Ch04: Lower Limb III — Gluteal Region & Posterior Thigh（臀区与股后区）

# Ch04: Lower Limb III — Gluteal Region & Posterior Thigh

Preparatory Mindset

The gluteal region is where intramuscular injections are given (must know the safe quadrant!), where the sciatic nerve is at risk (largest nerve in the body), and where the piriformis acts as the key landmark dividing the supra- and infrapiriform foramina. The posterior thigh is the hamstring compartment, innervated by the sciatic nerve.

Exam mindset: "To avoid injury to the sciatic nerve, IM injections should be given in which quadrant?" (upper lateral — answer E in the MCQ bank). "Which muscle is the most important landmark to identify vessels and nerves of the gluteal region?" (piriformis). These exact questions appear in 局解mc大全.

Core Concepts

1. Gluteal region — layers & muscles

• 1st layer: gluteus maximus (largest, covers the region; extends hip, lateral rotation)
• 2nd layer: gluteus medius, piriformis, superior gemellus, obturator internus (tendon), inferior gemellus
• 3rd layer: gluteus minimus, obturator externus
• Gluteus medius & minimus: abduct & medially rotate the thigh; both supplied by the superior gluteal nerve (L4-S1) — injury → Trendelenburg gait (pelvis drops on the unsupported side)

2. Piriformis — the key landmark (concept 16)

- Suprapiriform foramen (above piriformis): superior gluteal nerve, artery & vein (between gluteus medius & minimus; supply both) - Infrapiriform foramen (below piriformis): sciatic nerve, posterior femoral cutaneous nerve, inferior gluteal nerve, artery & vein (supply gluteus maximus), internal pudendal vessels & pudendal nerve

• The most important landmark for identifying gluteal vessels & nerves
• Passes through the greater sciatic foramen, dividing it into:

3. Lesser sciatic foramen

• Borders: sacrotuberous ligament, sacrospinous ligament, lesser sciatic notch
• Contents: internal pudendal vessels + pudendal nerve (re-enter the perineum), obturator internus tendon & nerve
• Key concept: the pudendal nerve & internal pudendal artery exit the pelvis via the greater (infrapiriform) foramen, hook around the ischial spine, and re-enter via the lesser sciatic foramen

4. Sciatic nerve (L4-S3) — largest nerve in the body

• Exits the pelvis through the infrapiriform foramen, deep to gluteus maximus
• Descends in the posterior thigh between the long head of biceps femoris and adductor magnus
• Divides in the distal thigh (or popliteal fossa) into tibial nerve (L4-S3) and common peroneal (fibular) nerve (L4-S2)
• Clinical: sciatic nerve is lateral to the ischial tuberosity — IM injections must avoid the medial-upper quadrant where the nerve could be struck; sciatica → radiating pain down the posterior thigh & leg

5. Posterior thigh — hamstring muscles

• Biceps femoris (lateral), semitendinosus & semimembranosus (medial)
• All arise from the ischial tuberosity (except short head of biceps from linea aspera)
• Actions: extend the hip & flex the knee
• Innervation: sciatic nerve (tibial division supplies semitendinosus, semimembranosus, long head of biceps; short head of biceps by common peroneal division)
• Clinical: hamstring strain; avulsion of ischial tuberosity apophysis (concept 17: avulsion fractures — e.g., hamstring avulsion, ASIS avulsion by sartorius, ischial tuberosity by hamstrings in adolescents)

6. Posterior femoral cutaneous nerve (S1-S3)

• Exits the infrapiriform foramen with the sciatic nerve; supplies skin of posterior thigh & upper calf

High-Yield Points

• Mnemonic for safe IM injection: "Upper Outer Quadrant (UOQ)" — the upper lateral quadrant of the buttock avoids the sciatic nerve & superior/inferior gluteal vessels. MCQ answer: E. upper lateral.
• Piriformis = key landmark. Above it: superior gluteal N/A/V (gluteus medius & minimus). Below it: sciatic n., posterior femoral cutaneous n., inferior gluteal N/A/V (gluteus maximus), pudendal n. + internal pudendal vessels.
• Trendelenburg gait: superior gluteal nerve injury (e.g., hip surgery, injection into wrong quadrant) → gluteus medius/minimus paralysis → pelvis drops on the unsupported side during stance
• Sciatic nerve divisions: tibial n. (L4-S3) & common peroneal n. (L4-S2); the tibial portion supplies hamstrings (except short head of biceps)
• Pudendal nerve pathway (mnemonic): "Exit greater, hook the spine, enter lesser" — a common exam trap
• Ischiorectal fossa relation: the pudendal canal (Alcock's) lies in the lateral wall of the ischiorectal fossa, carrying pudendal n. & internal pudendal a./v.

Topic Summary

The gluteal region is organized around gluteus maximus (superficial) and the piriformis (key landmark). The greater sciatic foramen is split by piriformis into the suprapiriform foramen (superior gluteal N/A/V → gluteus medius/minimus) and infrapiriform foramen (sciatic n., posterior femoral cutaneous n., inferior gluteal N/A/V → gluteus maximus, pudendal n. + internal pudendal vessels). The sciatic nerve then runs down the posterior thigh between biceps femoris and adductor magnus, innervating the hamstrings (via tibial division except short head of biceps), and divides into tibial & common peroneal nerves. Safe IM injection = upper lateral quadrant; piriformis is the landmark; Trendelenburg gait = superior gluteal nerve injury.