Peripheral Nerve Anatomy of the Lower Extremity

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by Everett Johnson, DC, ART

Peripheral Nerve Anatomy of the Lower Extremity.

Peripheral nerve entrapment lesions are a common clinical disorder. Knowledge of the pathway and relationships of the peripheral nerves increases the effectiveness of treatment of these conditions by allowing the practitioner to be more precise with their treatments. The following includes the nerves of the lower extremity, their pathways, relationships and common entrapment sites.

Lateral femoral cutaneous nerve:
LE-nerve-entrapments-femoral nerve1The nerve arises from the anterior rami of L2-L4 The nerve courses over the iliacus toward the ASIS as it emerges from the distal lateral aspect of the psoas major muscle. As the nerve reaches the ASIS it travels deep to the inguinal ligament, and pierces the fascia lata of the anterior thigh to take a position subcutaneously in the anterior and lateral aspects of the thigh. Entrapment of the nerve can occur as it passes deep to the inguinal ligament or where it pierces the fascia lata. Common presentation is burning, pain or tingling in the region of the anterolateral thigh that is worse when standing, walking or lying prone (hip extension).

Femoral nerve:
The femoral nerve arises from the posterior divisions of the anterior rami of L2-L4 nerves. The location of the roots of the femoral nerve are within the matrix of the psoas major muscle. The nerve emerges from the distal and lateral aspect of the psoas and descends in a groove between the psoas and iliacus muscle as they form the iliopsoas. It travels deep to the inguinal ligament as it crosses from the trunk into the lower extremity, just lateral the the femoral artery in the femoral triangle. In the triangle the nerve divides into several branches. It has muscular branches to the muscles of the anterior thigh, and a terminal cutaneous branch, the saphenous nerve.

LE-nerve-entrapments-femoral nerveanatomy1While the most common cause of femoral nerve neuropathy is diabetes, entrapment of the femoral nerve can occur at several sites along its pathway to the anterior thigh. Entrapment may occur inside the matrix of the psoas muscle or where it penetrates the muscles’ lateral border. The most common sites of entrapment occur below the inguinal ligament. This location does not provide adequate protection for the nerve and it is in close proximity to the femoral head, tendinous insertion of the vastus intermedius, tendon for the psoas and the hip joint capsule. LE-nerve-entrapments-saph nerve vasto add3Typically, if there is compression of the nerve in the inguinal region the patient may not present with hip flexion weakness, but may have sensory loss along the medial aspect of the leg, below the knee in the distribution of the saphenous nerve.

Symptoms of entrapment include pain in the inguinal area that is exacerbated by extension of the thigh and may be relieved somewhat by flexion and external rotation of the thigh. The patient may present with weakness of hip flexion or knee extension, and may complain of difficulty when walking or buckling of the knee.

Saphenous nerve:
The nerve is a terminal cutaneous branch of the femoral nerve, composed of fibers from L3 and L4 nerve roots. As the femoral nerve passes deep to the inguinal ligament, it begins to give off its branches. The saphenous nerve is one of these branches and traverses the femoral triangle in the anterior region of the thigh as it descends. It passes deep to the sartorius muscle as it enters the adductor canal on the medial aspect of the thigh.LE-nerve-entrapments-saph nerve vasto add2 The saphenous nerve travels deep along the same pathway with the sartorius muscle, and pierces it, or just distal to it, as the two structures approach the knee joint. Distally the saphenous nerve is in a subcutaneous position as it descends the medial aspect of the leg with the great saphenous vein. It ends along the medial arch of the foot, supplying cutaneous innervation.

Entrapment of this nerve most commonly occurs around the knee joint. Typically the nerve pierces the fascia covering the adductor canal or the sartorius just before it gets to the knee, which is the most common area for entrapment to occur. The dynamic forces created by the muscles in this region cause continuous contraction and relaxation of the tissues that will impinge on the nerve.Entrapment of saphenous nerve may cause a deep ache in the thigh or medial knee, and cutaneous disturbances along the nerves pathway from the knee to the foot. This nerve is a pure sensory nerve, so no muscular weakness should be observed with it. If weakness is present suspect femoral nerve involvement, as it is the source of saphenous nerve.

Obturator nerve:
LE-nerve-entrapments-obturator internusThe fibers from the anterior portions of the anterior rami of L2-L4 fuse to form the obturator nerve. They come together within the matrix of the psoas muscle on its medial aspect. The obturator nerve emerges from the muscle along its medial border, lateral to the sacrum and just deep to the common iliac blood vessels. As it descends it enters the lesser pelvis and continues along its lateral wall where it enters the obturator foramen. As the nerve descends from the pelvis into the lower extremity it gives anterior and posterior branches that travel around the obturator internus. The anterior branch is just superficial to the adductor brevis, while the posterior branch is just deep to the adductor brevis. The obturator nerve supplies motor innervation to the adductor muscles of the thigh and a patch of cutaneous sensation to the medial and distal aspect of the thigh the the knee.

