Dental crowding is the most common type of malocclusion, characterized by teeth that are crowded, misaligned, and irregularly arranged. The incidence of cavities and periodontal disease is higher in crowded teeth compared to normally aligned teeth. Additionally, dental crowding significantly affects appearance, and in severe cases, it can lead to difficulty in closing the lips, resulting in an open-lip and exposed-teeth condition.

bubble_chart Etiology

1. Genetic Factors Dental crowding exhibits clear hereditary characteristics. Whether it's the crowding or misalignment of individual teeth, multiple teeth, or even the degree of rotation of a specific tooth, identical manifestations can be observed between parents and offspring. This hereditary trait is objectively present, but its detailed genetic mechanism remains not fully understood to this day.

2. Problems During Tooth Replacement Issues during the tooth replacement period are a common disease cause of dental crowding. For example, premature loss of primary teeth can lead to adjacent teeth shifting forward to occupy the gap, resulting in insufficient space for permanent teeth to erupt, causing misalignment. Additionally, retained primary teeth can cause subsequent permanent teeth to erupt in incorrect positions.

3. Insufficient Jaw Development Inadequate jaw development leads to significant bone discrepancy, preventing teeth from aligning neatly within the dental arch and resulting in crowding and misalignment.

4. Oversized Teeth Excessive mesiodistal width of teeth can cause crowding and misalignment in dental arrangement.

5. Bad Habits Certain oral habits in children, such as thumb-sucking or mouth-breathing, can narrow the dental arch or affect jaw development, leading to crowded teeth. {|104|}

bubble_chart Clinical Manifestations

The clinical manifestation is crowded and misaligned teeth, primarily due to the tooth size exceeding the bone volume, resulting in abnormal dental arch morphology.

bubble_chart Diagnosis

1. Small bone mass: The mesiodistal width of the teeth is normal, but the dental arch length is insufficient. Especially, underdevelopment of the maxillary anterior region can easily lead to crowding of the maxillary anterior teeth.

3. Grading: For clinical diagnostic purposes, dental crowding is generally classified into three degrees based on severity: mild, moderate, and severe.

    Grade I crowding (Ⅰ°): The discrepancy per dental arch is less than 5mm.

    Grade II crowding (Ⅱ°): The discrepancy per dental arch is less than 10mm.

    Grade III crowding (Ⅲ°): The discrepancy per dental arch is more than 10mm.

bubble_chart Treatment Measures

Principles of Correction

The main mechanism of dental crowding is the discrepancy between tooth size and bone size, where the tooth size is relatively large and the bone size is relatively small. Therefore, the principle of correction is to achieve coordination between tooth size and bone size by either increasing bone volume or reducing tooth volume, thereby creating conditions for aligning the teeth. In other words, to align crowded teeth, space must be created for their arrangement.

1. Increasing Bone Volume: Various orthodontic appliances are used to expand the length and width of the dental arch to increase bone volume, thereby obtaining space. However, the space gained through this method is limited.

2. Reducing Tooth Volume: Space can be obtained by reducing tooth volume through methods such as interproximal reduction (IPR), tooth size reduction, or extraction. The space gained through tooth size reduction is also very limited and can easily lead to secondary caries, so it is less commonly used clinically. Extraction, on the other hand, can provide significant space and is the primary clinical method for correcting Grade II or Grade III crowding.

Whether through increasing bone volume or reducing tooth volume, crowded teeth can only begin to undergo force correction after sufficient space for alignment has been obtained. This is a critical condition for successful treatment.

Correction of Grade I Crowding

Clinically, Grade I crowding often manifests as twisted or misaligned incisors or Grade I labial displacement of canines. Generally, methods involving dental arch expansion are used.

Apparatus: ① Bands with buccal tubes; ② Extraoral arch with open-coil springs or arch springs at both ends; ③ Headgear for traction; ④ Elastic force rings.

Using an extraoral arch to push the maxillary first permanent molars distally to create space for aligning anterior teeth with Grade I crowding is a common clinical method. This approach is mainly used before the eruption of the second permanent molars, particularly suitable for cases with mesial drift or open bite, distal bite. Typically, each posterior tooth can be moved distally by 3–4 mm, with the extent of distal movement controlled based on the required space. However, the limit of distal movement should be until the molars begin to move mesially.

The mechanism of this appliance involves inserting the extraoral arch into the round tubes on the terminal bands and applying force through the headgear and elastic rings. The traction force compresses the coil springs at the ends of the arch, exerting pressure on the molars. The coil springs must have sufficient elasticity, and their length should allow the front part of the arch to remain 2–3 mm away from the incisors when fully compressed.

The appliance must be worn for at least 12 hours daily, including during sleep.

As the molars move distally, the anterior teeth with Grade I crowding gain space for alignment, which may either self-adjust or require other removable or fixed appliances for alignment.

2. Local Space Opening: Local space opening is a common orthodontic technique for expanding the dental arch to obtain space. It is mainly used for individual teeth with crowding and misalignment due to insufficient space.

Apparatus: Bands on both molars, brackets on the adjacent teeth of the crowded or misaligned tooth, a lingual arch for additional anchorage, a labial arch on the buccal side, and open-coil springs placed between the adjacent teeth of the crowded or misaligned tooth to locally open space (Figure 1).

Figure 1 Local Space Opening

The space source for partial dental arch expansion mainly comes from the increased curvature of the anterior dental arch. Therefore, attention should be paid to the anterior overbite during use, avoiding excessive increase in anterior overbite that could lead to deep overbite malocclusion. After resolving insufficient space in crowded teeth, the malpositioned teeth should then be corrected to their normal positions.

