Exposure of the Sciatic Nerve
The sciatic nerve is the largest nerve in the human body. It originates from the L4–S3 nerve roots, exits the pelvis through the sciatic notch covered by the piriformis and gluteus maximus muscles, and courses between the hamstring muscles of the posterior thigh.
Trauma may be caused by fracture or posterior dislocation of the hip; iatrogenic injury includes surgery and needle injury. An important etiology of entrapment is piriformis syndrome, which may result from inflammation of the piriformis muscle overpassing sciatic nerve, an abnormal course of the sciatic nerve through the muscle, or both.
For lesions that do not extend inferiorly into the thigh, the transgluteal approach may offer adequate exposure while being faster and potentially less structurally disruptive. However, given the challenging nature of this approach, it is best performed by a fellowship-trained peripheral nerve surgeon.
In the gluteal region, only one division of the sciatic nerve may be affected by a given pathology. In such cases, it may be possible to separate the nerve into its peroneal and tibial divisions by careful dissection.
One should be mindful about hemostasis in the gluteal area because injury to a gluteal artery may be difficult to deal with if its retraction into the pelvis occurs, requiring laparotomy, and possibly ligation of the internal iliac artery to reestablish hemostasis.
The sciatic nerve is composed of contributions from the L4–S3 nerve roots. Descending L4 combines with L5 anterior to the ala of the sacrum to form the lumbosacral trunk of the lumbar plexus. The trunk then joins with the first three sacral roots anterior to the origin of the piriformis muscle to form the sciatic nerve. Within the nerve, the tibial division runs medially, formed by the ventral division of L4–S3, and the peroneal division runs laterally, formed by the dorsal division of L4–S3.
As a flattened band ~2 cm wide and encased in fat, the sciatic nerve exits the pelvis through the sciatic notch (greater sciatic foramen) above the ischial spine and lateral to all other passing structures, including the superior and inferior gluteal artery and nerve, the posterior femoral cutaneous nerve, the pudendal nerve, and the internal pudendal vessels. It usually enters the gluteal region as a single band inferior to the piriformis, but may enter through the piriformis, or divide as it leaves the pelvis, in which case the tibial division usually enters inferior to the piriformis and the peroneal division may enter through or superior to the muscle. In each variant, the sciatic nerve remains deep to the gluteus maximus and superficial to the short external rotators of the hip (the obturator internus, superior and inferior gemelli, and quadratus femoris). It descends between the ischial tuberosity medially and greater trochanter laterally and continues between two hamstring muscles, the semitendinous medially and the short head of the biceps femoris laterally. It continues deep to the biceps femoris, as the muscle crosses from the ischial tuberosity to the head of the fibula, and superficial to the adductor magnus. The sciatic nerve typically then divides into the medial tibial nerve and the smaller lateral peroneal nerve in the lower third of the thigh or popliteal fossa, but may divide more proximally including before exiting the pelvis.
Importantly, the sciatic nerve does not provide innervation to any muscles in the gluteal region, but supplies all muscles of the posterior compartment of the thigh including the semimembranous, semitendinous, and biceps femoris. The tibial nerve supplies the posterior muscles of the leg and knee joint and the peroneal nerve gives off branches that supply the skin on the lateral posterior leg and muscles of the knee joint. Arterial supply of the sciatic nerve is provided by the inferior gluteal artery, perforating branches of the femoral artery, and the popliteal artery. The sensory distribution of the sciatic nerve comprises most of the lower leg, from the inferior part of the gluteal region downward.
A critical structure that must be identified during exposure of the sciatic nerve is the posterior femoral cutaneous nerve. This nerve branches out from the sciatic nerve, medially, in the sacral plexus prior to both emerging from the inferior border of the piriformis muscle. It then courses superficially under the deep fascia at the inferior border of the gluteus maximus and over the long head of the biceps femoris. It provides skin sensation to the inferior buttock, posterior and upper medial thigh, and lateral perineum.
Exploration of the sacral plexus and its contributions to the sciatic nerve is difficult from a retroperitoneal approach given that in the lateral position the pelvis slopes away from the surgeon. A transperitoneal approach with the patient supine provides better exposure and control of the vasculature. Lesions within the pelvis therefore typically necessitate laparotomy, which may require general surgical, colorectal, or gynecologic assistance.
