Surgical exposure of the peripheral nerves of the lower extremity is required less frequently than exposure of the nerves of the upper extremity. The surgical techniques employed are, therefore, often somewhat foreign to most surgeons. With the introduction of new surgical advances in the treatment of paraplegia below D8 and the need for nerve grafts, which is yet not feasible from donors to obtain cross-anastamoses, this area is regaining more attention, so as to harvest as maximally nerves as possible, without causing functional impairment of the lower limbs, in case of achieving regeneration of the targeted end organs.

In the anterior aspect of the thigh, two nerves of clinical importance are the lateral femoral cutaneous and the femoral nerves. The anatomical relationships and the surgical exposure of both of these nerves is relatively straightforward.

Lateral Femoral Cutaneous Nerve

The lateral femoral cutaneous nerve is commonly entrapped as it exits the pelvis. It typically courses through the pelvis along the lateral border of the psoas major muscle and then runs across the iliacus muscle obliquely toward the anterior superior iliac spine. At the anterior superior iliac spine it passes under the inguinal ligament and over the sartorius muscle. It then branches into a distinct mid-anterior and posterior branch. As the nerve courses around the anterior superior iliac spine, in the vicinity of the strong inguinal ligament, it can be entrapped and compressed. This is usually associated with swelling and/or neuroma formation, which may lead to the clinical syndrome meralgia paresthetica, consisting of pain, dysesthesia, and often hypalgesia. These symptoms occur more often in the distribution of the mid-anterior branch than the posterior branch. Both, however, may be involved. More than 90% of symptomatic patients with this condition are best managed by conservative means. Lateral femoral cutaneous neuropathy should be differentiated from far lateral disc herniation at the Ll-2 or L2-3 level. Disc herniation, however, is associated with muscle dysfunction. Electromyography (EMG) is helpful in differentiation since meralgia paresthetica is purely a sensory phenomenon.

Operative exposure of the lateral femoral cutaneous nerve is performed by 6-8 cm skin incision along the skin lines beneath the lateral extent of the inguinal ligament and the anterior superior iliac spine is carried down to fascia lata. A self-retaining retractor is placed in the wound. The fascia lata is then incised in the same direction as the skin incision. By dissecting bluntly beneath fascia lata, the nerve is identified. The most common site of nerve entrapment is at the level of the inguinal ligament. A rongeur is used to remove the medial edge of the iliac bone in order to create a tunnel for the nerve. The superior aspect of the inguinal ligament is preserved. The nerve occasionally divides proximal to the inguinal ligament. Most commonly, however, it branches in the leg. These variations may be important at the time of surgery. Rarely, the nerve may pass between the two leaves of the inguinal ligament which may cause similar symptomatology and may respond as well to surgery.

Surgical treatment of meralgia paresthetica is controversial. Simple section of the nerve has been suggested as the best treatment. Good results have been reported from exposing the nerve, pulling it down, and transecting it above the neuroma. Transecting the nerve is not required in all cases. If one simply provides a bony decompression in the path of the nerve as it courses around the anterior superior iliac spine, the continuity of the nerve can be preserved. The neuroma will often resolve and the patient's symptoms subside. The results are not instantaneous, however. Bone wax may be used to discourage local bone regrowth.

 

The Femoral Nerve

The femoral nerve is the largest branch of the lumbar plexus. It provides considerable sensory innervation to the anterior aspect of the thigh and knee, and motor innervation to the quadriceps muscles. As it emerges from the pelvis in a groove between the psoas and the iliacus muscles, it closely approximates the external iliac artery. Together with the artery, the nerve passes beneath the inguinal ligament in the femoral triangle. The vein and lymphatics are located further medially. High in the inguinal region, it gives a branch to the sartorius muscle. Clinically, it is sometimes difficult to ascertain sartorius muscle weakness. EMG studies may be helpful. If sartorius muscle dysfunction is demonstrated, a high femoral nerve injury is likely. Within 2-3 cm below the inguinal ligament, the femoral nerve divides into many branches. Superficially, the cutaneous branches may be divided into anterior, intermediate, and medial components, the most medial of which is the saphenous nerve that supplies cutaneous sensory innervation to the medial aspect of the lower leg. The saphenous nerve passes deep to the sartorius muscle and accompanies the femoral artery.

