# Functional Appliances Explained: What They Are and Who Needs Them Publisher: Virtual Consultant Online Author: Virtual Consultant Online Published: 2026-07-03 Updated: 0001-01-01 Canonical URL: https://virtualconsultant.online/blog/functional-appliances-explained Citation metadata: https://virtualconsultant.online/citation.json?url=https%3A%2F%2Fvirtualconsultant.online%2Fblog%2Ffunctional-appliances-explained Most people are familiar with braces — the brackets and wires that move teeth gradually into alignment. But there is a category of orthodontic appliance that works quite differently, not by moving teeth but by influencing the position and growth of the jaws themselves. These are functional appliances, and for the right patient at the right stage of development, they can achieve things that braces alone cannot. This guide explains what functional appliances are , how they work, the most common types used in the UK, who needs one and at what age, and what treatment with a functional appliance looks and feels like in practice. Key takeaway A functional appliance is an orthodontic device — usually removable but sometimes fixed — that works by positioning the lower jaw forward, stimulating changes in the jaw bones and joints during the growth period. It is used primarily for children and young teenagers with a significant overjet (protruding upper front teeth) caused by a retrusive (set-back) lower jaw. Functional appliance treatment typically precedes a phase of fixed braces and is only effective while the patient is still growing — it cannot reposition the jaw in adults. The most commonly used types in the UK are the Twin Block, the Clark Twin Block, and the Herbst appliance. ## What Is a Functional Appliance and How Does It Work? A functional appliance works by holding the lower jaw in a more forward (protruded) position than its natural resting position. By maintaining this forward posture over months, the appliance stimulates adaptive changes in the temporomandibular joint (the jaw joint), the muscles around the jaw, and — during active growth — the bone of the lower jaw itself. The mechanism is orthopaedic rather than orthodontic: it is influencing skeletal growth and jaw position rather than simply moving teeth through bone. This is why functional appliances are most effective when used during the period of active skeletal growth — around the pubertal growth spurt, typically between ages 10 and 14 in girls and 11 and 15 in boys. The timing is important: the same appliance in a patient who has finished growing will produce minimal skeletal effect and may only move teeth. ## Who Needs a Functional Appliance? The primary indication for a functional appliance is a Class II skeletal malocclusion — a jaw relationship where the upper jaw is positioned in front of the lower jaw, creating a retruded (set-back) lower face profile and a large overjet (the horizontal distance between the upper and lower front teeth). This is commonly described as an "overbite" by patients, though the clinical term for the horizontal protrusion is overjet. Functional appliances are typically recommended when: There is a significant overjet (typically 7mm or more) driven by a retrusive lower jaw The patient is in or approaching their pubertal growth spurt — the window of maximum skeletal responsiveness A skeletal (jaw-based) rather than purely dental (tooth-based) cause underlies the overjet The clinical aim is to redirect jaw growth to reduce the need for extractions or jaw surgery later An important additional reason to treat a significant overjet early — particularly with protrusive upper front teeth — is protection from trauma. Upper front teeth that protrude significantly are more vulnerable to injury from falls and impacts. The British Orthodontic Society notes this as a clinical justification for early treatment of large overjets, independent of the jaw growth benefit. 9–12 months Typical duration of functional appliance treatment before fixed braces begin Pubertal spurt The growth window during which functional appliances are most effective — timing treatment to coincide with this is key Phase 1 Functional appliance treatment is usually Phase 1 — fixed braces follow as Phase 2 to align the teeth ## Types of Functional Appliance Twin Block The most widely used functional appliance in the UK Two separate removable acrylic plates — one for the upper jaw, one for the lower — with interlocking ramps that hold the lower jaw forward Designed by Dr William Clark in the 1970s; now the standard in most UK NHS and private practices Worn full-time including during eating; removed for contact sport and cleaning Herbst Appliance A fixed functional appliance — cemented to the teeth Metal telescoping arms connect the upper and lower teeth and hold the lower jaw forward continuously Effective for patients unlikely to comply with a removable appliance Cannot be removed — all cleaning done in situ Has your child been referred for a functional appliance? Get a second view 😀 A free virtual assessment helps you understand whether the recommendation is appropriate and what to expect from treatment. Start free assessment → ## What Does Wearing a Functional Appliance Feel Like? Most children and teenagers find the Twin Block appliance significantly more manageable than they expect, though the first few days involve an adjustment period. Speech is commonly affected initially — particularly sounds requiring the tongue and front teeth — and most patients sound slightly different for the first one to two weeks until the tongue adapts. Eating with the appliance in place takes practice, particularly for the first few days. The most common reason functional appliance treatment fails or takes longer than expected is poor compliance. The Twin Block