Chapter 7: Extremitas Inferior: Lower Limb

The lower limb is built for stability and locomotion: it bears the body's weight, keeps it balanced, and moves it forward. Compared with the upper limb, its joints are deeper, its bones heavier and its ligaments stronger, so it trades range of movement for strength. It has two parts:

• Cingulum membri inferioris (pelvic girdle): the two hip bones, joined to the sacrum behind and to each other in front.
• Pars libera (free limb):
  • thigh (femur, with the patella);
  • leg (crus: tibia and fibula);
  • foot (pes: tarsals, metatarsals and phalanges).

Bones

Os coxae (hip bone)

The os coxae forms from three bones that fuse at the acetabulum, the socket for the femoral head: the ilium above, the ischium behind and below, and the pubis in front.

• Ilium:
  • the crista iliaca (iliac crest), ending in front at the spina iliaca anterior superior (SIAS/ASIS) and behind at the spina iliaca posterior superior (SIPS/PSIS);
  • below each spine, the spina iliaca anterior inferior and posterior inferior;
  • the fossa iliaca on the inner surface;
  • the facies auricularis (for the sacroiliac joint).
• Ischium:
  • the spina ischiadica (ischial spine), with the incisura ischiadica major above it and the incisura ischiadica minor below it;
  • the tuber ischiadicum, the bone you sit on.
• Pubis:
  • the body and superior and inferior rami;
  • the tuberculum pubicum;
  • the facies symphysialis (for the pubic symphysis).
• Foramen obturatum: the large opening framed by pubis and ischium, closed in life by the obturator membrane except for the small canalis obturatorius.

The ASIS, the iliac crest, the pubic tubercle and the ischial tuberosity can all be felt through the skin.

Femur

The longest and strongest bone of the body.

Proximal end:

• Caput femoris, with the fovea capitis (for the ligament of the head).
• Collum femoris: the neck, angled about 125° to the shaft.
• Trochanter major (lateral), with the fossa trochanterica on its medial side, and trochanter minor (posteromedial).
• The linea intertrochanterica (in front) and crista intertrochanterica (behind, with the tuberculum quadratum) join the trochanters.

Shaft:

• Linea aspera: a rough ridge down the back, where adductors and hamstrings attach.
• The tuberositas glutea (gluteal tuberosity) and linea pectinea run up from the linea aspera.
• Below, the linea aspera splits into the lineae supracondylares medialis and lateralis, enclosing the facies poplitea.

Distal end:

• Condylus medialis and condylus lateralis, each with an epicondyle. The tuberculum adductorium on the medial epicondyle is where adductor magnus inserts.
• The fossa intercondylaris behind, between the condyles.
• The facies patellaris in front, for the patella.

The patella is the largest sesamoid bone. It sits in the tendon of quadriceps and increases its leverage.

Tibia and fibula

The tibia is the medial, weight-bearing bone of the leg.

• Condylus medialis and condylus lateralis, whose flat upper surfaces form the facies articularis superior (tibial plateau).
• The eminentia intercondylaris, with medial and lateral tubercles.
• The tuberositas tibiae, where the patellar ligament inserts.
• A sharp margo anterior: the shin, directly under the skin.
• The linea musculi solei on the back.
• The incisura fibularis at the distal end.
• The malleolus medialis.

The fibula is the slender lateral bone. It bears little weight but anchors muscles and stabilises the ankle.

• Caput fibulae (with its apex) and collum fibulae. The common fibular nerve winds around the neck.
• The shaft, with its borders.
• The malleolus lateralis, which reaches lower than the medial malleolus.

Ossa pedis (bones of the foot)

• 7 tarsal bones:
  • talus: its trochlea tali articulates with the leg bones; it has a head and neck, and no muscle attaches to it;
  • calcaneus: the heel, the largest tarsal bone. It has the tuber calcanei (for the Achilles tendon) and the sustentaculum tali, a shelf that supports the talus;
  • os naviculare, on the medial side, in front of the talus;
  • os cuboideum, on the lateral side, in front of the calcaneus;
  • ossa cuneiformia mediale, intermedium and laterale.
• 5 metatarsals (I–V).
• 14 phalanges: the great toe (hallux) has two, the other toes three each.

Two joint lines are used in amputation and fracture description:

• Chopart's joint (transverse tarsal joint): the talonavicular and calcaneocuboid joints together.
• Lisfranc's joint: the tarsometatarsal joints.

