Key topics

• Model analysis
• Treatment strategies
• Expansion of dental arches to gain space
• Reduction of tooth material by extraction or enamel reduction of teeth

Learning objectives

• To be able, in a model analysis, to evaluate the arch length, arch width and form, amount of crowding and size of the apical base
• To understand treatment strategies for crowding of teeth in the primary, mixed and permanent dentition
• To know when and how to expand the dental arches to solve crowding of teeth
• To know when and how to solve crowding with extractions
• To know when and how to assess enamel reduction to solve crowding of teeth

Introduction

Crowding of teeth is one of the most common malocclusions with a frequency of about 25%. Crowding of teeth in the jaw is often generalised but can be local in the anterior or posterior region of the jaw, and then with the latest erupted teeth; for instance, the maxillary canine will be buccally displaced or a second premolar will be dislocated lingually in the dental arch. Moreover, premature loss of deciduous molars and the resulting mesial migration of permanent molars will develop crowding in the premolar and canine region.

Crowding of teeth is more frequent in the mandible than in the maxilla, and crowding in the mandible is often combined with deep bite. The need for treatment is often owed to aesthetic reasons, and evidence exists that untreated crowded anterior teeth will result in negative effects on oral-health-related quality of life.

Minor crowding of teeth is considered a normal condition; particularly, in the mandibular incisor region, minor crowding is found in almost all individuals and this crowding increases with age.

In treatment planning for crowding of teeth, there are several factors to consider:

• the severity of crowding;
• the form of the alveolar arches, including the apical bases;
• whether it is possible to expand the dental arch to gain space or whether reduction of tooth material should be performed;
• the skeletal relationships of the face;
• the remaining facial growth;
• the shape of the lips and the lip-seal;
• hereditary factors (small dental arches versus large-sized teeth).

Model analysis

To more precisely evaluate the extent of crowding, arch form and the apical bases of the jaws, it is recommended to take study casts of the jaws and make a model analysis. On the study casts, it is easy with the aid of a sliding calliper to measure arch length, arch width, amount of crowding of teeth, overjet and overbite and rotation of teeth (Figure 6.1). The model analysis also includes determination of the size of the apical base; dental arch form; the sagittal, vertical and transversal relation between the jaws; and the relation between the midlines in the maxilla and the mandible.

Over all, the crowding of teeth is closely related to the size of the teeth and the adequacy of bony support for the teeth in the jaws. The volume of the alveolar bone that is on a level with the root apices of the teeth is called the apical base. When the alveolar process of the jaw is undersized, and thereby does not have enough space for the roots of the teeth, the disproportion is called a small apical base, which often results in proclined incisors and crowding of the anterior teeth (Figure 6.2a). The opposite is a large apical base (Figure 6.2b), which means that the alveolar process of the jaw is oversized, resulting in vertical positions of the teeth and spacing (Lundström, 1923).

Crowding of teeth in the anterior part of the jaws can be evaluated by using Little’s irregularity index, whereby the distances between the contact points on adjacent teeth from canine to canine are measured (Little, 1975).

It must be emphasised, however, that in addition to the extent of crowding of teeth, arch form and the apical base, the face in profile, skeletal relationships of the face, the shape of the lips and lip seal, as well as remaining growth, must also be considered in treatment planning of crowding.

Orthodontic appliances

In the permanent dentition, both extraction and non-extraction treatments, require treatment with fixed appliances, multibracket techniques, with the brackets on the labial surfaces or on the lingual surfaces to achieve derotated teeth, parallel roots, Class I occlusion and a stable occlusion.

A great number of fixed appliance systems are on the market, as well as several different anchorage systems. Temporary anchorage devices (TADs), such as mini screws and fixed anchorage plates, were developed at the end of the 20th century. Furthermore, intermaxillary elastics such as Class II or Class III elastics may be necessary to attain stable occlusion and a good aesthetic result. The use of different appliances is described in detail in other textbooks, for example, multibracket techniques by Bennett and McLaughlin (2014).

Sometimes removable appliances such as the Invisalign technique can be used. This technique uses a series of very thin aligners. The practically invisible aligners are individually manufactured and are changed every 2 to 3 weeks to the next set of aligners. The Invisalign treatment is performed during 1 to 2 years, and is mainly recommended for adults or non-growing patients.

Treatment strategies

Primary dentition – mixed dentition

Treatment of crowding in deciduous dentition is seldom indicated. In mixed dentition, slight anterior crowding (2–3 mm) (Figure 6.3) at the time of eruption of the incisors can be followed without treatment, since self-correction of the crowding occurs. The normal growth of the jaws and increased arch length due to proclined eruption direction of the permanent incisors as well as increased intercanine distance contributes to the self-correction of crowding.

When moderate anterior crowding (3–4 mm) exists (Figure 6.4