The major motor branches of the femoral nerve lie directly adjacent to or beneath the target muscles, starting with the sizeable branch to the rectus femoris and vastus medialis muscles. Approximately 10 cm further along its course, the nerve divides into multiple branches. Due to this anatomical relationship, it is practically impossible to distinguish the neural anatomy following deep devitalizing injuries to the terminal region of the femoral nerve. The surgical approach to the proximal portion of the femoral nerve is achieved by using either a vertical incision or the preferred 'skin line' incision which is parallel to (and 2 cm below) the inguinal ligament. The latter is curved downward over the sartorius muscle. A small branch of the genitofemoral nerve may also be identified and dissected. Once the skin and subcutaneous tissues are divided, fascia lata is identified, incised and retracted widely. The deeper fascia is then incised over the femoral triangle. This fascial layer is an extension of fascia lata. The incision over fascia lata should run parallel to the medial margin of the sartorius muscle. Upon retracting this fascia, the femoral nerve, artery, and vein are visible. The femoral artery then can be dissected, freed and retracted medially.

 

 The saphenous nerve

 A vertical incision is made over the final fascial layer, the iliacus fascia. Extension and exposure of the femoral nerve between the iliacus and psoas muscles is performed proximally toward and underneath the inguinal ligament. The nerve then can be easily dissected distally to its multiple branches where the cutaneous branches tend to be more superficial and the muscle branches tend to be deeper. The distal saphenous nerve may be used for nerve grafting. It is harvested by tracing it distally to the medial aspect of the knee. The sural nerve may be used if the available saphenous nerve is inadequate. If the femoral nerve has been sharply transected, a single anastomosis can be achieved. Nerve length is gained by dissecting the nerve proximally underneath the inguinal ligament and distally to past its branching point. A primary anastomosis is then performed. Postoperatively, the patient should be placed in a hip spica orthosis with the leg flexed approximately 70°. It is difficult to differentiate between motor and sensory branches in extensively injured nerves. The most superficial branches tend to be sensory, and the deeper branches tend to be motor. In the author's experience, the major branches to the vastus medialis and vastus lateralis can be successfully anastomosed with cable grafts, facilitating return of a near normal gait. Distal sensory loss from permanent changes in the saphenous nerve is not a handicap.

Injury to the sciatic nerve is common and is often associated with a proximal neurologic deficit. Associated vascular injury is uncommon due to the lack of anatomically close vascular relationships. The goal of therapy is to restore muscle function if possible, and to obtain protective sensation.

The sciatic nerve arises from the lumbosacral plexus, from the fifth lumbar, first sacral, and second sacral nerves. The more superior and lateral aspect of the lumbosacral plexus gives rise to the common peroneal nerve, while the more medial and inferior aspect of the plexus contributes to the tibial nerve. Often, these two segments can be identified through nearly the entire length of the thigh. The common peroneal portion of the sciatic nerve is positioned laterally. The sciatic nerve exits the pelvis through the great sciatic foramen. Once it has left the confines of the pelvic structures, it is easily identified and dissected. To achieve exposure of the sciatic nerve at the level of the sciatic notch, the patient is placed in a prone position. A line from the posterior iliac spine toward the greater trochanter is imagined. The incision is made along the distal aspect of this line and curved underneath the crease of the gluteus maximus muscle. It is carried through the soft tissue to the muscle itself. The gluteus maximus muscle may be partly transected close to its insertion onto the femur, iliotibial band, and fascia lata.56