Joints

• Cruciate ligaments. The ACL prevents the tibia sliding forward on the femur; the PCL prevents it sliding backward. They cross like an X inside the knee.
• Sprained ankle. Most sprains are inversion injuries that tear the lateral ligaments, starting with the anterior talofibular ligament. The medial (deltoid) ligament is so strong that it is more likely to pull off a flake of the medial malleolus than to tear.

Muscles

Iliopsoas and the hip rotators

• M. iliopsoas (m. psoas major, m. iliacus; a small m. psoas minor may also be present):
  • course: from the lumbar vertebrae and iliac fossa to the lesser trochanter;
  • action: the strongest flexor of the hip;
  • nerves: femoral nerve (iliacus) and ventral rami of L1–L3 (psoas).
• Deep lateral rotators of the hip:
  • m. piriformis;
  • m. gemellus superior;
  • m. obturatorius internus;
  • m. gemellus inferior;
  • m. quadratus femoris;
  • m. obturatorius externus.

Piriformis is the landmark muscle of the gluteal region (see the topography section).

Gluteal region

The shape of the buttock comes mainly from m. gluteus maximus and the overlying fat. The region extends from the iliac crest above to the gluteal fold below.

If the superior gluteal nerve is damaged, the pelvis drops on the opposite side when the patient stands on the affected leg (Trendelenburg sign).

Intramuscular injections go into the upper outer quadrant of the buttock, well away from the sciatic nerve.

Thigh

Anterior compartment (extensors of the knee), supplied by the femoral nerve:

• M. quadriceps femoris: rectus femoris (from the AIIS, so it also flexes the hip), vastus lateralis, vastus medialis and vastus intermedius. All insert via the patella and lig. patellae into the tibial tuberosity, and together they extend the knee.
• M. sartorius, the longest muscle in the body, from the ASIS to the medial tibia. It flexes, abducts and laterally rotates the hip, and flexes the knee (the cross-legged position).

Medial compartment (adductors), mostly supplied by the obturator nerve:

All adduct the hip. Obturator externus is mainly a lateral rotator.

Posterior compartment (hamstrings), supplied by the sciatic nerve:

The hamstrings flex the knee and extend the hip. Biceps femoris also rotates the flexed knee laterally.

Leg (crus)

The leg has three compartments, each with its own nerve.

The deck lists extensor hallucis brevis and extensor digitorum brevis with the anterior leg. Both actually lie on the dorsum of the foot, but they share the same nerve (deep fibular).

Foot

• Dorsum pedis: m. extensor digitorum brevis and m. extensor hallucis brevis. The dorsal interossei are sometimes listed here too, though they belong to the sole's deepest layer. The dorsal muscles are supplied by the deep fibular nerve.
• Planta pedis (sole), in three groups:
  • medial (great-toe) eminence: m. abductor hallucis, m. flexor hallucis brevis;
  • intermediate: m. flexor digitorum brevis, m. quadratus plantae, the lumbricals and the interossei;
  • lateral (little-toe) eminence: m. abductor digiti minimi, m. flexor digiti minimi brevis (and a small m. opponens digiti minimi, not always present).

The sole's muscles are supplied by the medial and lateral plantar nerves, the terminal branches of the tibial nerve.

Topography

Foramen ischiadicum majus and minus

The sacrospinous and sacrotuberous ligaments convert the greater and lesser sciatic notches into two foramina.

• Foramen ischiadicum majus (greater sciatic foramen): m. piriformis passes through it and divides it into two.
  • Above piriformis (foramen suprapiriforme): the superior gluteal artery, vein and nerve.
  • Below piriformis (foramen infrapiriforme):
    • the sciatic nerve;
    • the inferior gluteal artery, vein and nerve;
    • the posterior femoral cutaneous nerve;
    • the pudendal nerve and internal pudendal vessels, which leave here only to re-enter through the lesser foramen.
• Foramen ischiadicum minus (lesser sciatic foramen): bounded by the sacrospinous ligament above and the sacrotuberous ligament below. It transmits the tendon of obturator internus, and the pudendal nerve and internal pudendal vessels on their way to the perineum.

Canalis obturatorius

A gap at the top of the obturator membrane, under the obturator groove of the pubis. It carries the obturator nerve, artery and vein from the pelvis into the medial thigh.

Lacuna musculorum and lacuna vasorum

The space under the inguinal ligament is divided by the arcus iliopectineus into two:

• Lacuna musculorum (lateral):

  • m. iliopsoas;
  • n. femoralis;
  • n. cutaneus femoris lateralis.

