APPLIED ANATOMY OF THE NERVES OF THE LOWER LIMB
Understand the concepts & associated principles, functional & clinical applications of:
1.
The site of intramuscular injections in the buttock (noting the structure endangered if the
correct quadrant is not selected).
Intramuscular Injections
The gluteal region is a common site for IM injection of drugs. Gluteal IM injections penetrate the
skin, fascia and muscles. The gluteal region is a common injection site because the muscles are thick
and large; consequently, they provide a substantial volume for absorption of injected substances by
IM veins.
It is important to be aware of the extent of the gluteal region and the safe area for giving injections.
Some people restrict the area of the buttock to the most prominent part. This misunderstanding can
be dangerous because the sciatic nerve lies deep to this area.
Injections of the buttock are safe only in the superior lateral quadrant of the buttock, or superior to
a line extending from the PSIS to the superior border of the greater trochanter (approx the superior
border of gluteus maximum).
IM injections can also be given safely into the anterolateral part of the thigh, where the needle
enters the tensor fascia latae as it extends distally from the iliac crest and ASIS. The index finger is
placed on the ASIS, and the fingers are spread posteriorly along the iliac crest until the tubercle of
the crest is felt by the middle finger. An IM injection can be safely made in the triangular area
formed between the fingers (just anterior to the proximal joint of the middle finger) because it is
superior to the sciatic nerve.
Complications of improper technique include nerve injury, haematoma, and abscess formation.
2.
i.
ii.
The mechanism and effects of injury to:
the common peroneal nerve;
the sciatic nerve.
Nerve Injuries of the Lower Limb
Common Peroneal Nerve Injury
The common peroneal nerve is in an exposed position as it leaves the popliteal fossa and winds
around the neck of the fibula to enter the peroneus longus muscle. It is commonly injured in
fractures of the neck of the fibula and by pressure from casts or splints. The following clinical features
are present:
Motor:

The muscles of the anterior and lateral compartments of the leg are paralysed, namely,
the tibialis anterior, the extensor digitorum longus and brevis, the peroneus tertius, the
extensor hallucis longus (supplied by the deep peroneal nerve), and the peroneus longus
and brevis (supplied by the superficial peroneal nerve). As a result, the opposing
muscles, the plantar flexors of the ankle joint and the invertors of the subtalar and
transverse tarsal joints, cause the foot to be plantar flexed (foot drop) and inverted, a
condition referred to as equinovarus.
Sensory:

Loss of sensation occurs down the anterior and lateral sides of the leg and dorsum of the
foot and toes, including the medial side of the big toe. The lateral border of the foot and
the lateral side of the little toe are virtually unaffected (sural nerve, mainly formed from
tibial nerve). The medial border of the foot as far as the ball of the big toe is completely
unaffected (saphenous nerve, a branch of the femoral nerve).
Sciatic Nerve Injury
The sciatic nerve (L4, L5, S1, S2, S3) curves laterally and downward through the gluteal region,
situated at first midway between the posterosuperior iliac spine and the ischial tuberosity, and lower
down, midway between the tip of the greater trochanter and the ischial tuberosity. The nerve then
passes downward in the midline on the posterior aspect of the thigh and divides into the common
peroneal and tibial nerves, at a variable site above the popliteal fossa.
Trauma
The nerve is sometimes injured by penetrating wounds, fractures of the pelvis, or dislocations of the
hip joint. It is most frequently injured by badly placed intramuscular injections in the gluteal region.
To avoid this injury, injections into the gluteus maximus or the gluteus medius should be made well
forward on the upper outer quadrant of the buttock.
Most nerve lesions are incomplete, and in 90% of injuries, the common peroneal part of the nerve is
the most affected. This can probably be explained by the fact that the common peroneal nerve
fibres lie most superficial in the sciatic nerve. The following clinical features are present:
Motor:

The hamstring muscles are paralyzed, but weak flexion of the knee is possible because of
the action of the sartorius (femoral nerve) and gracilis (obturator nerve). All the muscles
below the knee are paralyzed, and the weight of the foot causes it to assume the
plantar-flexed position, or footdrop.
Sensory:

Sensation is lost below the knee, except for a narrow area down the medial side of the
lower part of the leg and along the medial border of the foot as far as the ball of the big
toe, which is supplied by the saphenous nerve (femoral nerve).
The result of operative repair of a sciatic nerve injury is poor. It is rare for active movement to return
to the small muscles of the foot, and sensory recovery is rarely complete. Loss of sensation in the
sole of the foot makes the development of trophic ulcers inevitable.
Sciatica
Sciatica describes the condition in which patients have pain along the sensory distribution of the
sciatic nerve. Thus, the pain is experienced in the posterior aspect of the thigh, the posterior and
lateral sides of the leg, and the lateral part of the foot. Sciatica can be caused by:
i.
prolapse of an intervertebral disc, with pressure on one or more roots of the lower lumbar
and sacral spinal nerves,
ii.
pressure on the sacral plexus or sciatic nerve by an intrapelvic tumour, or
iii.
inflammation of the sciatic nerve or its terminal branches.