16.2 Lower Limb: Bones, Venous & Lymphatic Drainage, Thigh, Gluteal Region, Popliteal Fossa, Leg Compartments, Foot, Joints & Gait

Why this area matters

The TOS lists:

• bones of the lower limb;
• fascia, vessels and nerves (venous drainage, lymphatic drainage, cutaneous innervation);
• the anterior and medial thigh;
• the gluteal region;
• the posterior thigh;
• the popliteal fossa;
• the leg compartments;
• the foot;
• the joints (hip, knee, tibiofibular, ankle, foot, arches).

Elderly hip fractures, diabetic foot disease, varicose veins, injections and trauma make this region very practical.

Bones and fractures

• Hip bone: ilium (iliac crest, anterior superior iliac spine), ischium (ischial tuberosity, the bone you sit on), pubis. The acetabulum is formed by all three.
• Femur:
  • neck fractures (elderly women with osteoporosis): an intracapsular displaced fracture threatens the medial circumflex femoral artery and causes avascular necrosis of the head. The leg is shortened and laterally rotated.
  • shaft fractures bleed heavily, up to about 1 to 1.5 L.
• Tibia and fibula: the tibia is subcutaneous along its whole medial surface, so open fractures are common. The fibular neck carries the common fibular nerve.
• Foot:
  • tarsal bones: talus, calcaneus, navicular, cuboid and three cuneiforms;
  • metatarsals: the fifth metatarsal base is avulsed by fibularis brevis;
  • phalanges.

Fascia, veins, lymphatics and cutaneous nerves

• Fascia lata encloses the thigh. The iliotibial tract is its lateral thickening, and the saphenous opening lies below the inguinal ligament.
• Great saphenous vein:
  • begins at the medial end of the dorsal venous arch;
  • passes anterior to the medial malleolus, the classic venous cutdown site;
  • runs up the medial leg and thigh with the saphenous nerve below the knee;
  • joins the femoral vein at the saphenofemoral junction.
• Small saphenous vein: passes behind the lateral malleolus and joins the popliteal vein, with the sural nerve.
• Perforating veins connect superficial to deep veins. Valve failure causes varicose veins and venous ulcers near the medial malleolus. The calf muscle pump drives venous return, and immobility leads to deep vein thrombosis.
• Lymph: superficial lymph drains to the superficial inguinal nodes (horizontal and vertical groups), then to the deep inguinal and external iliac nodes. The popliteal nodes drain the lateral foot and calf (the small saphenous territory).
• Cutaneous nerves:
  • lateral femoral cutaneous nerve (compressed under the inguinal ligament in meralgia paresthetica);
  • saphenous nerve (medial leg);
  • sural nerve (lateral foot);
  • superficial and deep fibular nerves.

Anterior and medial thigh

• Anterior compartment: quadriceps femoris (knee extension) and sartorius, supplied by the femoral nerve (L2 to L4); iliopsoas flexes the hip.
• Medial compartment: the adductors (longus, brevis, magnus), gracilis and pectineus, mainly supplied by the obturator nerve.
• Femoral triangle: bounded by the inguinal ligament, sartorius and adductor longus. It contains, from lateral to medial, the femoral nerve, artery, vein and lymphatics ("NAVeL"); the vessels lie in the femoral sheath, whose medial part is the femoral canal.
  • The femoral artery is palpated at the mid-inguinal point (halfway between the ASIS and the pubic symphysis) for pulses, angiography and arterial blood gases.
  • The femoral vein lies just medial for central venous access.
• Adductor (Hunter) canal: carries the femoral vessels and saphenous nerve to the popliteal fossa.

Gluteal region and posterior thigh

• Muscles: gluteus maximus (inferior gluteal nerve; hip extension); gluteus medius and minimus (superior gluteal nerve; hip abduction that keeps the pelvis level); the lateral rotators (piriformis, obturator internus and gemelli, quadratus femoris).
• Piriformis is the key landmark: the superior gluteal vessels and nerve exit above it, while the sciatic nerve, inferior gluteal and pudendal structures exit below it.
• Safe intragluteal injection uses the upper outer quadrant of the buttock or, preferably, the ventrogluteal site (gluteus medius, between the ASIS and the iliac tubercle) to avoid the sciatic nerve.
• Posterior thigh (hamstrings): semitendinosus, semimembranosus and biceps femoris (long head) extend the hip and flex the knee, supplied by the sciatic nerve (tibial division, except the short head of biceps, which is fibular).