Options for the initial incision include a midline, transverse, or pelvic brim approach. Once the peritoneal contents are displaced superiorly, the posterior peritoneum can be transected to expose the lower lumbar and sacral plexuses. The proximal sacral plexus must be approached cautiously due to passing branches of the internal iliac artery and the superior gluteal, inferior gluteal, and internal pudendal arteries.
A limited exposure of the sciatic nerve can be obtained by a muscle-splitting approach through the gluteus maximus muscle. This approach receives limited discussion in the neurosurgical texts, but is the classic orthopedic posterior approach to the hip and may be appropriate for more focal lesions of the sciatic nerve in the gluteal region.
The patient may be positioned prone with all pressure points padded, legs slightly elevated, and a folded sheet beneath the pelvis on the operative side to induce mild hip flexion. Alternatively, the patient may be placed in a true lateral position with the operative side up, the hip mildly flexed, and a pillow between the knees.
An 8- to 10-cm curvilinear incision is made in line with the gluteal fibers and centered on the intertrochanteric crest, which may be palpated at the skin. The incision may begin ~5 cm below and just lateral to the posterior inferior iliac spine, extend laterally to the intertrochanteric crest, and continue inferiorly along the femoral shaft. It is also described in the reverse direction. Alternatively, a linear incision can be made with the hip flexed at 90 degrees so that the fibers of the gluteus maximus are aligned with the femoral shaft. The gluteal fascia and iliotibial tract are also divided along the course of the skin incision.
The gluteus maximus is then divided in line with its fibers and the skin incision beginning laterally and progressing medially. As splitting the muscle along its fibers will inevitably cross vascular planes, some bleeding should be expected. Bleeding can be minimized by staying in the watershed region between the superior and inferior gluteal arteries, which is delineated by the skin incision and the inferior border of the piriformis muscle deep to the gluteus maximus. Blood vessels become more prominent as the dissection moves more medially in the gluteus maximus and gentle blunt dissection is critical to identify and coagulate vessels. Caution is required at this point; vessels torn by dissection may retract into the pelvis and bleed, requiring laparotomy for hemostasis. Splitting the muscle fibers exposes the short external rotators and retraction of the medial portion of the gluteus maximus reveals the sciatic nerve.
The exposure can be extended more proximally up to the sciatic notch by dividing the piriformis at its femoral insertion and retracting medially. It is important when dividing the gluteus maximus muscle to expose sufficient length of the piriformis so that it can be cut through this exposure. For more-distal exposure, the fascial incision can be extended inferiorly into the fascia lata. As the gluteus maximus remains attached at the femur in the transgluteal approach, exposing the sciatic nerve in the posterior thigh requires a second incision.
The wound is closed by approximating the fascia and iliotibial tract. Interrupted, absorbable sutures are used to prevent muscle herniation. The skin and subcutaneous tissue may then be closed in layers.
The infragluteal approach involves the elevation and medial reflection of a myocutaneous flap known as the gluteal lid. Introduced by Stookey (1922) and further detailed by Henry (1957), it remains the predominant approach in neurosurgical texts. The approach is necessary if the sciatic nerve lesion extends beyond the gluteal region into the posterior thigh. By raising the gluteal lid, the approach also obviates the need for crossing vascular planes. This is the most common approach to the sciatic nerve. Nevertheless, it is a morbid operation because it requires detaching and reattaching the gluteus maximus.
The patient is positioned prone as described for the transgluteal approach. The entire buttock and leg are prepped if sural nerve harvest may be necessary. A question-mark incision (Fig. 97.1) begins just lateral to the posterior superior iliac spine, curves laterally along the superior border of the gluteus maximus or ~3 cm inferior and parallel to the iliac crest, and down to the greater trochanter. It then continues medially parallel with the femoral shaft to the level of the gluteal fold and sharply curves inferiorly to the midline of the thigh. If more-distal exposure of the sciatic nerve in the thigh is required, the incision can be continued inferiorly at the midline of the thigh. The posterior femoral cutaneous nerve must be identified and avoided as it travels down the midline of the posterior thigh under the deep fascia inferior to the edge of the gluteus maximus. The incision is carried through the gluteal fascia to expose the gluteus maximus muscle.
