Incisal edge resorption of an unerupted maxillary incisor: a previously unreported phenomenon

A healthy 8-year-old male presented to a new patient orthodontic clinic with an unerupted upper right central incisor (UR1). On clinical and radiographic examination, two supernumerary teeth were found. In keeping with Royal College of Surgeons Management of Unerupted Maxillary Incisors,¹ the initial treatment plan included the surgical removal of the supernumeraries under general anaesthetic, with an exposure and bonding of the UR1. A sectional upper fixed appliance was used to create space for the UR1. Following 16 months of orthodontic treatment (including 12 months of orthodontic traction), the UR1 failed to erupt. The case was referred to the orthodontic–restorative dentistry multidisciplinary (MDT) clinic for further investigation and treatment planning where a decision was made to extract the UR1. At the time of surgery, it was noted that the incisal edge of the crown was significantly resorbed.

This report describes the presentation, investigation and management of the ectopic, impacted UR1 with a discussion of the possible aetiology of the resorption of the crown.

Initial clinical assessment, radiographs and treatment

An 8-year-old male, with no relevant medical history, presented in the early mixed dentition with a Class I incisal relationship on a mild skeletal 2 base with average vertical proportions. The overjet was average and the overbite was increased and complete to tooth. There was a 2-mm centreline shift in the upper arch to the right-hand side, and the lower centreline was coincident. The molars were ½ unit II bilaterally. The lower arch was mildly crowded and the upper arch moderately crowded with an unerupted upper right central incisor (UR1). The upper right central incisor was palpable high and labially, at the level of the floor of the nose. The upper right lateral incisor (UR2) was rotated, but there were no other findings of note. Initial radiographic examination included an orthopantomogram (OPT) provided by the referring specialist orthodontist, an upper standard occlusal and a lateral cephalogram (Figures 1 and 2). This imaging confirmed the presence of all permanent teeth and two supernumeraries, one overlying the UR1 crown and the second positioned incisal to the unerupted upper right canine. The UR1 was located at the level of the nasal floor with the crown protruding labially (Figure 3). The root had an open apex and was straight (Figure 1). There was no evidence of resorption on the pre-treatment imaging.

The initial treatment plan included the surgical removal of the supernumeraries under general anaesthetic and the exposure and bonding of the ectopic UR1. In keeping with the Royal College of Surgeons guidelines¹ the patient initially underwent surgical removal of the two supernumeraries under daycase general anaesthetic with simultaneous exposure and bonding of the UR1. A gold pad and chain was bonded to the palatal aspect of the UR1 and eight links of gold chain remained attached There was no indication of abnormal morphology or pre-existent resorption noted in the operation notes from the original surgery. A sectional pre-adjusted edgewise appliance (3M Victory MBT prescription) was placed in the upper arch 3 months after surgery bonded to all permanent teeth. At this point, three links of excess gold chain were removed. A further link of excess gold chain was removed 6 weeks later. Open stainless steel coil was used to create space for the UR1 (the UL1 was 9.5 mm wide). Six months into treatment, space for the UR1 was 7.5 mm and traction was applied to the UR1 through the gold chain. Treatment progress was delayed through a breakage and a rescheduled appointment. Space for the UR1 had been lost and had to be re-opened. The length of the gold chain visible intraorally increased over appointments so it was assumed that the UR1 was close to eruption. There was never any notable distortion in the stainless steel archwire or loss of vertical anchorage suggestive of ankylosis.

Sixteen months following exposure of the UR1, including 12 months of traction, the tooth failed to erupt and new radiographs were taken (Figures 4 and 5). These showed that the position of the UR1 had not improved. It was agreed with the patient and family that a referral to the regional unit for CBCT and report would be made. In this specific case, a pre-operative scan would have been unlikely to change the initial management plan as, at the time of primary surgery there was no clinical sign of resorption, nor were there any radiographic or clinical signs of ankyloses. Although in hindsight it would have been preferable to obtain a pre-treatment CBCT scan, this was not practical for most of our patients and their families because our nearest scanner was located 120 miles away from our hospital. Local access is now available.

Secondary clinical assessment, radiographic findings and treatment rationale

The patient was reviewed on the orthodontic–restorative dental MDT clinic with the CBCT imaging (Figure 6). The CBCT confirmed that the UR1 was lying horizontally. There was no evidence of dilaceration although a shortened and blunted apex was noted, with a tiny lip of mesial root projecting from the coronal third of the root. The crown shape was reported to be normal but the tooth compromised. It was noted that the crown had an absence of surrounding bone which was consistent with the osteotomy required to access the UR1 at the time of the primary surgery. Definitive ankyloses was not reported but could not be entirely excluded. There was blunting of the upper left central incisor (UL1) in line with orthodontic treatment.