Popliteal fossa

• Boundaries: biceps femoris (superolateral), semimembranosus and semitendinosus (superomedial), and the two heads of gastrocnemius (inferior).
• Contents, from superficial to deep: the tibial nerve, then the popliteal vein, then the popliteal artery, which is deepest, lying on the femur and joint capsule. The artery is at risk with supracondylar femur fractures and posterior knee dislocation.
• The common fibular nerve follows biceps femoris laterally around the fibular neck.
• A Baker (popliteal) cyst is a distended semimembranosus-gastrocnemius bursa that can rupture and mimic deep vein thrombosis.

Lower Extremity Nerves, Functional Innervation & Pathological Gaits

Lumbosacral Plexus Nerves

1. Femoral Nerve ($L_2\text{--}L_4$):

   • Courses beneath the inguinal ligament lateral to the femoral artery within the femoral triangle (outside the femoral sheath).
   • Motor: Iliopsoas (hip flexion), pectineus, sartorius, quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius: knee extension).
   • Deficit: Inability to extend the knee; difficulty climbing stairs; knee instability ("buckling"); diminished/absent patellar tendon reflex ($L_3\text{--}L_4$).
   • Sensory: Anterior thigh and medial leg down to the medial malleolus via the saphenous nerve.

2. Obturator Nerve ($L_2\text{--}L_4$):

   • Passes through the obturator foramen. Injured in pelvic fractures or during pelvic lymphadenectomy.
   • Motor: Thigh adductors (adductor longus, adductor brevis, adductor magnus, gracilis, obturator externus).
   • Deficit: Severe weakness of thigh adduction; unstable lateral leg swing; sensory loss over a small patch on the medial medial thigh.

3. Superior Gluteal Nerve ($L_4\text{--}S_1$):

   • Exits greater sciatic foramen above the piriformis muscle. Innervates gluteus medius, gluteus minimus, and tensor fasciae latae.
   • Function: Abducts and medially rotates the thigh; during unipedal stance, contracts to prevent the contralateral unsupported pelvis from sagging downward.
   • Trendelenburg Sign: When the patient stands on the affected (lesioned) leg, the pelvis drops downward on the contralateral normal side. The patient compensates during ambulation by leaning the trunk toward the affected side (Trendelenburg gait).

4. Inferior Gluteal Nerve ($L_5\text{--}S_2$):

   • Exits greater sciatic foramen below the piriformis muscle. Innervates the gluteus maximus (chief hip extensor).
   • Deficit: Marked weakness in hip extension; severe difficulty rising from a seated position, climbing stairs, or jumping.

5. Sciatic Nerve ($L_4\text{--}S_3$):

   • Exits greater sciatic foramen below the piriformis; bifurcates in the popliteal fossa into the tibial and common fibular nerves.
   • Motor: Hamstring muscles (biceps femoris, semitendinosus, semimembranosus: knee flexion) and the hamstring portion of adductor magnus.
   • Vulnerable to posterior hip dislocations and misplaced intragluteal injections (safe injection site: upper outer quadrant or ventrogluteal region).

6. Common Fibular (Peroneal) Nerve ($L_4\text{--}S_2$):

   • Winds superficially around the lateral neck of the fibula.
   • Most frequently injured nerve in the lower extremity: caused by proximal fibular fractures, lateral knee blunt trauma, tight lower extremity casts, or prolonged crossed-leg sitting.
   • Bifurcates into:
     • Deep Fibular Nerve: Innervates anterior compartment muscles (tibialis anterior, extensor hallucis longus, extensor digitorum longus) $\rightarrow$ mediates ankle dorsiflexion and toe extension. Sensory: web space between the 1st and 2nd toes.
     • Superficial Fibular Nerve: Innervates lateral compartment muscles (fibularis longus and brevis) $\rightarrow$ mediates foot eversion. Sensory: anterolateral lower leg and the dorsum of the foot.
   • Clinical Deficit: Foot Drop: Inability to dorsiflex or evert the foot; patient adopts a compensatory steppage gait (lifting the knee excessively high to prevent dragging toes on the ground, followed by an audible foot slap).