Fig. 97.1 Three different possible incisions to access the sciatic nerve through the infragluteal, midthigh, or distal thigh approach. P, piriformis muscle; GM, gluteus maximus muscle; ST, semitendinosus muscle; BF, biceps femoris muscle.
The gluteus maximus inserts on the iliac crest and sacrum rostrally and the iliotibial tract and gluteal tuberosity laterally. The muscle may be freed from the iliotibial tract to reveal its attachments to the femur. Prior to transecting the gluteus maximus, the sciatic nerve is identified in the upper thigh between the semitendinous muscle medially and the biceps femoris muscle laterally. A finger or sponge can be placed superficial to the nerve under the inferior border to the gluteus maximus to protect it during transection (Fig. 97.2). The muscle is then split superiorly, near the insertion into the gluteal tuberosity, leaving a collar for reattachment. As an interval between the gluteus maximus and the underlying gluteus media is developed, the gluteus maximus can be lifted and reflected medially. As the gluteal lid is retracted, tension is avoided on the gluteal arteries and nerve and the posterior femoral nerve. If a gluteal artery is severed, it may retract into the pelvis requiring laparotomy and possibly ligation of the internal iliac artery to reestablish hemostasis. Part of the rostral attachment can be separated from the iliac crest anteriorly to allow a more-medial exposure. Edges of the cut muscle are marked with suture. The sciatic nerve can now be visualized emerging caudally from the sciatic notch and the piriformis muscle, superficial to the short external rotators (Fig. 97.3).
Fig. 97.2 Exposure of the sciatic nerve through the infragluteal approach. The patient is in the prone position. A curved incision is used to expose the sciatic nerve at a buttock level. After curving along the lateral insertion of the gluteus maximus muscle, the incision angles inferiorly onto the posterior thigh to allow for exposure of the nerve inferior to the border of the gluteal musculature (A). The inferior border of the gluteus maximus muscle is elevated to show the sciatic nerve at the pelvic outlet (B). PSIS, posterior superior iliac spine; IC, iliac crest; GM, gluteus maximus muscle; ITB, iliotibial band; TFL, tensor fascia latae; IGA, inferior gluteal artery, nerve; SGA, superior gluteal artery; GT, greater trochanter; SN, sciatic nerve; VL, vastus lateralis muscle; FPA, femoral perforating artery; BF, biceps femoris muscle; PFCN, posterior femoral cutaneous nerve; SR, short rotators; P, piriformis muscle; AM, adductor magnus muscle; GMt, gluteus maximus cut tendinous insertion; ST, semitendinosus.
Fig. 97.3 (A) Intraoperative photograph following incision and retraction of the gluteal muscles, showing the subjacent sciatic nerve and neurofibroma. The patient is in the prone position (superior is at the top of the photo). The inferiorly placed Penrose drain delineates normal sciatic nerve distal to the lesion. The rostral portion of the lesion is located at the outlet of the pelvis. (B) Artist’s rendering depicting details of the relevant anatomy. 1. Sciatic nerve; 2. perineural fat; 3. biceps femoris; 4. semitendinosus.
To expose the nerve more proximally into the pelvis the piriformis muscle is divided just as in the transgluteal approach. Bone and ligament around the sciatic notch can be removed for further exposure (Fig. 97.4).
Fig. 97.4 (A) Intraoperative photograph and (B) artist’s depiction showing a magnified view of the rostral portion of the sciatic nerve and neurofibroma at the pelvic outlet. The patient is in the prone position (superior is at the top of the photo). Sectioning of the piriformis muscle (tagged with the suture and elevated with the retraction) enables entry into the pelvic outlet at this level. 1. Tibial division; 2. popliteal fossa; 3. scatic nerve; 4. peroneal nerve.
To close, the gluteus maximus is reattached both laterally to the femur and superiorly to the iliotibial tract. Poor reapproximation can lead to gluteal dysfunction. Subcutaneous tissue and skin are closed in layers.
