Introduction
Types of orthosis
What is thermoplastic?
Making of an orthosis
Common examples of orthosis
Orthosis is a device that is applied in direct contact with the body to serve the following purposes:
Support the upper or lower limb or the spine
Provide stability to a paralysed, weakened or unstable limb or spine
Relief pain due to instability or pressure points arising from deformity
Relief pressure
Accommodate deformity
Control deformity due to postural or abnormal muscle forces
Depending on the nature of the material used, orthosis may be classified into the following types:
Rigid orthosis. Made from a firm material, usually a thermoplastic. The device is moulded to the body part concerned and provides structural support and controls the function of the part.
Soft orthosis. Made of soft, compressible material. Used to attenuate shock, improve balance or take pressure off uncomfortable or sore spots.
Semi-rigid orthosis. Constructed of layers of soft material reinforced with rigid material. Designed to permit comfortable and dynamic balance of the limb during motion.
Most materials change from solid to liquid state when heated to its melting point. Thermoplastic is a polymer that turns to liquid when heated and freezes to a firm glassy state on cooling. Unlike common materials, the change in state does not occur at a sharp melting or freezing point. When a thermoplastic in its glassy state is heated, there is a range of temperature above which it becomes flexible. The mid point of this range is called the glass transition temperature. Note that thermoplastic is different from thermosetting material in that it can be re-melted and re-moulded by altering the temperature. Thermoplastic has crystalline regions alternating with amorphous regions. The crystalline region contributes to strength and rigidity while the amorphous region contributes to its elasticity. It is the change in proportion with temperature that accounts for its change in physical property and strength.
Making use of this property, it is possible to fabricate an orthosis using thermoplastic with a glass transition temperature above room temperature. The orthosis can fit exactly to the contour of the limb when heated. On cooling to room temperature, the shape is retained but it becomes rigid and provides the mechanical strength required.
Orthosis may be tailor-made for each individual or prefabricated into different sizes that are chosen off the shelf for an individual depending on the requirement.
To tailor-make orthosis, 2 kinds of thermoplastics are commonly employed:
Low temperature thermoplastic. Its glass transition temperature is within the range that a patient can tolerate without thermal injury. The thermoplastic comes in solid sheets at room temperature. It is cut into the shape of the body part concerned and then put into hot water until it becomes elastic. This is then applied directly onto the skin so that it can fit to the contour of the body part. When the thermoplastic cools, an exact fit orthosis is obtained. Velcro straps are then added to facilitate the wearing of the orthosis. Low temperature thermoplastic has the advantage that it is very convenient to make. However, the material is usually not as durable or as strong as high temperature thermoplastic.
High temperature thermoplastic. The glass transition temperature is higher than a patient can safely tolerate. Therefore, the orthotist has to make a mould of the body part with plaster of Paris first. Then, the mould is taken to a laboratory where plaster is put into the mould to produce a replica of the body part on which the high temperature thermoplastic orthosis can be made.
Orthoses are commonly used in the lower limbs. Ankle foot orthosis (AFO) is used for patients suffering from drop foot. Orthosis are frequently used to relief pain caused by pressure points resulting from deformities arising from injury or arthritis.
In the upper limb, orthoses are frequently employed to splint the hand for rehabilitation after repair of tendons or ligaments. For the spine, a brace can be fitted in the treatment of scoliosis.
Dr. YEUNG, Sai-hung