The sciatic nerve is the largest nerve in the body. Distally it branches medially to the Tibial Nerve and laterally to the Common Peroneal Nerve. It is formed from the ventral rami of the fourth lumbar to third sacral spinal nerves and is a continuation of the upper part of the sacral plexus.
It leaves the Pelvis through the greater sciatic foramen, inferior to the Piriformis muscle, and descends between the greater trochanter of the Femur and the ischial tuberosity. Initially deep to piriformis, it runs inferiorly and laterally posterior to the ischium, crossing over the nerve to Quadratus Femoris.
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| Sciatic Nerve |
Root
L4, 5, S1, 2, 3
Branches
Tibial nerve
Common peroneal nerve
Function
Motor
- Hamstrings
- Biceps femoris
- Semitendinosus
- Semimembranosus
- Adductor magnus
Indirectly innervates (via common peroneal and tibial nerves) the muscles of the leg and foot.
Sensory
Indirectly innervates (via common peroneal and tibial nerves) the skin of the lateral leg, heel, and both the dorsal and plantar surfaces of the foot.
Clinical Relevance
There is some variability between the relationship of the sciatic nerve, piriformis muscle and short external rotators.
In approximately 85% of cases the sciatic nerve exists as described above. In 11% of individuals a portion of the piriformis muscle splits the common peroneal nerve and tibial nerve.
Piriformis Syndrome
Youngman described 'Piriformis Syndrome' in 1928 as compression of the sciatic nerve by the piriformis muscle. This is associated with acute trauma to the buttock and occurs when the sciatic nerve exits posterior to the piriformis.
The patient's may report that sitting and activities that require hip flexion or internal rotation as aggravate the pain. The pain radiates in a sciatic nerve distribution.
Objective examination reveals tenderness directly over the piriformis or in the gluteal area, and the pain can be replicated by internal rotation of the extended thigh. This finding is called the 'Pace's sign'.
There is sometimes weak abduction or external rotation against resistance. The pain may also be reproduced by rectal or vaginal examination.
Initial medical management is oral anti-inflammatory medication. Steroid injection and surgical exploration may be considered for chronic cases.
Hamstring Syndrome
This pathology commonly affects athletes who present with localised and radiating pain near the ischial tuberosity. The pathophysiology is thought to be that of an insertional tendopathy at the ischium but there may also be involvement of sciatic nerve compression.
The pain in hamstring syndrome radiates down the posterior thigh or popliteal region and is exacerbated when the hamstrings are on tension. This is often seen in sprinters or hurdlers.
On examination there is exquisite tenderness over the ischial tuberosity and percussion in that region may reproduce the sciatic distribution of pain. Treatment involves rest, anti-inflammatory agents and steroid injections.
Hip Dislocation
Hip dislocations are most commonly associated high-energy trauma and therefore are often associated with multi-system injuries.
It is important to examine for sciatic nerve injuries. The common peroneal division of the sciatic nerve is most commonly involved. Care is needed durring assessment as a femoral fracture can mask otherwise obvious hip dislocation.
Surgical Approach for viewing of sciatic nerve injury post hip dislocation
Kocher-Langenbeck Approach (Posterior Approach)
'Kocher-Langenbeck' approach is the standard posterior approach to the hip joint.
Performed in a lateral decubitous position without use of a traction table.
To access the sciatic nerve the gluteus maximus muscle is split, where the nerve can be identified and examined.
After identifying the sciatic nerve, the tendinous insertions of the piriformis muscle and the obturator internus are identified.
When surgeons perform capsulotomies they take care not to injure the acetabulum labrum. The joint can then be assessed, any fracture fragments removed or stabilized and the entire area thoroughly washed out.
Anterior Approach
The anterior approach to the hip for injuries can be performed through the 'Smith Peterson' or 'Watson Jones' approaches.
These can be performed with the patient in the semi-lateral or lateral dicubitous position.
Intramuscular Injections
The path of the sciatic nerve must be considered when administering intramuscular injections into the gluteal region. The region can be divided into quadrants using 2 lines, marked by bony landmarks:
One line descends vertically from the highest point on the iliac crest.
The other horizontal line passes through the vertical line half way between the highest point on the iliac crest and ischial tuberosity.
The sciatic nerve passes through the lower medial quadrant. To avoid damaging the sciatic nerve therefore, intramuscular injections are given only in the upper lateral quadrant of the gluteal region.
Neurological Examination
Myotomes
- Ankle dorsiflexion (L4)
- Extension of the big toe (L5)
- Eversion of the ankle, contraction of buttock and knee flexion (S1)
- Knee flexion and toe standing (S2)
- Pelvic floor, bladder and genital function (S3)
Dermatomes
- Over the medial malleolus (L4)
- On the dorsum of the foot at the third metatarsophalangeal joint (L5)
- On the lateral aspect of the calcaneus (S1)
- At the midpoint of the popliteal fossa (S2)
- Over the tuberosity of the ischium or infragluteal fold (S3)
Reflexes
- Knee jerk (L3 and 4)
- Ankle jerk (S1)
Straight leg raise
The Straight Leg Raise Test (SLR) test is a neurodynamic test. Neurodynamic tests check the mechanical movement of the neurological tissues as well as their sensitivity to mechanical stress or compression.
These tests, along with relevant history and decreased range of motion, are considered by some to be the most important physical signs of disc herniation, regardless of the degree of disc injury.
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