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The Lower Limb - A Complete Organized Guide

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Overview: Regions of the Lower Limb

• Femoral triangle - pyramid-shaped depression in the proximal thigh, where major blood supply and the femoral nerve enter from the abdomen under the inguinal ligament
• Popliteal fossa - diamond-shaped region posterior to the knee, where major vessels and nerves pass between thigh and leg
• Tarsal tunnel - posteromedial ankle canals formed by bones and the flexor retinaculum, through which nerves, vessels, and flexor tendons pass between leg and foot

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Part 1: Bones of the Lower Limb

1.1 The Pelvis / Hip Bone (Pelvic Girdle)

• Iliac crest - the superior rim, ending anteriorly as the anterior superior iliac spine (ASIS) and posteriorly as the posterior superior iliac spine (PSIS)
• Anterior inferior iliac spine (AIIS) - attachment for the rectus femoris
• Ischial tuberosity - the bony prominence you sit on; attachment for the hamstring muscles
• Acetabulum - the cup-shaped socket for the femoral head (hip joint)
• The gluteal surface of the ilium is divided by three curved lines (inferior, anterior, and posterior gluteal lines) that define the attachment zones for gluteus minimus, medius, and maximus

1.2 The Femur

• Head - articulates with the acetabulum; blood supply comes primarily from an arterial ring formed by the medial and lateral circumflex femoral arteries around the femoral neck
• Neck - connects the head to the shaft at an angle; fractures here can interrupt blood supply to the femoral head (avascular necrosis risk)
• Greater and lesser trochanters - prominent projections for muscle attachments
• Intertrochanteric line (anterior) and intertrochanteric crest (posterior) - connect the trochanters
• Linea aspera - the major posterior ridge of the shaft; a key site for multiple muscle attachments. In the proximal femur it splits into the pectineal line (medially) and gluteal tuberosity (laterally)
• The shaft descends at 7 degrees from vertical, so the distal end is closer to the midline than the proximal shaft
• Medial and lateral condyles distally - articulate with the tibia

1.3 The Patella

• The largest sesamoid bone in the body, embedded within the quadriceps tendon
• Protects the knee joint anteriorly and improves the mechanical advantage of the quadriceps

1.4 Tibia and Fibula (Leg Bones)

• Medial and lateral condyles proximally articulate with the femoral condyles
• Tibial plateau - the superior articular surface
• Tibial tuberosity - anterior prominence, attachment for patellar ligament
• Medial malleolus distally forms part of the ankle mortise

• Mainly for muscle attachment and ankle stability
• Lateral malleolus distally forms the lateral wall of the ankle mortise

1.5 Bones of the Foot

• Tarsal bones (7): Talus, calcaneus (heel bone), navicular, cuboid, and 3 cuneiforms (medial, intermediate, lateral). Organized in two rows with the navicular as an intermediate bone on the medial side
• Metatarsals (5): One per digit; connected distally by deep transverse metatarsal ligaments
• Phalanges: Three per toe except the great toe (digit I) which has only two (proximal and distal)

• Medial longitudinal arch (higher, more flexible) - supported by the calcaneus, talus, navicular, cuneiforms, and metatarsals I-III
• Lateral longitudinal arch (flatter) - supported by calcaneus, cuboid, and metatarsals IV-V
• Transverse arch - formed across the width of the foot at the level of the cuneiforms and cuboid
• These arches act as shock absorbers and reduce the energy cost of walking

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Part 2: Muscles of the Lower Limb

2.1 Gluteal Region Muscles

The gluteus medius and minimus are the primary hip abductors - they hold the pelvis level during walking (a positive Trendelenburg sign means they are weak or their nerve is damaged).

2.2 Thigh Muscles

All three hamstrings originate from the ischial tuberosity (except short head of biceps).

2.3 Leg Muscles

• Gastrocnemius - powerful plantarflexion; also flexes knee (two heads from femoral condyles)
• Soleus - plantarflexion (deeper, from tibia/fibula)
• Plantaris - weak plantarflexion (vestigial)
• Together gastrocnemius + soleus form the triceps surae, attaching via the calcaneal (Achilles) tendon to the calcaneus

• Tibialis posterior - plantarflexion + inversion; supports medial arch
• Flexor digitorum longus - flexes toes 2-5
• Flexor hallucis longus - flexes great toe; supports medial arch

2.4 Foot Muscles (Intrinsic)

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Part 3: Joints of the Lower Limb

