The deep fibular nerve, also known as the deep peroneal nerve, is a branch of the common fibular nerve. It primarily runs along the front of the lower leg. This nerve originates near the knee, specifically from the bifurcation of the common fibular nerve, and travels down the anterior compartment of the leg. It passes deep to the muscles of the leg, running alongside the anterior tibial artery, and continues into the dorsum (top) of the foot. Its path extends between the first and second toes, where it provides sensory innervation to the skin.
Structure and Anatomy
Origin
The deep fibular nerve originates from the bifurcation of the common fibular (peroneal) nerve, which is itself a branch of the sciatic nerve. This bifurcation occurs near the neck of the fibula, where the common fibular nerve divides into the superficial fibular nerve and deep fibular nerve.
Course in the Leg
After originating from the common fibular nerve, the deep fibular nerve enters the anterior compartment of the leg by passing deep to the fibularis longus muscle. It then pierces the extensor digitorum longus and descends along the anterior surface of the interosseous membrane. As it descends, the nerve runs in close proximity to the anterior tibial artery, following its course along the leg.
Anatomical Relationships
- Proximal Leg: Initially, the deep fibular nerve is located deep to the extensor digitorum longus muscle.
- Mid-Leg: The nerve becomes more superficial and lies between the tibialis anterior and extensor hallucis longus muscles.
- Distal Leg: At the lower part of the leg, it runs anterior to the ankle joint between the tendons of the extensor hallucis longus and extensor digitorum longus muscles.
Passage into the Foot
As the deep fibular nerve approaches the ankle, it crosses deep to the extensor retinaculum and enters the dorsum of the foot. Upon entering the foot, the nerve divides into a lateral and medial branch.
Branches
- Lateral Branch: This branch supplies innervation to the extensor digitorum brevis muscle and the tarsal joints.
- Medial Branch: The medial branch continues along the dorsum of the foot and passes between the first and second toes, providing sensory innervation to the web space between these toes.
Anatomical Landmarks
- Proximal Landmarks: The nerve begins near the fibular neck.
- Mid-Leg Landmarks: It runs between the tibialis anterior and extensor hallucis longus muscles, alongside the anterior tibial artery.
- Distal Landmarks: The nerve crosses the ankle joint and enters the dorsum of the foot beneath the extensor retinaculum.
Terminal Branches
The deep fibular nerve’s terminal branches are found in the dorsum of the foot. The medial branch provides sensation to the web space between the first and second toes, while the lateral branch primarily innervates the intrinsic muscles of the foot.
Relations to Other Structures
Throughout its course, the deep fibular nerve is closely associated with the anterior tibial artery, particularly in the leg’s distal portion. It also lies deep to the extensor retinaculum at the ankle, a structure that holds down the tendons of muscles in the anterior compartment.
Surrounding Musculature
- Tibialis Anterior Muscle: The nerve passes medial to this muscle in the upper leg.
- Extensor Hallucis Longus: The nerve runs between this muscle and the tibialis anterior as it travels downward.
- Extensor Digitorum Longus: The nerve passes deep to this muscle in the proximal leg before becoming more superficial.
Function
The deep fibular nerve has both motor and sensory functions. It innervates muscles responsible for dorsiflexion of the foot and extension of the toes, while also providing sensory innervation to specific areas on the dorsum of the foot. Below is a detailed breakdown of its functions.
Motor Functions
The deep fibular nerve primarily innervates muscles in the anterior compartment of the leg and certain muscles in the dorsum of the foot, enabling critical movements such as dorsiflexion and toe extension.
Muscles of the Anterior Compartment of the Leg
These muscles are involved in lifting the foot upwards (dorsiflexion) and extending the toes.
- Tibialis Anterior: This muscle is the main dorsiflexor of the foot. It lifts the foot towards the shin and inverts the foot, which is important in walking and running.
- Extensor Hallucis Longus: This muscle extends the big toe and assists in dorsiflexion of the foot. It is critical for actions such as raising the foot off the ground during walking.
- Extensor Digitorum Longus: Responsible for the extension of the lateral four toes, this muscle also contributes to dorsiflexion of the foot.
- Fibularis Tertius: A small muscle that helps with dorsiflexion and eversion of the foot, assisting in stabilizing the foot during activities like walking or running.
Muscles of the Dorsum of the Foot
In the foot, the deep fibular nerve supplies small intrinsic muscles.
- Extensor Digitorum Brevis: This muscle aids in the extension of the toes, particularly the second through fourth toes.
- Extensor Hallucis Brevis: Responsible for extending the big toe, this muscle works in conjunction with the extensor hallucis longus.
Sensory Functions
In addition to its motor functions, the deep fibular nerve provides sensory innervation to a specific region of the foot.
Sensory Supply to the Foot
Dorsal Web Space (First Interdigital Cleft): The deep fibular nerve is responsible for providing sensory innervation to the web space between the first and second toes. This is a small, but highly specific, region of the dorsum of the foot.
Exclusion of Other Areas
Unlike the superficial fibular nerve, which supplies the majority of sensory innervation to the dorsum of the foot, the deep fibular nerve’s sensory function is limited to the narrow region between the first and second toes. It does not supply sensory information to the majority of the foot or leg.
Clinical Significance
The deep fibular nerve plays a vital role in both motor control and sensation in the lower leg and foot, and its injury or dysfunction can lead to significant clinical problems.
Foot Drop
One of the most notable consequences of deep fibular nerve damage is foot drop, a condition where a person is unable to dorsiflex the foot (lift it upward). This results in the dragging of the toes while walking, leading to a characteristic “high-stepping” gait. Foot drop can be caused by trauma, nerve compression, or systemic conditions like diabetes.
Anterior Compartment Syndrome
The deep fibular nerve is vulnerable in conditions like anterior compartment syndrome, where increased pressure in the anterior compartment of the leg can compress the nerve. This can lead to pain, weakness in dorsiflexion, and loss of sensation in the first web space between the toes. Early diagnosis and intervention are critical to prevent permanent nerve damage.
Nerve Entrapment (Ski Boot Syndrome)
Deep fibular nerve entrapment, often called Ski Boot Syndrome, occurs when the nerve is compressed at the ankle, commonly by tight footwear such as ski boots. Symptoms include pain and tingling over the dorsum of the foot, particularly between the first and second toes. In severe cases, motor deficits can also occur.
Surgical and Trauma Considerations
In surgeries involving the leg, such as fixation of fractures around the knee or ankle, the deep fibular nerve is at risk of iatrogenic injury. Direct trauma, fractures, or penetrating injuries to the knee or shin may also damage the nerve, causing motor or sensory deficits.
