4.6 Hip, Thigh, Gluteal Region, Leg, Joints & Foot

Hip Bone and Femur

The hip (coxal) bone is formed by the fusion of three bones — ilium, ischium, and pubis — at the acetabulum (the cup-shaped socket for the femoral head), meeting at the triradiate cartilage in childhood. The two hip bones plus the sacrum and coccyx form the pelvis. Landmarks: iliac crest (the subcostal and iliohypogastric nerves cross its posterior portion), anterior superior iliac spine (ASIS) (attachment of sartorius and the inguinal ligament), ischial tuberosity (origin of the hamstrings, weight-bearing when seated), ischial spine (an obstetric pelvic landmark), pubic symphysis, and the obturator foramen (closed by the obturator membrane, traversed by the obturator nerve and vessels in the obturator canal).

The femur is the longest bone. Its head bears a small fovea capitis (attachment of the ligamentum teres, carrying a minor artery to the head), the neck angles about 125° to the shaft and is the most common fracture site in the elderly (subcapital), the greater trochanter (gluteal insertions) and lesser trochanter (iliopsoas insertion), the intertrochanteric line (anterior) and crest (posterior), the linea aspera (muscle attachments on the posterior shaft), and the medial and lateral condyles separated by the intercondylar fossa. The patella (the largest sesamoid bone) sits in the quadriceps tendon.

Gluteal Region and Sciatic Nerve

The gluteal muscles lie posterior to the hip joint:

• Gluteus maximus — the largest; extends and laterally rotates the hip; arises from the posterior ilium, sacrum, and coccyx and inserts on the gluteal tuberosity and the iliotibial tract; inferior gluteal nerve (L5–S2). Used for standing up from sitting and climbing stairs.
• Gluteus medius and minimus — abduct the hip and medially rotate; attach from the ilium to the greater trochanter; superior gluteal nerve (L4–S1). They stabilize the pelvis in the coronal plane during the stance phase of gait — a Trendelenburg sign (the pelvis drops on the unsupported side) indicates weakness or superior gluteal nerve injury.
• Tensor fasciae latae — anterior hip, abducts and medially rotates; superior gluteal nerve.
• Piriformis — a lateral rotator; originates on the anterior sacrum and exits the pelvis through the greater sciatic foramen. It is the key landmark for gluteal anatomy: the sciatic nerve typically exits the pelvis inferior to piriformis ("piriformis syndrome" arises when an anomalous or tight piriformis compresses it). The six lateral rotators — piriformis, gemellus superior, obturator internus, gemellus inferior, quadratus femoris, and obturator externus — are mostly supplied by small nerves to these short rotators.

The sciatic nerve (L4–S3) is the largest nerve in the body. It exits the pelvis through the greater sciatic foramen below piriformis, descends under gluteus maximus midway between the ischial tuberosity and the greater trochanter (intramuscular gluteal injections are given in the upper outer quadrant to avoid it), and divides in the distal thigh (or at the apex of the popliteal fossa) into the tibial nerve and the common fibular (peroneal) nerve. The sciatic nerve supplies the hamstrings (via its tibial component, except the short head of biceps femoris, which is supplied by the common fibular component) and all of the leg and foot via its terminal branches.

Thigh Compartments

Three fascial compartments:

• Anterior — quadriceps (rectus femoris, vastus medialis, lateralis, and intermedius) extends the knee; sartorius flexes, abducts, and laterally rotates the hip and flexes the knee. Innervated by the femoral nerve (L2–L4). The saphenous nerve is its sensory continuation down the medial leg.
• Posterior (hamstrings) — semitendinosus, semimembranosus, biceps femoris (long head and short head). Extend the hip and flex the knee. Supplied by the tibial part of the sciatic nerve (L5–S2), except the short head of biceps femoris by the common fibular part.
• Medial (adductors) — adductor longus, brevis, and magnus (the magnus also has a hamstring, sciatic-supplied part), gracilis, pectineus, and obturator externus. Supplied by the obturator nerve (L2–L4), except pectineus (femoral) and the adductor magnus hamstring part (tibial).

Leg Compartments

• Anterior (extensor/dorsiflexor) — tibialis anterior (dorsiflexion and inversion), extensor hallucis longus, extensor digitorum longus, and fibularis tertius. Deep fibular (peroneal) nerve (L4–S1). Injury produces foot drop and a high-steppage gait.
• Lateral (evertor) — fibularis longus and brevis; superficial fibular nerve (L5–S1); eversion and weak plantarflexion.
• Posterior (plantarflexor) — superficial: gastrocnemius (medial and lateral heads from the femoral condyles) and soleus (from the tibia and fibula) join to form the Achilles (calcaneal) tendon, plus plantaris; deep: tibialis posterior (inversion and plantarflexion), flexor digitorum longus, flexor hallucis longus, and popliteus (unlocks the knee). Supplied by the tibial nerve (L4–S3); tibial nerve injury prevents plantarflexion and inversion and loses sensation on the sole.

Joints of the Lower Limb

Hip joint — ball-and-socket, multiaxial. The acetabulum is deepened by the acetabular labrum (fibrocartilage). Ligaments: iliofemoral (the "Y-ligament of Bigelow," the strongest, prevents hyperextension), pubofemoral, ischiofemoral, and the weak ligamentum teres (to the fovea capitis). The femoral head's blood supply comes primarily from the medial and lateral circumflex femoral arteries (branches of the profunda femoris) that anastomose around the neck — a femoral neck fracture disrupts this supply and risks avascular necrosis of the head; the artery of the ligamentum teres contributes negligibly in adults.

Knee joint — the largest joint, a modified hinge. Articulations: the femoral condyles with the tibial plateau, and the patella with the femur. Major ligaments: anterior cruciate ligament (ACL) (from the posterior femur to the anterior tibia; prevents anterior tibial translation — torn in pivoting injuries, positive anterior drawer and Lachman tests), posterior cruciate ligament (PCL) (prevents posterior tibial translation), medial collateral ligament (MCL) (resists valgus stress, fused with the medial capsule), and lateral collateral ligament (LCL) (resists varus, independent of the capsule). The medial meniscus is C-shaped and firmly attached to the MCL and capsule — therefore more commonly torn; the lateral meniscus is O-shaped and mobile. The "unhappy triad" (ACL + MCL + medial meniscus) classically results from a lateral blow to a planted, twisting knee.

Ankle (talocrural) joint — a hinge. The talus sits in the mortise formed by the tibial plafond plus the medial malleolus (tibia) and lateral malleolus (fibula). The strong deltoid (medial) ligament resists eversion; the weaker lateral ligament complex (anterior talofibular, calcaneofibular, posterior talofibular) resists inversion — inversion (lateral) ankle sprains are far more common, with the ATFL the first to tear.

Foot Arches and Muscles

The foot has medial longitudinal, lateral longitudinal, and transverse arches, supported by bony shape, ligaments (the plantar aponeurosis, the long plantar ligament, and the short plantar [plantar calcaneocuboid] ligament acting like tie-rods), and active muscular support — most importantly the tibialis posterior (supports the medial arch and eccentrically controls pronation) and the fibularis longus (its tendon crosses the sole to insert on the 1st metatarsal and medial cuneiform, supporting the transverse and longitudinal arches). Intrinsic muscles lie in four layers on the plantar surface; the abductor hallucis, flexor digitorum brevis, and abductor digiti minimi form the superficial layer. Plantar fasciitis is inflammation of the plantar aponeurosis at its calcaneal origin — one of the most common causes of heel pain.