Retention of the compromised UR1 was considered a poor long-term option, with continued traction of the UR1 potentially leading to further resorption of the adjacent UR2 and UL1 roots. Alternative management options were therefore explored with the patient and his mother. An agreement was reached that the best option was to arrange for the loss of the UR1 under GA. Fixed appliance treatment would continue to open the UR1 space to 9.5 mm to facilitate the provision of a resin-retained bridge to replace the UR1, with the UL1 as the abutment tooth. This would reduce the risk of resorption to the adjacent tooth roots as the intrusive forces would no longer be present in response to traction and extrusion of the UR1.

On raising the flap intra-operatively, it was noted that the crown of the UR1 was lying submucosally, free from bone, and that the incisal edge was severely resorbed (Figure 7). No unusual findings, such as anomalous morphology or resorption, were observed with regard to the UR1 crown at the time of the original surgery 18 months earlier, nor was there evidence of unusual crown form on the pre-treatment radiographs. There was no evidence of ankyloses and the tooth readily mobilized intra-operatively.

Fixed appliance treatment was continued post-operatively, by which stage the patient was in the late mixed dentition. Following a further 7 months of treatment, the fixed appliance was removed and an upper Essix retainer incorporating a prosthetic UR1 was fitted for daytime wear. The patient was given a second Essix retainer with no prosthetic tooth for nighttime wear.

The patient is currently undergoing restorative dental treatment in the hospital for a resin-retained bridge supported on the UL1.

Aetiology

The aetiology of unerupted teeth can be due to hereditary or environmental factors.¹ In this case, there was no history of trauma so the failed eruption of the UR1 was thought to be associated with the presence of the two supernumerary teeth.

The UR1 failed to erupt despite orthodontic traction and the reason for this is unknown. The apical blunting on the root of the UR1 could be a result of the orthodontic traction, although plain radiographic imaging shows little change in the appearance of the root over time.

The incisal resorption of the UR1 seen on removal of the tooth is of unknown aetiology and previously unreported in the literature. While the supernumeraries may have triggered the crown resorption, there was no clinical evidence of resorption at the time of primary surgery when the UR1 was exposed and bonded.

Root resorption is a well-recognized complication of trauma and impactions, particularly canine impactions on incisors.² The coronal resorption of unerupted teeth is very rarely reported in the literature, with only a few cases reported over the last 40 years.^3,4,5,6,7 The available published case reports of coronal resorption describe internal resorption, most frequently of unerupted canines, rather than external resorption of the crown. Incisal edge resorption of anterior teeth does not appear to have been previously reported in the literature.

The most widely recognized classification system for dental resorption was described by Andreasen,⁸ and is broadly divided into internal and external resorption. Further sub-classification to surface, inflammatory and replacement resorption has been described.⁹

Aidos et al proposed a two-stage process for dental resorption whereby there is both injury and persistent stimulus.⁹ Darcey and Qualtrough² described several mechanisms for dentine resorption, but there appears to be little discussion in the literature of enamel resorption mechanism. Boyle et al linked the resorptive process seen in teeth to osteoclastic activity.¹⁰ Sasaki concluded that the mineralized components of bone and teeth resorb by a common mechanism, odontoclasts being primarily differentiated from osteoclasts by location.¹¹ Osteoclasts and odontoclasts can be found in Howship's lacunae on calcified hard surfaces including bone and dentine. These multinucleate giant cells form a sealed environment in which hydrogen ions are released within the ruffled border of odontoclasts and osteoclasts, together with proteolytic enzymes, to create a highly acidic environment leading to the dissolution of the associated hard tissue surface.² RANKL (a cytokine that regulates osteoclast formation, activation and survival in normal bone modelling and remodelling) is considered fundamental to the formation and function of osteoclasts¹² and may be stimulated by interleukin 1B, a chemical mediator essential to the inflammatory process, including the resorption of dental hard tissues in periodontal and peri-apical disease processes. Andreasen et al¹³ observed that bacteria have also been associated with the transformation of leukocytes into osteoclasts, in particular Porphyromonas, Prevotella, Treponema and some Gram-positive species. In the presence of tissue insult or injury, cytokines may be released that stimulate repair, including stimulation of RANKL, leading to resorption. If tissue is irreparably damaged, progressive resorption may occur.² Thermal injury and exposure of dentinal tubules have also been suggested as risk factors for dental resorption, although evidence is reported to be limited.^2,14

Summary

Considering the presentation and history of the case in question, several mechanisms may have contributed to the destruction of the crown, including subclinical resorption secondary to the position of the supernumerary tooth, inadvertent injury to the crown at the time of the osteotomy, post-surgical inflammation, orthodontic traction and possibly an enamel defect in the crown resulting in exposed and therefore susceptible dentine. There were no clinical signs or symptoms of infection at any stage. This does not, however, rule out the possibility of subclinical bacterial presence within the surgical site.

In considering the outcome of the case, it is important to note that the incidence of incisal resorption has not previously been published in the English language literature and that coronal resorption of unerupted teeth in general is a very rarely reported phenomenon. However, the case does highlight the importance of continued clinical and radiographic monitoring, as well as the importance of re-evaluating the treatment plan, should progress not be observed within normal time limits. The role of multidisciplinary review and appropriate use of CBCT is also advocated.