Sacral Plexus Branches Applied
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Direct answer
Ask what stops working, and each branch of the sacral plexus answers with its own deficit pattern. The superior gluteal nerve (L4 to S1) fails as a Trendelenburg lurch; the inferior gluteal nerve (L5 to S2) as inability to rise from a chair; the sciatic nerve (L4 to S3) as foot drop with hamstrings wasted; the pudendal nerve (S2 to S4) as perineal numbness and incontinence. Between them run the small named nerves students forget — to obturator internus, to quadratus femoris, the posterior cutaneous nerve of the thigh and the perforating cutaneous nerve — each with a root value and a recognisable clinical signature.
What you must remember
- Superior gluteal nerve (L4-S1): the only nerve leaving the greater sciatic foramen above piriformis; supplies gluteus medius, minimus and tensor fasciae latae; injury gives a positive Trendelenburg sign and an abductor lurch.
- Inferior gluteal nerve (L5-S2): below piriformis, supplies gluteus maximus alone; injury blocks hip extension — no stairs, no chairs, no rising from sitting without pushing off.
- Sciatic nerve (L4-S3): the thickest nerve in the body; injured by misplaced gluteal injections, posterior hip dislocations and hip surgery; divides into tibial and common peroneal nerves, so complete palsy wastes hamstrings and drops the foot.
- Nerve to quadratus femoris (L4-S1): runs deep against the hip capsule, also supplying inferior gemellus; at risk in posterior hip approaches.
- Nerve to obturator internus (L5-S2): leaves below piriformis with the internal pudendal artery, supplies superior gemellus, then turns through the lesser sciatic foramen to obturator internus.
- Pudendal nerve (S2-S4): crosses the sacrospinous ligament at the ischial spine into Alcock's canal; blocked there for operative delivery; stretched in prolonged second stage causing later incontinence.
- Posterior cutaneous nerve of thigh (S1-S3): the "small sciatic", sensory to the posterior thigh and lower gluteal skin.
- Perforating cutaneous nerve (S2-S3): pierces the sacrotuberous ligament to reach the lower medial buttock; direct muscular twigs supply piriformis, levator ani and coccygeus from S3 and S4.
Localising three palsies by branch anatomy
A woman delivers with forceps after a prolonged second stage and cannot dorsiflex her foot afterwards. Two possibilities sit in the same pelvis. If the lumbosacral trunk (L4-L5) was compressed by the fetal head or blade against the pelvic brim, the deficit is a flaccid foot drop with inverted foot, weak hip abduction and an intact hamstring reflex pattern — an obstetric palsy of the plexus itself. If instead the common peroneal division was compressed at the fibular neck by stirrups in lithotomy position, the foot drops but hip abduction and knee flexion are normal. The branch map decides between plexus and periphery without a single test.
Two more patterns complete the set. A patient with a misplaced lower-quadrant gluteal injection develops burning pain down the leg then foot drop: the sciatic nerve's common peroneal division lies laterally against the greater trochanter side of the buttock and takes the needle first. A cyclist with perineal numbness and erectile difficulty has pudendal nerve compression in Alcock's canal against the ischial tuberosity — the same nerve an obstetrician blocks deliberately at the ischial spine.
Where students slip
The nerves to obturator internus and quadratus femoris are listed without their gemelli passengers — the first supplies superior gemellus, the second inferior gemellus — a standing short-question half-mark. The posterior cutaneous nerve of the thigh is mistaken for a motor nerve because it travels with the sciatic; it is purely sensory. And students place the pudendal block "at the greater sciatic foramen": the needle is guided to the ischial spine, just medial to which the nerve hooks around the sacrospinous ligament — the spine is the palpable landmark of the whole procedure.
Frequently asked questions
Which sacral plexus branch leaves above piriformis and what does its injury cause?
The superior gluteal nerve (L4-S1); injury paralyses gluteus medius and minimus, giving a positive Trendelenburg sign and abductor lurch.
What deficit follows inferior gluteal nerve injury?
Loss of gluteus maximus power — the patient cannot extend the hip to climb stairs, rise from sitting or straighten up from bending, and pushes off with the arms.
Why does foot drop occur after forceps delivery?
The lumbosacral trunk (L4-L5) may be compressed by the fetal head or forceps blade against the pelvic brim, an obstetric palsy distinct from common peroneal palsy at the fibular neck from lithotomy position.
Which branches supply the gemelli?
The nerve to obturator internus supplies superior gemellus; the nerve to quadratus femoris supplies inferior gemellus along with quadratus femoris.
What is the surface landmark for a pudendal nerve block?
The ischial spine, palpated transvaginally; the nerve hooks around the sacrospinous ligament just medial to it before entering Alcock's canal.
What is the posterior cutaneous nerve of the thigh?
A purely sensory branch (S1-S3), the "small sciatic nerve", supplying skin of the buttock, posterior thigh and upper calf as it travels beside the sciatic nerve.