Contusion of the common peroneal nerve resulting in foot drop
I saw an interesting case of post-trauma foot drop (peroneal palsy) this week. The patient was a 24 y/o female who described falling against her car and hitting her right leg on the license plate. She presented to the emergency department for a small laceration of the lateral knee. She also described the inability to lift her foot while walking. The laceration was dressed in the ED and the patient referred to me for follow-up.
The patient is of healthy BMI, good health and an everyday smoker. Upon examination, she showed a well-healed laceration of the lateral right knee. Muscle strength testing was strong on the left leg noting dorsiflexion, plantarflexion, inversion, and eversion all 4/4 with resistance. The injured right leg though showed 4/4 plantar flexion with 2/4 inversion and eversion. Dorsiflexion was present but limited at 1/4. Findings were suggestive of an injury to the common peroneal nerve. Sensation of the top of the foot (distribution of the common peroneal nerve) was intact. Gait exam noted adequate but weak dorsiflexion of the right foot in the swing phase of gait. The patient did relate two falls since the injury due to instability of the ankle. Percussion of the injury site noted a positive Tinel’s sign to the dorsal right foot.
Anatomy of the common peroneal nerve
The common peroneal nerve is a branch of the sciatic nerve that wraps around the lateral aspect of the knee just below the head of the fibula. The common peroneal nerve branches into the superficial and deep branches. The deep branch supplies the innervations to the two dorsiflexors of the ankle, the extensor digitorum longus and the tibialis anterior muscles.
Innervation of the extensor digitorum longus and tibialis anterior by the deep branch of the common peroneal nerve enables dorsiflexion of the foot at the ankle. Dorsiflexion at the ankle is an important part of the biomechanics of walking. In the swing phase of gait, when the foot is not touching the ground but swinging forward at the hip, the innervations of these muscles lifts the foot so that it doesn’t drag on the ground. Injury to the common peroneal nerve results in foot drop or ‘palsy’. Inability to lift the foot at the ankle necessitates lifting of the leg at the hip and low back to complete a cycle of gait.
Neuropraxia of the common peroneal nerve
Injuries of peripheral nerves are broken down into three primary types by the Seddon classification of peripheral nerve injuries. My patient sustained the mildest of injury called neuropraxia. Neuropraxia should respond in weeks to months as the nerve regains its ability to send both motor and sensory signals.
Treatment of peroneal palsy
What was interesting in this particular case was the fact that the laceration was several centimeters proximal to the common peroneal nerve. This knowledge was comforting in that we could assume that there was not a direct laceration of the common peroneal nerve. Palpation of the nerve at the head of the fibula produced a tingling sensation in the top of the foot called a Tinel’s sign. A positive Tinel's sign meant that the common peroneal nerve was intact.
The initial injury was treated with ice, elevation, and compression. Since the common peroneal nerve was found clinically to be intact, there was no need for surgical exploration or reapposition of a lacerated nerve. Physical therapy has been helping with muscle strengthening, preserving existing muscle strength while the nerve regenerates.
How quickly will the nerve regenerate? There are a number of variables in this case that include;
- Age of the patient
- Smoking status
- Severity of the injury to the nerve
Based on our clinical findings this week, I think the patient will go on to a full recovery.