A labial inversely impacted maxillary central incisor is a relatively rare occurrence. The crown of the tooth is directed upward, and its palatal aspect is facing labially. This typical orientation can be the result of trauma to the deciduous incisor transmitted to the palatal side of the crown of the permanent incisor. Labial inversely impacted maxillary central incisors are most commonly combined with labial-lingual root dilacerations. The aim of this study was to examine factors that affect the surgical-orthodontic treatment duration of labial inversely impacted maxillary central incisors.
The records of 35 consecutively treated patients were retrospectively evaluated. Cone-beam computed tomography images were taken before treatment, and factors affecting treatment time, including age, length, crown-root angle, crown height and depth, angle of inversion, rotation to axial plane, and distance and angle to midline, were measured using Dolphin Imaging software (version 11.8; Patterson Supply, St Paul, Minn). Treatment duration was evaluated for each patient, and logistic regression analyses were applied.
The 15 boys and 20 girls had a mean age 8.36 ± 1.36 years. The mean orthodontic traction duration was 11.28 ± 3.08 months. Multiple regression analyses indicated that factors resulting in a longer duration were age (β = 0.779; P = 0.043), crown height (β = 0.344; P = 0.007), crown-root angle (β = −0.037; P = 0.018), and tooth length (β = −0.623; P = 0.038).
Surgical-orthodontic correction of labial inversely impacted maxillary incisors requires an average of 1 year. Assessments of age, crown height, root dilaceration, and length of incisor can help the orthodontist to better predict treatment duration during consultations with patients and parents.
We analyzed the variables that affect treatment duration of labial inversely impacted maxillary central incisors.
The average treatment duration for 35 patients was 11.28 ± 3.08 months.
Age, crown height, crown root angle, and incisor length were the factors with significant effects on treatment duration.
The labial inversely impacted maxillary central incisor, a special and rarely occurring type of impacted incisor, has its crown directed upward and its palatal aspect facing labially ( Fig 1 ). This typical orientation can be the result of trauma to the deciduous incisor transmitted to the palatal side of the crown of the permanent incisor, causing the partially developed and unerupted tooth to rotate upward and labially. Further root development continues in the initial direction; thus, labial inversely impacted maxillary central incisors are most commonly combined with labial-lingual root dilacerations.
Impaction is initially suspected when a tooth fails to emerge into the dental arch after the normal age of eruption or when its contralateral tooth has already erupted for at least 6 months with a fully developed root. The tooth is diagnosed to be labial inversely impacted when the long axis of its crown pointing to the palatal plane (line drawn from posterior nasal spine to anterior nasal spine) has an angulation of more than 0° when seen in the sagittal view on radiographic examination ( Fig 1 ).
The literature reports a relatively low treatment success rate. This rare type of impacted tooth has a complicated and lengthy treatment, since different approaches may be warranted because of the special morphology, unique characteristics, and location. Lack of evidence-based information, due to relatively rare occurrence rates of 0.06% to 0.2%, and use of conventional 2-dimensional (2D) imaging were limitations of previous studies; hence, many patients, general dentists, and orthodontists opted for surgical repositioning or extraction with prosthodontic rehabilitation to treat these patients.
However, attitudes seem to have changed recently, due to the introduction of cone-beam computed tomogrpahy (CBCT), which provides undistorted images with submillimeter resolution, eliminating superimpositions, for more precise and accurate assessment of the tooth, its root, and its spacial relationships relative to adjacent structures during 3-dimensional (3D) radiographic analysis of these impacted teeth. Recent CBCT studies on labial inversely impacted maxillary central incisors by Sun et al have shown that dilacerations are most common in late dental age groups and that early treatment would provide continuous root development to a better morphology with reduced alveolar bone loss on the labial side. In a 2-year follow-up, Hu et al concluded that labial inversely impacted maxillary central incisors have an overall good prognosis after early treatment. There is still uncertainty about duration of treatment of labial inversely impacted maxillary central incisors among practitioners due to a lack of research. Treatment duration of this type of impacted tooth is of interest to orthodontists seeking to allay concerns of parents during preinitiation treatment consultations, since monthly appointments may be time-consuming and costly. Previous studies were carried out to measure some variables that could alter treatment duration of various kinds of ectopic maxillary central incisors, using both 2D and 3D imaging, although some measurements were performed on 2D views. However, measurements done on 2D radiographs have limitations and lack precision considering the 3D angulation and typical dilacerated root of the labial inversely impacted incisor. The aim of our study was to use CBCT to more accurately examine factors that affect treatment duration in patients who are specifically affected by unilateral labial inversely impacted maxillary central incisors.
Material and methods
A total of 43 patients were treated for impaction of maxillary central incisors, between January 2010 and May 2017 at the Department of Orthodontics, School and Hospital of Stomatology, Wenzhou Medical University, in China. Of these, 35 patients (15 boys, 20 girls) were recruited for our study. These patients were relatively young, ranging in age from 5.9 to 11 years old. They met the following criteria: (1) successfully treated healthy subjects diagnosed with a unilateral labial inversely impacted maxillary central incisor and the contralateral tooth having normal crown and root formation and orientation; (2) clear pretreatment CBCT images; (3) regular attendance for orthodontic appointments; (4) full clinical documentation of orthodontic appointments, date of bonding, date of beginning of traction of teeth toward normal position; and debonding date; (5) compliance of the patient and parents with the treatment plan; and (6) informed consent signed by parents or guardians of all children. The exclusion criteria were (1) systemic diseases; (2) serious oral and maxillofacial diseases and craniofacial anomalies, cysts, or cleft lip and palate; and (3) frequent failure to be punctual for appointments. Approval was obtained for this clinical study from the ethics committee of the School and Hospital of Stomatology of Wenzhou Medical University.
