Foot drop is a complex problem that has been defined as a substantial weakness in toe and ankle dorsiflexion, preventing the patient from adequately lifting the toe of the foot clear off the ground whilst walking.
The dorsiflexor muscles for the ankle and foot include the extensor digitorum longus (EDL), the extensor hallucis longus (EHL) and the tibialis anterior (TA). These muscles act together to help the foot swing clear of the floor during normal gait and to control the foot plantar flexion during the initial ‘foot to the ground’ contact.
What does foot drop mean to the patient?
In order to understand the impact of foot drop on the patient, it is necessary to understand the normal gait cycle, which is divided into three phases. Normal gait commences with the swing phase (SW), which is characterised by the foot being lifted off the ground and moving forwards, swinging clear of the ground. This is followed by initial contact (IC), also known as heel strike, when the foot (usually the heel) contacts the ground to commence the stance phase. Following this moment of stance, the normal walking gait has the terminal contact (TC) or foot / toe off phase where the foot is lifted off the ground and commences forward swinging motion into the SW phase.
In drop foot gait, these three phases are far more exaggerated. SW requires a greater flexion of the knee and sometimes an outward swing of the hip, in order to accommodate the patient’s inability to lift the toes (dorsiflex). This often results in the patient adopting a stair climbing movement with one leg as the foot is lifted higher to clear the ground. During the IC phase, patients with foot drop will not follow the normal heel to toe foot strike. In normal gait, plantar flexion, defined as movement of the ankle joint to point the toes downward and away from the leg, is controlled by the lengthening of the TA, which absorbs the impact of the heel contacting the ground during gait.
However, in patients with foot drop, this extension of the TA does not occur, resulting in either the entire foot contacting the ground at the same time or the foot slapping the ground in an uncontrolled manner. The TC phase is often quite different in drop foot patients due to the muscle weakness preventing them from supporting their own body weight. As such, they may significantly shorten the usual stance phase and commence toe off at a much earlier point than normal gait. Due to the irregular gait, patients with drop foot often require a walker or a support cane to assist with balance.
Any weakness in the dorsiflexor muscle group can cause this steppage gait, also known as equinovarus deformity. As such, patients will find it difficult to lift the toe of the foot clear off the ground, often scuffing or dragging the toes along the floor, which can lead to an increased incidence of trips and falls. In addition, because the direction of the ground reaction vector is behind the centre line of the knee and ankle, the heel strike or IC force exceeds body weight resulting in plantar flexion of the foot and the characteristic uncontrolled slap of the foot on the ground. Foot drop can occur in any age group and affect the left or right side with equal frequency, however, foot drop more commonly affects males than females with a ratio of 2.8 : 1 (male : female).
What are the causes of foot drop?
Foot drop occurs if there is any direct injury to the dorsiflexor muscle group or if there is any damage to the neural pathway that controls these muscles. Rupture of the anterior tibial tendon, following minor trauma when the foot is in plantar flexion, has also been reported. Neurological causes have been attributed to mononeuropathies of the sciatic, common peroneal or deep peroneal nerves.
As such, foot drop has been associated with a number of conditions including motor neuron disease, stroke, diabetes, drug toxicities, lumbar radiculopathy, lumbosacral plexopathy, cerebral lesions in the parasagittal cortical or subcortical regions and other neuropathies as well as peripheral nerve and dorsiflexor injuries. Foot drop causes are, therefore, generally divided into three categories: muscular, neurologic and anatomic. Treatment of foot drop varies between causes.
What is the prognosis?
The short and long term outcome of foot drop very much depends on the underlying cause. For example, recovery times of around 3 months have been reported for peripheral compressive neuropathies and good long term prognosis has also been reported for partial peroneal nerve palsy following total knee replacement. However, in situations of complete postoperative palsy, outcomes are more variable as reinnervation occurs only through proximal to distal axonal growth. As this axonal growth averages around 1 mm per day, injuries where the nerve remains close to the target muscle resolve faster and have more favourable outcomes than those of total nerve / muscle separation or where there is rupture or damage to the myelin sheath surrounding the nerve fibres.
What treatments are available for foot drop?
Common foot drop treatments include physiotherapy, ankle / foot prostheses, electrical nerve stimulation or surgery. However, treatment is very much dependent on the underlying cause of the problem. Surgical options can be adopted in severe cases where there is permanent muscle paralysis and movement loss. One commonly used surgical technique is tendon transference from stronger leg muscles to the anterior tibial or extensor muscles in order to lift the foot. Alternative surgical options include the fusion of the foot and ankle bones to aid in stabilisation of the joint.
Electrical nerve stimulation uses electrical currents to activate the nerves that are damaged due to injury or illness. Two electrode patches, with the leads connected to a battery operated stimulator, triggered by a sensor in the patient’s shoe, are positioned on the patient’s skin, one over the centre of the muscle and the other close to the muscle’s nerve supply. Electrical impulses pass through the electrodes and stimulate the nerves to shorten the muscle, thereby inducing an improved gait.
Physiotherapy options often include strengthening exercises for the leg muscles using resistance training and range of motion exercises for the knee and ankle joints to prevent stiffness, particularly in the heel. Whilst these exercises are focused on restoring normal movement patterns, it is vital that the patient remains stable at all times. Where the patient is unable to maintain stability when weight bearing on the affected foot, an ankle / foot orthosis (AFO) is often used.
An AFO is specifically used to provide dorsiflexion of the foot during the SW phase of gait or to aid lateral stability of the ankle during the stance phase. The AFO therefore keeps the ankle / foot joint at 90 degrees, preventing the toe from dropping downwards when the leg is lifted. This allows the patient to adopt a more even gait. There are a large number of AFOs currently in use dependent on the specific needs of the patient. These include articulated or solid ankle and posterior leaf spring AFOs, which are usually constructed from polypropylene.
The articulated and posterior leaf spring ankle AFOs allow a small range of motion and can encourage and assist dorsiflexion and push-off during the TC phase of gait. Solid ankle AFOs are used in patients with little or no ankle or knee stability. However, an AFO can only be used in patients who can achieve normal plantigrade position (sole of the foot on the ground) when standing; therefore, equinus (limitation of the upward bend of the ankle joint) would preclude the successful use of an AFO.