Landmarks at the gluteal region:
- the gluteus maximum
- the ischial spine
- pudendal vessels
Landmarks at the upper thigh:
- greater trochanter of the femur
- ischial tuberosity
- common tendon of hamstring muscles
Landmarks at the popliteal fossa:
- popliteal artery and vein
The patient is prone on the examination table.
Palpate the anatomical bony reference structures: the greater trochanter of the femur and the ischial tuberosity.
Then place the transducer in the transverse plane on the ischial tuberosity,
where the common tendon of Hamstring muscles can be seen.
Then move the transducer laterally (between the ischial tuberosity and greater trochanter): the Sciatic nerve is seen lateral to the ischial tuberosity overlying the Quadratus Femoris muscle.
To easily identify the sciatic nerve,
also in obese patients,
use the low-frequency curved probe to identify both the greater trochanter and ischial tuberosity,
then use a high-frequency linear probe to increase resolution.
The ischial tuberosity (medial) and the greater trochanter (lateral) should be visible as hyperechoic curved structures on the right and left sides of the ultrasound screen.
The Sciatic nerve is between these two landmarks and appears as a hyperechoic fasciculate oval structure.
Follow the Sciatic nerve in the posterior thigh: it descends between and deep to the Biceps Femoris (lateral) and Semitendinosus and Semimembranosus muscles (medial) and superficial to the Adductor Magnus muscle (Fig.3 and 4).
Then the Sciatic nerve reaches the proximal margin of the popliteal fossa,
where it branches into the Common Peroneal and Tibial nerves.
Identify the point of bifurcation of the sciatic nerve and the popliteal artery and vein deep to the nerve (Fig.
5).
Turn the probe by 90° to examine the sciatic nerve in long-axis (longitudinal).
With more difficulty due to its deep position,
the sciatic nerve can be identify with a gluteal approach: it emerges from the greater sciatic foramen under the piriformis muscle and lies deep to the Gluteus Maximus muscle and superficial to the inner muscle layers.
Always with the patient supine,
palpate the posterior superior iliac spine,
the sacral hiatus and the greater trochanter.
Place the probe in a transverse axis in a mid-gluteal position (the midpoint of the line joining the sacral hiatus and the greater trochanter): the Sciatic nerve is scanned in its short axis as a triangular hyperechoic bundle under the Gluteus Maximus; the ischial bone is identified as a continuous linear hyperechoic structure.
Move the probe caudally to identify the ischial spine,
which is seen as a curved hyperechoic line,
with the pudendal vessels above it.
Use the color Doppler to help in the identification of the pudendal vessels .