The sciatic nerve
The sciatic nerve (L4, 5, S1–3) is the largest nerve in the body . It is broad and flat at its origin, although peripherally it becomes rounded. The nerve emerges from the greater sciatic foramen distal to piriformis and under cover of gluteus maximus, crosses the posterior surface of the ischium, crosses obturator internus, with its gemelli, quadratus femoris and descends on adductor magnus (Figs 183, 184). Here it lies deep to the hamstrings and is crossed only by the long head of biceps. The sciatic nerve terminates by dividing into the tibial and common peroneal nerves . The level of this division is variable—usually it is at the mid-thigh, but the two nerves may be separate even at their origins from the sacral plexus
Branches
The trunk of the sciatic nerve supplies the hamstring muscles (biceps, semimembranosus, semitendinosus) and also the adductor magnus, the latter being innervated also by the obturator nerve.
All the muscle branches apart from the one to the short head of biceps arise on the medial side of the nerve; its lateral border is therefore the side of relative safety in its operative exposure.
Clinical features
1◊◊The sciatic nerve may be wounded in penetrating injuries or in posterior dislocation of the hip associated with fracture of the posterior lip of the
acetabulum, to which the nerve is closely related (Fig. 168).
Damage to the sciatic nerve is followed by paralysis of the hamstrings
and all the muscles of the leg and foot (supplied by its distributing branches); there is loss of all movements in the lower limb below the knee joint with foot
drop deformity. Sensory loss is complete below the knee, except for an area along the medial side of the leg, over the medial malleolus and down to the hallux, which is innervated by the saphenous branch of the femoral nerve.
2◊◊The sciatic nerve is accompanied by a companion artery (derived from the inferior gluteal artery) which bleeds quite sharply when the nerve is divided during an above-knee amputation. The artery must be neatly isolated and tied without any nerve fibres being incorporated in the ligature, since this would be followed by severe pain in the stump.
The tibial nerve
The tibial nerve (L4, 5, S1–3) is the larger of the two terminal branches of thesciatic nerve; it traverses the popliteal fossa superficial to the popliteal vein and artery, which it crosses from the lateral to the medial side.
Branches
a) in popliteal fossa
•◊◊muscular—to gastrocnemius, soleus and popliteus;
•◊◊cutaneous— the sural nerve, which descends over the back of the calf, behind the lateral malleolus to the 5th toe; it receives a communicating branch from the common peroneal nerve and supplies the lateral side of the leg, foot and 5th toe;
•◊◊articular—to the knee joint. It then descends deep to soleus, in company with the posterior tibial vessels, passes on their lateral side behind the medial malleolus to end by dividing into the medial and lateral plantar nerves.
b) in the leg
The tibial nerve supplies flexor hallucis longus, flexor digitorum longus and tibialis posterior. Its terminal plantar branches supply the intrinsic muscles and skin of the sole of the foot, the medial plantar nerve having an equivalent distribution to that of the median nerve in the hand, the lateral plantar nerve being comparable to the ulnar nerve.
The common peroneal (fibular) nerve
The common peroneal nerve (L4, 5, S1, 2) is the smaller of the terminal branches of the sciatic nerve. It enters the upper part of the popliteal fossa, passes along the medial border of the biceps tendon, then curves around the neck of the fibula where it lies in the substance of peroneus longus and divides into its terminal branches, the deep peroneal and superficial peroneal nerves
Branches While still in the popliteal fossa, the common peroneal nerve gives off the lateral cutaneous nerve of the calf, a peroneal (sural) communicating branch and twigs to the knee joint, but has no muscular branches.
The deep peroneal (fibular) nerve
The deep peroneal nerve pierces extensor digitorum longus, then descends, in company with the anterior tibial vessels, over the interosseous membrane and then over the ankle joint. Medially lies tibialis anterior, while laterally lies first extensor digitorum longus, then extensor hallucis longus. Its branches are:
•◊◊muscular — to the muscles of the anterior compartment of the leg — extensor digitorum longus, extensor hallucis longus, tibialis anterior, peroneus tertius—and extensor digitorum brevis;
•◊◊cutaneous — to a small area of skin in the web between the 1st and 2nd toes.
The superficial peroneal (fibular) nerve
The superficial peroneal nerve runs in the lateral compartment of the leg.
Its branches are:
•◊◊muscular— to the lateral compartment muscles (peroneus longus and brevis);
•◊◊cutaneous—to the skin of the distal two-thirds of the lateral aspect of the leg and to the dorsum of the foot (apart from the small area between the 1st and 2nd toes supplied by the deep peroneal nerve).
Clinical features
The common peroneal nerve is in a particularly vulnerable position as it winds around the neck of the fibula. It may be damaged at this site by the pressure of a tight bandage or plaster cast or may be torn in severe adduction injuries to the knee. Damage to this nerve is followed by foot drop (due to paralysis of the ankle and foot extensors) and inversion of the foot due to paralysis of the peroneal muscles with unopposed action of the foot flexors
and invertors). There is also anaesthesia over the anterior and lateral aspects of the leg and foot, although the medial side escapes, since this is innervated by the saphenous branch of the femoral nerve.
