Biomechanical abnormalities of post-polio patients and the implications for orthotic management.

Muscle weakness resulting from the combined effects of acute and late motor neuron pathology is the basic cause of post-polio dysfunction. Through their normal sensation and moter control, post-polio patients minimize their disability by useful substitutions. Orthoses are needed only when these substitutions either are inadequate or result in muscle or joint overuse. The areas most commonly showing late disability are the lower extremities, shoulders and low-back. In the lower extremities, the major muscle groups are the hip extensors and abductors, the knee extensors (quadriceps), ankle plantar flexors and dorsi flexors. Each group has a specific function which relates to one of the basic tasks of walking, weight acceptance, single limb support and swing. To determine orthotic needs, polio gait deviations representing useful substitutions must be differentiated from symptomatic dysfunction. Weight acceptance utilizes the quadriceps, hip extensors and hip abductors to establish a stable limb and provide shock absorbing mechanics. Substitutions to preserve weight bearing stability include sacrifice of normal shock absorbing knee flexion for quadriceps weakness, backward or lateral trunk lean for hip extensor and abductor weakness. Knee pain, excessive hyperextension and flexion contractures are indications for orthotic assistance with a KAFO. Orthotic designs relate to the type of knee joint (off-set, free, locked) and completeness of the AFO component. Low-back pain from hip substitutions or over use of the hip muscles requires a walking aid. Single limb support is the period when the limb and body advance over the supporting foot. The key muscle group is the soleus-gastrocnemius complex. Swing involves lifting and advancing the unloaded limb. While all three joints flex simultaneously, the hip flexors and ankle dorsi flexors are the critical muscles. A drop foot from ankle dorsiflexor weakness is the common disability. Excessive hip flexion is the usual substitution. An orthosis which assists dorsiflexion without obstructing loading response plantar flexion is the most functional design.