In this systemic review, different kinds of literature were evaluated regarding MIE in the form of resolving crowding among non-growing orthodontic patients; that included their long-term stability, relapse rates, profile, and esthetic outcomes. Therefore, in addition to different treatment modalities such as non-extraction forms of arch expansion, premolar extraction, and interproximal reduction (IPR), MIE alongside and its outcomes were also considered. All articles that were included in this paper, were provided with level I & II evidence, which is considered a great strength. The full quality assessment is mentioned in (Table 2).

All eight articles included the topic of mandibular incisors extraction that matches the focus question of the current systemic review. Four described mandibular incisor extraction [8-13]. Three were compared in terms of premolar extraction and mandibular incisor extraction and their outcomes [14-16]. Two showed various extraction patterns and MIE as well as different treatment approaches, including IPR and dental arch expansion [10, 17]. All information regarding the author, year, setting, sample, retention, treatment appliance used, parameters measured, study type, post retention, and strength of evidence, was collected from the included articles and described in (Table 3).

Burçak Kaya et al. compared cephalometric measurement, maxillary and mandibular space analysis and irregularities, Bolton excess in mandible, and skeletal, dental, and soft tissue. As a result, mandibular lower incisor extraction can be a more conservative approach than four premolar extractions in cases that demands localized treatment and little change in the dental arch. Since it showed at the end of both treatments no significance sagittal skeletal changes and overjet and overbite remained unchanged, but the study lacked in mentioning the relapse rate of both treatments [14].

On the other hand, different outcomes were found in a study focused on the long-term stability in 3 different treatment modalities in resolving mandibular incisor crowding among 3-time points: before the treatment, after the active treatment, and ≥2years following retention with a mean of 3.5 years. They found that relapse of crowding was obvious in all treatments, and there wasn’t a major correlation between different treatment approaches and post-treatment relapse [15].

Gısli Vilhjalmsson et al. shed light on significant objectives that concern the dentist while following this approach; black triangles, tooth discrepancies, and patient concern of the visible site of extraction. They described how to overcome those by simply lingual tipping of the mandibular incisor before extraction, which showed an almost 100% success rate with patients under 20 that had no black triangles before the treatment. Yet this approach may extend the treatment time 2- 6 months—none the less this study was localized to the Icelandic population and lacked other populations. If different populations that are known for their poor oral hygiene were included, they might have different results, since black triangles are periodontal multifactorial [9].

Multiple studies used the Peer Assessment Rating (PAR) index which is considered a reliable and valid tool in orthodontics; to assess the pre- and post-treatment dental casts after treatment modalities [8, 16]. Though PAR index had some limitations; it lacked consideration of patient’s satisfaction, periodontal and dental health, functional occlusion, cephalometric changes, and soft tissue profiles [4].

Sherry Lee et al. analyze clinical records pre- and post-treatment study casts, intraoral photographs, and wax setups to compare treatment attractiveness between non-extraction controls and MIE cases. The results indicated that in carefully selected cases, the extraction of a mandibular incisor might lead to proper outcomes that are considered as attractive as those handled without extraction, and with a longitudinal evaluation which was an advantage to the study [8]. While J. Antoszewska-Smith et al. assessed the reliability of Little's Irregularity Index and established an effective algorithm for the treatment of adult patients with crowding in the mandibular front area [10, 18].

Waheed Ullah Khan et al. determined the prevalence of crowding, frequency, and pattern of extraction [17]. Extraction of a tooth and treatment planning for orthodontic treatment can be influenced by many factors such as good patient cooperation, appliance selection, and management of the treatment for achieving functional, stable occlusion and aesthetic outcome. The main indications to extract the mandibular incisor are lower anterior crowding or protrusion anomalies in the number, size, class III malocclusion, Class I malocclusion with anterior tooth size discrepancies and severe mandibular anterior segment crowding, improve aesthetic, edge-to-edge occlusion, ectopic eruption, open bite and crossbite of anterior teeth, periodontally compromised incisors, lower anterior Bolton’s excess greater than 4mm and for Class II Mandibular single incisor extraction should be combined with maxillary premolar extractions in order to establish normal occlusion and overjet [9, 11, 14-17, 19, 20].

Moreover, MIE contraindications are anterior maxillary tooth size excess, deep overbite, periodontal diseases, and triangular-shaped mandibular incisors [19]. The advantages of one MIE are: maintaining the overall arch form, reduce cost, diminish the relapse in the anterior region, minimizing the change in profile and treatment time [8, 11, 14, 16, 21, 22]. However, clinical experience, diagnostic wax setup, and initial records are factors that should be considered before the final decision to extract the mandibular incisor [23].

Nevertheless, the disadvantages are acceptable aesthetic, midline discrepancy, differences between adjacent teeth in the shade, increases in the loss of the interdental gingival papillae, space reopening, crowding recurrence, unsatisfactory posterior occlusion, and overbite in the mandibular anterior region. In case there is no Bolton discrepancy exist, an increase in the overjet will exist [8, 14, 17, 21].

Conclusion

Several conclusions can be drawn on the indications and different impacts of mandibular incisor extraction (MIE) as an alternative for orthodontic purposes.

There was a significant agreement about the most frequent indication of MIE in moderate to severe mandibular anterior crowding, as well as a camouflage of skeletal class III cases with a mild anterior crossbite, mainly when an excellent posterior intercuspation is present.

An unclear correlation was detected between MIE or PME in terms of post-treatment relapse rate. However, multiple studies, showed that MIE could be a more effective and conservative alternative to PME in cases where limited treatment and little changes in the dental arch are required, particularly among adults.

Furthermore, the major problem with MIE is black triangles (that are caused by loss of interdental papilla height), tooth size discrepancy, and patient concern of visible site of extraction. This was simply solved by lingual tipping of the mandibular incisor before extraction. Clinicians should be aware of the factors that influence the choice of teeth extraction in orthodontic cases, in order to achieve proper treatment management with excellent patient cooperation, as well as achieving best esthetics, functional, and stable occlusion.

Acknowledgments: The authors thank the Institutional Review board and the research center at Riyadh Elm University for their continuous support.

Conflict of interest: None

Financial support: None

Ethics statement: Ethical review and approval were obtained from the institutional review board (IRB) of Riyadh Elm University, Riyadh, KSA and a registration no SRS/2020/8/189 is assigned.