What are crowded teeth?
When teeth erupt (come through the gum into the mouth), they may twist, stick out, drop back, or overlap if there is not enough space in the mouth. Losing baby teeth early from tooth decay or trauma can lead to crowded permanent teeth. If crowded teeth affect a child's self-esteem or cause pain, damage or chewing problems, the child may be referred to a specialist dentist known as an orthodontist to correct them. Orthodontics is about the growth of the jaws and face, and development of the teeth and bite.
What is orthodontic treatment?
Crowded teeth can be prevented or corrected using braces if crowding is mild (less than 4 mm). Removal of some teeth (extraction) may also be needed if crowding is moderate (4 to 8 mm) or severe (more than 8 mm). Fixed braces are used on permanent teeth. Removable braces can be used on baby or permanent teeth, or both. Baby or permanent teeth can be extracted.
Fixed braces
Fixed braces attach parts to each tooth using dental glue, with brackets holding a wire that puts a force on the teeth, to move and straighten them. The wire is secured with metal ties, small rubber bands or a clip that is built into the bracket ('self-ligating').
A lower lingual arch (LLA) or lip bumper (LB) retains the lower back teeth (molars) while allowing the lower front teeth to straighten and move forwards. To remove pressure on the teeth, a LLA wire lies on the inner side of the teeth; a LB wire lies on the outer side. The 0.9 mm stainless steel wires are attached to metal bands around the back (molar) teeth at either end, with the LB wire having a plastic coating at the front.
Sometimes extra items are used with fixed braces, such as headgear (straps attached to a frame outside the mouth), vibrating plates or lacebacks (thin wires holding teeth together).
Removable braces
Removable braces are usually made from hard plastic that joins together active parts that move the teeth and clips that secure the brace. Some removable braces are made from moulded flexible plastic.
The Schwarz appliance has a screw that is turned once a week by parents, to widen the arch of the lower jaw and make more space into which the permanant teeth can move.
The eruption guidance appliance guides permanent teeth into a better position as they erupt. It is a combined upper and lower brace that holds the lower jaw forwards, and has guiding slots to align the front teeth and improve the side teeth bite.
Extraction
Baby eye teeth (canines) are extracted when children have a mix of adult and baby teeth, to provide space into which other teeth can move.
Wisdom teeth (third molars) may be removed any time from when they form (early teenage years) until adulthood, to help prevent them putting forward pressure on the other teeth.
What did we want to know?
We aimed to evaluate scientific research on the effectiveness of orthodontic treatments (fixed braces, removable braces, tooth extraction) used to prevent or correct crowded teeth in children aged 16 years old or younger. We searched for studies that compared these treatments against no treatment, delayed treatment, placebo (pretend treatment) or another orthodontic treatment.
What studies did we find?
We included 24 studies that presented results from 1314 children aged from 7 to 16 years in different countries. Twenty studies tested fixed braces, two tested removable braces and two tested extractions.
What were the main results?
Fixed braces and related items
A lower lip bumper may prevent crowding when the adult teeth are starting to come through. Nickel-titanium wires may correct crowding better than wires made of copper nickel-titanium, and twisted multi-stranded (co-axial) nickel-titanium archwires may be better than single-stranded ones. However, we cannot be sure of these findings.
For the other comparisons evaluated, it was not possible to show that one group did better or worse than the other for reducing crowding.
Removeable braces and related items
The Schwarz appliance may reduce crowding in the lower arch, when measured at nine months. Use of an eruption guidance appliance, for a year, may reduce likelihood of crowding, but there may be other explanations for this. Again, we cannot be sure of these findings.
Extraction
Taking out wisdom teeth (third molars) does not seem to affect crowding later in life, while taking out the pointy baby teeth (canines) from the lower jaw, may reduce crowding in the short term, but we cannot be sure of this. There are probably other explanations for this finding.
What are the limitations of the evidence?
The evidence is uncertain. It consists of small, individual studies testing different treatments. Some of them have problems with how they were carried out. We cannot be sure about our findings and future research may change them.
How up to date is the evidence?
The evidence is up to date to January 2021.
Most interventions were assessed by single, small studies. We found very low-certainty evidence that lip bumper, used in the mixed dentition, may be effective for preventing crowding in the early permanent dentition, and a Schwarz appliance may reduce crowding in the lower arch. We also found very low-certainty evidence that coaxial NiTi may be better at reducing crowding than single-stranded NiTi, and that NiTi may be better than copper NiTi. As the current evidence is of very low certainty, our findings may change with future research.
