Studies show that 46 per cent of children between 6 and 12 years old and 85% of children between 12 and 17 years old have crowding. This is because, if action is not taken, leeway space is lost, and consequently, a mild and easily resolved crowding in mixed dentition becomes stable and significant in permanent dentition
[9][10][11][12]. During the third phase of the permutation, when deciduous canines and molars are replaced by permanent canines and premolars, there is a decrease in the length of the dental arch perimeter. The maximum increase in size of both dental arches occurs in the first 2 years of life; after that, the arch length increases up to 13 years in the maxilla and up to 8 years in the mandible; after this age, length decreases in both arches
[13]. Crowding is thus considered a malocclusion that never self-corrects and instead worsens over time
[14][15][16][17]. If present in deciduous teeth, it will worsen over the next two dental stages
[18][19][20]. It generally involves 50% of individuals who were exempt in the first decade of life
[21]. An orthodontic treatment cannot ignore the concept of the “guide arch”, which concerns the lower one, because of the objective difficulty in increasing its perimeter; the bone structure of the lower jaw is more compact than that of the upper one. The identification of factors that contribute to mandibular tooth crowding in mixed dentition is critical for treatment planning
[22]. Several factors can be assumed to affect the development and severity of crowding, such as the direction of mandibular growth, the early loss of deciduous molars, mesiodistal tooth and arch dimensions, oral and perioral musculature and incisor and molar inclination
[23]. The correction of severe mandibular crowding in mixed dentition could be carried out by extraction, distalization and surgical and non-surgical expansion of the mandible
[18][24]. It is useful to emphasize that the clinician is confronted not only with crowding but also with clinical situations that are predictive of it or represent a different aspect, such as the lack of diastemas in deciduous dentition, the early loss of deciduous canines, the loss of arch length due to destructive caries, the early loss of the deciduous second molar, the appearance of teeth in ectopic position and protrusion or the accentuated retrusion of the incisors
[25]. The early detection of mandibular teeth crowding is critical for interceptive orthodontic treatment planning.
2. Diagnostic Methods
Regarding the diagnosis, the lack of space in the primary dentition is a predictor to crowding. The inability to accommodate the size difference between the primary and permanent incisors is also due to a “closed” primary dentition, which prevents the mesial shift in the erupting permanent molars into a class I molar relationship during the closing of the private space
[26]. Measurements for diagnosing crowding are usually taken on the plaster model with a digital caliper, both before and after treatment. Tooth size is the sum of the mesiodistal diameters of all teeth
[26]. Arch length is calculated as the perpendicular distance between a line that connects the medial contact point of the first permanent molars and the most vestibular point between the lower central incisors. Crowding is measured as the difference between tooth size and arch length
[26]. As result, crowding is associated with both bigger teeth and a smaller dental arch. The degree of crowding is influenced by the direction of mandibular development, early loss of primary molars, arch size, oral musculature and incisor and molar inclination
[22]. Many studies have found a correlation between crowding and the direction of mandibular rotation. Extreme mandibular rotation has been linked to increased crowding, and crowding is also brought on by some growth/skeletal patterns at the start of adolescence
[27]. Other factors that should not be underestimated for the diagnosis of crowding are the changes in facial morphology brought on by growth or orthodontic treatment
[28]. Dental crowding is also caused by several reasons, including the impact of environmental and genetic variables on dental arch measurements such arch width, arch length and arch perimeter. Further factors influencing mesiodistal tooth width include racial characteristics, sex and inherited traits
[18]. Lateral cephalograms can be used for skeletal parameters contributing to dental crowding such as effective maxillary and mandibular length, mandibular plane angle, Y axis, lower anterior face height and dental parameters such as axial inclination of the lower incisor, inclination of the lower incisor to the mandibular plane and interincisal angle
[29]. Some authors have found that the use of 2D lateral cephalograms or profile photos for orthodontic measurements may not be adequately accurate. Three-dimensional computed tomography provides better frontal and three-quarter profile data for diagnosis, allowing structures to be precisely measured, aiding in analysis and diagnosis
[18]. A template applied for the measurement of crowding is Little’s Irregularity Index. It denotes the total linear displacement of the six mandibular anterior teeth’s anatomical contact locations, represented in millimeters: 0 indicates perfect alignment, 1–3 mm indicates the least amount of irregularity, 4–6 mm indicates moderate abnormality, 7–9 mm indicates severe irregularity and 10 mm or more indicates very severe misalignment
[30]. For treatment planning, it is crucial to identify the causes of anterior mandibular tooth crowding in mixed dentition
[22]. If malocclusion is characterized by a deep bite, the cause could be skeletal, dental or both
[31].
Two clinical disorders can coexist as a result of altered tooth eruption, which is the failure of a tooth to erupt into the proper location in the arch: transmigration and inclusion
[32].
3. Prophylaxis
Primary dentition is essential for influencing the eruption of permanent teeth. Early primary tooth loss can result in undesired tooth motions and space loss in the permanent dentition
[2].
It is recommended to maintain the primary dentition in the arch until exfoliation; nevertheless, if early loss is inevitable, it should be managed to minimize the negative consequences on the developing occlusion
[23].
Space maintainers can be utilized for this purpose.
Early primary molar loss might result in a reduction in arch length, increasing the severity of crowding/malocclusion; therefore, in the affected patient, every effort should be made to preserve the natural leeway space
[33]. The unilateral loss of a primary canine or first molar can result in a significant centerline disparity and mesial migration of the buccal segments, which is another critical clinical condition to preserve space
[29]. Space maintainers are classified into three types: fixed unilateral appliances, fixed bilateral appliances and removable partial dentures. The band and loop space maintainer is one of the most prevalent permanent unilateral appliances
[34]. The appliance has a band that cements to the primary second molar. It also has a loop that contacts the distal surface of the primary canine
[2].