  (The deck lists the two nerves with the prefix "M.", which would make them muscles.)

• Lacuna vasorum (medial):

  • the femoral artery and vein;
  • the femoral branch of the genitofemoral nerve;
  • lymph vessels and the lacunar lymph node.

Anulus femoralis, canalis femoralis and hiatus saphenus

• The femoral canal is the most medial compartment of the femoral sheath. It contains only fat, lymph vessels and a lymph node (Cloquet's node).
• Its upper opening is the anulus femoralis (femoral ring), bounded by:
  • the inguinal ligament in front;
  • the lacunar ligament (of Gimbernat) medially;
  • the femoral vein laterally;
  • the pectineal ligament behind.
• A loop of bowel pushed through the ring forms a femoral hernia, which is more common in women. Because the ring is narrow and rigid, it is prone to strangulation.
• The hiatus saphenus (saphenous opening) is a gap in the fascia lata below and lateral to the pubic tubercle. The great saphenous vein passes through it to join the femoral vein, along with the superficial external pudendal vessels and lymph vessels.

Trigonum femorale (femoral triangle)

• Boundaries:

  • superior: the inguinal ligament;
  • lateral: the medial border of sartorius;
  • medial: the medial (or lateral) border of adductor longus (texts differ).

• Roof: fascia lata.

• Floor: iliopsoas, pectineus and adductor longus.

• Contents, from lateral to medial:

  • the femoral nerve;
  • the femoral artery;
  • the femoral vein;
  • the femoral canal, with lymph vessels and nodes.

  Mnemonic: "NAVEL". The lateral femoral cutaneous nerve runs near the lateral edge. The femoral pulse, taken at the midinguinal point, is used for arterial access (catheterisation); the femoral vein lies just medial to it.

Canalis adductorius (Hunter's canal)

A tunnel in the middle third of the thigh, leading from the apex of the femoral triangle to the adductor hiatus in adductor magnus.

• Roof (anterior): sartorius.
• Posterior/medial wall: adductor longus and adductor magnus.
• Lateral wall: vastus medialis.
• Contents:
  • the femoral artery and vein, which pass through the hiatus to become the popliteal vessels;
  • the saphenous nerve;
  • the nerve to vastus medialis.

Fossa poplitea

The diamond-shaped hollow behind the knee.

• Boundaries:
  • superomedial: semitendinosus and semimembranosus;
  • superolateral: biceps femoris;
  • inferomedial and inferolateral: the medial and lateral heads of gastrocnemius.
• Roof: the popliteal fascia.
• Floor: the popliteal surface of the femur, the joint capsule and popliteus.
• Contents, from superficial to deep:
  • the tibial and common fibular nerves, the most superficial;
  • the popliteal vein;
  • the popliteal artery, the deepest, against the bone. Its pulse is hard to feel for that reason.
  • the small saphenous vein, which enters from below.

Canalis tarsi

The tunnel formed between the talus and calcaneus, filled mainly by the strong interosseous talocalcaneal ligament.

It is different from the tarsal tunnel, behind the medial malleolus under the flexor retinaculum. That tunnel carries, from front to back:

• the tibialis posterior tendon;
• the flexor digitorum longus tendon;
• the posterior tibial artery and vein;
• the tibial nerve;
• the flexor hallucis longus tendon.

Mnemonic: "Tom, Dick And Very Nervous Harry".

Neurovasculature

Arteries

• The a. iliaca externa becomes the a. femoralis under the inguinal ligament.
• The femoral artery gives off the a. profunda femoris (deep femoral artery), which supplies most of the thigh through its perforating branches and the medial and lateral circumflex femoral arteries.
• The femoral artery becomes the a. poplitea at the adductor hiatus. Below the knee this divides into:
  • the a. tibialis anterior, which runs in the anterior compartment and continues on the foot as the a. dorsalis pedis. The dorsalis pedis pulse is felt lateral to the extensor hallucis longus tendon.
  • the a. tibialis posterior, which runs in the deep posterior compartment. Its pulse is felt behind the medial malleolus. It gives off the a. fibularis and ends as the medial and lateral plantar arteries.
• The aa. glutea superior and inferior (from the internal iliac artery) supply the buttock. The a. obturatoria supplies the medial thigh.

