Superficial peroneal nerve entrapment (SPNE) occurs when the superficial peroneal nerve becomes compressed or pinched. This entrapment happens where the nerve passes through a tough band of fibrous tissue, known as the deep crural fascia, in the lower third of the leg. When compressed, the nerve can no longer function properly, leading to chronic pain and altered sensation along the outside of the lower leg and the top of the foot.
The Role of the Superficial Peroneal Nerve
The superficial peroneal nerve begins as a branch of the common peroneal nerve near the knee. It travels down the lateral compartment of the lower leg, situated between the peroneus longus and the extensor digitorum longus muscles. This nerve provides minor motor function by innervating the peroneus longus and brevis muscles, which are responsible for turning the foot outward (eversion).
The nerve’s primary function is sensory. It supplies sensation to the skin of the distal two-thirds of the lateral leg and the entire top surface (dorsum) of the foot. The only area of the foot’s dorsum it does not supply is the web space between the first and second toes.
The most common location for entrapment is approximately 10 to 15 centimeters above the ankle. Here, the nerve pierces the deep crural fascia to become superficial. This fascial opening can become a tight bottleneck, squeezing the nerve as it exits. Symptoms are often purely sensory due to the location of the compression.
Triggers for Nerve Entrapment
Compression of the superficial peroneal nerve can be caused by various events. Traumatic causes frequently involve acute injuries that stretch the nerve or cause significant swelling in surrounding tissues. A common scenario is an ankle sprain, particularly an inversion injury, which can pull and tension the nerve at its fascial exit point.
Direct blunt trauma or fractures in the area can lead to inflammation and scar tissue formation, narrowing the space around the nerve. Pressure may also build up internally, as seen in chronic exertional compartment syndrome. Here, muscle swelling during activity compresses the nerve against the unyielding fascia.
Mechanical and iatrogenic triggers involve external or surgical factors. Repetitive motions, such as long-distance running or marching, can cause chronic microtrauma where the nerve passes through the fascia. Tight-fitting boots, restrictive casts, or braces can exert direct pressure, contributing to compression. Entrapment can also occur following surgery in the lower leg or ankle, where post-operative swelling or scar tissue (fibrosis) constricts the nerve’s path.
Recognizing the Signs and Confirming Diagnosis
The characteristic presentation of SPNE involves altered sensations along the nerve’s distribution. Patients report burning pain, tingling, or numbness located along the outside of the lower leg and across the top of the foot. These symptoms often worsen with specific activities like walking, running, or standing for long periods.
Rest often provides relief from the symptoms, which helps distinguish this condition from other types of persistent pain. A physical examination includes specific maneuvers designed to provoke the symptoms, such as the positive stretch test. This test involves forcibly turning the foot inward (inversion) and pointing the toes downward (plantar flexion), which stretches the nerve and reproduces the patient’s pain.
Tinel’s sign involves lightly tapping the skin over the nerve’s exit point, about 10 to 15 centimeters above the ankle. If this tapping causes a shooting or tingling sensation that radiates down into the foot, the sign is positive.
Confirming the diagnosis requires distinguishing SPNE from conditions like nerve compression in the lumbar spine (radiculopathy) or other nerve entrapments. Nerve conduction studies (NCS) and electromyography (EMG) assess nerve function and localize the compression site. A diagnostic nerve block, involving the injection of a local anesthetic around the suspected site, provides strong evidence if it results in temporary relief of symptoms.
Managing Superficial Peroneal Nerve Entrapment
Management of SPNE starts with conservative measures. This involves modifying activities to avoid movements or pressures that aggravate symptoms, coupled with rest. Nonsteroidal anti-inflammatory drugs (NSAIDs) may be used to reduce inflammation and nerve irritation in the early stages.
Physical therapy incorporates stretching, strengthening, and techniques like nerve gliding to promote nerve mobility. Changes to footwear or padding can also alleviate external compression. If symptoms persist, a targeted injection of a corticosteroid near the entrapment site may reduce localized swelling and inflammation.
Surgical intervention is considered when conservative treatments fail to provide lasting relief, or when symptoms severely impact daily life. The most common surgical procedure is a fascial release, or neurolysis. This procedure physically frees the nerve from the tight band of fascia that is compressing it. The goal is to widen the narrow opening where the nerve passes through the deep crural fascia, thereby decompressing the nerve and restoring its function.
The surgical release is often performed on an outpatient basis. Following the procedure, patients are advised to keep the leg elevated and limit walking for several weeks to manage swelling and promote healing. Physical therapy is resumed post-surgery to assist recovery, with a full return to activity expected within a couple of months.