Functional Appliances UK: Complete Guide
Functional orthodontic appliances are used in growing children and teenagers to harness jaw growth — correcting skeletal jaw discrepancies that cannot be addressed by braces alone. This guide explains the main types used in the UK (twin block, Herbst, palate expander), how they work, what they treat, and what to expect from treatment.
What are functional appliances?
Functional appliances are orthodontic devices designed to modify jaw growth in children whose jaws are still developing. Unlike braces, which move individual teeth within the existing jaw structure, functional appliances work by repositioning the lower jaw and stimulating bone remodelling — changing the relationship between the upper and lower jaws.
They are most effective during the pubertal growth spurt — typically ages 10–14 — when jaw growth is most active. Used at the right time, they can reduce or eliminate skeletal discrepancies that would otherwise require jaw surgery in adulthood.
Types of functional appliance used in the UK
Twin Block Brace
Removable (two interlocking acrylic blocks)
Class II malocclusion — protruding upper teeth and receding lower jaw (overjet, overbite)
10–14 years (during active growth)
9–18 months functional phase
Yes — if IOTN eligible
The twin block is the most commonly used functional appliance in the UK. It consists of two removable plastic appliances — one for the upper and one for the lower arch — with interlocking bite blocks that hold the lower jaw in a forward position. When worn full-time, this forward posture stimulates the lower jaw to grow forward and the upper jaw to be restrained slightly. It is comfortable once adapted to and is highly effective when worn as instructed during the growth spurt.
Herbst Appliance
Fixed (metal rods connecting upper and lower jaws)
Class II malocclusion — similar indications to twin block but for patients where compliance with removable appliances is a concern
10–16 years
9–12 months
Yes — if IOTN eligible
The Herbst appliance is a fixed version of functional jaw positioning — metal telescoping rods are cemented to upper and lower molars, holding the lower jaw in a permanently advanced position 24 hours a day. Because it is fixed, it does not rely on patient compliance. It is bulkier and less comfortable than a twin block initially, and requires more careful eating and oral hygiene, but produces reliable results regardless of patient cooperation.
Rapid Maxillary Expander (Palate Expander / RME)
Fixed (cemented to upper molars with a central screw)
Narrow upper arch, posterior crossbite, mild to moderate crowding in growing children
8–14 years (before mid-palatal suture fuses)
3–9 months expansion, then retention period
Yes — if IOTN eligible
A palate expander is a fixed appliance cemented to the upper back teeth. A central screw is activated by turning with a key — usually once or twice daily — which gradually separates the two halves of the palate, widening the upper jaw. This creates space for crowded teeth and corrects a narrow upper arch. After the expansion phase, the expander is left in place for a retention period to allow new bone to fill in the gap. Only effective in children while the mid-palatal suture remains open — typically before age 14–16.
Removable Expansion Plate
Removable (acrylic plate with expansion screw and wire clasps)
Mild upper arch narrowing, minor crowding, crossbite in growing children
7–13 years
6–18 months
Yes — if IOTN eligible
A removable expansion plate is a traditional NHS appliance — an acrylic plate fitting the roof of the mouth, held in place by wire clasps around the back teeth, with an expansion screw that is turned periodically to widen the arch. Less precise and slower than a fixed RME, but effective for mild cases. Relies entirely on patient compliance — must be worn 22+ hours per day and can be lost or damaged easily.
Forsus Fatigue Resistant Device
Fixed (spring mechanism attached to fixed braces)
Class II malocclusion — used during fixed brace treatment to correct overjet without elastics
12–18 years (during fixed brace phase)
3–6 months
Yes — if IOTN eligible
The Forsus appliance is used during fixed brace treatment to advance the lower jaw and reduce overjet. It consists of a spring mechanism attached between the upper and lower brace brackets, working continuously without elastic compliance. Less visible than a twin block and does not require a separate functional phase — the jaw correction happens simultaneously with tooth alignment.
How much do functional appliances cost in the UK?
| Appliance | NHS cost | Private cost (est.) |
|---|---|---|
| Twin block brace | Free for under-18s (if IOTN eligible) | £1,500–£3,000 (Phase 1 only) |
| Fixed palate expander (RME) | Free for under-18s (if IOTN eligible) | £800–£2,000 |
| Removable expansion plate | Free for under-18s (if IOTN eligible) | £500–£1,500 |
| Herbst appliance | Free for under-18s (if IOTN eligible) | £1,500–£3,000 |
| Full Phase 1 + Phase 2 (functional + braces) | Free for under-18s (NHS covers complete course) | £3,500–£7,000 |
On the NHS, orthodontic treatment is free for under-18s, who are exempt from dental charges, and one course of treatment covers any functional phase and the fixed brace phase that follows. Private costs are typically quoted per phase, and the total two-phase treatment cost can be substantial.
What to expect from functional appliance treatment
Speech changes initially: Removable functional appliances, especially twin blocks, affect speech significantly in the first 2–4 weeks. Reading aloud helps adaptation. Most patients adapt within a month.
Compliance is critical: Removable appliances only work if worn as instructed — typically 22 hours per day. Part-time wear slows or prevents the desired jaw changes. Fixed functional appliances (Herbst, Forsus) remove this compliance issue.
Jaw discomfort is normal: The appliance repositions the lower jaw, which can cause aching jaw muscles, particularly in the first weeks. This is normal and settles.
Duration is typically 9–18 months: Functional phase treatment typically takes 9–18 months. This is usually followed by a fixed brace phase to detail tooth alignment.
Results depend on growth timing: The same appliance worn by a child in the middle of their growth spurt will achieve more than one worn too early or too late. Your orthodontist will assess skeletal maturity (often using a hand/wrist X-ray or cervical vertebrae staging) to time treatment appropriately.
Frequently asked questions
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