/treatment can then be determined.

Retention and occlusal build-up

Where the level of infra-occlusion is not severe or progressive, the tooth is caries free, with good root length/morphology and the adjacent teeth are not tipped, an occlusal build-up can be considered in an attempt to re-establish occlusion and prevent overeruption of the opposing tooth and tipping of adjacent teeth. Retention of the primary molar has been shown by Bjerklin and Bennett to be a viable option, and 90% of those primary molars retained in their study survived into adulthood.³⁶ The patient should be seen in a multidisciplinary clinic that includes an orthodontist and a paediatric/restorative dentist. Methods of occlusal build-up include a simple direct composite resin build-up, indirect composite or gold onlays and stainless-steel crowns (Figure 11). The patient must be made aware of the potential for future loss of the retained primary molar, and of the potential for failure of any restorative work undertaken.³⁴

Extraction and space closure or maintenance

Where an extraction is necessary as part of orthodontic treatment to align the dentition, it is common to extract the retained primary tooth. In some cases, the arch is well aligned, but the prognosis of the primary tooth is poor due to severe infra-occlusion, root resorption, caries or abscess formation, and an extraction is recommended.

In a malocclusion where orthodontic space closure may be undesirable or challenging, consideration can be given to reducing the size of the space and prosthetic replacement with a resin-bonded bridge or an implant-retained prosthesis.

In some instances where the adjacent teeth have tipped over the infra-occluded primary molar, orthodontic treatment may be required prior to an extraction to upright tipped adjacent teeth to facilitate an easier extraction with less alveolar bone removal. The patient at this point should ideally been seen on a joint clinic by an orthodontist and a surgical colleague. Uprighting of a tooth can be achieved with a removable appliance incorporating a finger spring, or with a sectional fixed appliance (Figure 12).

Restoration of residual infra-occluded primary molar spaces by resin-bonded bridges has several advantages; they are minimally invasive; of low cost; and are not directly dependent on bone volume in the edentulous area. The obvious disadvantage is the need for life-long maintenance. The 5-year survival of resin-retained bridges in a recent systematic review has been reported as 87.5%.³⁷ A higher success rate of 96.8% has been reported for crowns supported by an implant.³⁸ However, often where the permanent teeth have failed to develop and the primary tooth is infra-occluded, there is a lack of alveolar development, which may complicate implant placement, necessitating ridge augmentation and increasing the morbidity and cost of the procedure.^39,40

Alternatively, the extraction space can be accepted in poorly motivated patients. This option may also be considered for patients who do not show dark spaces on smiling, and where there is minimal risk of tipping of adjacent teeth and overeruption of the opposing tooth.

Decoronization

Another treatment option that was highlighted in 1984, but which has not been widely adopted, is that reported by Malmgren et al⁴¹ and latterly by Smalley.⁴² Rather than extraction of the ankylosed primary tooth, they suggested decoronation of the primary tooth instead. The decoronation is undertaken to a depth of 2 mm beneath the cervical bone margin and the surgical site is left open and is not sutured. The benefits of this approach allow a new periosteum to form over the remaining roots and subsequent reorganization of the interdental fibres, which would continue to stimulate normal alveolar bone formation, thus maintaining alveolar bone width and encouraging further vertical growth.⁴¹ This is an interesting concept, but which has not yet been widely adopted in the UK.

Summary

Important factors to consider when managing infra-occluded primary molars are:

• Early diagnosis is the key to avoiding complications with infra-occluded primary molars;¹⁹
• Identifying the presence/absence of the permanent successor, in most cases the infra-occluded primary molar will exfoliate normally when the permanent successor is present;
• Progressive ankylosis can occur with continued skeletal growth therefore, regular monitoring is strongly recommended;
• A 6-month delay in exfoliation is thought to be acceptable; however, longer delays could lead to impaction of the permanent tooth and future alignment issues;
• Interceptive orthodontic treatment can facilitate the extraction of severely infra-occluded teeth and encourage normal dental development.

In conclusion, if an infra-occluded or ankylosed primary tooth is observed, it should not be neglected. Early recognition and the appropriate treatment will potentially reduce the complexity of any required treatment plan with long-standing results.⁴²