Ch06: Lower Limb V — Summary & High-Yield Injuries（下肢总结与高发损伤）

# Ch06: Lower Limb V — Summary & High-Yield Injuries

Preparatory Mindset

This chapter consolidates the lower limb through the lens of classic clinical injuries — the exact scenarios the 100-Concepts deck and 局解mc大全 test: avascular necrosis of the femoral head, unhappy triad of the knee, ankle joint injury, gluteal (sciatic) injury, avulsion fractures, femoral sheath/hernia, Achilles rupture, fibular neck fracture.

Exam mindset: each injury = (mechanism) → (structure damaged) → (clinical finding). Memorize the triplet, not just the name.

Core Concepts

1. Avascular necrosis of the femoral head (concept 13)

• Mechanism: transcervical (intracapsular) fracture of the femoral neck — common in elderly women with osteoporosis
• Vascular basis: the head of the femur is supplied by retinacular arteries from the medial circumflex femoral artery (the main supply); the artery of the ligamentum teres (from obturator a.) is inadequate alone
• Consequence: if the retinacular supply is disrupted (intracapsular fracture tears them) → avascular necrosis of the femoral head
• Clinical: hip pain, inability to weight-bear, late collapse; seen on X-ray/MRI; treatment often hip replacement
• Note: the medial circumflex femoral artery is the dominant supply to the head — its injury (dislocation, fracture) causes AVN

2. Knee joint injury — the Unhappy Triad (concept 14)

1. Tibial (medial) collateral ligament (TCL/MCL) — most frequently torn ligament at the knee 2. Medial meniscus (torn as the femur & tibia compress it) 3. Anterior cruciate ligament (ACL)

• Mechanism: lateral blow to the knee (e.g., football tackle) → valgus force → tears, in order:
• Mnemonic: "O'Donoghue's triad" = MCL + medial meniscus + ACL
• Clinical: effusion, instability, positive anterior drawer/Lachman test
• Note: the lateral side is struck more often → the medial side is injured (that's why the TCL is the most common torn ligament)

3. Ankle joint injury (concept 15)

• Inversion injury (most common): tears the lateral ligaments — anterior talofibular ligament (first & most commonly torn) → calcaneofibular → posterior talofibular (rarely)
• Eversion injury: tears the deltoid (medial) ligament — strong, rarely torn; may avulse the medial malleolus instead
• Clinical: lateral ankle swelling & pain; ligament sprain vs malleolar fracture (Ottawa rules)

4. Injury of the gluteal region (concept 16)

• Sciatic nerve injury: may occur with hip dislocation/posterior approach to hip, misplaced IM injection
• Superior gluteal nerve injury: → Trendelenburg gait (gluteus medius & minimus paralysis)
• Inferior gluteal nerve injury: → weak hip extension (gluteus maximus), difficulty rising from a chair/climbing stairs
• Injection safety: upper lateral quadrant avoids all these

5. Avulsion fractures (concept 17)

- ASIS avulsion — by sartorius (with sudden sprinting/kicking) - AIIS avulsion — by rectus femoris - Ischial tuberosity avulsion — by hamstrings (biceps femoris, semitendinosus, semimembranosus) - Base of 5th metatarsal — by peroneus brevis (Jones vs avulsion fracture)

• Definition: a tendon/muscle pulls off its bony attachment — common in adolescents (apophyses not yet fused)
• Examples:

6. Femoral sheath & femoral hernia (concept 18) — recap

• Femoral sheath = transversalis + iliacus fascia; 3 compartments: artery / vein / canal
• Femoral canal contents: lymphatics + deep inguinal node; femoral ring medial border = lacunar ligament
• Femoral hernia: below & lateral to pubic tubercle, more common in women, high strangulation risk (rigid ring)
• MCQ (局解mc大全): "The femoral canal contains the:" → B. Deep inguinal lymph node(s)
• MCQ: elderly woman, hernia in right femoral triangle + weak adduction → which nerve compressed? → A. Obturator (the hernial sac may compress the obturator nerve medial to it → referred pain to knee/weak adduction)

7. Rupture of the Achilles tendon (concept 19)

• Mechanism: sudden forceful plantar flexion (sprinting, jumping); middle-aged "weekend warrior"
• Structure: Achilles tendon (gastrocnemius + soleus → calcaneus)
• Clinical: sudden snap in the calf, inability to plantarflex/push-off; Thompson's test (squeeze calf → no plantar flexion)
• Note: the plantaris tendon may be intact, preserving weak plantar flexion

8. Fracture of the fibular neck (concept 20)

• Mechanism: direct blow, twisting injury
• Structure at risk: common peroneal nerve winding around the fibular neck
• Clinical: foot drop (deep peroneal n. → loss of dorsiflexion), loss of eversion (superficial peroneal n.), sensory loss over the dorsum of the foot & lateral leg; often with tibial shaft fracture (Maisonneuve-type)

High-Yield Points

• AVN of femoral head: transcervical fracture in elderly women → retinacular arteries (medial circumflex femoral) torn → head dies
• Unhappy triad (O'Donoghue): MCL + medial meniscus + ACL — valgus blow from the lateral side
• Ankle: inversion → anterior talofibular ligament (first torn); eversion → deltoid (rarely torn)
• Gluteal injection: upper lateral quadrant only
• Avulsion fractures: ASIS (sartorius), AIIS (rectus femoris), ischial tuberosity (hamstrings), 5th metatarsal base (peroneus brevis)
• Femoral hernia MCQ: femoral canal = deep inguinal lymph node; compression of obturator nerve → weak adduction + medial thigh/knee pain
• Fibular neck fracture → common peroneal → foot drop
• Achilles rupture: Thompson's test negative squeeze = rupture

Topic Summary

The classic lower-limb injuries map to specific anatomy: femoral neck fracture → AVN of the femoral head (retinacular arteries from medial circumflex femoral); valgus knee blow → unhappy triad (MCL, medial meniscus, ACL); inversion ankle → anterior talofibular ligament; gluteal injection/hip surgery → sciatic or superior/inferior gluteal nerve injury (Trendelenburg, weak extension); apophyseal avulsions (ASIS–sartorius, AIIS–rectus femoris, ischial tuberosity–hamstrings); femoral sheath/hernia (canal = lymphatics, strangulation by lacunar ligament, obturator nerve compression sign); Achilles rupture (Thompson's test); fibular neck fracture → common peroneal → foot drop. Each pairs a mechanism with a structure and a sign — the format the exam rewards.