Lower Limb Anatomy: Hip Joint, Femoral Triangle, and Sciatic Nerve Pathway

The hip joint is a ball-and-socket joint between the femoral head and the acetabulum, stabilized by three powerful ligaments (iliofemoral, pubofemoral, ischiofemoral) and deepened by a fibrocartilaginous labrum. Its blood supply — primarily from the medial and lateral circumflex femoral arteries via retinacular branches — is clinically critical because femoral neck fractures can disrupt these vessels and cause avascular necrosis of the femoral head. The femoral triangle (bounded by the inguinal ligament, sartorius, and adductor longus) contains, from lateral to medial, the femoral Nerve, Artery, Vein, Empty space, and Lymphatics (mnemonic: NAVEL). The sciatic nerve (L4-S3) exits the pelvis through the greater sciatic foramen below the piriformis muscle and descends through the posterior thigh before dividing into the tibial and common fibular (peroneal) nerves at the popliteal fossa.

The hip joint is the most stable joint in the body, and it needs to be — it bears the entire weight of the upper body during standing and walking, experiencing forces of 3-5 times body weight during running. The femoral head sits deep within the acetabulum (unlike the humeral head in the shoulder, which sits on a shallow glenoid). The acetabular labrum deepens the socket further, creating a near-seal around the femoral head. Three ligaments reinforce the capsule: the iliofemoral (the strongest ligament in the body — Y-shaped, running from the AIIS to the intertrochanteric line), the pubofemoral (prevents excessive abduction), and the ischiofemoral (prevents excessive internal rotation). Together, they spiral around the femoral neck in a way that tightens during extension and loosens during flexion. This is why you can flex the hip to 120+ degrees but can only extend it about 20 degrees — the ligaments are at maximum tension in extension. The blood supply is where clinical anatomy gets real. The femoral head receives blood from three sources: the medial circumflex femoral artery (supplies most of the femoral head via retinacular arteries that run along the femoral neck under the capsule), the lateral circumflex femoral artery (supplies the anterior femoral head), and the artery of the ligamentum teres (small, inconsistent, supplies only a tiny patch of the femoral head in adults). A femoral neck fracture — especially an intracapsular fracture — can tear the retinacular arteries, cutting off blood supply to the femoral head. Without blood, the bone dies. This is avascular necrosis (AVN), and it is why displaced femoral neck fractures in elderly patients often require hip replacement rather than fixation — even if you pin the fracture back together, the blood supply may already be destroyed. AnatomyIQ has 3D hip joint models that let you rotate the femur and trace the retinacular arteries along the neck to understand exactly why fracture location determines treatment. This content is for educational purposes only and does not constitute medical advice.

The femoral triangle is a depression on the anterior thigh just below the inguinal ligament. It is one of the most clinically important regions in the body because it contains the major neurovascular structures supplying the lower limb — and because it is where you access them for procedures. Boundaries: the inguinal ligament superiorly (the line from ASIS to pubic tubercle), the medial border of sartorius laterally, and the lateral border of adductor longus medially. The floor is formed by the iliopsoas (laterally) and pectineus (medially). The roof is the fascia lata. Contents from lateral to medial (NAVEL): Femoral Nerve, Femoral Artery, Femoral Vein, Empty space (femoral canal — the site of femoral hernias), and Lymphatics (the deep inguinal lymph node of Cloquet sits in the femoral canal). The femoral artery is the key landmark. It is palpable just below the midinguinal point (halfway between the ASIS and the pubic symphysis) — this is where you feel the femoral pulse and where vascular access is obtained for cardiac catheterization, angiography, and femoral line placement. The femoral nerve is immediately lateral to the artery (which is why femoral nerve blocks are performed just lateral to the arterial pulse). The femoral vein is immediately medial to the artery (which is why femoral venous access targets medial to the pulse). The femoral canal is the most medial compartment. It is a potential space that normally contains fat and lymph tissue. Its clinical significance: it is the site of femoral hernias. Abdominal contents can push through the femoral ring (the opening of the canal) and into the thigh. Femoral hernias are more common in women (wider pelvis = wider femoral ring) and have a high incidence of strangulation because the femoral ring is rigid and does not stretch.

The sciatic nerve (L4, L5, S1, S2, S3) is the largest and longest nerve in the body — about the width of your thumb in the gluteal region. It is actually two nerves bundled in a common connective tissue sheath: the tibial nerve (from the anterior divisions of L4-S3) and the common fibular nerve (from the posterior divisions of L4-S2). They travel together through the gluteal region and posterior thigh, then separate at or above the popliteal fossa. The nerve exits the pelvis through the greater sciatic foramen, passing inferior to the piriformis muscle (in about 85% of people — anatomic variants where the nerve splits around or through the piriformis are relevant to piriformis syndrome). In the gluteal region, it lies deep to the gluteus maximus, resting on the posterior surface of the ischium, the obturator internus tendon, and the quadratus femoris. In the posterior thigh, the sciatic nerve descends between the hamstring muscles (which it innervates — the tibial division supplies the semimembranosus, semitendinosus, and the long head of biceps femoris; the common fibular division supplies the short head of biceps femoris). The nerve then enters the popliteal fossa and typically divides into its two terminal branches. The tibial nerve continues down the posterior leg, passing through the tarsal tunnel behind the medial malleolus to supply the sole of the foot. The common fibular nerve wraps around the neck of the fibula (where it is extremely superficial and vulnerable to injury from casts, fractures, or prolonged pressure from crossing legs) and divides into superficial and deep branches. Common fibular nerve injury causes foot drop — inability to dorsiflex the foot — because the deep fibular nerve supplies tibialis anterior and the toe extensors. This is one of the most commonly tested nerve injuries in anatomy.

Intracapsular femoral neck fracture in an elderly patient: think avascular necrosis risk. The retinacular arteries are disrupted. If the fracture is displaced, hip replacement (arthroplasty) is usually preferred over internal fixation because the blood supply is likely already compromised. Femoral pulse location: midinguinal point (halfway between ASIS and pubic symphysis). This is different from the midpoint of the inguinal ligament (halfway between ASIS and pubic tubercle) — exams love to test this distinction. The femoral artery is at the midinguinal point. The deep inguinal ring (relevant for inguinal hernias) is at the midpoint of the inguinal ligament. Foot drop after a fibular fracture or tight cast: common fibular nerve injury at the fibular neck. The nerve is superficial and vulnerable here. Test by asking the patient to dorsiflex the foot (pull toes toward shin) and evert the foot. Loss of both = common fibular nerve. Loss of dorsiflexion only = deep fibular nerve. Positive Trendelenburg sign (pelvis drops on the unsupported side during single-leg stance): indicates weakness of the gluteus medius and minimus on the standing side, usually from superior gluteal nerve injury (L4-S1). The superior gluteal nerve exits above the piriformis — the only structure to do so through the greater sciatic foramen. Damage during intramuscular gluteal injections (which is why injections go in the upper outer quadrant to avoid this nerve) is a classic cause.