knee-ankle-foot orthosis (KAFO)
/ KAFO = KAY-fo /
Some legs cannot be trusted at the knee. When the muscles that lock the knee straight are too weak, the knee buckles the moment weight goes through it, and the person collapses. An ankle-foot brace alone cannot help, because the problem is higher up. The knee-ankle-foot orthosis, or KAFO, is a long leg brace that reaches from the thigh down past the knee to the foot, controlling the knee as well as the ankle.
Its central purpose is usually to stabilise a weak or unstable knee so the leg can bear weight without giving way. The simplest design has a knee joint that locks straight during standing and walking; the wearer releases the lock to sit. More sophisticated stance-control KAFOs are clever: they lock the knee only while weight is on the leg and unlock it automatically to let the knee bend during the swing phase, giving a far more natural and less tiring walk. The thigh and calf shells carry the load, and a three-point pressure arrangement keeps the knee aligned.
KAFOs are used when knee instability is the limiting problem — after polio, in certain incomplete spinal cord injuries, in some muscle diseases, or after major nerve injury. They are heavier and more demanding to walk in than an AFO, and locking the knee straight for every step costs a great deal of energy and produces a stiff, swinging gait. That trade-off — stability bought at the price of effort and naturalness — is the central clinical conversation, and it is why a lighter device is always preferred if the knee can be safely left free.
A man whose quadriceps was paralysed by polio in childhood walks with a locked-knee KAFO. The brace holds his knee dead straight so the leg never buckles, but he swings the stiff leg out with each step and tires quickly — the price of the stability he needs.
A long leg brace that stabilises an unreliable knee — at the cost of weight and walking energy.
Locking the knee straight for every step is energy-expensive and gives a stiff gait, so a KAFO is used only when the knee genuinely cannot be left free. Stance-control designs reduce, but do not erase, this cost.