Entrapment of the obturator nerve most commonly occurs at the anterior branch as it is crossing just superficial to the adductor brevis muscle, usually due to an inflammatory process. Entrapment may occur with the posterior branch in athletes experiencing issues with their adductor magnus, as obturator nerve is located between the adductor brevis and adductor magnus. Typically this nerve is damaged due to some type of trauma or fracture to the pelvis or during delivery due to compression of the head of the fetus against the pelvis. Symptoms of entrapment of the obturator nerve usually involve difficulty with walking or a feeling of instability in the thigh. If the anterior branch is entrapped the patient typically describes groin pain that appears to be exercise related. The pain is typically in the location of the pubic bone and increases with activity and may radiate down the medial aspect of the thigh and knee.

Sciatic nerve:
The sciatic nerve arises from the anterior rami of L4 through S2 nerves. In the pelvis these roots from the lumbo-sacral plexus lay on the anterior surface of the piriformis muscle as they exit the pelvis through the greater sciatic notch. The sciatic nerve emerges typically from just deep to the piriformis and from its inferior aspect. It LE-nerve-entrapments-tibialis anteriordescends in the gluteal region just deep to the gluteus maximus, passes lateral to the ischial tuberosity and sacrotuberous ligament and superficial to the obturator internus, gemelli muscles and quadratus femoris. As the nerve approaches the superior aspect of the posterior thigh it travels deep to the common origin of the hamstring muscles where it travels just superficial to the adductor magnus. As the sciatic nerve reaches the superior aspect of the popliteal fossa (split of biceps femoris and semimembranosus), it splits into tibial and common fibular branches.

Entrapment of the sciatic nerve is most commonly described as at the piriformis muscle. The area where the sciatic nerve exits the greater sciatic notch just inferior to the piriformis muscle is a site of compression of the nerve due to inflammation or dysfunction of the muscle. The sciatic nerve may also pierce the matrix of the piriformis muscle, which may cause increased problems for that population of people. The nerve may be involved with adhesion formation with any of the related structures along its pathway, and may be subject to compression with inflammation of the sacrotuberous or sacrospinous ligaments.

Piriformis syndrome is a term used to describe sciatic nerve type symptoms without a spinal cause or component. The piriformis muscle has trapped the sciatic nerve in some way. Approximately 15-30% of the population present with a sciatic nerve that will pierce the matrix of the piriformis muscle, which may put them at an increased risk of this condition. The symptoms usually consist of pain along the pathway of the nerve as it descends down the posterior thigh and into the posterior leg and foot. Symptoms may be alleviated somewhat if the patient externally rotates their hip, which will decrease the tightness of the muscle and stress on the nerve.

Tibial nerve:
Tibial division of sciatic is composed of L4-S3 spinal nerves. The tibial nerve continues through the popliteal fossa, where it travels in the mid-line of the posterior aspect of the knee. It crosses the knee joint and descends into the posterior leg just deep to the soleus as it creates a tendinous arch for passage of the tibial nerve and the popliteal blood vessels. The tibial nerve continues to descend the posterior leg between the superficial and deep compartments with the tibial blood vessels. As the tibial nerve descends toward the medial malleolus, it lies posterior to it and deep to the flexor retinaculum and abductor hallucis muscle. At this point the nerve splits to its terminal branches, the medial and lateral plantar nerves. The medial plantar nerve travels on the medial plantar aspect of the foot in a groove between the flexor digitorum brevis and the abductor hallucis muscle. The lateral plantar nerve crosses from the medial foot to the lateral aspect of the plantar surface, deep to the flexor digitorum brevis muscle. It lies in a groove between the lateral aspect of the flexor digitorum brevis and the abductor digiti minimi and flexor digiti minimi brevis. The calcaneal nerve emerges from either the tibial nerve or the medial plantar nerve and supplies sensation to the medial and posterior heel.

Entrapment of the tibial nerve can occur anywhere along its pathway. One area of concern is in the popliteal fossa at the tendinous arch of the soleus, where the tibial nerve travels deep to the soleus to continue between the superficial and deep layers of the posterior compartment. The most common area for entrapment of the nerve is distal to the ankle and is commonly referred to as tarsal tunnel syndrome. Entrapment of the lateral plantar nerve either under the abductor hallucis muscle or at the LE-nerve-entrapments-post tibial nerve tend arch2medial margin of the quadratus plantae and its facia are considered the most common causes of tarsal tunnel syndrome. Patients with entrapment of the tibial nerve proximal to the ankle typically have a more diffuse pain pattern with vague pain or discomfort. They may exhibit symptoms of burning, numbness or tingling on the plantar aspect of the foot. Symptoms are typically worse with standing or walking for extended periods of time, that will improve with rest. Typically this issue is insidious in onset, but may be from a traumatic event.