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(1) The ends of the labial arch are fitted with omega loops. When these loops abut against the tube ends, the anterior part of the archwire is separated from the anterior dental arch. After ligation into the bracket slots, the archwire's elasticity is utilized to expand the dental arch.

(2) Vertical force units: Vertical loops are placed on the teeth requiring arch expansion to widen the dental arch.

4. Full Arch Expansion with Split Spring Removable Appliance

Device: A split spring or screw expander is placed in the maxillary baseplate. The split spring or screw expander is used to widen the dental arch through the baseplate, creating the necessary space to alleviate anterior crowding.

When using such appliances, it is generally necessary to switch to other removable or fixed appliances after obtaining the required space through arch expansion to align the crowded and malpositioned teeth. There are limits to the use of these appliances; the dental arch cannot be expanded indefinitely to avoid deep overbite or locked occlusion, which could lead to occlusal contact issues.

Treatment of Grade II and Grade III Crowding

Grade II and Grade III crowding are typically treated with extraction combined with orthodontic appliances.

1. Treatment of Labially Displaced Canines: Labially displaced canines with insufficient space, commonly known as "vampire teeth," are a frequent malocclusion in clinical practice. When the labial displacement and crowding exceed half the width of the canine crown, extraction is often considered.

(1) Selection of Extraction Teeth: For labially displaced canines, the canine itself is usually not extracted because canines play a crucial role in maintaining arch form and facial fullness. Removing a canine can lead to facial asymmetry. Additionally, canines have the strongest roots in the dentition, making them valuable as abutments for prosthetic restorations. The preferred extraction site is the first premolar, as removing one premolar has minimal impact on masticatory function.

However, if the first premolar is healthy while the second premolar or first permanent molar is decayed, damaged, or developmentally abnormal, the decayed tooth should be extracted instead of the first premolar. Although this complicates treatment, it preserves the healthy first premolar.

In special cases where the canine is labially displaced on the labial side and has a lingual crossbite or is decayed or developmentally abnormal, extraction may also be considered. Afterward, the malposition can be corrected, and the shape can be modified post-treatment to resemble that of a lateral incisor.

Before finalizing the extraction site, if the second primary molar is still present, an X-ray should be taken to check the tooth germ. If the second premolar germ is congenitally missing, dislocated, or developmentally abnormal, the unerupted primary molar should be the extraction site.

(2) Treatment Methods

1) Manual Pressure Method: For growing children, after extracting the first premolar, if there is ample space for the canine and the displacement is not severe, the canine can be aligned without an appliance using manual pressure. The patient can press the displaced into the arch with their fingers three times daily, 40–50 presses each time. The natural pressure from the lip muscles can also help reposition the canine.

2) Orthodontic appliance correction: Removable appliances can be used, with distal arch springs or vertical springs welded onto the labial arch. Alternatively, attachments with hooks can be bonded on the labial side to use elastic rings for alignment into the dental arch.

When using fixed appliances for correction, either edgewise or Begg appliances can be employed. Stainless steel or nickel-titanium archwires are ligated to full-arch brackets to intrude the teeth into the dental arch.

2. Correction of Severe Crowding in Upper and Lower Anterior Teeth Severe crowding of upper and lower anterior teeth often manifests as labial displacement of canines, accompanied by crossbite and insufficient space, with varying degrees of tooth rotation. The treatment plan typically involves extraction of teeth before correction.

(1) Removable Appliance Correction: Commonly used upper and lower removable appliances include double-loop labial arches, arch springs, and lingual springs, with the upper appliance featuring a bite plate. First, the arch spring is used to push the teeth distally to create space for the crowded anterior teeth. Then, with the bite plate elevating the occlusion, the lingual spring is used to push the lingually displaced teeth labially to correct the crossbite and further align the anterior teeth.

(2) Edgewise Appliance Correction

1) Bonding bands and brackets, where crowded incisors may hinder bracket placement. The canines are first retracted distally, during which the crowded incisors will also move distally, reducing the crowding.

2) Once the canines have moved distally and sufficient space is available for alignment, brackets are bonded to the upper and lower incisors, and nickel-titanium archwires are used for alignment.

3) With edgewise appliances, due to the use of continuous force and the patient's habitual rest position, even without a bite plate to elevate occlusion, crossbite teeth can still move labially for alignment.

4) While aligning the anterior teeth, intra-arch or inter-arch traction is adjusted based on molar relationships to ensure a neutral molar relationship.

5) When the molar relationship is neutral and the upper and lower anterior teeth are aligned, any remaining space in the arch is closed using elastic thread or continuous elastic chains to ligate the full arch.

Principles of Extraction for Crowding Correction:

1. Before deciding on extraction, dental model measurements and analysis should be conducted, especially through setup tests to determine the space required for alignment and to calculate the Bolton index of the upper and lower teeth.

2. Generally, X-rays should be taken for upper and lower anterior teeth to assess periodontal and root conditions, such as alveolar bone resorption or short-root anomalies. If possible, a panoramic X-ray should also be taken to check for impacted teeth or congenital missing teeth.

3. For anterior crowding, the first premolars are often considered for extraction due to their proximity to the anterior teeth and minimal impact on the full arch (with four pairs of first premolars). However, if there are severely decayed or malformed teeth in the arch, these should be prioritized for extraction.

4. After calculating the space needed to align crowded teeth, the potential loss of anchorage during treatment (leading to mesial movement of anchorage teeth) should be added to the required space. Generally, fixed appliances lose about 2mm of anchorage per side, while removable appliances lose even more.