Crooked teeth: what causes crowding, why teeth shift with age and how they can be straightened

What are crooked teeth, and what is crowding?

Crooked teeth is an everyday term for teeth that are rotated, sit in front of the arch or overlap. The most common cause is dental crowding, meaning too little space in the arch. The American Association of Orthodontists (AAO) defines crowding as insufficient space for the teeth.

The American Academy of Pediatric Dentistry (AAPD) describes it as a discrepancy between the space needed to align teeth in the arch and the space available. The opposite of crowding is excess space, or gaps between the teeth. A mismatch in size between upper and lower teeth can be a separate problem.

Crooked front teeth are the most visible, but crowding can affect any part of the arch and often accompanies other abnormalities. A separate article on malocclusion explains the classification of bite problems, such as Class II and Class III malocclusion.

What causes crooked teeth?

Crooked teeth most often result from a mismatch between tooth size and arch size. The AAO lists large teeth, baby teeth lost too early or too late, and insufficient space in the jaws among the causes of crowding.

A 2021 Cochrane review notes that early loss of baby teeth through decay or injury can lead to crowding of the permanent teeth. The AAPD adds that premature loss of baby teeth can shorten the arch, which is why guidelines prioritise treating decayed baby teeth whenever possible.

The AAPD also points out that studies find shorter arches, more frequent incisor crowding and slightly larger teeth in children today than in children measured decades ago. In children with a mixture of baby and permanent teeth, however, incisor crowding can be temporary, as discussed in one of the questions at the end of this article.

Crooked lower front teeth: why do teeth shift with age?

Crooked lower central incisors in adults are a common sign of late crowding, which increases with age. A 2021 Cochrane review states that crowding usually worsens over the years, especially in the lower jaw, and the AAPD says skeletal ageing leads to further crowding and rotation of the incisors.

This also affects people who have had orthodontic treatment. The AAPD notes that lower incisors are highly prone to recurrent rotation and crowding. In a follow-up study by Little et al. (1988), only 10% of patients had acceptable lower incisor alignment 20 years after retention ended.

In adults with periodontitis, teeth can move for a different reason. A review by Brunsvold (2005) reports that pathological tooth migration affects 30.03 to 55.8% of patients with periodontal disease, with bone loss around the roots playing the main role. A new gap or protruding front teeth in an adult is a reason to have the gums checked.

How common are crooked teeth?

Dental crowding is among the most common orthodontic abnormalities in Polish children. A study of 653 children aged 9 to 12 in Gdańsk and Sopot (Kalinowska et al., 2025) found crowding in 37.98%, while a study of 738 adolescents aged 12 and 13 in West Pomerania (Milona et al., 2025) found it in 19.51%.

Both studies were regional and used different definitions, so their results should not be compared directly. A meta-analysis of 77 studies from around the world (Lombardo et al., 2020) indicates that crowding becomes more common in the permanent dentition than at earlier stages.

Prevalence does not in itself mean treatment is needed. In the Gdańsk study, half of the children wearing a removable appliance (50.26%) had no or minimal treatment need according to the IOTN index. The authors stress that not every diagnosed malocclusion requires an appliance.

What problems can crooked teeth cause?

Crooked teeth rarely cause direct harm to health, but can make cleaning harder and affect wellbeing. The AAO points out that overlapping teeth are more difficult to clean thoroughly.

The link with gum disease is weaker than is often assumed. A systematic review by Bollen (2008) found an association between malocclusion and periodontal disease, but the studies were of poor quality. It also noted that an association does not prove causation; the findings do not justify orthodontic treatment solely to prevent gum problems.

The effect on quality of life is better documented. A systematic review by Dimberg et al. (2015) found strong evidence that malocclusion, especially in the front teeth, impairs oral health-related quality of life in children and adolescents, mainly in emotional and social aspects.

Crooked teeth are not, however, a proven cause of temporomandibular joint pain. The US National Institute of Dental and Craniofacial Research (NIDCR) says current evidence does not support a strong link between malocclusion and these symptoms.

How are crooked teeth straightened?

Crooked teeth are straightened with fixed braces or aligners. Depending on the case, space for alignment is created by reducing enamel, widening the arch or extracting a tooth. The orthodontist measures the degree of crowding on models or scans. One example is Little’s irregularity index, the sum of five displacements between adjacent contact points of the lower front teeth, from canine to canine.

The British Orthodontic Society (BOS) considers aligners very effective when only mildly irregular teeth need aligning, and a review of 18 systematic reviews (Yassir et al., 2022) rated them a good option for mild and moderate problems. In a study of 2 184 patients (Jackson et al., 2017), tooth extraction became more likely as crowding increased.

In a review of 8 studies (Gómez-Aguirre et al., 2022), interproximal enamel reduction did not increase decay or sensitivity. The AAPD, meanwhile, cautions against extracting teeth to relieve crowding without a full space analysis and treatment plan. The table is illustrative; the orthodontist uses measurements to determine the boundaries between degrees of crowding.

Crooked teeth in adults: is it too late to straighten them?

There is no upper age limit for straightening crooked teeth in adults: the AAO says adults can start treatment at any time. A meta-analysis by Abbing et al. (2020) found no significant difference in fixed brace treatment duration between adults and adolescents.

In adults, however, planning starts with the gums. European Federation of Periodontology (EFP) guidelines for advanced periodontitis recommend orthodontic treatment only after the goals of periodontal therapy have been reached, followed by lifelong retention. The article on braces for adults gives more detail.

For mild and moderate crowding, a review of 10 studies (Alhamwi et al., 2024) found no difference in treatment duration between aligners and fixed braces, with low-certainty evidence. Typical adult orthodontic treatment is private, as the standard orthodontic package of the NFZ, Poland’s National Health Fund, covers removable appliances for children up to their 12th birthday. One exception may be patients over 18 continuing treatment begun in the orthodontic care programme for children with congenital craniofacial defects, if they meet its criteria (legal status as of 24 October 2026).

Can teeth become crooked again after braces?

Teeth can become crooked again after braces if retention is stopped, with the risk particularly affecting the lower incisors. In a follow-up study by Little et al. (1981), two-thirds of patients treated with premolar extractions had unsatisfactory lower front tooth alignment at least 10 years after retention ended.

These data come from people who stopped wearing a retainer and show why recommendations have shifted towards long-term retention. A 2016 Cochrane review concludes that almost everyone needs some form of retention after orthodontic treatment. The article on retainers after braces describes the different types.

How crooked teeth are assessed at DentalBlue

At DentalBlue, crooked teeth are assessed at an orthodontic consultation, which costs 300 zł in the clinic’s price list. Panoramic X-rays, cephalometric X-rays and, when indicated, CBCT scans are taken in the clinic’s own radiology unit. Treatment with fixed braces or Clear Aligner trays is planned digitally using cephalometric analysis.

“Plan leczenia z analizą cefalometryczną” (treatment plan with cephalometric analysis) costs 380 zł, and “Skany diagnostyczne” (diagnostic scans) 300 zł. The final scope and cost are agreed after diagnostic assessment, and prices may change.