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1 and 2. If a person has knee hyperextension ROM, they can use a KAFO with an offset knee joint and an unlocked knee to allow free motion into knee flexion during swing phase and during sit-to-stands. They will rely on their knee hyperextension for stance stability – the KAFO will not block hyperextension unless it has an additional stop installed. The patient will require adequate voluntary muscle control to hyperextend their knee during stance phase. The KAFO hinge would be aligned posterior to the patient’s knee joint. When the person hyperextends their knee, it moves the ground reaction force anterior to the knee, creating an extension moment.

Here’s another explanation from this article (https://www.sciencedirect.com/topics/nursing-and-health-professions/knee-ankle-foot-orthosis):
“An offset joint is a hinge placed posterior to the midline of the leg so that the patient’s weight line falls anterior to the joint. This stabilizes the knee during the early stance phase of gait when the wearer is on a level surface and does not hamper knee flexion during swing or sitting. The joint may, however, flex inadvertently when the wearer walks down a steep ramp. An offset joint should not be used by the patient with a knee flexion contracture because the contracture will make the floor reaction force pass posterior to the knee.”

3. Someone may be hyperextending their knee/plantarflexing their ankle during stance phase as a compensation for quad/PF weakness to prevent uncontrolled tibial advancement during stance phase. A DF stop can control tibial advancement and may allow the person to avoid knee hyperextension.