3.1 Hip Joint

• Type: Ball-and-socket (multiaxial synovial joint)
• Articular surfaces: Head of femur + acetabulum (deepened by the fibrocartilaginous acetabular labrum)
• Movements: Flexion, extension, abduction, adduction, medial/lateral rotation, circumduction
• Key ligaments:
  • Iliofemoral ligament (Y-ligament of Bigelow) - strongest in the body; prevents hyperextension
  • Pubofemoral ligament - limits abduction and extension
  • Ischiofemoral ligament - limits medial rotation and extension
  • Ligament of the head of the femur (ligamentum teres) - carries a small artery to the femoral head (minor blood supply in adults)
• Blood supply to femoral head: Primarily from medial and lateral circumflex femoral arteries forming a ring around the femoral neck; these are vulnerable in femoral neck fractures

3.2 Knee Joint

• Type: Modified hinge joint (allows flexion/extension + limited rotation when flexed)
• Articular surfaces: Medial and lateral femoral condyles + tibial condyles + patella (patellofemoral component)
• Menisci: Medial and lateral C-shaped fibrocartilage pads that improve congruency and act as shock absorbers. The medial meniscus is attached to the medial collateral ligament, making it more prone to injury. Tears present with joint line pain, locking, clicking, and swelling; diagnosed by MRI.

• Cruciate ligaments (inside the joint):
  • Anterior cruciate ligament (ACL) - prevents anterior translation of tibia on femur; taut in extension
  • Posterior cruciate ligament (PCL) - prevents posterior translation; the stronger of the two
• Collateral ligaments (outside the joint):
  • Medial (tibial) collateral ligament (MCL) - resists valgus stress
  • Lateral (fibular) collateral ligament (LCL) - resists varus stress
• The "unhappy triad" of knee injury: ACL + MCL + medial meniscus (commonly torn together in contact sports)
• Screw-home mechanism: Terminal extension of the knee involves medial rotation of the femur, "locking" the joint for efficient standing. Unlocking requires the popliteus muscle.

3.3 Ankle Joint (Talocrural Joint)

• Type: Hinge joint (uniaxial synovial)
• Articular surfaces: Distal tibia + medial malleolus + lateral malleolus forming a mortise around the trochlea of the talus
• Movements: Dorsiflexion (tibia and fibula) and plantarflexion (talus)
• Key ligaments:
  • Medial (deltoid) ligament - broad, strong; connects medial malleolus to tarsal bones; resists eversion
  • Lateral ligaments - three bands (anterior talofibular, calcaneofibular, posterior talofibular); resist inversion; most commonly sprained (anterior talofibular is torn first in inversion sprains)

3.4 Subtalar & Midfoot Joints

• Subtalar (talocalcaneal) joint + transverse tarsal joint = responsible for inversion and eversion of the foot
• Tarsometatarsal joints - allow limited sliding movements only
• Metatarsophalangeal joints - allow flexion, extension, abduction, and adduction

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Part 4: Nerves of the Lower Limb

Major Nerves and Their Distributions

The common fibular nerve is particularly vulnerable as it winds around the neck of the fibula - a fracture there, or pressure from a tight cast, causes foot drop (inability to dorsiflex).

Dermatomes - Key Test Points

Myotomes & Reflexes

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Part 5: Blood Supply

5.1 Arteries

Aorta → Common iliac → External iliac → Femoral artery (in femoral triangle)
         → Deep femoral artery (profunda femoris) → perforating arteries to thigh
         → Popliteal artery (posterior to knee)
              → Anterior tibial artery → Dorsalis pedis artery (dorsum of foot)
              → Posterior tibial artery → Medial and lateral plantar arteries (sole)
              → Fibular (peroneal) artery

• Superior and inferior gluteal arteries (from internal iliac) - supply the gluteal region
• Obturator artery (from internal iliac, through obturator canal) - supplies medial thigh
• A rich anastomotic network around the hip (formed by circumflex femoral, gluteal, and obturator arteries) provides collateral circulation when one vessel is blocked

5.2 Veins

• Great (long) saphenous vein - originates from the medial dorsal venous arch, ascends the medial leg, knee, and thigh, drains into the femoral vein just below the inguinal ligament (at the saphenofemoral junction). Longest vein in the body; used for coronary artery bypass grafts
• Small (short) saphenous vein - originates from the lateral dorsal venous arch, ascends the posterior leg, penetrates deep fascia and drains into the popliteal vein behind the knee

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Part 6: Function & Gait

Body Weight Support

• Slightly posterior to the hip joints (so the hip is passively extended - no muscle needed)
• Anterior to the knee and ankle joints (so these are passively stabilized in extension)
• This arrangement allows standing with minimal energy expenditure

Determinants of Gait

1. Pelvic rotation - reduces stride length drop
2. Pelvic tilt - the non-stance hip drops slightly (requires active gluteus medius on stance side)
3. Knee flexion in the stance phase
4. Foot and ankle mechanics - heel strike, then plantarflexion at toe-off

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Part 7: Clinical Correlates Summary

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