Crowded teeth develop when there is not enough space in the jaws into which the teeth can erupt. Crowding can affect baby teeth (deciduous dentititon), adult teeth (permanent dentition), or both, and is a common reason for referral to an orthodontist. Crowded teeth can affect a child's self-esteem and quality of life. Early loss of baby teeth as a result of tooth decay or trauma, can lead to crowded permanent teeth. Crowding tends to increase with age, especially in the lower jaw.
To assess the effects of orthodontic intervention for preventing or correcting crowded teeth in children.
To test the null hypothesis that there are no differences in outcomes between different orthodontic interventions for preventing or correcting crowded teeth in children.
Cochrane Oral Health's Information Specialist searched four bibliographic databases up to 11 January 2021 and used additional search methods to identify published, unpublished and ongoing studies.
We included randomised controlled trials (RCTs) that evaluated any active interventions to prevent or correct dental crowding in children and adolescents, such as orthodontic braces or extractions, compared to no or delayed treatment, placebo treatment or another active intervention. The studies had to include at least 80% of participants aged 16 years and under.
Two review authors, independently and in duplicate, extracted information regarding methods, participants, interventions, outcomes, harms and results. We resolved any disagreements by liaising with a third review author. We used the Cochrane risk of bias tool to assess the risk of bias in the studies. We calculated mean differences (MDs) with 95% confidence intervals (CI) for continuous data and odds ratios (ORs) with 95% CIs for dichotomous data. We undertook meta-analysis when studies of similar comparisons reported comparable outcome measures, using the random-effects model. We used the I2 statistic as a measure of statistical heterogeneity.
Our search identified 24 RCTs that included 1512 participants, 1314 of whom were included in analyses. We assessed 23 studies as being at high risk of bias and one as unclear.
The studies investigated 17 comparisons. Twenty studies evaluated fixed appliances and auxiliaries (lower lingual arch, lower lip bumper, brackets, archwires, lacebacks, headgear and adjunctive vibrational appliances); two studies evaluated removable appliances and auxiliaries (Schwarz appliance, eruption guidance appliance); and two studies evaluated dental extractions (lower deciduous canines or third molars).
The evidence should be interpreted cautiously as it is of very low certainty. Most interventions were evaluated by a single study.
Fixed appliances and auxiliaries
One study found that use of a lip bumper may reduce crowding in the early permanent dentition (MD −4.39 mm, 95% CI −5.07 to −3.71; 34 participants). One study evaluated lower lingual arch but did not measure amount of crowding.
One study concluded that coaxial nickel-titanium (NiTi) archwires may cause more tooth movement in the lower arch than single-stranded NiTi archwires (MD 6.77 mm, 95% CI 5.55 to 7.99; 24 participants). Another study, comparing copper NiTi versus NiTi archwires, found NiTi to be more effective for reducing crowding (MD 0.49 mm, 95% CI 0.35 to 0.63, 66 participants). Single studies did not show evidence of one type of archwire being better than another for Titinol versus Nitinol; nickel-titanium versus stainless steel or multistrand stainless steel; and multistranded stainless steel versus stainless steel.
Nor did single studies find evidence of a difference in amount of crowding between self-ligating and conventional brackets, active and passive self-ligating brackets, lacebacks added to fixed appliances versus fixed appliances alone, or cervical pull headgear versus minor interceptive procedures.
Meta-analysis of two studies showed no evidence that adding vibrational appliances to fixed appliances reduces crowding at 8 to 10 weeks (MD 0.24 mm, 95% CI −0.81 to 1.30; 119 participants).
Removable appliances and auxiliaries
One study found use of the Schwarz appliance may be effective at treating dental crowding in the lower arch (MD -2.14 mm, 95% CI −2.79 to −1.49; 28 participants). Another study found an eruption guidance appliance may reduce the number of children with crowded teeth after one year of treatment (OR 0.19, 95% CI 0.05 to 0.68; 46 participants); however, this may have been due to an increase in lower incisor proclination in the treated group. Whether these gains were maintained in the longer term was not assessed.
Dental extractions
One study found that extracting children's lower deciduous canines had more effect on crowding after one year than no treatment (MD −4.76 mm, 95 CI −6.24 to −3.28; 83 participants), but this was alongside a reduction in arch length. One study found that extracting wisdom teeth did not seem to reduce crowding any more than leaving them in the mouth (MD −0.30 mm, 95% CI −1.30 to 0.70; 77 participants).