The sciatic nerve (L4, 5, S1–3) is the largest nerve in the body . It is broad and flat at its origin, although peripherally it becomes rounded. The nerve emerges from the greater sciatic foramen distal to piriformis and under cover of gluteus maximus, crosses the posterior surface of the ischium, crosses obturator internus, with its gemelli, quadratus femoris and descends on adductor magnus (Figs 183, 184). Here it lies deep to the hamstrings and is crossed only by the long head of biceps. The sciatic nerve terminates by dividing into the tibial and common peroneal nerves . The level of this division is variable—usually it is at the mid-thigh, but the two nerves may be separate even at their origins from the sacral plexus
Branches
The trunk of the sciatic nerve supplies the hamstring muscles (biceps, semimembranosus, semitendinosus) and also the adductor magnus, the latter being innervated also by the obturator nerve.
All the muscle branches apart from the one to the short head of biceps arise on the medial side of the nerve; its lateral border is therefore the side of relative safety in its operative exposure.
Clinical features
1◊◊The sciatic nerve may be wounded in penetrating injuries or in posterior dislocation of the hip associated with fracture of the posterior lip of the
acetabulum, to which the nerve is closely related (Fig. 168).
Damage to the sciatic nerve is followed by paralysis of the hamstrings
and all the muscles of the leg and foot (supplied by its distributing branches); there is loss of all movements in the lower limb below the knee joint with foot
drop deformity. Sensory loss is complete below the knee, except for an area along the medial side of the leg, over the medial malleolus and down to the hallux, which is innervated by the saphenous branch of the femoral nerve.
2◊◊The sciatic nerve is accompanied by a companion artery (derived from the inferior gluteal artery) which bleeds quite sharply when the nerve is divided during an above-knee amputation. The artery must be neatly isolated and tied without any nerve fibres being incorporated in the ligature, since this would be followed by severe pain in the stump.
The tibial nerve
The tibial nerve (L4, 5, S1–3) is the larger of the two terminal branches of thesciatic nerve; it traverses the popliteal fossa superficial to the popliteal vein and artery, which it crosses from the lateral to the medial side.
Branches
a) in popliteal fossa
•◊◊muscular—to gastrocnemius, soleus and popliteus;
•◊◊cutaneous— the sural nerve, which descends over the back of the calf, behind the lateral malleolus to the 5th toe; it receives a communicating branch from the common peroneal nerve and supplies the lateral side of the leg, foot and 5th toe;
•◊◊articular—to the knee joint. It then descends deep to soleus, in company with the posterior tibial vessels, passes on their lateral side behind the medial malleolus to end by dividing into the medial and lateral plantar nerves.
b) in the leg
The tibial nerve supplies flexor hallucis longus, flexor digitorum longus and tibialis posterior. Its terminal plantar branches supply the intrinsic muscles and skin of the sole of the foot, the medial plantar nerve having an equivalent distribution to that of the median nerve in the hand, the lateral plantar nerve being comparable to the ulnar nerve.
The common peroneal (fibular) nerve
The common peroneal nerve (L4, 5, S1, 2) is the smaller of the terminal branches of the sciatic nerve. It enters the upper part of the popliteal fossa, passes along the medial border of the biceps tendon, then curves around the neck of the fibula where it lies in the substance of peroneus longus and divides into its terminal branches, the deep peroneal and superficial peroneal nerves
Branches While still in the popliteal fossa, the common peroneal nerve gives off the lateral cutaneous nerve of the calf, a peroneal (sural) communicating branch and twigs to the knee joint, but has no muscular branches.
The deep peroneal (fibular) nerve
The deep peroneal nerve pierces extensor digitorum longus, then descends, in company with the anterior tibial vessels, over the interosseous membrane and then over the ankle joint. Medially lies tibialis anterior, while laterally lies first extensor digitorum longus, then extensor hallucis longus. Its branches are:
•◊◊muscular — to the muscles of the anterior compartment of the leg — extensor digitorum longus, extensor hallucis longus, tibialis anterior, peroneus tertius—and extensor digitorum brevis;
•◊◊cutaneous — to a small area of skin in the web between the 1st and 2nd toes.
The superficial peroneal (fibular) nerve
The superficial peroneal nerve runs in the lateral compartment of the leg.
Its branches are:
•◊◊muscular— to the lateral compartment muscles (peroneus longus and brevis);
•◊◊cutaneous—to the skin of the distal two-thirds of the lateral aspect of the leg and to the dorsum of the foot (apart from the small area between the 1st and 2nd toes supplied by the deep peroneal nerve).
Clinical features
The common peroneal nerve is in a particularly vulnerable position as it winds around the neck of the fibula. It may be damaged at this site by the pressure of a tight bandage or plaster cast or may be torn in severe adduction injuries to the knee. Damage to this nerve is followed by foot drop (due to paralysis of the ankle and foot extensors) and inversion of the foot due to paralysis of the peroneal muscles with unopposed action of the foot flexors
and invertors). There is also anaesthesia over the anterior and lateral aspects of the leg and foot, although the medial side escapes, since this is innervated by the saphenous branch of the femoral nerve.
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