Orthotic Devices and Biomechanics
Scope
Orthoses correct, support or protect a body segment; the exam tests principles, device selection and biomechanics.
Biomechanics
- Three-point pressure: two forces on the convex side and one on the concave side create a correcting moment; a longer lever arm needs less force.
- Total contact and broad pads reduce pressure, since pressure = force ÷ area.
- Correct at the joint, not only at the deformity apex, and avoid pressure on bony prominences.
- Tone matters: a spastic limb may need tone management or serial casting before a rigid device.
Devices
AFO for foot drop and ankle instability; KAFO for knee and ankle control; HKAFO for hip; TLSO/LSO and cervical orthoses for the spine; foot orthoses for arch support and pressure redistribution. Stance-control and locked knees serve different purposes.
Worked example
A knee orthosis pad applies 40 N over 20 cm²: pressure = 40 ÷ 0.002 = 20 kPa. Over 5 cm² the same force gives 80 kPa, likely to cause skin breakdown. Keep force low and area large, and place counter-pads away from the fibular head.
Traps
Confusing stance-control with locked knees; forcing correction on a spastic limb; skipping skin checks; fitting an AFO with the ankle not at 90°; forgetting the device must fit inside the shoe.