This study aimed to introduce the first 3D classification of difficulty degree for M2M impaction, after describing the difference between 2D and 3D diagnostic records for the subsequent therapeutic choices. Precise diagnosis is imperative to ascertain the most suitable treatment among available therapeutic modalities and the preoperative assessment of M2M impaction require collaborative effort between oral surgeons and orthodontists to establish an accurate diagnosis and evaluate the complexity of each case [1, 19]. Identifying this condition promptly is crucial to initiate treatment at the opportune age [1, 11]. Early adolescence has been recognized as the optimal period for addressing impacted M2M due to incomplete root development and the third molar being in its germinal stage [10–12, 19, 20]. While some authors have reported favorable outcomes in adult patients, younger individuals have shown superior results with quicker improvement of their clinical condition [3, 7, 9–12, 16, 21]. As reported by La Monaca and colleagues, teeth movement in adolescents are easier and more appropriate than in adults who are often intolerant to the fixed orthodontic therapy [7]. The incomplete root apexification likely plays a significant role in the success of treatment, enabling the advantageous utilization of the residual eruptive force of the molar.
The initial findings of this analysis confirmed distinctions between 2D and 3D diagnostic records. Specifically, only the measurement of M2M height to the alveolar crest and M2M depth to the M1M exhibited satisfactory evaluation with orthopantomography. Conversely, for all other characteristics, the assessment favored the use of 3D examinations, offering superior visualization and greater clinical relevance. Undoubtedly, the three-dimensional analysis with the evaluation of M2M impaction in the three spatial axes is fundamental both for the surgeon, who must avoid intra-operative damage to the surrounding structures, and for the orthodontist, who has to manage the most favourable biomechanics. Cone-beam CT scans influenced clinical decisions across all examined aspects, particularly when considering the patients and their parents’ education. The final decision depends on diagnostic features, patient-related items, and operator-related skills. Diagnostic factors encompass the available diagnostic tools, the degree of impaction severity, the presence of any pathological bone abnormalities, and the condition of both affected and neighboring teeth. Patient-specific items include the patient's age and their cooperation during treatment. Operator-related factors involve the skills of the orthodontist in using the available tools, the surgeon's expertise in executing the most appropriate procedure, and their prior experience.
In terms of therapeutic approaches, many impacted M2Ms underwent successful treatment through a combination of surgical intervention and orthodontic traction [1, 6, 7, 12, 16, 19, 22–26]. M2M impaction can show a partial or total mucosal coverage, or a bone impaction for the most severe cases. A direct consequence of its depth is the relationship between M2M and the mandibular canal because a closer position with the tooth can result in alveolar nerve injuries during surgical manoeuvres. In particular, surgical uprighting, surgical repositioning, and extractions should be performed with caution because the tooth luxation could cause severe post-operative complications [1]. Orthodontically, the difficulty increases with a more horizontal position of M2M where a deeper surgical approach is mandatory, and a wider distal tipping of the tooth is necessary. As observed by most authors, the angulation can influence the therapeutic outcomes [7, 10]. Mesial inclination is the most common position for impacted M2M, mainly due to an abnormal eruption path [7, 10, 27]. This position often allows a less complex treatment for the M2M repositioning because the tooth usually has a potential of eruption yet [10]. As reported in literature, the angulation of impacted M2M usually ranged between 13° and 75° [28]. For this reason, 45° was considered the cut-off value for distinguishing M2M impaction from simple to moderate difficulty. Although several authors stated that periodontal defects didn’t worsen after orthodontic uprighting of impacted M2M, an accurate pre-operative evaluation should be mandatory to support both surgeon’s and orthodontist’s decisions [1, 6, 29]. Their final evaluation should also include the buccal-lingual position in relation to the cortical plates. If M2M is buccally positioned or in the middle between buccal and lingual plates, a buccal access can be performed, with less surgical risk. On the contrary, in the complicated cases of transverse position, surgical treatment can require a lingual access or a double access (buccally and lingually), mainly in cases of tooth extraction. With these approaches, iatrogenic injury of the lingual nerve can compromise the therapeutic purpose. Orthodontically, anatomical and biomechanical considerations guide the difficulty assessment of impacted M2M. Because of cortical plates are the hardest part of the mandibular bone, higher forces are needed to achieve the tooth movement. Furthermore, it is mandatory to consider that, in the mandible, orthodontic traction can be applied only on the buccal side, providing better outcomes if M2M is parallel to the cortical walls and increasing the difficulty if it is oblique or completely perpendicular to them. Finally, a particular focus should be reserved to the posterior eruption space. An adequate retromolar space should be able to accommodate the lower second molar and the third molar is not always an obstacle for M2M recovery, mainly if it is completely distal to the impacted tooth [1, 7]. As reported by Kim et al., the development of the retromolar space can be predicted considering age and sex because it increases by 1.5 mm per year until 14 and 16 years of age for girls and boys, respectively [18]. Furthermore, as showed by Padwa et al., also surgical procedures could improve the mandibular bone remodelling in adolescents [6]. This anamnestic data can be very important during the diagnostic phase, influencing the clinical management.
During the validation process of this new classification, the analysis confirmed the absence of any discernible inter- or intra-operator bias in score assignment. The assessment of M2M impaction was objectively conducted by both orthodontists and surgeons (Fig. 6). In the pursuit of a simplified method for calculating M2M difficulty, a minimal adequate model was proposed. This model introduces a new simplified scoring classification system, highlighting the significant impact of specific factors such as height compared to the alveolar crest, angle between M2M and M1M, relationship with the mandibular third molar, and relationship with the cortical plate. Despite potential statistical errors attributed to exponential calculations, this new scoring system enables clinicians to establish an easy algebraic relationship, categorizing M2M impaction into low-, middle-, and high-risk levels. This proposed approach is both straightforward and swift, demonstrating stronger agreement with the initial scoring method based on the summation of individual parameters.
The primary limitation of this study is the small sample size of this study which serves as a pilot for validating the proposed classification system, stemming from the low incidence and prevalence of this clinical condition. Furthermore, statistical permutation tests were employed, confirming the statistical validity of these results due to the reduced variability of these data. Additionally, no data about treatment choices were reported, as this study solely focused on improving the diagnostic criteria for M2M impaction.
In conclusion, three-dimensional evaluation of impacted M2Ms could enhance diagnostic accuracy, surpassing the limitations of 2D radiographic records. A 3D classification system was proposed and validated by a group of experienced surgeons and orthodontists, who play key roles in the treatment of this clinical condition. Future research should encompass prospective analyses involving larger, multicenter datasets to thoroughly assess the significance of this difficulty score system in guiding clinical management of M2M impaction. Furthermore, future avenues may explore the integration of machine learning and artificial intelligence to generate comprehensive data inputs, potentially aiding in diagnosis and, ideally, enhancing the final decision-making protocol.