Femoral head blood supply. The head is supplied mainly by branches of the medial circumflex femoral artery, which run up along the neck. A displaced fracture of the femoral neck can tear them (see avascular necrosis below).

Veins

• Deep veins accompany the arteries.
• Superficial veins:
  • the v. saphena magna (great saphenous vein) begins on the medial side of the foot. It passes in front of the medial malleolus (a reliable site for a cut-down), runs up the medial leg and thigh, and joins the femoral vein through the saphenous opening. It is the longest vein in the body and is used for coronary bypass grafts.
  • the v. saphena parva (small saphenous vein) begins on the lateral side of the foot. It passes behind the lateral malleolus, runs up the back of the calf, and joins the popliteal vein.
• Perforating veins with valves carry blood from the superficial to the deep system. When those valves fail, the superficial veins dilate into varicose veins.

Nerves

• Plexus lumbalis (L1–L4), supplying the groin and front and medial thigh:
  • n. iliohypogastricus and n. ilioinguinalis;
  • n. genitofemoralis;
  • n. cutaneus femoris lateralis;
  • n. femoralis (anterior thigh: quadriceps, sartorius, iliacus); its continuation, the n. saphenus, supplies the skin of the medial leg;
  • n. obturatorius (medial thigh: adductors).
• Plexus sacralis (L4–S4), supplying the buttock, back of the thigh, leg and foot:
  • the nn. glutei superior and inferior;
  • the n. cutaneus femoris posterior;
  • the n. pudendus;
  • the n. ischiadicus (sciatic nerve, L4–S3), the largest nerve in the body. It supplies the hamstrings and usually divides above the knee into:
    • the n. tibialis: posterior leg and sole of the foot;
    • the n. fibularis (peroneus) communis, which winds around the neck of the fibula and divides into the superficial fibular nerve (lateral compartment, skin of the dorsum of the foot) and the deep fibular nerve (anterior compartment, dorsal foot muscles, first web space).

Clinical correlations

1. Congenital talipes equinovarus (CTEV, club foot). A congenital deformity in which the foot is plantar flexed (equinus), inverted and adducted (varus). It is usually treated from birth by serial casting (the Ponseti method).
2. Foot drop and claw foot.
   • Foot drop: loss of dorsiflexion, usually from injury to the common fibular nerve at the neck of the fibula (e.g. a fibular neck fracture, a tight plaster cast, prolonged leg-crossing). The toes drag, so the patient lifts the knee high (steppage gait). The foot also cannot evert, and sensation is lost over the dorsum of the foot.
   • Claw foot: hyperextension at the metatarsophalangeal joints with flexion at the interphalangeal joints, from weakness of the intrinsic foot muscles (e.g. neuropathy or a tibial nerve lesion).
3. Rupture of the Achilles tendon. Typically a sudden snap during sprinting or jumping. The patient cannot plantar flex strongly or stand on tiptoe. Squeezing the calf does not plantar flex the foot (positive Thompson test).
4. ACL and PCL rupture.
   • The ACL is torn by twisting on a planted foot or by hyperextension, often with a pop and rapid swelling. The tibia can be pulled forward excessively (anterior drawer and Lachman tests).
   • The PCL is torn by a blow to the front of the tibia with the knee flexed (the "dashboard injury"), giving a posterior drawer sign.
5. Avascular necrosis of the femoral head. A displaced (intracapsular) femoral neck fracture tears the retinacular branches of the medial circumflex femoral artery, so the head loses its blood supply and dies. This is why displaced neck fractures in older patients are often treated with hip replacement.
6. Genu varum and genu valgum.
   • Genu varum (bow legs): the knees are apart and the ankles together, which loads the medial compartment.
   • Genu valgum (knock knees): the knees are together and the ankles apart, which loads the lateral compartment.

Mild varum in infants and valgum in toddlers are normal stages of development. 7. Compartment syndrome. Swelling within a closed fascial compartment (most often the anterior compartment of the leg, after a tibial fracture or crush injury) raises the pressure until it cuts off the blood supply to the muscles and nerves.

• Signs: pain out of proportion to the injury, pain on passive stretch of the compartment's muscles, tense swelling, then numbness and weakness. Loss of pulses is a late sign.
• Treatment is urgent fasciotomy.

Key teaching point

Tie every region of the lower limb to its nerve:

Then learn the three places where a structure is most vulnerable:

• the sciatic nerve below piriformis (injections);
• the common fibular nerve at the fibular neck (foot drop);
• the femoral neck vessels (avascular necrosis).