Lower Limb Anatomy: Function & Subcutaneous Structures

Lower Limb Overview

• The lower limb is an extension of the lower trunk.
• It's responsible for accepting and redistributing weight from the upper body towards the ground.
• The architecture of the innominate (hip bones) distributes weight from the sacrum to the femur.
• The angle of inclination of the femoral neck aids in weight redistribution.
• The neck of the femur is a site of great bone strain and common fractures, especially in the elderly with osteoporosis.
• These fractures, often called "broken hips," are typically distal to the hip joint.
• Functions include supporting body weight during standing, locomotion, and balance.
• Balance is maintained through muscle contractions and joint position adjustments to keep the center of gravity over the base of support.

Subcutaneous Structures of the Lower Limb

• Fascia lata is a tough, fibrous fascia enveloping the thigh, acting like a surgical stocking to compress deep tissues and prevent blood pooling.
• Septa from the fascia lata divide the thigh into anterior, medial, and posterior compartments.
• The iliotibial band (ITB) is a thickening on the lateral side of the fascia lata, serving as an aponeurosis for the gluteus maximus and tensor fascia lata muscles, and it inserts on the lateral tibia.
• The saphenous opening is an oval region superomedially where the fascia lata thins, allowing cutaneous structures to communicate with deep structures.
• Crural fascia is the deep fascia of the leg, continuous superiorly with the fascia lata.
• Invaginations of the crural fascia form intermuscular septa, dividing the leg into compartments; a transverse intermuscular septum further divides the posterior compartment.
• The crural fascia is thickest over the anterior compartment and forms extensor retinacula at the ankle joint.
• Cutaneous venous drainage involves numerous unnamed venous tributaries.
• The great saphenous vein, the longest continuous vein in the body, originates on the dorsomedial foot, ascends anterior to the ankle, crosses posterior to the knee, and merges with the femoral vein at the saphenous opening.
• It is the main tributary for venous return in the superficial leg, receiving many smaller tributaries.
• Portions of the great saphenous vein are used as bypass vessels in coronary artery bypass graft surgery.
• The small saphenous vein is a major tributary on the posterolateral leg aspect.
• It ascends posterior to the ankle, pierces the deep fascia along the posterior knee (popliteal fossa), and drains into the deep venous network.
• Cutaneous lymphatic drainage involves lymph vessels accompanying superficial veins toward the saphenous opening.
• These vessels drain into the superficial inguinal lymph nodes and then connect with deep lymphatic nodes/vessels in the pelvis and upper thigh.
• Lymph vessels alongside the small saphenous vein drain through the popliteal fossa and deep lymphatics there.
• Cutaneous nerves branch extensively from distinct main branches.
• Anterior femoral cutaneous nerve branches from the femoral nerve to supply the anterior thigh.
• The lateral femoral cutaneous nerve directly branches from the lumbar plexus to supply the lateral thigh.
• The posterior femoral cutaneous nerve directly branches from the sacral plexus to supply the posterior thigh.
• Superior/inferior clunial nerves branch from varying origins to supply the gluteal region.
• The superficial fibular nerve branches from the common fibular nerve to supply the anterior leg and dorsum of the foot.
• The saphenous nerve, a branch of the femoral nerve, emerges proximally to the knee, runs with the saphenous vein, and supplies the medial leg.
• The sural nerve supplies the lateral/posterior leg and is typically formed by the fusion of the lateral and medial sural cutaneous nerves, both from the sciatic nerve.

Hip and Thigh Osteology

• This region is transitional between the trunk and lower limb, anatomically part of the trunk, with muscles acting on the hip joint.
• Superior boundary: iliac crest level
• Inferior boundary: gluteal folds
• Lateral boundary: greater trochanter
• The hip is bound to the sacrum by strong ligaments: the sacrospinous (sacrum to ischial spine, demarcating the greater sciatic foramen) and the sacrotuberous (sacrum to ischial tuberosity, demarcating the lesser sciatic foramen).
• The os coxae (pelvic bone) is the primary framework for the pelvis.
• It articulates with the sacrum proximally and the femur distally and consists of three bones joined by hyaline cartilage, fusing by age 25.
• The ilium, the largest pelvic bone, lies superiorly, featuring the iliac crest, the anterior superior iliac spine (ASIS), the posterior superior iliac spine (PSIS) and the auricular surface for articulation with the sacrum.
• The ischium is the posteroinferior bone of the pelvis.
• Includes the ischial spine, the ischial ramus (forming the lower border of the obturator foramen), and the ischial tuberosity (weight-bearing in sitting and attachment for hamstring muscles).
• The pubis is the anteroinferior bone of the pelvis, articulating medially at the symphysis pubis.
• Its superior and inferior rami form the obturator foramen margins.
• The sacroiliac joint is a compound joint.
• It features an anterior synovial joint between the sacrum and ilium's auricular surfaces and a posterior syndesmosis between tuberosities.
• It's reinforced by sacroiliac, sacrotuberous, and sacrospinous ligaments, decreasing rigidity and providing shock absorption.
• The ligaments limit sacrum movement on the ilium and prevent anterior rotation of the sacrum during weight-bearing.
• The femur, the longest bone in the body, accepts weight from the os coxae and redirects it medially.
• It articulates with the os coxae via the femoral head, which has the fovea centralis for ligament attachment and nutrient artery passage.
• The femoral neck projects inferolaterally from the head at an angle of inclination.
• The greater and lesser trochanters serve as muscle attachment sites.
• The femoral shaft is directed medially towards the knee and is partially bowed to absorb impact forces.
• The gluteal tuberosity and linea aspera on the posterior shaft serve as attachments for the gluteus maximus and quadriceps femoris, respectively.
• The shaft terminates in medial and lateral condyles for knee articulation.
• The medial and lateral epicondyles provide attachment points for collateral ligaments.

