Case report: cone-beam CT imaging in the management of a double tooth.
A double tooth is a descriptive term used to describe the developmental anomaly where two teeth appear joined together. The degree of union is variable and may involve the crown, the roots, or both. It is unusual for teeth to be united by enamel only, with union of the dentine and pulp chamber being much more frequent. The aetiology of the condition is either fusion or gemination. Fusion is the union between dentine and/or enamel of two or more separate developing teeth, whilst gemination is the partial development of two teeth from a single tooth bud following incomplete division. Concrescence on the other hand is an acquired disorder in which the roots of one or more crowns are united by cementum alone after formation of the crowns [Soames and Southam, 2005]. The incidence of double teeth is 0.5-1.6% in the primary dentition of Caucasian populations and 0.1-0.2% in the permanent dentition [Crawford and Aldred, 2005]. More detailed epidemiological information is now widely available [Soames and Southam, 2005; Ammari et al., 2008; Sivolella et al., 2008].
Cone-Beam Computed Tomography (Cone-beam CT) was developed in the late 1990's [Mozzo et al., 1998; Arai et al., 1999] and, therefore, is a relatively recent addition to the imaging armamentarium for use in Maxillofacial Radiology. Whilst Cone-beam CT shares a number of similarities with conventional CT, one of the most notable and clinically important differences is the marked reduction in radiation dosage from Cone-beam scanners; a dose reduction, typically of a factor of 20 times, is offered compared with conventional CT imaging of the maxillofacial region.
The dose reduction for Cone-Beam CT is very much dependent on the field of view irradiated, the voxel size and the time selected for the scan. The dosage received from a bimaxillary Cone-beam CT scan is approximately equivalent to a full-mouth periapical series or 3-10 standard orthopantomograms [MacLeod and Heath, 2008]. Cone-beam scanners are based on a cone-shaped beam of x-rays rotating around the object of interest giving a volume of data. The image receptor in this case study was a flat panel detector (for the iCat); other Cone-Beam CT systems include image intensifier detector systems.
The technique involves a single 360-degree scan in which the x-ray source and reciprocating area detector synchronously move around the patient's head. At certain degree intervals, single projection images, known as 'basis' images are acquired. Software programs are applied to these image data to generate a 3D volumetric dataset, which is used to provide primary reconstruction images in three orthogonal planes (axial, sagittal and coronal) as well as 3-dimensional images [Scarfe et al., 2006]. Additionally, this dataset can then be employed in 3D stereolithography to produce a life-size 3D model of the object for use in surgical planning.
The diagnosis and management strategies of double teeth are well reported in the dental literature. Diagnosis has largely relied on clinical examination and conventional radiographic findings. Management strategies include either extraction of the whole unit, separation and retention of both units, hemisection and extraction of one unit or reshaping the crown of the tooth [Karacay et al., 2006]. Endodontic and periodontal treatment may also be necessary [Aryanpour et al., 2002; Crawford et al. 2006]. However, the authors are unaware of any published work that has demonstrated the use of Cone-beam CT or 3D stereolithography in the diagnosis and surgical planning of the management of double teeth. Therefore the aim of this report is to present a case in which Cone-beam CT was used in the assessment and surgical planning of the management of a double maxillary lateral incisor.
A healthy 11-year old girl was referred by her orthodontist regarding fusion of tooth 22 with associated buccal displacement of 23 (Figure 1a). Clinical examination showed all permanent teeth to be erupted with the exception of 16 and 46 that had previously been extracted and third permanent molars that were unerupted. The maxillary left lateral incisor (22) was present as a double tooth. Mild premolar crowding was present in both arches. Orthopantomogram and plain film periapical radiographs of the maxillary left incisors had been provided by the referring orthodontist which revealed a 2D representation of the double tooth (22) with a large apparent overlap of the dentine at the mid root level. No obvious dental or bony pathology was evident, however no clear assessment of the joining tissues could be appreciated from this radiograph (Figure 1b).
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Radiographic diagnosis and assessment. This was undertaken by Cone-beam CT to augment the findings of the plain film radiographs. The Cone-beam CT revealed the point of union between the teeth (22) in 2D (Fig 2a) and the all-important third dimension was appreciated in the axial and sagittal sections (Fig 2b). In figure 2b the join was noted to occur at the level of the cervico-enamel junction and involve the dentine and enamel. The potential difficulties in terms of cleansing were explained to the patient and her parents using these images.
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The 3D volumetric dataset provided by the Cone-beam CT (Imaging Sciences International Version 3.1.62. 120KVp, 23.87mAs, voxel size 0.4mm, field of view 4cm) was manipulated using Maxilim (Medicim NV, Belgium) radiological software to construct a 3D stereolithographic model of the double tooth via a 3D printer (Z130 Plus; manufactured by Z Corporation, USA). This model was then employed in surgical planning to allow direct 'extra-alveolar' assessment of the entire double tooth unit to accurately determine the most appropriate approach for surgical resection (Figure 3).
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Following discussion of treatment options with the patient and guardian, a plan for surgical resection was made. The patient was appointed to have the double tooth surgically divided in situ and the distal portion extracted under local analgesia. The coronal extent of the fusion was sectioned with a diamond fissure bur and air turbine prior to raising a minimal localised mucoperiosteal flap, buccally and palatally, to fully expose the double tooth (Figure 4a). The remaining junction between the fused elements was removed with a tungsten carbide fissure bur in a straight surgical hand piece. The tooth was definitively separated and the distal portion extracted with a Couplands elevator (Figure 4b). No pulpal communication was evident clinically and the exposed dentine of the retained mesial portion was temporarily covered with flowable Revolution composite resin (SDS Kerr Corporation, Orange, CA), which was bonded with Prime&Bond NT (Dentsply Ltd, Surrey, UK). Wound closure was achieved with 4/0 Vicryl Rapide interrupted sutures (Ethicon, New Jersey, USA) [Figure 4c].