The distal shoe is another permanent unilateral appliance. A stainless-steel wire extends in front of the unerupted permanent first molar to guide it into position as it grows in. Distal shoes can only be placed on one tooth
[2].
Bilateral space maintainers are used once teeth on both sides of the mouth are lost. Common types include Lingual Holding Arch, Nance Arch and Transpalatal Arch.
Removable dentures are often used for cosmetic purposes rather than to avoid space loss, particularly when anterior (front) teeth are lost
[2].
4. Treatment
The lower jaw is considered the guiding arch in crowding therapy because it is difficult to modify its perimeter due to the more compact bone structure and the continuity with the mandibular branch, which does not allow for distalization
[28][35]. In addition, the symphysis cartilage ossifies in the first year of life, so it is not possible to perform an orthopedic bone expansion, as in the upper jaw, working at the level of the median palatine suture
[35][36]. The modalities of space recovery in mixed dentition are: arch perimeter increment, reduction in mesiodistal widths of teeth and serial extractions
[2][37][38]. In the upper arch, the expansions are quite stable, but inferiorly, it is universally recognized that the expansion of the intercanine diameter always recurs, whereas expansion at the molar level is quite stable, which should be considered
[26][39][40][41]. The space that can be recovered in the lower arch depends on the type of sector: in general, in the posterior sector, utilizing a lip bumper-style device, it is possible to recover a maximum of 2 mm of space per molar distalization in the posterior regions
[42]. At the molar level in the lateral sectors, the arch length could increase by about 0.4 mm; at the canine level, the arch length increases about 0.7 mm
[43]. The vestibularization of one millimeter of the incisal margin in the anterior sector results in the gain of one millimeter of arch space, or roughly a ratio of one to one (changing the arch form)
[41]. Schwarz’s appliance (
Figure 2) and lip bumper (
Figure 3) are two commonly used appliances for increasing lower dental arch dimensions
[44]. In their study, Vincenzo Quinzi et al. compared the effects of these appliances on reducing mandibular crowding by increasing lower arch dimensions
[26]. The study included twenty subjects (10 males and 10 females). Ten patients were treated with Schwarz’s appliance, and ten with lip bumper. The Schwarz appliance was more effective in increasing intercanine arch dimensions and arch perimeter, although the lip bumper reached a greater increase in arch length
[26]. Since the 1970s, there have been reports of spontaneous changes in mandibular dentition caused by maxillary expansion
[45][46]. Di Ventura et al. assessed the consequences of rapid maxillary expansion (RME) anchored to primary molars on the mandibular arch. A total of 54 patients were recruited for this study and divided into two groups: a test group (21 patients, 7.4 ± 1.2 years) who underwent RME, and a control group (17 patients, 7.3 ± 1.1 years old) who did not receive any treatment. The results of this study showed a significant increase in interdental width in the lower arch after 9 months of RME therapy
[45]. Olivia Griswold et al. evaluated the changes in sagittal mandibular incisors’ position in response to lip bumper therapy using CBCT
[47]. In this study, the authors compared a group that was treated only with rapid maxillary expansion (experimental group) and an RME + LB (lip bumper) group (control group)
[47]. The CBCTs were placed in 3D on the mandibular structure, and the angular and linear alterations in the mandibular incisors throughout LB therapy were assessed. In the investigation, there was no statistically significant difference in the degree of mandibular incisor protrusion between the two groups; the lip bumper did not generate substantial proclination, protrusion or extrusion of the mandibular incisors.
[13]. Air-rotor stripping (ARS) (
Figure 4) is a technique for creating space during the mixed dentition period by reducing interproximal enamel thickness. Yahya B. Nakhjavani et al. assessed the efficacy of the mesial stripping of mandibular deciduous canines for the correction of rotated and lingually erupted lateral incisors in 42 patients with <3 mm mandibular crowding
[48]. In this study, the mesial stripping of mandibular primary canines resulted in full crowding correction; in just few cases, the amount of crowding did not reach zero, and a small crowding in the range of 0.06 to 0.1 mm remained
[48]. The extraction of all the first premolars with subsequent orthodontic treatment is the most used method to relieve dental crowding
[49]. The importance and timing of extraction as a component of orthodontic therapy for late incisor crowding have been well investigated
[50]. No difference in late incisor crowding is shown by the data, regardless of whether serial extraction or early or late premolar extraction is performed prior to orthodontic treatment. Additionally, selecting a non-extraction orthodontic procedure has been linked to post-retention crowding
[50]. Maurits Persson et al. investigated changes in the mandibular incisor area from early adolescence to late adulthood in patients with a class I crowding malocclusion treated in the mixed dentition by the extraction of all first premolars without subsequent orthodontic treatment
[50]. The extraction group included 24 subjects who had all their first premolars extracted at a mean age of about 11.5 years to treat a class I space deficiency malocclusion. The control group included 21 subjects who had normal occlusions at the age of 13 years
[50]. The extraction group showed no improvements in lower incisor irregularity, and a significant increase in lower tooth space insufficiency into adulthood. Lower incisor irregularity and space shortage developed significantly in the control group throughout late adulthood
[50]. Premolar extraction is the sole treatment option for severe crowding in a class I occlusion, allowing for spontaneous adjustments and more stable incisor alignment in late adulthood, according to the authors
[50].
Figure 2. Schwarz’s appliance.
Figure 4. Air-rotor stripping (ARS).