only works while it is in the mouth — hours it is sitting in a glass of water are hours it is not doing anything. Most orthodontists recommend wearing it for a minimum of 22 hours per day, removing it only for contact sport, swimming, and cleaning. A common question from parents is whether functional appliances are painful. They are not painful in the same way as fixed braces — there is no wire pressure. The jaw muscles may feel fatigued or ache initially as they adapt to the new jaw position, and the joints may feel different. These sensations typically settle within two to four weeks. ## What Happens After the Functional Appliance Phase? Functional appliance treatment is almost always Phase 1 of a two-phase treatment plan. After the overjet has been reduced and the jaw relationship improved, fixed braces are fitted to align the teeth into their final positions. Phase 2 (fixed braces) typically takes 12 to 18 months and produces the final result. Some patients ask whether they could skip the functional appliance and just have braces. In mild cases, this may be an option — the overjet can be corrected using elastics worn with fixed braces. In significant skeletal cases, however, skipping the functional phase typically means either extraction of teeth to compensate for the jaw discrepancy or a future risk of relapse, as the underlying skeletal mismatch has not been addressed. Important Functional appliances are effective only during the growth period. The sooner treatment begins in relation to the growth spurt, the more skeletal change can be achieved. A child referred for functional appliance treatment who waits a year or two before starting may miss the window for significant jaw modification, meaning the same overjet may require extractions and longer treatment later. If your child has been told they need a functional appliance, acting promptly on the referral is clinically important. The early orthodontic treatment guide explains how to identify which children benefit most from acting quickly. Explore Your Child's Orthodontic Options A free virtual assessment lets a qualified clinician review your child's teeth and advise whether a functional appliance, early interceptive treatment, or waiting until the teenage years is the right approach. Start Free Assessment → ## FAQs ### What is a functional appliance in orthodontics? A functional appliance is an orthodontic device — usually removable — that holds the lower jaw in a more forward position than its natural resting position. By maintaining this posture during active growth, it stimulates changes in the jaw joint, muscles, and bone that help bring the lower jaw forward. It is used primarily for children and teenagers with a significant overjet (protruding upper front teeth) caused by a set-back lower jaw. The most common type used in the UK is the Twin Block appliance. ### What is a Twin Block brace? A Twin Block is a type of removable functional appliance consisting of two separate acrylic plates — one fitting over the upper teeth and one over the lower — with interlocking ramps that hold the lower jaw forward when the teeth are together. It is the most widely prescribed functional appliance in the UK and is available on the NHS. It is typically worn full-time (including during eating) for nine to twelve months before fixed braces follow to align the teeth. ### How long does a child need to wear a functional appliance? Functional appliance treatment typically lasts between nine and twelve months, though this varies depending on the severity of the jaw discrepancy and the patient's growth response. For the appliance to be effective, it must be worn consistently — most orthodontists recommend 22 or more hours per day. After functional treatment is complete, fixed braces are typically fitted for 12 to 18 months to align the teeth in the improved jaw relationship. ### At what age do children get a functional appliance? Functional appliances are most effective when used during the pubertal growth spurt — approximately ages 10 to 14 in girls and 11 to 15 in boys. Timing treatment to coincide with this growth window maximises the skeletal effect the appliance can achieve. Starting too early (before the growth spurt begins) means the treatment effect is limited; starting too late (after growth has largely completed) means the appliance can only move teeth rather than modify jaw position. An orthodontist will assess growth stage, often using a hand X-ray (skeletal maturity indicator), to determine optimal timing. ### Is a functional appliance available on the NHS? Yes — functional appliances (including the Twin Block) are available on the NHS for children who meet the IOTN clinical threshold, which a significant overjet typically does. NHS referrals for functional appliance treatment are made by the child's dentist to an NHS orthodontic practice. There may be a waiting list, which is one reason acting promptly on a referral is important — a delay of a year or more can mean the growth window narrows and less skeletal change becomes possible. ### Can adults have a functional appliance? Functional appliances in adults produce primarily dental effects (tooth movement) rather than skeletal change, because the jaw bones and joints are no longer growing and cannot be repositioned with these forces. Adults with a significant jaw discrepancy who want skeletal correction typically require jaw surgery (orthognathic surgery) combined with orthodontic treatment, rather than a functional appliance. In mild adult Class II cases, elastics worn with fixed braces can achieve modest tooth-based correction of an overjet without surgery. ### What is a functional appliance in orthodontics? A functional appliance is an orthodontic device — usually removable — that holds the lower