Posterior Crossbite Treatment UK

What is a posterior crossbite?

A posterior crossbite exists when one or more upper premolar or molar teeth sit inside their corresponding lower teeth when the jaws are closed — the opposite of the normal relationship where upper back teeth overlap the outside of the lower teeth. In a normal bite, the upper arch is slightly wider than the lower arch, and the upper back teeth cover the outside of the lower back teeth. In a posterior crossbite, the reverse is true: the upper arch is narrow relative to the lower, causing the upper teeth to bite into the inside of the lower arch. This can affect one side (unilateral) or both sides (bilateral), and may involve a single tooth or multiple teeth across a section of the arch.

Unilateral vs bilateral posterior crossbite

A bilateral posterior crossbite affects both sides of the mouth equally and is typically caused by a genuinely narrow upper arch. Treatment involves symmetric expansion of the upper arch to widen it proportionally on both sides. A unilateral posterior crossbite — where only one side is in crossbite — is more commonly the result of a functional shift rather than a true unilateral skeletal asymmetry. In a functional shift, the upper arch is slightly narrow on both sides, but as the patient closes, the jaw deflects laterally to find a more comfortable biting position, making the crossbite appear one-sided. Distinguishing between a true unilateral skeletal crossbite and a functional shift is an important part of the orthodontic assessment because the treatment approach and the prognosis differ significantly.

Functional shift and mandibular deviation

When a posterior crossbite is accompanied by a functional shift, the mandible habitually deviates to one side on closing. Over time, asymmetric loading of the temporomandibular joints (TMJ) can contribute to facial asymmetry, differential condylar growth, and TMJ loading on the deviated side. Early correction of the crossbite — ideally before or during the pubertal growth phase — can reduce or eliminate the functional shift and allow the mandible to close in a more symmetric, centric relationship. If a functional shift is left untreated into adulthood, the resulting facial asymmetry may become skeletal and require orthognathic surgery to correct fully. This is one of the primary reasons orthodontists recommend early assessment and treatment of posterior crossbites in children.

Why early intervention before growth completion is most effective

The midpalatal suture remains cartilaginous and responsive to orthodontic expansion forces until approximately the mid-teens in girls and slightly later in boys. In growing patients, expansion appliances can achieve true skeletal arch widening by gradually separating the two halves of the palate. Teeth move through suture separation rather than by tipping, producing a stable skeletal result. The evidence base consistently supports early intervention — ideally between ages 7 and 14 — as producing the most stable outcomes with the least relapse risk. Correction before growth completion also allows functional shift correction early enough to minimise the risk of secondary facial asymmetry. Waiting until growth is complete does not preclude treatment but significantly complicates it.

Treatment options — quad helix fixed expander

The quad helix is a fixed palatal appliance consisting of four helical wire coils soldered to bands fitted around the upper first molars. It delivers a continuous gentle expansion force across the upper arch without the need for patient activation. The appliance is cemented by the orthodontist and adjusted at recall appointments by bending the helical coils to increase or redirect expansion force. The quad helix is well-suited to mild to moderate arch width deficiencies and functional crossbites. It is particularly useful where patient compliance is a concern because it is fully fixed and cannot be removed or forgotten. Treatment typically takes six to twelve months of active expansion followed by a retention phase.

Treatment options — Hyrax and rapid palatal expander

The Hyrax is a tooth-borne rapid palatal expander (RPE) consisting of a central expansion screw connected to bands on the upper first molars and often the premolars. Parents or patients activate the appliance at home by inserting a small key and turning the screw a specified number of quarter-turns per day — each quarter-turn delivers approximately 0.25mm of expansion. This rapid activation rate is intended to separate the midpalatal suture faster than it can calcify, producing skeletal rather than purely dental widening. Hyrax expansion is particularly effective in growing patients aged 8 to 14 and is commonly used for moderate to severe arch width deficiencies. A diastema opens between the upper front teeth during expansion — this is a positive sign that skeletal separation is occurring — and typically closes spontaneously or with orthodontic treatment once expansion is complete.

URA with coffin spring or expansion screw

For milder cases, a removable upper arch appliance (URA) incorporating a midline expansion screw or coffin spring can achieve limited arch widening. These appliances are less powerful than fixed expanders and rely entirely on consistent patient wear — typically 22 or more hours per day. The coffin spring is a U-shaped palatal wire that delivers gentle lateral forces when activated. Removable appliances are often used in younger children as a first-phase treatment before fixed appliances. They are less suitable for significant arch width deficiencies or where compliance is likely to be inconsistent. Some orthodontists use removable expanders as interim appliances while waiting for enough permanent teeth to erupt to support a fixed expander.

How activations work and what to expect

Whichever expander is used, the activation protocol involves applying gradual incremental forces over weeks to months. Fixed appliances such as the quad helix are adjusted by the orthodontist at appointments. Removable or Hyrax-type appliances are activated by the patient at home following written instructions provided at the fitting appointment. Patients and parents should expect mild discomfort and a sensation of pressure in the roof of the mouth and around the upper teeth for one to two days following each activation. Eating and speaking may feel slightly different during the active expansion phase. A gap between the upper front teeth during expansion with a Hyrax is normal and expected. Most patients and parents adapt quickly to the routine of activation.

Retention with transpalatal arch

Following active expansion, the palate needs time to consolidate — new bone must fill the expanded suture before the appliance is removed. In most cases, the active expansion appliance is left in place passively for three to six months. Once removed, a transpalatal arch (TPA) — a rigid wire connecting the upper first molars across the palate — is usually fitted to maintain arch width. The TPA is typically worn for six to twelve months after the expander is removed. In younger patients where comprehensive fixed appliance treatment follows, the TPA may be replaced by the molar bands used for full orthodontic treatment. Long-term retention of arch width is also supported by bonded retainers and removable retainers following completion of all orthodontic treatment.

Adult correction and NHS eligibility

Correction of posterior crossbite in adults requires different techniques because the midpalatal suture is fused (see SARPE and MSE options). Fixed appliances with TAD anchorage can achieve limited dental compensation, and surgical expansion remains the gold standard for skeletal correction in adults. NHS eligibility for posterior crossbite treatment in growing children depends on the IOTN assessment, but functionally significant crossbites with associated shift typically score at IOTN grade 4 or 5 and are usually eligible for NHS treatment. Adults receive no NHS orthodontic treatment for crossbite correction and must fund treatment privately. Early referral — around age 7 to 8 at the first sign of a developing crossbite — gives the best chance of NHS treatment and optimal clinical outcomes.

Useful related pages

This guide is for general information only. It is not a diagnosis, treatment plan, or substitute for advice from a registered dentist or orthodontist.