A foot drop refers to a variety of conditions that lead to the inability to dorsiflex the foot and big toe. The term dorsiflex, in relation to ankle movement, refers to when the top of the foot is brought closer to the shin. The specific nerves involved in dorsiflexion include the branches of the common peroneal nerve: the superficial and deep peroneal nerves. These peroneal nerves supply portions of the skin with sensation and muscles of the lower leg with impulses to contract. There are three major categories of causes for foot drop deformity, including neurologic, muscular or anatomic due to compression of the nerves. Combined causes exist as well. The foot drop can be an isolated injury or a symptom of a disease.
Anatomy
The common peroneal nerve courses around the knee near the head of the fibula on the lateral knee. This is a site where there can be injury to the nerve that supplies the muscles that flex the foot/ankle. These muscles prevent foot drop when working normally. According to "Clinical Anatomy," an article by Dr. William Ryan of Trinity College's Anatomy Department in Dublin, Ireland, refers to this anatomic location as the "fibular tunnel."
Congential Causes
Patients can be born with neurologic, muscular or structural deformities and conditions that can lead to a congenital form of foot drop. Cerebral palsy is a common disorder than can lead to the inability to properly flex the foot and big toe. These conditions can be related to abnormal central or peripheral nervous system and muscular tone. Conditions related to the development of the spine, including spina bifida and related conditions, can often have associated foot drop.
Direct Injury
The most common direct injury to the dorsiflexors occurs from trauma. There can be a direct tearing of the tendons that can lead to the inability to dorsiflex. Fractures or penetrating trauma can often directly injure the structures that are responsible for properly flexing the foot and big toe.
Compression/Entrapment
The most common site of compression of the peroneal nerve is as it courses around the fibular head at the "fibular tunnel". It is the most common lower extremity peripheral neuropathy. Usually it is a unilateral injury. It can occur because of casting or bracing for fractures, or for no known reason. An accessory bone known as the fabella (not present in everyone) that is located in the back of the knee can often cause direct compression to the common peroneal nerve.
Post-Surgical
When surgery is performed around the knee, there is a risk of injury to the structures. With total knee arthroplasty, for example, the knee is manipulated leading to the possibility for irritation of the nerve, ischemia (lack of blood flow), or direct laceration to the nerve. If the lateral (outside) knee is not protected well with padding during any surgery, there is the risk of compression injury to the nerve. In the Anatolian Journal of Cardiology, there is a case series study by Dr. Berrin Durmaz of the Department of Physical Medicine and Rehabiliation at Ege University in Turkey describing post-cardiac surgery foot drop.
Compartment syndrome
Compartment syndrome can be acute or chronic. The term compartment syndrome refers to increased pressure in an anatomic area that can lead to vascular or neurologic changes typically by compression. Acute causes can be seen after trauma or gangrene. Chronic causes are typically due to exercise where exercise can transiently increase the compartment pressures to levels that can cause neurovascular deficits including foot drop.
Neurologic Causes
Foot drop can stem from a variety of syndromes affecting either the peripheral nerves. A neuroma or other benign lesion on a nerve can often affect the function of the nerve, leading to an isolated nerve palsy/neuropathy. Lumbar compression from a disk, canal stenosis (spinal stenosis), or arthritis can lead to a foot drop if it occurs at the nerve root for L5. Systemic illnesses such as amyotrophic lateral sclerosis (ALS, Lou Gehrig's Disease), Guillain Barre, Myasthenia Gravis, Lambert-Eaton, and a myositis can all cause a foot drop, however there will typically be other neurologic symptoms as well.
Behavioral
An example of a common behavior that can lead to a foot drop is crossing one's legs. The compression on the lateral aspect, or outside of the leg, can lead to nerve damage to the peroneal nerves.
Metabolic/Systemic
Any disease that causes a polyneuropathy can lead to a foot drop. Polyneuropathy refers to the simultaneous development of multiple perpipheral nerves having dysfunction. Diabetes commonly causes a "stocking and glove" type of neuropathy, when in advanced stages, it can lead to a foot drop and other motorsensory symptoms. Patients with deficiencies of vitamin B12 can also develop a polyneuropathy that can include a foot drop. Processes such as sarcoidosis can also cause nerve palsies.
Oncologic/Cancer
Tumors and other neoplastic processes can often present with a nerve compression injury because of a tumor itself or as a nerve palsy due to a paraneoplastic cause. Blood cancers such as Waldenstrom's Macroglobulinemia and leukemias can present as an isolated nerve palsy.
Infectious
Many infectious agents can cause peripheral neuropathy including Mycobacterium leprae (leprosy).
Combination
An example of when the cause of foot drop is multifactorial is in the case of Charcot foot. The Charcot joint is a a result of neuropathy that leads to the destruction of the bony architecture of the foot. A foot drop is often found in patients with a Charcot joint.
References
- Wheeless' Textbook of Orthopaedics: Peroneal Nerve Palsy
- "The Anatolian Journal of Cardiology"; Bilateral common peroneal nerve palsy following cardiac surgery.; B Durmax et al; 2008.
- "Clinical Anatomy"; Relationship of the common peroneal nerve and its branches to the head and neck of the fibula; W Ryan et al; 2003.