7. Tibial Nerve ($L_4\text{--}S_3$):

   • Descends vertically through the popliteal fossa; passes deep to the flexor retinaculum at the medial malleolus (tarsal tunnel).
   • Motor: Posterior compartment muscles (gastrocnemius, soleus, plantaris, popliteus, tibialis posterior, flexor digitorum longus, flexor hallucis longus) $\rightarrow$ mediates ankle plantarflexion, foot inversion, and toe flexion; innervates all intrinsic plantar foot muscles.
   • Clinical Deficit: Inability to plantarflex the foot ("tiptoe" walking is impossible); loss of toe flexion; diminished/absent Achilles tendon reflex ($S_1\text{--}S_2$); sensory anesthesia over the sole of the foot. (High-Yield Board Mnemonic: PED: Peroneal Everts and Dorsiflexes; TIP: Tibial Inverts and Plantarflexes).

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Knee Joint Stability, Ligamentous Integrity & Meniscal Pathologies

The knee joint is a complex synovial hinge joint whose stability is governed by primary ligamentous restraints:

1. Anterior Cruciate Ligament (ACL):

   • Arises from the anterior intercondylar area of the tibia and extends superiorly, posteriorly, and laterally to attach to the medial surface of the lateral femoral condyle (mnemonic: LAMP — Lateral condyle = ACL; Medial condyle = PCL).
   • Prevents anterior translation of the tibia relative to the femur and resists rotational pivot forces.
   • Mechanism of injury: Non-contact deceleration, sudden directional change, hyperextension, or pivoting on a planted foot; accompanied by an audible "pop", acute hemarthrosis, and instability.
   • Diagnostic tests: Lachman test (most sensitive and specific), Anterior Drawer test, and Pivot-Shift test.

2. Posterior Cruciate Ligament (PCL):

   • Arises from the posterior intercondylar area of the tibia and extends superiorly, anteriorly, and medially to attach to the lateral surface of the medial femoral condyle.
   • Prevents posterior translation of the tibia relative to the femur.
   • Mechanism of injury: Direct anterior impact to the proximal tibia with the knee in $90^\circ$ flexion ("dashboard injury" during a motor vehicle collision).
   • Diagnostic tests: Posterior Drawer test and the Posterior Sag Sign (tibial plateau drops backward relative to femoral condyles).

3. Collateral Ligaments & Menisci:

   • Medial Collateral Ligament (MCL): Broad band extending from the medial epicondyle of the femur to the medial tibial condyle. Resists valgus stress (abduction blow to lateral knee). Firmly attached to the medial meniscus.
   • Lateral Collateral Ligament (LCL): Cord-like ligament extending from the lateral femoral epicondyle to the fibular head. Resists varus stress (adduction blow to medial knee). Completely separated from the lateral meniscus by the popliteus tendon.
   • Menisci: Crescentic fibrocartilaginous shock absorbers. The medial meniscus is C-shaped, larger, and less mobile (firmly anchored to MCL); the lateral meniscus is circular (O-shaped) and more mobile. Meniscal tears present with localized joint-line tenderness, clicking, and "locking"; diagnosed clinically via the McMurray test (pain/click on axial loading and rotation during extension).
   • The Unhappy Triad of O'Donoghue: Classical description of severe traumatic knee derangement resulting from a forceful lateral blow to a flexed, planted knee: simultaneous rupture of the Anterior Cruciate Ligament (ACL), Medial Collateral Ligament (MCL), and Medial Meniscus (modern biomechanical studies demonstrate that acute lateral meniscal tears are statistically more frequent, but the classical triad remains a classic board exam benchmark).

