Oral Health Group

Diagnosis and Treatment Planning of Mandibular Crowding in the Mixed Dentition

January 5, 2017
by Bruno L. Vendittelli, DDS, D. ORTHO, FRCD(C); Tracey J. Hendler, DDS, D. Ortho, MSD, FRCD(C)

Crowding, or tooth size arch length deficiency, is one of the most common reasons that people seek orthodontic treatment for themselves or their children. As permanent mandibular anterior teeth erupt a primary tooth may become over-retained leading to a double row of “shark teeth”, causing parents a great deal of concern. While this may be easily remedied with removal of the over-retained primary tooth, some other space management issues are not as straightforward. What about crowding that is more significant? Should you treat all of these patients with early interceptive orthodontic treatment or do you wait until the permanent dentition? If you are to treat them at an early stage, what is the treatment? What happens if you wait?

It is important to understand the pattern of growth and development of the untreated arch as it transitions from the primary to the permanent dentition. In a normal maxillary and mandibular arch there are primate spaces. It is well established that 1-2 mm of lower incisor crowding is normal before closure of the canine primate space, otherwise known as the incisor liability. 1 Incisors will often use up the primate space in order to drift into better alignment. The width of the mandibular arch is set with the eruption of the permanent canines, around age nine. Finally, the arch length in the mandible decreases when the E’s are lost and the first molars drift mesially into this space around age 11-12, (i.e. the late mesial shift). Leeway space in the mandibular arch is roughly 4.5-5 mm, accounting for the difference in size between the second primary molars and the second premolars that replace them. 2 When the leeway space is closed, this leads to a natural loss in arch length during the transition from primary to permanent dentition. 3 It seems counter-intuitive to many people that the arch length of a six-year-old is larger than that of an 18-year-old, but it is true!

Arnold reported crowding in 85% of mandibular models examined with an average of 4.5 mm of crowding in the mixed dentition. 4 In a study of 150 children in the mixed dentition, 89% of patients who had crowding in the mixed dentition had crowding in the full permanent dentition. The patients whose crowding self-corrected had very minimal crowding of less than 0.5 mm to start with. 5 From this one can conclude that there is some good news and some bad news: the bad news is that crowding generally gets worse as the dentition transitions from primary to permanent. The good news is that leeway space in the mandibular arch is roughly 4.5- 5 mm 2, which is the average amount of crowding from Arnold’s study. 4 Perhaps one can take advantage of this space?

It is clear that without intervention crowding will get worse with time. Aside from the fact that straight teeth are easier to maintain, there is a psychosocial benefit to straighter teeth as well. 6 The strategies with which to treat mandibular crowding will be discussed below.


Treatment (Table 1)
Mandibular anterior crowding is initially identified as a discrepancy between the mesiodistal tooth widths of the four permanent incisors and the available space in the anterior part of basal bone. There are varying treatment modalities to address lower incisor crowding. The rationale for treating this condition include allowing for more effective cleaning of the teeth, increasing long-term stability, minimizing the need for adult tooth removal and simplifying orthodontic treatment at a later date. The type of treatment provided can be based on the amount of crowding. To follow is a summary of treatment strategies:

In situations where lower incisor crowding measures in the 1-3 mm range, monitoring is the most viable solution. This amount of crowding typically does not pose any issues. Moorrees and Reed 7,8 studied a series of 184 casts at three years and 16 to 18 years of age. They found that, in the mandible, an average of 1.6 mm of crowding in boys and 1.8 mm of crowding in girls will recover to 0.0 mm by the age of eight years. They attributed this recovery to a phenomenon termed secondary spacing, or closing of the primate spaces, which facilitates lateral incisor eruption. This process occurs when the mandibular lateral incisors emerge and push the mandibular primary canines laterally. This also causes the maxillary primary canines to move laterally by occlusal force, creating spaces for the maxillary lateral incisors. In situations where there is not resolution of crowding, this amount of crowding can be dealt with at a later date using various types of orthodontic appliances.

Discing of the Mesial of the Lower C’s
In situations where crowding is in the 3-4 mm range, and where cleaning of the teeth is difficult and periodontal inflammation exists, discing is a conservative measure to allow for improvement of lower incisor crowding (Fig. 1). The lower incisors will ‘spread’ into the created space. It is important to monitor these cases and make a decision whether space maintenance via a lower holding arch is indicated prior to the exfoliation of the E’s.