jaw in a more forward position than its natural resting position. By maintaining this posture during active growth, it stimulates changes in the jaw joint, muscles, and bone that help bring the lower jaw forward. It is used primarily for children and teenagers with a significant overjet (protruding upper front teeth) caused by a set-back lower jaw. The most common type used in the UK is the Twin Block appliance. ### What is a Twin Block brace? A Twin Block is a type of removable functional appliance consisting of two separate acrylic plates — one fitting over the upper teeth and one over the lower — with interlocking ramps that hold the lower jaw forward when the teeth are together. It is the most widely prescribed functional appliance in the UK and is available on the NHS. It is typically worn full-time (including during eating) for nine to twelve months before fixed braces follow to align the teeth. ### How long does a child need to wear a functional appliance? Functional appliance treatment typically lasts between nine and twelve months, though this varies depending on the severity of the jaw discrepancy and the patient's growth response. For the appliance to be effective, it must be worn consistently — most orthodontists recommend 22 or more hours per day. After functional treatment is complete, fixed braces are typically fitted for 12 to 18 months to align the teeth in the improved jaw relationship. ### At what age do children get a functional appliance? Functional appliances are most effective when used during the pubertal growth spurt — approximately ages 10 to 14 in girls and 11 to 15 in boys. Timing treatment to coincide with this growth window maximises the skeletal effect the appliance can achieve. Starting too early (before the growth spurt begins) means the treatment effect is limited; starting too late (after growth has largely completed) means the appliance can only move teeth rather than modify jaw position. An orthodontist will assess growth stage, often using a hand X-ray (skeletal maturity indicator), to determine optimal timing. ### Is a functional appliance available on the NHS? Yes — functional appliances (including the Twin Block) are available on the NHS for children who meet the IOTN clinical threshold, which a significant overjet typically does. NHS referrals for functional appliance treatment are made by the child's dentist to an NHS orthodontic practice. There may be a waiting list, which is one reason acting promptly on a referral is important — a delay of a year or more can mean the growth window narrows and less skeletal change becomes possible. ### Can adults have a functional appliance? Functional appliances in adults produce primarily dental effects (tooth movement) rather than skeletal change, because the jaw bones and joints are no longer growing and cannot be repositioned with these forces. Adults with a significant jaw discrepancy who want skeletal correction typically require jaw surgery (orthognathic surgery) combined with orthodontic treatment, rather than a functional appliance. In mild adult Class II cases, elastics worn with fixed braces can achieve modest tooth-based correction of an overjet without surgery. ## References - British Orthodontic Society — Functional appliances for children (https://www.bos.org.uk/patients-parents/orthodontic-treatment/children-and-orthodontics) - BOS guidance on functional appliances in orthodontic treatment, covering the clinical indications, types of appliance used in the UK (particularly the Twin Block), the optimal timing in relation to the pubertal growth spurt, and the evidence base for their effectiveness in reducing overjets in growing patients. - NHS — Orthodontics: Types of brace (https://www.nhs.uk/conditions/orthodontics/types-of-brace/) - NHS guidance on functional appliances as part of the range of orthodontic appliance types available to children in the UK, including information on the Twin Block appliance, what the treatment experience involves, and how Phase 1 functional treatment relates to subsequent Phase 2 fixed brace treatment. - Oral Health Foundation — Children and orthodontics: Functional appliances (https://www.dentalhealth.org/braces-and-orthodontics) - Oral Health Foundation patient and parent information on functional appliances in children's orthodontic treatment, covering what a functional appliance is, who needs one, compliance expectations, and what parents should expect during the treatment period before fixed braces begin. - British Orthodontic Society — Functional appliances for children (https://www.bos.org.uk/patients-parents/orthodontic-treatment/children-and-orthodontics) - BOS guidance on functional appliances in orthodontic treatment, covering the clinical indications, types of appliance used in the UK (particularly the Twin Block), the optimal timing in relation to the pubertal growth spurt, and the evidence base for their effectiveness in reducing overjets in growing patients. - NHS — Orthodontics: Types of brace (https://www.nhs.uk/conditions/orthodontics/types-of-brace/) - NHS guidance on functional appliances as part of the range of orthodontic appliance types available to children in the UK, including information on the Twin Block appliance, what the treatment experience involves, and how Phase 1 functional treatment relates to subsequent Phase 2 fixed brace treatment. - Oral Health Foundation — Children and orthodontics: Functional appliances (https://www.dentalhealth.org/braces-and-orthodontics) - Oral Health Foundation patient and parent information on functional appliances in children's orthodontic treatment, covering what a functional appliance is, who needs one, compliance expectations, and what parents should expect during the treatment period before fixed braces begin.