Course
The sciatic nerve runs from the pelvis to the popliteal fossa, passing through the gluteal region and the posterior compartment of the thigh.
Origin
It forms on the anterior surface of piriformis from the ventral rami of L4 to S3. It is really two nerves bound in one sheath: a tibial part from the anterior divisions of L4–S3 and a common fibular (peroneal) part from the posterior divisions of L4–S2.
In the gluteal region
The nerve leaves the pelvis through the greater sciatic foramen, usually below piriformis. It curves downward and laterally deep to gluteus maximus, lying first on the posterior surface of the ischium and then crossing, in turn, superior gemellus, the tendon of obturator internus, inferior gemellus and quadratus femoris. It passes between the ischial tuberosity and the greater trochanter, closer to the tuberosity.
In the thigh
Below gluteus maximus the nerve lies on adductor magnus and is crossed obliquely from medial to lateral by the long head of biceps femoris. It descends in the midline of the back of the thigh, covered by the hamstrings.
Termination
It usually divides into the tibial and common fibular nerves at the apex of the popliteal fossa. The level varies widely: the division can occur anywhere in the thigh, and in some people the two parts leave the pelvis separately, with the common fibular part passing through or above piriformis.
Branches and distribution
In the thigh the sciatic nerve gives only muscular branches; all its skin territory is reached through its two terminal branches.
| Part | Muscles supplied in the thigh | Continues as |
|---|---|---|
| Tibial part | Semitendinosus, semimembranosus, long head of biceps femoris, hamstring (ischiocondylar) part of adductor magnus | Tibial nerve: posterior leg and sole |
| Common fibular part | Short head of biceps femoris | Common fibular nerve: anterior and lateral leg, dorsum of foot |
The muscular branches arise from the medial side of the nerve, apart from the branch to the short head of biceps. The lateral side of the nerve is therefore the safer side in posterior surgical exposures of the thigh.
The sciatic nerve supplies no gluteal muscle. Gluteus maximus is supplied by the inferior gluteal nerve, and the skin of the back of the thigh by the posterior femoral cutaneous nerve (S1–S3), both of which leave the pelvis alongside it below piriformis.
Relations and surface marking
The sciatic nerve is the most lateral structure passing below piriformis, which makes it the structure at risk from lateral hip approaches and misplaced buttock injections.
- Below piriformis, medial to the nerve: the posterior femoral cutaneous nerve, inferior gluteal nerve and vessels, internal pudendal vessels, pudendal nerve and the nerve to obturator internus.
- Deep: the posterior ischium, the gemelli, obturator internus tendon, quadratus femoris and then adductor magnus.
- Superficial: gluteus maximus above, then the long head of biceps femoris.
- Blood supply: a slender artery to the sciatic nerve from the inferior gluteal artery runs on or within it, joined lower down by twigs from the perforating arteries.
To mark the nerve on the skin, draw a curved line from a point midway between the posterior superior iliac spine and the ischial tuberosity, through a point between the ischial tuberosity and the greater trochanter, then straight down the middle of the back of the thigh to the apex of the popliteal fossa.
Clinical relevance
Sciatic nerve injury paralyses all muscles below the knee and weakens knee flexion, producing foot drop and a flail foot.
Causes
- Posterior dislocation of the hip and acetabular fractures stretch the nerve over the displaced femoral head. The common fibular part is injured more often than the tibial part.
- Hip arthroplasty, especially through a posterior approach.
- Intramuscular injection in the lower or medial buttock. Injections belong in the upper outer quadrant, or at the ventrogluteal site over gluteus medius, well away from the nerve.
- Penetrating trauma, prolonged pressure in the unconscious patient, and tumours of the nerve sheath.
Signs
Knee flexion is weakened but not lost, because sartorius and gracilis still act. There is no active movement at the ankle or in the toes, the ankle jerk (S1) is lost, and sensation is absent below the knee except along the medial leg and foot, which is saphenous territory.
Sciatica
Sciatica is pain radiating down the back of the leg in a root distribution, usually from a lumbar disc prolapse compressing L5 or S1. It is a root lesion rather than an injury to the sciatic nerve itself. Piriformis syndrome, compression of the nerve by piriformis, is proposed as a less common cause and remains debated.