--- title: "Functional Appliances UK 2026 | Twin Block & Expanders" url: https://virtualconsultant.online/guides/functional-appliances-uk description: "Functional appliances such as twin blocks and Herbst appliances use jaw growth in children to correct bite problems braces cannot. UK types, NHS and costs." updated: 2026-09-18T00:00:00Z publisher: Virtual Consultant Online --- # 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. [Children's Orthodontics](https://virtualconsultant.online/guides/childrens-orthodontics-uk) [Get Assessed](https://virtualconsultant.online/evaluations/quote) ## 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. Functional appliances do not work in adults — once jaw growth is complete (typically late teens), the skeletal changes they produce in growing patients are no longer possible. This is why timing of treatment is critical. ## Types of functional appliance used in the UK Twin Block Brace Type Removable (two interlocking acrylic blocks) Treats Class II malocclusion — protruding upper teeth and receding lower jaw (overjet, overbite) Age range 10–14 years (during active growth) Duration 9–18 months functional phase NHS available 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 Type Fixed (metal rods connecting upper and lower jaws) Treats Class II malocclusion — similar indications to twin block but for patients where compliance with removable appliances is a concern Age range 10–16 years Duration 9–12 months NHS available 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) Type Fixed (cemented to upper molars with a central screw) Treats Narrow upper arch, posterior crossbite, mild to moderate crowding in growing children Age range 8–14 years (before mid-palatal suture fuses) Duration 3–9 months expansion, then retention period NHS available 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 Type Removable (acrylic plate with expansion screw and wire clasps) Treats Mild upper arch narrowing, minor crowding, crossbite in growing children Age range 7–13 years Duration 6–18 months NHS available 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 Type Fixed (spring mechanism attached to fixed braces) Treats Class II malocclusion — used during fixed brace treatment to correct overjet without elastics Age range 12–18 years (during fixed brace phase) Duration 3–6 months NHS available 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 What is a twin block brace and does it hurt? A twin block is a removable functional appliance consisting of two interlocking acrylic plates — one for the upper arch, one for the lower — that hold the lower jaw in a forward position to stimulate jaw growth. In the first 1–2 weeks, patients typically experience soreness in the jaw muscles (similar to the ache from chewing for a long time), difficulty speaking clearly, and increased saliva production. These effects settle as the muscles and jaw adapt. Most patients find the twin block very comfortable after the initial adjustment period. How long does a child need to wear a twin block brace? A twin block should be worn for 22 hours per day — all the time except during contact sports and swimming. Wear time is critical: a twin block worn for only 12–14 hours per day may produce little or no skeletal change. The active functional phase typically lasts 9–18 months, after which the overjet is corrected and fixed braces are usually placed to align the individual teeth. Total treatment time including fixed braces is often 2–3 years. Can adults have a palate expander? In skeletally mature adults (typically over 17–18), the mid-palatal suture has fused and a conventional palate expander will not widen the bone — it will only tip the teeth outward. For adults who need genuine palatal expansion, surgically assisted rapid palatal expansion (SARPE) — where the suture is surgically released under general anaesthetic before the expander is activated — is required. This is a specialist hospital procedure. For most adults, alternative approaches to creating arch space (IPR, extractions, or accepting narrower arch form) are considered first. Will my child need braces after a functional appliance? In most cases, yes. Functional appliances correct the skeletal jaw relationship but do not detail tooth alignment. After the functional phase, fixed braces are usually placed to align teeth precisely within the corrected jaw framework. This is called a two-phase treatment: Phase 1 (functional) and Phase 2 (fixed braces). The good news is that by correcting the jaw relationship first, the subsequent fixed brace phase is often simpler and shorter than it would have been with braces alone. Is a functional appliance available on the NHS? Yes — functional appliances are part of NHS orthodontic treatment for eligible patients under 18 with a sufficient IOTN score. If a child's overjet, jaw discrepancy, or crossbite means they meet the IOTN threshold, the NHS should fund the full course of treatment including any functional phase and subsequent fixed braces. Under-18s are exempt from NHS dental charges, so the whole course is free regardless of how many phases or appliances are involved. What happens if a functional appliance is not worn enough? The correction simply does not happen, and unlike fixed braces there is no partial credit. A functional appliance works by holding the lower jaw forward, and it only does that while it is in the mouth, so inconsistent wear does not slow progress proportionally, it can stall it entirely. Because these appliances rely on a growth spurt that does not wait, months lost to poor wear can mean the window closes and the option is gone, leaving camouflage treatment or surgery later. This is why orthodontists are direct about compliance before starting rather than afterwards. ## Has your child been assessed? An online assessment can indicate whether functional treatment might be appropriate and what to expect from an orthodontic consultation. [Start Assessment](https://virtualconsultant.online/evaluations/quote) [More Guides](https://virtualconsultant.online/guides) ## Related guides [Children's orthodontics UK](https://virtualconsultant.online/guides/childrens-orthodontics-uk) [IOTN eligibility explained](https://virtualconsultant.online/guides/iotn-score-explained-uk) [How to get NHS braces](https://virtualconsultant.online/guides/how-to-get-nhs-braces) [All brace types compared](https://virtualconsultant.online/guides/types-of-braces-uk) [Jaw surgery and orthodontics](https://virtualconsultant.online/guides/jaw-surgery-orthodontics-uk) Citation details Title Functional Appliances UK: Twin Block, Herbst & Palate Expanders — Guide 2026 Publisher Virtual Consultant Online Published 2024-06-01 Updated 2026-09-18 URL [https://staging.virtualconsultant.online/guides/functional-appliances-uk](https://staging.virtualconsultant.online/guides/functional-appliances-uk)