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Leg Compartments & Acute Compartment Syndrome

The Four Fascial Compartments of the Lower Leg

The crus (lower leg) is partitioned into four rigid, inextensible osteofascial compartments bounded by the tibia, fibula, interosseous membrane, and anterior/posterior intermuscular septa:

Acute Compartment Syndrome

• Pathophysiology: Acute intracompartmental bleeding or post-ischemic edema (following closed tibial shaft fractures, crush trauma, or arterial reperfusion) elevates tissue pressure within a closed, non-yielding osteofascial space. When intracompartmental tissue pressure exceeds microvascular capillary perfusion pressure (approx. $30\text{ mmHg}$), capillary collapse produces progressive tissue ischemia, cellular hypoxia, and irreversible muscle necrosis within 4 to 6 hours and permanent neurological paralysis within 8 hours.
• The Clinical "Ps":
  1. Pain out of proportion to exam: The earliest, most sensitive, and most reliable diagnostic indicator. Characteristically elicited or massively aggravated by passive stretching of the ischemic muscles traversing the affected compartment (e.g., passive plantarflexion of the ankle stretching ischemic anterior compartment muscles).
  2. Paresthesia: Early sign of nerve ischemia in the distribution of traversing nerves (e.g., numbness in the first dorsal web space from deep fibular ischemia).
  3. Pallor: Skin appears cold, pale, or shiny.
  4. Poikilothermia: Temperature disequilibrium (limb feels cold compared to unaffected side).
  5. Paralysis: Late, catastrophic finding indicating established muscular infarction.
  6. Pulselessness: An extremely late finding; distal arterial pulses (dorsalis pedis, posterior tibial) typically remain palpable because systemic systolic pressure ($120\text{ mmHg}$) far exceeds compartment pressure ($30\text{--}45\text{ mmHg}$). The presence of palpable pulses never excludes acute compartment syndrome.
• Diagnostic Objective Criteria: Direct intracompartmental needle pressure measurement showing an absolute pressure $> 30\text{ mmHg}$ or a Delta Pressure (Diastolic Blood Pressure minus Intracompartmental Pressure) $\le 30\text{ mmHg}$ is diagnostic.
• Definitive Treatment: Immediate, emergency four-compartment surgical fasciotomy executed via two long vertical skin incisions (anterolateral incision decompressing anterior and lateral compartments; posteromedial incision decompressing superficial and deep posterior compartments).

Foot, arches and other joints

• Arches: the medial longitudinal arch (the highest; supported by the plantar aponeurosis, spring ligament and tibialis posterior), the lateral longitudinal arch and the transverse arch. Collapse leads to flat foot (pes planus). Plantar fasciitis causes heel pain on the first steps in the morning.
• Hip joint:
  • a stable ball-and-socket joint, deepened by the labrum;
  • the iliofemoral ligament is the strongest;
  • posterior dislocation (dashboard injury) leaves the limb shortened, adducted and medially rotated, and can injure the sciatic nerve;
  • in children, developmental dysplasia is screened with the Ortolani and Barlow tests.
• Ankle (talocrural) joint: a hinge for dorsiflexion and plantarflexion.
  • The lateral ligaments (anterior talofibular, calcaneofibular, posterior talofibular) are weaker. Inversion sprains tear the anterior talofibular ligament first.
  • The strong medial (deltoid) ligament may avulse the medial malleolus instead of tearing.
• Subtalar and transverse tarsal joints: inversion and eversion.
• Tibiofibular joints: a proximal synovial joint and a distal syndesmosis ("high ankle sprain").

Gait

Normal gait alternates stance and swing phases. Key abnormal gaits are:

• Trendelenburg gait: superior gluteal nerve injury, or weak hip abductors.
• Steppage (high-stepping) gait with foot slap: common fibular nerve injury causing foot drop.
• Gluteus maximus lurch: the trunk leans back.
• Antalgic gait: a short stance phase on a painful limb.
• Circumduction: in hemiplegia after stroke, the leg swings outward because of spastic extension.