 The gluteus maximus muscle is then retracted superiorly and hinged medially to expose the sciatic nerve as it exits beneath the piriformis muscle and leaves the pelvis. The sciatic nerve is identified by carrying the incision distally and following the nerve proximally. Many variations occur in the sciatic nerve's relationship to the piriformis muscle: the piriformis muscle occasionally passes between the two divisions of the nerve, but most commonly the nerve courses below this muscle. The inferior gluteal artery should be identified and protected because it contributes to the blood supply of the sciatic nerve. In the posterior thigh, the sciatic nerve is easily exposed with a vertical incision. In the mid­thigh, the biceps femoris muscle passes over the nerve. The integrity of this muscle should be preserved. In the distal aspect of the thigh, the biceps femoris muscle is retracted laterally. More proximally, the biceps muscle is retracted medially. The posterior femoral cutaneous nerve is maintained by medial biceps femoris muscle retraction, thus leading to no additional sensory loss. Once the sciatic nerve is dissected, an attempt to divide its two major branches, the common peroneal and tibial nerves is undertaken. This facilitates repair by allowing for two separate anastomoses. In many cases, only one part of the nerve requires repair, while the other simply requires a careful neurolysis. By using the techniques of intraoperative nerve action potential recording, intraoperative decision-making is facilitated. The more distal aspect of the sciatic nerve in the thigh is easily exposed through a curvilinear incision over the popliteal fossa. The common peroneal and tibial branches are readily identified between the biceps femoris muscle laterally and the semitendinosus and semimembranosus muscles medially. By dissecting under the biceps femoris muscle and working proximally, the nerve can be completely freed from surrounding tissue. The femur in this region is directly below the nerve and can be readily palpated.

In the distal leg, the common peroneal and tibial nerves have already divided. The common peroneal nerve hugs the border of the biceps femoris muscle as it descends on the fascial attachments next to the fibula. The nerve then passes around to the lateral aspect of the proximal fibula. The common peroneal nerve is palpable just below the prominence of the fibular head. It passes below the extensor digitorum longus and the peroneus muscles. The superficial branch maintains a lateral course and innervates the cutaneous aspect of the dorsum of the foot.The deep peroneal nerve is the more impor­tant branch. It innervates the extensors of the toes. The tibial nerve descends through the midportion of the popliteal fossa between the heads of the gastrocnemius muscle into the back of the leg. Two important cutaneous branches are found at the point of bifurcation of the sciatic nerve into distinct entities of the common peroneal nerve and the tibial nerve. The lateral cutaneous sural nerve arises from the common peroneal nerve, and the medial cutaneous sural nerve arises from the tibial nerve. Often these two branches will rejoin as the common sural nerve. If these branches are observed to be intact during the exposure of this region, they should be preserved.

Lateral Calf

Exposure of the common peroneal nerve in the popliteal fossa is accomplished with a curvilinear incision. In this manner, the incision does not directly cross the region of the popliteal fossa. The nerve's course, once the subcutaneous tissue is penetrated and flaps are raised, is easily identified on the medial and inferior aspect of the biceps femoris muscle. It then passes lateral to the lateral head of the gastrocnemius muscle. Once beneath the fibular head, a small recurrent articular branch is observed. It then divides into its two major branches: (1) the deep peroneal nerve supplies the tibialis anterior muscle and the extensors of the toes; (2) the superficial peroneal nerve provides two small branches to the peroneus longus and brevis muscles and is responsible for sensation on the dorsum of the foot. The most common location for peroneal nerve injury is just below the fibular head and below the large attachment for the biceps femoris muscle. Beneath the fibular head, there is a groove under which the common peroneal nerve passes. It is in this groove that the nerve is commonly injured. The injury may be as trivial as a minor blow, may be caused by squatting for a prolonged period of time, or may be a result of cast compression. The common peroneal nerve is very vulnerable to injury as it passes over (around) the fibular head.

