As clinicians prescribing orthoses, the stiffness is vitally important for our prescriptions to achieve their goals. Understanding what we are trying to control allows us to choose materials that will exert an external force onto a person and influence a particular movement pattern.
Link to paper
When prescribing orthoses what do we take into consideration before we can think about material stiffness?
Diagnosis, age, weight, range of motion, muscle tone and what we want the orthosis to do. The materials available to us have evolved over time from traditional leather or metal orthoses to homo and copolymers, silicone, dynamic elastomeric fabrics, carbon composites and now nylons because we can 3D print
The take home points from the paper are:
- All AFOs will deform to some extent under sufficient load
- Rigid AFOs are designed to resist peak plantarflexion and dorsiflexion loads
- Comparison of AFO stiffness between USA and UK manufacturers to establish thresholds for AFO-FC algorithms
- 9 AFOs were used (image 1). 6 from the UK and 3 from the USA. All made from polypropylene of varying thickness (3-5mm) for patients with cerebral palsy or spina bifida. 2 USA AFOs in design B and 1 design C. UK AFOs 2 represented in all 3 design categories:
- Design A – fixed ankle free MTPJs
- Design B – fixed ankle with fixed MTPJs
- Design C – Fixed ankle, fixed MTPJs and anterior tibial shell
- Various unique design features for each AFO are described in table 1 (see paper)
- Overall UK AFOs were found to be stiffer than the USA AFOs, with comparable stiffnesses in design group B