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PNS Anatomy #15
#15
P amake
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This lecture about Sciatic Nerve, I will be concentrating what the Doctor concentrated and I |will
PNS Anatomy #15
clear as much as possible every point the doctor said it is important. As all anatomy lectures I will be
exam oriented but as I know every student don’t have time to study all the sources so I will make
understandable as much as I can and also try to cover most of the topic.
Sciatic nerve (You need to know a lot about it.)
Branch of the sacral plexus (L4-S3), lies posterior wall
of the pelvic cavity anterior to piriformis muscle and
emerges from the pelvis through the lower part of the
greater sciatic foramen and go to gluteal region. It is
the largest nerve in the body and consists of the tibial
and common peroneal (fibular) nerves bound together
with fascia.
Tibial and common peroneal (Febular) nerves are
separate nerves from the beginning and they have
different origin in sacral plexus (Tibial >>> Ventral
division of L4-S3, Febular >>> Dorsal division of L4-S2),
observe in the pic.
Golden piece of information:
L3/L4 and L4/L5 >>> common place for prolapsed disk (herniated disk).
L4 >> lumbosacral tract (a nerve that is connecting Lumbar plexus to sacral plexus) V.imp >> because
during late pregnancy when the baby is in pelvic brim, baby`s head will compress the L4
The greater sciatic foramen is divided into superior and inferior by the piriformis muscle so the
sciatic nerve passes inferior to piriformis muscle most of the population but there is very few people
that is passes superior to the piriformis muscle. This point the doctor said its important twice, so it’s
very very important.
Attention please:
In the pelvic cavity >>> Anterior surface of piriformis muscle.
In the sciatic foramen >>> Passes Inferior to the Piriformis muscle.
Piriformis Syndrome
when the piriformis muscle shortens or spasms due to trauma or overuse, it can compress or
strangle the sciatic nerve beneath the muscle. It causes pain, tingling and numbness in the buttocks
and along the path of the sciatic nerve descending down the lower thigh and into the leg.
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In the gluteal region,
the sciatic nerve emerges between the ischial tuberosity medially and the greater trochanter of
the femur laterally.
It descends in the plane between the superficial and deep group of gluteal region muscles.
In passes posterior to deep group of muscles except Piriformis and Anterior to superficial group of
muscles except gluteus maximus. It passes over obturator internus and quadratus femoris.
Deep group: the gemellus superior, the obturator internus, the gamellus inferior, quadratus
femoris and piriformis.
Superficial group: the gluteus minimus, medius and maximus & tensor fascia lata.
Very imp:
Medial to the sciatic nerve at gluteal region there is many structures: the posterior cutaneous
nerve, the inferior gluteal artery and nerve and the pudendal artery and nerve.
The sciatic nerve touches the bone in between ischial tuberosity and posterior superior iliac spine
(PSIS) posterior to the acetabulum, so the hip is dislocated posteriorly, the sciatic nerve will be
injured. Hip joint is very stable joint but huge force can dislocate it; most common example is car
accidents. In car accident when you are sitting with your hip flexed and a great force comes and
pushes the femur back then the head of the femur will dislocate posteriorly and damage the sciatic
nerve.
Posterior compartment of the thigh
At the lower margin of the quadratus femoris muscle, the sciatic nerve enters the posterior
compartment of the thigh, it lies on the adductor magnus muscle, and is covered by the long head
of the biceps femoris and semimembranosus.
The greater sciatic nerve bifurcates in the posterior compartment of the thigh into its terminal
branches, the tibial and the common fibular nerves. The point of bifurcation varies, it is usually
hand breadth above the knee, but it could be as high as the greater sciatic notch.
Clinical note: Herpes Zoster virus infection starts at the nerve root and it spreads along the
distribution of that nerve only. Here comes the importance of dermatomes and the doctor
repeated many times to know well the dermatomes and showed us some cases:
Case 1: gluteal cleft >>> its root is S3 , PYQ >> S2
Obturator nerve >> L3, we sit on S3 and S4
Case 2: lateral of the thigh >>> L3
Please make sure to refer other source to study the dermatomes well because its huge and
important as well.
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PNS Anatomy #15
Tibial nerve
The tibial part of the sciatic nerve, either before or after
its separation from the common fibular nerve, supplies
branches to all muscles in the posterior compartment of
thigh (long head of biceps femoris, semimembranosus,
and semitendinosus) except the short head of biceps
femoris, which is innervated by the common fibular
part.
The tibial nerve descends through the popliteal
fossa with popliteal artery and vein >> enters the
posterior compartment of leg >> and continues into
the sole of the foot.
Important: in popliteal fossa the arrangement of the
vessels and nerve is opposite to most of body, the
nerve (Tibial) is the most superficial while in other
body regions the arrangement from superficial to
deep is VAN.
Tibial nerve is close to tibia and its injury cause
Conscious walking, which means you are aware
where you will put your foot.
It desends to the ankle by passing through the tarsal tunnel behind the medial malleolus,) lateral
to the tibial artery, and deep to the flexor retinaculum. In the ankle it will give a superficial sensory
branch to the heel, called the medial calcaneal nerve.
Midway between the medial malleolus and the heel, the
tibial nerve bifurcates to its terminal branches;
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Large medial plantar nerve; that will give a
muscular branch and cutaneous branch to the
medial part of the plantar surface of the foot.