Entrapment of the tibial nerve distal to the ankle joint typically have chronic heel pain that may have been present for a year or more and may appear to be the same symptomatology as plantar fasciitis. Typically, this condition causes pain with and without weight bearing. Systemic diseases such as diabetes, thyroid disease and alcoholism should be ruled out, as they cause an increase risk for developing peripheral entrapment neuropathies.

Fibular nerves:
LE-nerve-entrapments-super peroneal nerve peroneus longThe common fibular nerve arises from the split of sciatic into its branches at the superior margin of the popliteal fossa. The nerve travels laterally through the fossa under cover of the long and short heads of the biceps femoris toward the head of the fibula. The nerve innervates the short head of biceps femoris along its pathway. The nerve then couses around the head of the fibula and through a fibrous tunnel of the fibularis longus muscle, referred to as the fibular tunnel. Distal to this tunnel the common fibular nerve divides into two branches, the superficial and deep fibular nerves.

Compression or entrapment of the common fibular nerve may occur at any point along its pathway. Its course from the popliteal fossa to the fibular head leaves the nerve exposed for traumatic and compressive forces. Most of these injuries occur at the region of the fibular head. The superficial fibular nerve comes from the common fibular just distal to the fibular tunnel. It descends in the lateral compartment of the leg with the fibularis longus. Approximately mid-way down the leg it emerges superficially from between the fibularis longus and brevis on their anterior aspect and continues to descend to the distal leg and to the dorsal aspect of the foot.

LE-nerve-entrapments-saph nerve vasto add1This nerve is usually symptomatic due to compression from traumatic injury, though it may become entrapped as it emerges from deep beneath the fibularis brevis to take its subcutaneous position. The nerve may be compressed due to compartment syndromes or from trauma to the ankle.Patients typically present with vague pain over the dorsal aspect of the foot. They may also present with pain in the lateral leg or numbness and tingling over the distribution of the nerve. The deep fibular nerve emerges from the common fibular nerve after the fibular tunnel. It crosses into the anterior compartment of the leg and travels inferiorly to the ankle between the tibialis anterior muscle and the extensor digitorum longus in the proximal half of the leg and between the tibialis anterior and extensor hallucis longus in the distal half of the leg with the anterior tibial artery. It crosses the ankle joint anteriorly to supply innervation to the dorsum of the foot. Just proximal to the ankle joint the nerve can be found between the extensor hallucis longus and extensor digitorum longus.

Entrapment of the deep fibular nerve occurs most commonly at the ankle joint where the nerve is exposed to mechanical stress from the extensor retinaculum and may be referred to anterior tarsal tunnel syndrome. It may be compressed by the tendon of the extensor hallucis longus crossing superficial to the nerve. The nerve may also be compressed at this tight from over tightening of shoe laces. Chronic anterior compartment syndrome may affect the nerve at its proximal location around the head of the fibula. Symptoms associated with distal entrapment of the deep fibular nerve include vague pain, cramping or burning on the dorsal aspect of the foot and between the first two digits (web space). Tenderness is usually present over the entrapped segment at or below the ankle joint. Proximal entrapment of the deep fibular nerve may present with foot drop or weakness in dorsiflexion of the foot, or difficulty walking on heels.

Resources:
1. Clinically Oriented Anatomy. Moore 6th ed. Williams and Wilkins

2. Grays Anatomy, The anatomic basis of clinical practice. 40th ed. Churchill Livingstone.

3. Carai A, Fenu G, Sechi E, Crotti FM, Montella A. Anatomic variability of the lateral femoral cutaneous nerve:findings from a surgical series. Clin Anat. Apr 2009;22(3):365-70.

4. Anselimi SJ. Common peroneal nerve compression. J Am Podiatr Med Assoc. Sep-Oct 2006; 96(5):413-7

5. Kopell HP, Thompson WA. Knee pain due to saphenous nerve entrapment. NEngl J Med. 1962; 266:16-9.

6. Keck C. The tarsal-tunnel syndrome. J Bone Joint Surg. 1962;44(1):180-2.

7. Styf J. Entrapment of the superficial peroneal nerve. Diagnosis and results of decompression. J Bone Joint Surg Br. Jan 1989;71(1):131-5.

8. Dellon AL. Deep peroneal nerve entrapment on the dorsum of the foot. Foot Ankle. Oct 1990;11(2)73-80.

9. Bradshaw C, McCrory P. Obturator nerve entrapment. Clin J Sport Med. Jul 1997;7(3):217-9.

10. McCrory P, Bell S, Bradshaw C. Nerve entrapments of the lower leg, ankle and foot in sport. Sports Med. 2002;32(6):371-91.

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