The sciatic nerve may be explored in the thigh by inferior extension of the infragluteal question-mark incision in the midline of the posterior thigh or by an independent longitudinal incision beginning at the gluteal fold (Fig. 97.5). The incision continues down the midline of the thigh approximately over the division between the semitendinous and biceps femoris muscles. Care is taken not to injure the posterior femoral cutaneous nerve as it courses over the long head of the biceps femoris beneath the deep fascia. The deep fascia is opened and the semitendinous and the biceps femoris muscles (Fig. 97.6) are separated to reveal the sciatic nerve under the long head of the biceps femoris, superficial to the adductor magnus (Fig. 97.7). Closure is generally straightforward because the hamstring muscles naturally close over the operative field and do not need to be sutured separately.
Fig. 97.5 (A) Intraoperative photograph and (B) artist’s depiction demonstrating exposure of the right sciatic nerve at a midthigh level. The patient is in the prone position (superior is at the top of the photo). Dissection through the subcutaneous tissues reveals the medial (M) and lateral (L) hamstring muscles, separated by a fatty plane. The sciatic nerve is located deep within this plane, between the muscles. IGA, inferior gluteal artery; GF, gluteal fold; ST, semitendinosus muscle; PFCN, posterior femoral cutaneous nerve; BF, biceps femoris muscle; AM, adductor magnus muscle; SN, sciatic nerve.
Fig. 97.6 Intraoperative photograph demonstrating exposure of the right sciatic nerve at a midthigh level. The patient is in the prone position (superior is at the top of the photo). Dissection between the hamstring muscles reveals the sciatic nerve. A nerve sheath tumor is shown within the sciatic nerve.
Fig. 97.7 (A) Intraoperative photograph and (B) artist’s depiction demonstrating exposure of the left sciatic nerve at the distal thigh level. The patient is in the prone position (superior is at the top of the photo). The sciatic nerve splits into the tibial (T) and peroneal (P) nerves at this level. The tibial nerve continues along the midline of the leg and enters the popliteal fossa, whereas the peroneal nerve courses laterally toward the fibular head. SM, Semimembranosus muscle; AM, adductor magnus muscle; BF, biceps femoris muscle; PA, popliteal artery; PV, popliteal vein; TN, tibial nerve; CPN, common peroneal nerve; LHG, lateral head of the gastrocnemius muscle; MSCN, medial sural cutaneous nerve.
The sciatic nerve may be explored in the distal thigh by placing the incision a bit more laterally from the midline as one gets closer to the fibular head. Once the hamstrings are split, as for the midthigh exposure, the sciatic nerve becomes visible. Within the distal thigh above the popliteal fossa, the sciatic nerve splits into the tibial and peroneal divisions. Slightly more distally, each of these divisions will give off a sural cutaneous branch, which later coalesces to form the sural nerve in the calf.
During transgluteal and infragluteal approaches, it is important to avoid vascular injuries to the gluteal arteries and their branches, because the vessels of these regions (superior and inferior gluteal arteries and their branches) can retract into the pelvis if torn.
To avoid injuries to the gluteal arteries during transgluteal approach, it is important to bluntly dissect gluteus maximus in the line of the skin incision, which corresponds to the virtually bloodless plane between the two gluteal arteries.
The posterior femoral cutaneous nerve innervates the skin of the perineum and posterior aspect of the thigh and leg. It is at big risk during the initial phases of the infragluteal approach because it travels on the midline under the deep fascia. Care needs to be taken to avoid damaging it, which will result in obvious unpleasant sensory disturbances.
If, during the transgluteal or infragluteal approach, branches of the gluteal arteries are accidentally injured and retract into the pelvis, it is important to abandon the procedure and drape the patient for an immediate emergency laparotomy with the aid of a general or vascular surgeon.
Outcome depends on the extension of the injury itself as well as the anatomic location. Lesions that are more proximal have worse outcomes than those located distally. Likewise, pathology requiring nerve grafting typically recovers less function than those that only need simple decompression (neurolysis).
3 Murovic JA. Lower-extremity peripheral nerve injuries: a Louisiana State University Health Sciences Center literature review with comparison of the operative outcomes of 806 Louisiana State University Health Sciences Center sciatic, common peroneal, and tibial nerve lesions. Neurosurgery. 2009;65(4) Suppl:A18-A23.