The incisor was diagnosed as impacted and confirmed to be in a labial inversely position using a 3D CBCT scan (NewTom, Verona, Italy). This examination was performed on all patients before treatment at the following machine parameters: 110 kV, 1-20 mA (pulse mode), 26-second scanning time with an axial thickness of 0.25 mm, 15 × 15 cm field of view, and 0.30 × 0.25 mm voxel size. The data generated in DICOM format were imported into Dolphin Imaging 3D software (version 11.8; Patterson Supply). To correct the errors caused by different head inclinations while taking the CBCT images, 1 examiner (C.B.) first adjusted the 3 reference planes as defined by Togashi et al ( Figs 2-4 ). The skeletal and soft tissue landmarks used are given in Table I .
|Porion||Po||Highest point on the roof of the external auditory meatus|
|Orbitale||Or||Deepest point on the infraorbital margin|
|Basion||Ba||Midpoint on the anterior margin of the foramen magnum|
|Nasion||N||Intersection of the frontal bone and 2 nasal bones|
|Frontozygomatic suture||FZ||Point at the medial margin of the orbital rim at the zygomatic frontal suture|
Head reorientation was carried out on the CBCT images by 1 examiner (C.B.) using the Dolphin Imaging software. The following measurements were made on the impacted teeth using the measurement tools with a precision of 0.1 mm for factors affecting success rate and treatment time.
Crown height, defined as the distance between the highest located tip of the impacted incisor to the same level as the incisal edge of the adjacent erupted central incisor, ie, the distance to which the impacted tooth must move to be in its correct position.
Crown depth, defined as the horizontal distance between the midpoint of the incisal edge of the ectopic incisor and a vertical line drawn along the tip of the normal contralateral incisor, ie, the horizontal distance which the impacted tooth must move to be in its normal position.
Angle of inversion, defined as the angle between the long axis of the crown (a line drawn from the mesial portion of the incisal edge to the middle point of the cementoenamel junction) and the axial plane. The wider the angle, the greater the inversion.
Distance to the midline, defined as the horizontal distance between the sagittal plane and the most mesial part of the mesial marginal ridge of the crown, having a negative value for crowns crossing the midline and a positive value for crowns away from the midline.
Angle to midline, defined as the angle between the long axis of the crown and the sagittal plane, when viewed on an axial slice (>0° if the crown is directed away from the midline; <0° if the crown is directed toward the midline) as shown in Figure 5 .
Degree of rotation, defined as the angle between the mesiodistal line of the incisal edge of the crown and the axial plane.
The impacted tooth was then reoriented by the same examiner so that both the mesial and distal portions of the tooth were on the axial plane, and the sagittal plane coincided with the long axis of the tooth to correct any rotation or inclination, minimizing errors during the following 2 measurements.
Incisor length, following the same measurement methods as Sun et al, defined as the sum of distances between the following points as illustrated in Figure 6 and Table II : the mesial portion of the incisal edge, the middle point of the line connecting the cementoenamel junction on the mesial marginal ridge to the cementoenamel junction on the distal marginal ridge, the middle point on the dilaceration angle if there are any dilacerations, and the root apex.Table II
Reference point Definition A Incisal edge of the maxillary central incisor B Middle point of the line connecting e and f C Middle point of the line connecting g and h D Middle point of the root apex of the maxillary central incisor e Cementoenamel junction at the labial side f Cementoenamel junction at the palatal side g Dilaceration point at the labial side h Dilaceration point at the palatal side i Labial part of developing root apex j Palatal part of developing root apex
Angle between the crown and root, using the same methods as Sun et al, defined as the angle between lines AC and CD in Figure 6 : ie, the angle between the 2 dilacerated portions of the root. The smaller the angle, the greater the dilaceration.
To reduce method error, all measurements were repeated by the same investigator at 2-week intervals and the values averaged.
All patients were treated by the same dental specialist (R.H.) using surgical exposure with the closed eruption technique to achieve overall good esthetic results and continuous root growth. Anchorage preparation and orthodontic reopening of incisor space were done when necessary. The impacted incisors were exposed with a full-thickness mucoperiosteal flap after careful CBCT examination of the position of the impacted incisor relative to its adjacent structures. An eyelet attachment was then bonded to the palatal aspect of the exposed impacted incisor to which a 0.009-in stainless steel ligature wire was attached to connect it to the dental arch. After 1 week, the sutures were removed, and traction was initiated toward the Guide Rod appliance using an elastomeric chain with a light extrusive traction force of 35 to 60 mg based on reports by Oyama et al and Nanekrungsan et al that dilacerated roots are more prone to root resorption than are normal roots during orthodontic movement. The Guide Rod appliance, invented by Rongdang Hu, is a further modification of the modified Nance arch ( Fig 7 ), consisting of 2 molar bands connected by a stainless steel wire and fixed in a palatal plate, providing added anchorage during traction of the incisor toward the adjustable hook ( Fig 8 ). Patients were then seen at monthly intervals by the same dental specialist for replacement of the elastic chain, maintaining an appropriate correction force according to the method of Kim et al, who reported a steady force loss in stretched elastomeric chains. Depending on the crown position at each appointment, 3D repositioning of the hook was performed when needed to produce movement of the impacted incisor in a new direction, thus achieving a better periodontal result ( Fig 9 ).