Surgical healing was allowed to take place prior to definitive restoration with Z100 composite resin (3M, Berkshire, UK) of the residual portion of the double tooth to resemble normal lateral incisor architecture. The disto-buccally displaced canine was then to be allowed to drift mesially into its normal anatomical position. However, due to the patient developing a poor attendance pattern, the mesial drift of the permanent canine occurred prior to composite resin build up of the retained lateral incisor (Figure 5a). Fortunately, the physiological mesial drift of the canine resulted in 7.5mm of space remaining between canine and central incisor to build up the retained lateral incisor; matching the mesio-distal width of its contra- lateral counterpart, thus giving a successful aesthetic outcome (Figure 5b).
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Following the active phase of treatment, the vitality of the retained mesial portion was monitored clinically, including cold thermal and electrical testing, and radiographically to assess pulpal sensibility (Figure 6). At ten month clinical and radiographic follow up, the retained portion has continued to maintain vitality, there are no signs or symptoms of pathology and the patient remains pleased with her dental aesthetics.
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Many case reports exist in the literature detailing the diagnostic management of double teeth. Radiological examination is an essential component of the necessary clinical work-up. Conventional radiography is almost universally used, e.g. periapical radiographs [Sivolella et al., 2008], occlusal radiographs [Ammari et al., 2008] and panoramic radiographs [Karacay et al., 2006]. However, different modalities have been combined to obtain a more specific diagnosis. Danesh et al.  described the use of an orthopantomogram, a lateral cephalogram, intra-oral radiographs and a magnetic resonance tomogram to confirm the exact path of the root canals of a fused maxillary central incisor with dens evaginatus.
The amount of information gained from conventional film and digitally captured periapical radiographs is limited by the three-dimensional anatomy of the area in question being conveyed as a two-dimensional image. As a result of superimposition, periapical radiographs also reveal limited aspects of the three-dimensional anatomy. In addition, there may be geometric distortion of the anatomical structures being imaged [Grondahl and Huumonen, 2004]. These problems can be overcome using Cone-beam CT imaging techniques that can produce three-dimensional images of individual teeth and the surrounding tissues. Root morphology can be visualised in three dimensions, as can the number of root canals and the level at which they converge or diverge from each other [Patel et al., 2007].
To plan a suitable management strategy for double teeth it is essential to know the extent of pulpal communications if they exist. It is also essential to ascertain the extent and position, in an apical-coronal direction, of hard tissue union to ascertain whether surgical division is possible and if so, whether this can be achieved in situ or whether intentional extraction, resection and subsequent re-implantation is necessary. This can sometimes be achieved via conventional radiography alone [Tsurumachi and Kuno, 2003; Karacay et al., 2006]. However, this is not always the case.
According to Hulsmann et al. , the severity of fusion between two teeth is properly assessed at the time of sectioning. This implies that the need for endodontic treatment only becomes evident after resecting the tooth. This was further evidenced in a more recent report in which the authors were unable to tell from a periapical radiograph if a communication existed between the pulp chambers of a geminated maxillary lateral incisor. After sectioning, a small exposure was found and sealed with mineral trioxide aggregate (MTA) [Hong et al., 2006]. Cone-beam CT may have helped to ascertain the need for pulpal intervention pre-operatively.
In some cases, Cone-beam CT may help to avoid exploratory surgery that was deemed necessary in the case of another fused maxillary lateral incisor [Sivolella et al., 2008]. It was not completely clear from the pre-operative periapical radiograph if the fused tooth was simply an adjacent supernumerary tooth and surgical exploration was carried out to reach a definitive diagnosis. The advantage to the patient of avoiding such procedures is clear.
Furthermore, when hemisection with retention of one unit is planned, Cone-beam CT may help avoid or reduce treatment times by allowing the clinician to assess pre-operatively which unit to retain based on the three-dimensional anatomy of the double tooth. In a report by Braun et al., , both roots of a geminated maxillary central incisor were root treated pre-operatively, as the decision as to which root to retain could only be made intra-operatively. Again, conventional radiography was inadequate.
Cone-beam CT is being utilised in an ever-increasing array of clinical applications. Whilst taking into account the potential implications of imaging a relatively large area of the craniofacial anatomy in order to investigate one tooth, the greater amount of radiological information that this technique provides revolutionises decision making in this clinical situation. This imaging modality can offer not only improved pre-operative and surgical planning, but the 3D models provided can be used to further educate and inform patients and guardians in the management of double teeth.
The authors would like to take the opportunity to thank Dr John Whitters for the construction of the 3D stereolithographic model, the Department of Medical Illustration for provision of photographic imaging and the Maxillofacial Radiographers for radiographic image acquisition. All are based at the research authors' institution.
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S. Lucey, N. Heath, R.R. Welbury, G. Wright.
Dept. Paediatric Dentistry, University of Glasgow Dental Hospital and School, 378 Sauchiehall Street, Glasgow Scotland
Postal address: Dr. G. Wright. Paediatric Dentistry Department, Falkirk and District General Hospital, Majors Loan, Falkirk, Scotland. FK1 5QE.