Diagnosis & Considerations
Tooth-jaw size discrepancy (Primary Crowding)
There is a discrepancy between the permanent teeth size and the size of the jaw/arch. Teeth size are usually too large relative to the jaw size. This is often genetically determined. (Rodrigues et al. 2011)
Loss of arch length (Secondary Crowding)
Secondary crowding is caused by environmental factors. This can be the premature loss of primary molars or changes in the tongue, lip or buccal musculatures. For example, the early extraction of primary molars due to caries can lead to mesial drifting of the permanent molars and this will cause a loss of leeway space. All of the above factors lead to crowding by the reduction of arch length. (Cobourne and DiBiase 2010, Rodrigues et al. 2011)
Late incisor crowding (Tertiary Crowding)
Tertiary crowding occurs in the lower incisor segment in late adolescence and into adulthood. It develops because of a net lingual and constricting force on the lower incisors over a long period of time. Factors which cause this are: mandibular growth rotations, anterior occlusal force, lip pressure, mesial drifting, PDL changes, diet and a lack of interproximal wear (Cobourne and DiBiase 2010, Rodrigues et al. 2011). The contribution of third molars is now known to be minimal and their prophylactic removal to prevent tertiary crowding is not recommended (Cobourne and DiBiase 2010).
Clinical Considerations
Soft tissue equilibrium
The dental arch and tooth position are defined by the resting soft tissue pressure. It is usually at an equilibrium between the lips and cheeks on one side and the tongue on the other (Proffit 1978). Alterations of the soft tissue form, such as hyperactive lip muscles can lead to displacement of teeth lingually – i.e. Class II div 2 incisors are retroclined and crowded due to stronger lip muscles.
Do wisdom teeth cause crowding?
Many patients ask this question as the eruption or pain of their wisdom teeth occurs at a similar time to the onset of late incisor crowding (tertiary crowding). Wisdom teeth eruption can cause posterior crowding (crowding around the molars), however it has been shown to not cause incisor crowding in the majority of cases (Cobourne and DiBiase 2010, Proffit et al. 2013). Wisdom teeth prophylactic extractions for prevention of incisor crowding is not evidence-based or recommended practice (Cobourne and DiBase 2010).
Late incisor crowding can occur regardless of whether there are wisdom teeth present (Sinclair and Little 1985). Factors which influence late incisor crowding include mandibular growth rotation, anterior occlusal force, mesial drifing of teeth and changes in soft tissue forces.
Transient crowding?
Some crowding is transient during the mixed dentition phase and will resolve on its own. Such can be said about mild crowding during the early mixed dentition phase when the permanent incisors are larger than the size of the jaw at that point of growth. The jaws will continue to grow whilst the tooth dimensions do not, hence mild crowding can resolve on its own. Other transient crowding examples are the displacement of upper incisors as the upper canines are erupting through its osseous pathway next to the lateral incisor roots – traditionally known as the “ugly duckling” phase. The displacement of the incisors tend to resolve as the upper canines erupt into the oral cavity.
Treatment timing
Treatment timing for crowding can be developed into early treatment in the mid to late mixed dentition (approx. 9 to 11 years) and comprehensive treatment in the full permanent dentition (approx. 12 years and over). See treatment indications for more information.
