Assessing inter- and intra-examiner reliability of orthodontists in devising incisor position objectives on cephalograms: a comparative study between senior and junior practitioners

The position of incisors is important for dentofacial aesthetics and orthodontic treatment planning. A number of cephalometric analyses put a significant weight on incisors, for example the Downs analysis [1], the Tweed triangle [2], and McNamara analysis [3]. Among these, the Tweed triangle focuses on evaluating the position of lower incisors. Charles Tweed emphasized the significant role of the Frankfort mandibular incisor angle (FMIA) in facial aesthetics and should be varied according to Frankfort mandibular angle (FMA) [4]. However, the normal parameters of FMIA have been found to vary among different racial and gender groups [4‐6], which limits its applicability. Cecil Steiner thought that compromises of incisors must be made to successfully camouflage the discrepancy of jaw relationship. He proposed a series of “acceptable arrangement” values to guide incisor arrangement for different ANB angle [7]. According to his acceptable compromises, the linear and angular values of the maxillary and mandibular incisors should be changed with the ANB angles. However, it has been revealed that the data of so-called acceptable arrangements stem from a geometric scheme rather than being derived from a comprehensive survey of individuals, thus failing to reflect true biologic camouflage [8]. Hence, it is deemed insufficient for precise incisor positioning to use Steiner’s acceptable arrangements as the only reference to devise the incisors position. In contrast, Will Alan Andrews found that populations with pleasing profiles have their maxillary central incisors positioned between the glabella and the FFA point (the midpoint between trichion and glabella for foreheads with flat contour or the midpoint between superion and glabella for foreheads with rounded or angular contour). He suggested that the forehead be used as a landmark of anteroposterior positioning of maxillary incisors to improve facial harmony [9]. This finding provides a new perspective to evaluate the anteroposterior position of maxillary incisor and acts as a supplement tool to cephalometric analysis and repose soft tissue analysis. However, it is a guidance aiming to improving profile aesthetics, regardless of function, stability and safety. Researchers have found that the incidence of lingual alveolar bone defects, such as dehiscence and fenestration, significantly increased in anterior teeth after orthodontic treatment [10], especially in four premolars extracting cases [11]. Bone defects were associated with corresponding reduction in alveolar bone thickness after tooth movement [10]. Besides the risk of bone defect, root resorption is another common detrimental side effect, and the risk is highly related to the direction of tooth movement [12] and the amount of apical displacement [13]. In order to prevent side effects, the IPO must be carefully and precisely devised and controlled within the physiological range, according to each patient’s anatomic structure. With the evolution of customized orthodontic appliances, teeth can be moved exactly the same as digital arrangement designed [14‐17], calling for accurate designs of teeth movement from the very beginning of orthodontic treatment.

Any treatment plan is a prediction of change. Due to the lack of visual treatment objectives, devising an IPO largely depends on the orthodontists’ subjective prediction of treatment results. The positioning of incisors is a problem that needs to be considered from many aspects, such as aesthetics improvement, function establishment and side-effect prevention. The devising of optimal incisor positions may differ among orthodontists, owing to their different background knowledge, aesthetic perception, and clinical experience.

The aim of this study was to introduce a new method of devising IPO on cephalograms and to test the intra-examiner reliability among orthodontists in the same group, as well as the inter-examiner reliability between different groups. The null hypothesis was that there would be no significant difference in the IPO measurements within each group over time, and there would be no significant difference in IPO measurements between different groups at each time point.

The SmartOrtho software was used to visualize and quantify the orthodontic treatment objectives of incisors to facilitate the comparison among different orthodontists. The location of original incisors for each patient was identified and marked by a single orthodontist (author: Xue Li), and copies were distributed to all the examiners to ensure that different orthodontists start the IPO devising from the same location for each patient.

Permission to perform this study was approved by the Ethics Committee of the West China Hospital of Stomatology, Sichuan University (WCHSIRB-CT-2021-501). The pre-treatment lateral cephalograms (Veraviewepocs, Morita, Kyoto, Japan) of 60 patients (25 Class I, 32 Class II, and 3 Class III skeletal malocclusions according to the ANB angle, with an average age of 27.2 ± 6.4, range: 20–45 years) were included in the study. Inclusion criteria were: (1) Adult patients; (2) No missing anterior teeth before treatment; (3) Protrusive lips according to the E-line; and (4) Treatment involved four premolars extraction. Exclusion criteria were: (1) Severe facial asymmetry; (2) History of craniofacial defects or syndromes, e.g., cleft lip and palate; and (3) Severe skeletal discrepancy requiring orthognathic surgery.

The cephalograms were imported into a cephalometric analysis software – the SmartOrtho (Sichuan University, Chengdu, China), and oriented according to the Frankfort plane. An incisor template and a rectangular coordinate system were generated by marking the original incisor’s incisal edge, labial cementum-enamel junction, lingual cementum-enamel junction and root apex (Fig. 1). The incisor template was allowed to rotate around the origin (simulating the incisor torque movement) and translate along the X-axis (simulating the incisor retraction/protrusion) and Y-axis (simulating the incisor extrusion/intrusion). The type and amount of incisor movement relative to the initial position could be directly read and exported by the SmartOrtho software.

Ten orthodontists were divided into to two groups based on their clinical experience: the junior group (N = 5) with clinical experience of less than 3 years, and the senior group (N = 5) with clinical experience of more than 10 years. Average duration of clinical experience for junior group is 2.4 ± 0.5 years, they were orthodontic residents who had more than 2 years of their residency program. The average duration of clinical experience for senior group is 11.0 ± 0.7 years, they were considered experts, at least 500 orthodontic patients were treated.

Before the formal experiment, all orthodontists practiced positioning/moving the incisor templates on another 10 different cephalograms to familiarise themselves with the SmartOrtho software. Subsequently, for the formal experiment, each orthodontist was asked to use the 60 included cephalograms and position the upper and lower incisor templates to the location where they thought optimal on the cephalograms, without any reference to the data of cephalometric analysis. The operation was repeated by the orthodontists again after a 2-week interval.