Gluteal Region

• It is transitional between the trunk and lower limb where muscles act on the hip joint.
• Superior boundary: level of the iliac crest.
• Inferior boundary: gluteal folds.
• Lateral boundary: greater trochanter.
• It is bound to the sacrum via the sacrospinous and sacrotuberous ligaments.
• The superficial muscle layer originates off external surface of sacrum/ilium.
• It produces extension, abduction, and medial rotation.
• Abduction is particularly important during the stance phase of walking, preventing sagging of the contralateral side; disruption can indicated Trendelenburg's sign.
• The gluteus maximus is the thickest muscle of the body.
• A safe region for intramuscular injections is found laterally, void of neurovascular structures.
• It originates from the lateral sacrum, ilium (medial to posterior gluteal line), and sacrotuberous ligament.
• Fibers project inferolaterally to insert on the gluteal tuberosity and ITB.
• Forceful extension and lateral rotation of the thigh is the responsibility of the gluteus maximus.
• It sees greater use in flexed hip positions (e.g. sit-to-stand, climbing stairs).
• It receives its vascular suppl fromr superior and inferior gluteal vessels.
• Innervation: inferior gluteal nerve
• The gluteus medius originates off gluteal surface of the ilium, between posterior and anterior gluteal lines.
• It inserts on the posterior aspect of the greater trochanter
• It is responsible for abduction, and medial rotation of the hip.
• Vascular supply and innervation: superior gluteal vessels/nerve
• The gluteus minimus originates off the gluteal surface of the ilium, deep to gluteus medius, inferior to inferior gluteal lines.
• It inserts on anterior aspect of the greater trochanter.
• It is responsible for the abduction and medial rotation of the hip.
• Vascular supply and innervation: superior gluteal vessels/nerve
• The tensor fascia lata is fusiform and originates off the iliac crest, inferior to the ASIS.
• Epimysium of muscle converges on the ITB.
• It pulls the fascia of the anterior compartment tight, assisting with flexion of the thigh.
• It is a part of the gluteal region due to its neurovascular supply (superior gluteal artery/nerve).
• The deep muscular compartment is a collection of small muscles originating off the internal surface of the pelvis.
• They insert on/around the intertrochanteric fossa.
• They are principally involved in lateral rotation, stabilization of the hip joint.
• The piriformis is a pear-shaped muscle.
• Originates off the internal surface of the sacrum
• Inserts on the greater trochanter
• Important landmark:
  • The superior gluteal neurovascular bundle projects above.
  • The inferior gluteal neurovascular bundle, sciatic nerve projects below.
• Piriformis syndrome involves a tight piriformis muscle (resulting from repetitive contraction of gluteal muscles in sports such as skating, cycling) that may impinge on the sciatic nerve; more common when the sciatic nerve penetrates the piriformis.
• The obturator internus originates off internal surface of the obturator membrane.
• Its tendon projects posteriorly, curves at right angle, projecting laterally to insert on trochanteric fossa.
• Innervation: nerve to obturator internus
• The superior/inferior gemellis originate off ischial spine/tuberosity and insert on the obturator internus muscle.
• Innervation: superior innervated by nerve to oburator internus, inferior by nerve to quadratus femoris
• The quadratus femoris originates from the furrow between the ischial tuberosity and acetabular fossa.
• It inserts on the quadrate tubercle between the greater and lesser trochanters.
• Innervation: nerve to quadratus femoris

Hip Joint

• It is a synovial ball-and-socket joint between the acetabulum of the os coxae and the head of the femur.
• The femoral head articulates with the lunate surface within the superior margin of the acetabulum.
• The surface area of the acetabulum is increased by the presence of acetabular labrum.
• Acetabular labrum is continuous with transverse acetabular ligament.
• More than half of the femoral head is covered at a given position.
• The joint capsule is anchored to the acetabulum external to the labrum.
• It projects anteriorly in spiral fashion to attach to the intertrochanteric line anteriorly.
• It provides strong resistance to hyperextension.
• Lack of posterior anchor allows the synovial membrane to protrude posteriorly, forming a bursa to cushion the overlying muscle.
• Three thickenings form intercapsular ligaments: the iliofemoral, ischiofemoral, and pubofemoral.
• The iliofemoral ligament runs from AIIS and the acetabular rim to the intertrochanteric line. Extremely strong, it resists hyperextension of the hip joint through a corkscrew mechanism.
• The ischiofemoral ligament runs from the posterior acetabular rim. Relatively weak, it resists hyperextension.
• The pubofemoral ligament originates inferiorly off the obturator crest of the pubic bone and prevents hyperabduction of the joint.
• The synovial membrane anchors to the periphery of the articular surface of the femoral head, with numerous folds for increased mobility.
• It surrounds the ligamentum capitis femoris (ligament to the head of the femur) to conduct the artery to the head of the femur for vascular supply.
• Disruption of this artery leads to avascular necrosis and degeneration of the femoral head.
• The hip joint is a multi-axial joint that permits flexion/extension, abduction/adduction, circumduction, and rotation.
• Ligaments limit extension through a corkscrew mechanism.
• Flexion is only limited through soft tissue apposition (thigh against abdomen).
• Lateral rotators are stronger than medial rotators, providing greater stabilization posteriorly.

Overview of the lower limb's role in weight distribution and balance. Discussion of the fascia lata, a tough, fibrous fascia enveloping the thigh, which acts like a surgical stocking to compress deep tissues and prevent blood pooling.