Smaller lateral plantar nerve; that will give a
muscular branch and cutaneous branch to the
lateral part of the plantar surface of the foot.
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PNS Anatomy #15
Tibial nerve innervates:
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all muscles in the posterior compartment of thigh (by giving branches before or after separation
from fibular nerve) except the short head of biceps femoris, which is innervated by the common
fibular nerve.
all muscles in the posterior compartment of leg;
all intrinsic muscles in the sole of the foot except for the first two dorsal interossei muscles, which
are innervated by the deep fibular nerve;
Skin on the posterolateral side of the lower half of the leg and medial side of the ankle, foot, and
little toe, and skin on the sole of the foot and toes.
The common peroneal (fibular) nerve
One of the terminal branches of sciatic nerve, it passes in the
posterior compartment of the leg without innervating
anything there. It enters the popliteal fossa from above
directly under the margin of the biceps femoris muscle.
It exits by following the biceps femoris tendon goes to
the lateral side of the leg where it swings around the
neck of the fibula and enters the lateral compartment of
the leg.
Common problem(Clinical); common peroneal is very superficial around the neck of the fibula so
when you are crossing the street, its on the level of car pump so its prone to injury. And it will cause
Foot drop which means absence of dorsiflexion so when the patient is walking he will show special
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PNS Anatomy #15
way of gait (steppage gait) >>> abnormal lifting of the thigh, to avoid crushing the toes to the
ground.
Causes of foot drop:
1- L4-L5 disk herniation >> herniated disk compressing L5 nerve root may cause foot drop.
2- Lumbosacral plexus injury >> which result from pelvic injury.
3- Sciatic nerve injury >> caused by hip dislocation, the common peroneal division of the sciatic
nerve is commonly injured by any dislocation or fracture involving the hip.
4- injury to the knee >> in knee dislocation, its important to check common peroneal nerve and
popliteal artery injury.
5- Established compartment syndrome >> foot drop is a late finding. Irreversible nerve and muscle
ischemia occurs in patients if fasciotomy is not performed.
-Fasciotomy should be done early 4 to 6 hours.
-4 hours of ischemia may be tolerated but by 8 hours the damage will irreversible.
Treatment of common peroneal nerve / foot drop:
1- if the disk herniation of the low back is causing symptoms of foot drop, then it should be treated
or removed.
2- obtain EMG and nerve studies for patient, EMG is obtained after 3 weeks and we look for Early
Large Polyphasic waves >>> its good sign if we find it.
3- Recovery takes 12 to 18 months and the big toe is the last to recover (PYQ)
4- if the recovery is not achieved after reasonable amount of time; explore the nerve for repair,
graft or tendon transfer.
Here is the link of foot drop video, thanks to our colleague Amira Aftiha for sharing it:
https://www.youtube.com/watch?v=0uFJNgPoa8U
Most superficial nerves of the body: (PYQ)
-Common peroneal nerve at the level of the head of the fibula.
-Ulnar nerve at the level of medial epicondyle of the humerus.
-Cutaneous branch of the radial nerve.
The common peroneal nerve passes anteriorly around the fibular neck between the attachments of
the fibularis longus to the fibular head and shaft. Here the common fibular nerve divides into its
two terminal branches;
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The superficial fibular nerve
The deep fibular nerve.
The superficial fibular nerve descends in the lateral compartment deep to the fibularis longus and
innervates the fibularis longus and fibularis brevis.
It then penetrates the deep fascia in the lower leg and enters the foot (between the extensor
digitorum longus and the peroneal brevis) where it divides into medial and lateral cutaneous
branches.
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The deep peroneal nerve, which originates in the
lateral compartment of the leg, will pass into the
anterior compartment of the leg by penetrating the
intermuscular septum that separates the anterior
and lateral compartments of the leg. It then passes
deep to the extensor digitorum longus.
It innervates all the muscles in the anterior
compartment of the leg. So its antagonist to tibial
nerve which innervates all muscles in posterior
compartment of the leg.
It continues into the dorsal aspect of the foot, where it innervates the extensor digitorum brevis,
contribute to the innervation of the first two dorsal interossei muscles, and supply the skin
between the great and second toe (the 1st cleft). Important
Sural nerve: Dr not talked about it so its just copy paste from Lajneh book.
It is a branch of mainly the tibial nerve, it also receives contributions from the common fibular
nerve (through the sural communicating nerve).
Numerous small branches arise from the sural nerve to supply the skin of the calf and the back of
the leg. The sural nerve accompanies the small saphenous vein posterior to the lateral malleolus
and is distributed to the skin along the lateral border of the foot and the lateral side of the little toe.
Note;
The sural nerve penetrates the deep fascia approximately in the middle of the leg "superficial to
fascia".
It passes between the two heads of the gastrocnemius, superficial to the belly of the muscle
municating and lateral Sural nerves
Along the way of the fibular nerve on the medial margin of the biceps femoris tendon and over the
lateral head of the gastrocnemius muscle, it gives origin to two cutaneous branches that descend in
the leg;
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The Sural communicating nerve, which joins the Sural branch of the tibial nerve and
contributes to innervation of skin over the lower posterolateral side of the leg.
Lateral Sural cutaneous nerve, which innervates the skin over the upper lateral leg.
The END
Sorry for any possible mistake
Best of luck Awn
Your friend; Cilmi Faradheere.
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