The most frequent operation on the common peroneal nerve in this area is a release of the distal fascia over the nerve with an accompanying external neurolysis. A curvilinear incision is used for the exposure of the nerve, beginning in the popliteal fossa. It is curved downward on the lateral aspect of the thigh. A cutaneous flap is raised. An external neurolysis may then be performed distally to where it branches into its deep and superficial components. The common peroneal nerve is essential for dorsiflexion of the foot. While foot drop can be treated with a cock-up splint and brace, aggressive attempts at the restoration of nerve function is desirable in younger patients. A more distal exposure is needed for anastomotic repair of the common peroneal nerve. The knee joint can be flexed to as much as 70°- 80° in order to obtain a relaxed nerve. The leg is casted for 3 weeks in this position. The cast is then removed and a splint applied so that the leg is not overextended for the next 2-3 weeks. Physical therapy is used in order to obtain full mobility of the knee. On rare occasion, the deep peroneal nerve can be injured focally. This nerve is located on the lateral border of the fibula and adjacent to the extensor digitorum longus muscle. At this point the anterior compartment of the leg is vulnerable to pressure and compression. A syndrome of anterior tibial compartment compression is characterized by severe pain, swelling, and discoloration over the anterior aspect of the leg and over the foot with strenuous activity. Weakness of foot dorsiflexion may be present. The treatment of this condition it to enlarge the osseous fascial compartment where the nerve is entrapped along with the artery. Following decompression, the patient should be relieved of the symptoms of the so-called "shin splints".

The incision for this exposure is over the palpable tibialis anterior muscle. The dissection is carried distally between the tibialis anterior and the extensor hallucis longus muscles. Once the deep peroneal nerve is exposed, the anterior tibial artery and vein are visualized. By simply separating the strong fascial planes in this area, a neurolysis can be achieved.

Medial Calf

The tibial nerve lies directly in the midline of the popliteal fossa. It courses between the two heads of the gastrocnemius muscle to lie between the flexor digitorum longus and the flexor hallucis longus muscles. Just distal to the knee posteriorly, the tibial nerve divides into many branches. These include two branches to the gastrocnemius muscle and branches to the popliteus muscle and the plantar flexors of the foot. Further distally, the branches to the flexor digitorum longus and hallucis longus muscles become apparent. The deep aspect of the tibial nerve continues distally. Near the ankle, it may be entrapped in a similar manner to median nerve entrapment in the carpal tunnel syndrome.

The approach to the tibial nerve in the popliteal fossa is best made through a Z-plasty type of exposure. Further distally, an incision along the medial border of the gastrocnemius muscle can be used for exposure of the tibial nerve in the dorsal midline. Once the incision is carried through the fascia, the medial border of the gastrocnemius muscle can be retracted to expose the popliteus and soleus muscles. It is necessary to dissect through some of the fibers of the soleus muscle to visualize the medial aspect of the transverse intermuscular septum. Once this fascial layer is opened vertically, the tibial nerve is easily exposed directly behind the tibia. Following its exposure, distal dissection of the tibial nerve is carried out with ease.

At the level of the ankle, the tibial nerve can be entrapped in the "tarsal tunne1." The nerve is simply exposed around the medial malleolus through the thick fascial planes which support the ankle. The strong flexor retinaculum on the medial aspect of the ankle needs to be opened in order to fully decompress the nerve in this area. Rarely is it necessary to take down the muscular attachments of the gastrocnemius muscle in order to obtain exposure of the nerve in the popliteal fossa and in the more medial aspect of the leg itself. The tibial nerve, as it leaves the popliteal fossa, passes under a tendinous arch of the soleus muscle. This should be opened. The nerve then descends beneath the transverse intermuscular septum where it overlies the tibialis posterior muscle proceeding toward the medial malleolus. The distal aspect of the tibial nerve can be entrapped under the retinaculum beneath the medial malleolus.

Summary

There are many anatomical variations of peripheral nerves of the lower extremity. Only by studying a variety of textbooks, both old and new, can the patient's particular problems often be appreciated. This allows for the best possible patient care. It is important to notice that the sural and the saphenous nerves can bridge the gap between three-four levels of anastamosis, eliminating during that the need for two donors in the surgical treatment of paraplegia below D8. It is important to mention that the targeted nerves, must be long, minimally branching and cause no harm to the patient after harvest.

 

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