The Dorsiflexion (anterior) stop will prevent knee buckling (or a sudden, unwanted flexion of the knee leading to imbalance) because it assists the knee joint into extension.
An ankle foot orthosis (AFO) consists of a proximal calf band, two metal uprights, ankle joints and an attachment to the shoe to anchor the AFO. By design, it does not extend to the knee nor does it cross the knee joint. Therefore, both anterior and posterior stops work by influencing the moments (flexion or extension) occurring around the knee joint.
Plantar Stops (AKA Posterior Stop) – is commonly set at 90 degrees with insertion of the pin in the posterior channel of the ankle joint of the AFO. It used to control plantar spasticity and to help incrementally stretch plantar contractures. A plantar stop at 90 degrees produce a knee flexion moment during heel strike while the proximal posterior portion of the AFO exert a forward push on the proximal leg to increase knee flexion moment after heel strike.
At toe-off an extension moment is created at the knee. The greater the plantar flexion resistance, the greater the flexion moment at the knee at heel strike and the greater need for active hip extensors to prevent body from collapsing forward on buckling knee. To remedy this, the posterior stop is set at the minimal amount of plantar flexion to reduce the flexion (bending) moment at the knee after heel strike.
Dorsiflexion Stops (AKA Anterior Stops) – substitutes for gastroc-soleus complex and is usually set at 5 degrees of dorsiflexion. It assists in push-off and also assists the knee joint into extension. An earlier dorsiflexion stop would be useful in the added presence of quadriceps weakness and would cause a greater extension moment at knee. However, an extension moment at the knee that is too great or too long may result in genu recurvatum.
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