Fig. 1
Crowding of 3-4 mm. Lower Occlusal (masked).

Extraction of the Lower C’s with Lingual Holding Arch
In situations where crowding is in the 4-7 mm range, extraction of the lower C’s allows for self alignment of the lower incisors (Figs. 2a, 2b). This allows for better cleaning of the area and resolution of periodontal problems such as gingival inflammation. It has also been demonstrated that this can improve the long-term stability of incisor alignment. 9

Fig. 2a
Crowding of 4-7 mm.

Fig. 2b
Spontaneous alignment after extraction of the lower Bs & Cs.

It is imperative to understand that this technique ‘borrows space’ from the leeway space and transfers the crowding from the anterior to the posterior segment making the placement of a lingual holding arch essential. A holding arch can be placed shortly after extraction of the lower C’s if there is a desire to prevent lingual tipping of the lower incisors or placement can be deferred to a later date and prior to the exfoliation of the E’s (Fig. 3). Given that the leeway space is in the range of 4.5-5.0 mm 2, preservation of this space is able to prevent adult tooth extraction in most cases of crowding of this severity. In certain cases, crowding of up to 7 mm can be dealt with via maintenance of the leeway space, judicious development of the lower arch (through no greater than 1.0 mm expansion at the canines and incisor proclination) and interproximal reduction as required.

Fig. 3
Lingual holding arch.

Serial Extractions
In situations where the crowding is greater than 7 mm, serial extraction can be considered (Figs. 4, 5, 6). In comparing the differences in a sample of orthodontically treated children with serial extraction to those treated with extraction of four bicupids at the time of appliance placement, the following was found: 1) There was no difference in the end orthodontic result, 2) it does not result in greater long-term stability and 3) the time spent in fixed appliances for the serial extraction group was six months less. 10,11 It is imperative that if a decision to proceed with serial extractions is contemplated, all other criteria be met.

Fig. 4a
Serial extractions: extraction of 14 and 24.

Fig. 4b
Serial extractions: extraction of 34 and 44.

Fig. 4c
Serial extractions: post-extractions.

Fig. 5 
Right Buccal (masked).

Fig. 6
Left Buccal (masked).

Rapid Maxillary Expansion
In situations where there is dual arch constriction, the lower arch buccal segment was shown to upright or develop with sutural expansion of the upper arch. The effect on lower crowding incisor is minimal. 12 To this end, this technique should be aimed at the correction of true maxillary arch constriction and not for the elimination of lower incisor crowding.

Lower Arch Expansion
There is a school of thought that lower arch crowding can be eliminated via expansion of the lower arch with appliances such as a Schwartz plate. This is often done concomitantly with rapid maxillary expansion. Since the mandible lacks a suture, it must be understood that expansion is achieved entirely through dental tipping. Increasing the lower arch width and in particular, lower inter-canine width beyond 1 mm, has been shown to be unstable. Patients treated with this method have poor long-term stability. 13

An orthodontist will always weigh the risks and benefits of when to initiate treatment. On one hand, it can be difficult to watch a problem develop knowing that it will be more complex later on. In this vein, certain malocclusions or developing skeletal disharmonies greatly benefit from an early phase of treatment. In other cases, the desire to treat early must be weighed against the tendency to over-treat, committing patients to prolonged orthodontic treatment plans. Extended time in appliances leads to patient burnout, increased risks of decalcification and gingivitis and the additional burden of cost. In all cases, long term retention with fixed or removable retainers are essential for stability. 14 Monitoring of growth and development by an orthodontist is essential for determining if and when it is the appropriate time to initiate treatment.

1. Up to 85% of people have crowding in the mixed dentition.
2. Many modalities exist for the treatment of lower incisor crowding.
3. Timing is important: leeway space.
4. Crowding, if left untreated, will get worse over time. OH

Oral Health welcomes this original article.

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About the Authors
Drs. Vendittelli and Hendler are certified orthodontists who maintain a private practice in midtown Toronto. Both doctors are associate professors at the University of Toronto, Faculty of Dentistry. Dr. Vendittelli practices at the Hospital for Sick Kids on the cleft lip and palate team. They treat both children and adults and love making people smile! www.foresthillortho.com