An Ankle-Foot Orthosis is a specific type of assistive device that, as its name suggests, covers the foot and ankle. There are three main types of AFOs: flexible, rigid and jointed. Each device has specific advantages and disadvantages. The goal of the orthotist is to create an apparatus that ultimately enables the patient to achieve their highest functional level. Thus, the device should encourage maximal compliance of use and provide assistance for the patient to achieve their highest functional level - it should not hinder the patient.[1]
There are many different types of AFOs which will be discussed below. Each device has indications and contraindications, so an accurate assessment of the patient and their needs is vital. An AFO can be custom made or off-the-shelf, and both have their place in the rehabilitation journey.
Types of AFO
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Orthotic Assessment
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Passive Assessment:
Active Assessment:
Gait Cycle
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When the normal gait cycle is effected by a pathological condition, common complications emerge. These do vary from person to person, but often patterns can be found. The images below show a normal gait cycle and four pathological complications.
Normal Gait Cycle:
Pathological Gait Cycle:
There are various pathological gait conditions (see image below), including:[1]
Types of Pathological Gait.
Changes in gait can be due to:
Goal Setting / Prescription
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Three-Point Force System
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This system is used to stabilise a joint or segment and it can reduce angular rotation. A force is often applied medio-laterally or antero-posteriorly.[2]
The Three-Point Force System (also known as the Three-Point Pressure System) is paramount to creating an orthosis.[3] In this system, a primary force is applied, generally at the joint to be corrected, and counterforces are applied both above and below the primary force. The sum of these three forces applied should be zero. The amount of force, and the position at which the primary force is applied, should be the point where movement needs to be increased or decreased. This will vary depending on the assessment and functional requirements of the patient, along with the established treatment goal.[4][5]
When applying a force, it is important to remember the scientific equation of: Pressure equals Force over Area.[6]
When creating an orthotic device, the longer the lever arm or the longer the orthosis is, and the further away the points of force are, the bigger the correction will be. As there is more surface area to spread the force over, the pressure can be reduced using this principle. This principle can be used to reduce discomfort when wearing an orthosis.
When designing an AFO, the goal is to achieve the best foot and ankle position in conjunction with patient requirements. This means:
Casting Procedure
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When creating an AFO, a mould of the patient's foot and ankle is made using a plaster cast, a process requiring experience and skill. The cast is removed, creating a negative cast, which can be scanned or filled with plaster to create a positive model of the foot/ankle.. The custom device can be created using this new positive cast.
3D printing is occasionally used to create orthoses. However, a significant challenge associated with 3D printing is that it is difficult to maintain the optimum foot position long enough to take an accurate scan, which is needed to create an accurate device. Once this technology is improved, 3D printing might be an effective way of creating an AFO.[1]
1. Flexible AFO
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Flexible ankle-foot orthosis (AFO).
A flexible AFO has a certain amount of flexibility around the ankle area. This is achieved by cutting away part of the device around the ankle, reducing the surface area. This loss of surface area reduces the effect of the Three-Point Pressure System and as a result, this approach is not suitable for patients with mediolateral instability. A flexible AFO is useful in patients who have increased, uncontrolled movement in the ankle joint with good mediolateral stability. This type of device can allow for some roll over which encourages a natural gait pattern. It is easier to get out of chairs, and go up and down stairs. It is also easier for children to play on the floor and move around.
Prescription Criteria for a Flexible AFO
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Common Conditions Include
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Off-the-Shelf Flexible AFO
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Also known as a posterior leaf spring (PLS) as they essentially have a spring at the back of the AFO.[10]
These AFOs are commonly used in the short-term while a custom splint is being designed or while the patient's condition improves. These are commonly used in hospital or trauma-type settings to maintain range.
2. Rigid AFO
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A rigid AFO is an orthotic device, custom made for the individual, that allows no movement. They are often used in more severe situations and in conditions with some mediolateral instability. The rigid AFO allows the Three-Point Pressure System to function in the optimal way. It also influences the ground-reaction forces.
It is important to conduct a gait analysis when constructing these devices because we need to now what type of correction to apply.[11] If the AFO moves the ground-reaction force in front of the knee joint at mid-stance, it will create a hyperextension moment. Alternatively, if the ground-reaction force is moved or remains behind the knee joint, it will create a flexion moment at mid-stance.
A ground-reaction AFO (GRAFO) is essentially the same as a rigid AFO. The difference is that an anterior shell is attached, which will spread the load that is applied as part of the Three-Point Pressure System to the front of the shin. The force is applied at the front of the shin to extend the knee and to maintain the ankle in a plantigrade position.
Prescription Criteria for a Rigid AFO
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Varus deformity and orthosis.
Common Conditions
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3. Jointed AFO
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A jointed AFO is made with a moving part, or joint, which hinges at the ankle joint. It is designed to allow motion at the ankle joint, but still offers the correction that the Three-Point Pressure System offers. The jointed AFO is a good tool to try to optimise a gait pattern.
Disadvantages:
Advantages:
Prescription Criteria for a Jointed AFO
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Prescription criteria for Jointed AFO and DAFO/SMO
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Tuning / Optimising AFOs
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Once the AFO has been manufactured and fitted, optimisation can done in conjunction with footwear.[13] Walking in an AFO without footwear is dangerous and slippery. An AFO is always prescribed in conjunction with an appropriate type of footwear. This footwear will influence how the patient functions in the AFO and their gait pattern.[14] The orthotist needs to look at the footwear closely, and then optimise the angles of the AFO to influence the ground-reaction force.[15][16]
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