Lower Limb — Femoral Triangle, Popliteal Fossa, Sciatic Nerve & Knee Joint

Femoral Triangle — Boundaries, Floor, and Contents

The femoral triangle is an important superficial space in the anterior thigh through which major neurovascular structures pass en route to and from the lower limb. It is delineated by clear anatomical boundaries: the inguinal ligament forms the superior border; the sartorius muscle forms the lateral border; and the adductor longus muscle (or pectineus in some descriptions) forms the medial border. The apex is formed where the sartorius and adductor longus converge. The floor is composed (from lateral to medial) of the iliopsoas, the pectineus, and the adductor longus muscles. The triangle is covered superficially by the fascia lata, which is continuous with the inguinal ligament.

Within the femoral triangle, the femoral sheath — a fascial extension from the abdominal aorta — encloses the femoral artery, femoral vein, femoral canal, and associated lymphatics. The femoral canal is the most medial compartment of the sheath and contains lymphatics and fat; it is the potential space through which femoral hernias protrude.

The mnemonic “NAVA” (Nerve, Artery, Vein, Nails/Lymphatics) from lateral to medial helps recall the arrangement of contents. The femoral nerve (L2–L4) lies most laterally, supplying the quadriceps and giving off cutaneous branches to the anterior and lateral thigh. The femoral artery — the continuation of the external iliac artery — enters the thigh beneath the inguinal ligament at the midpoint of the inguinal ligament. It is the source of the deep artery of thigh (profunda femoris) and the superficial femoral artery, and is commonly used for cardiac catheterization access. The femoral vein — the continuation of the popliteal vein — accompanies the artery and receives the great saphenous vein at the saphenofemoral junction. The most medial structure is the femoral canal containing deep inguinal lymph nodes and lymphatics.

Clinical correlations: The femoral pulse is palpable below the inguinal ligament at its midpoint. A femoral hernia presents as a bulge inferolateral to the pubic tubercle and carries a high risk of strangulation because of the rigid boundaries of the femoral canal. The femoral nerve can be blocked at the femoral triangle for analgesia of the anterior thigh. During cardiac catheterization via the femoral artery, the common femoral artery is the preferred access point above the femoral bifurcation; the superficial femoral artery is the continuation below the profunda femoris origin.

Popliteal Fossa — Anatomy of the Posterior Knee

The popliteal fossa is a diamond-shaped space posterior to the knee joint, bounded superiorly by the biceps femoris (superolateral) and the semimembranosus and semitendinosus muscles (superomedial), and inferiorly by the lateral and medial heads of the gastrocnemius (inferolateral and inferomedial respectively). The floor is formed by the popliteal surface of the femur, the knee joint capsule, and the popliteus muscle.

The contents from superficial to deep are arranged with the tibial nerve most superficial, the popliteal vein in the middle (receiving the small saphenous vein), and the popliteal artery deepest. The common fibular nerve exits superiorly through the apex between the two heads of the gastrocnemius, lying lateral to the tibial nerve. The popliteal lymph nodes are embedded within the fat.

The popliteal artery — the continuation of the femoral artery — is the deepest and most surgically important structure. At the lower border of the popliteus, it terminates by dividing into the anterior tibial artery (which pierces the interosseous membrane) and the tibiopopliteal trunk, which divides into the posterior tibial and fibular arteries. The popliteal vein — formed by the union of the anterior and posterior tibial veins — accompanies the artery.

Clinical correlations: Popliteal aneurysm (a pulsatile swelling with or without pain) can cause compression of the tibial nerve, leading to calf pain, or may thrombose, leading to acute limb ischemia. Popliteal artery occlusion by atherosclerosis or embolism results in severe leg ischemia with absent popliteal and distal pulses. Baker’s cyst (a semimembranosus-gastrocnemius bursa herniation) presents as a swelling in the popliteal fossa and can rupture, mimicking deep vein thrombosis. The popliteal pulse is the deepest palpable arterial pulse and is felt with the knee slightly flexed to relax the fascia.

Sciatic Nerve — Course, Divisions, and Clinical Implications

The sciatic nerve (L4–S3) is the largest nerve in the body. It emerges from the greater sciatic foramen below the piriformis muscle and descends through the gluteal region deep to the gluteus maximus. It runs posterior to the hip joint, the quadratus femoris, and the gemelli, and then descends between the ischial tuberosity and the greater trochanter. In the posterior thigh, it lies deep to the biceps femoris and is crossed by the long head of biceps femoris. It terminates at the upper border of the popliteal fossa by dividing into the tibial nerve and common peroneal nerve, though this division may occur at any point from the sacral plexus to the lower thigh.

The tibial nerve (L4–S3) is the larger medial terminal branch. It descends through the popliteal fossa, passes deep to the tendinous arch of the soleus muscle, and continues into the posterior compartment of the leg, where it divides into the medial and lateral plantar nerves. It innervates all muscles of the posterior compartments of the thigh (hamstrings — long head of biceps femoris, semitendinosus, semimembranosus) and leg, plus the plantigrade muscles of the foot. The common peroneal nerve (L4–S2) is the smaller lateral terminal branch. It wraps superficially around the neck of the fibula — a point of extreme vulnerability — and divides into the deep peroneal nerve (anterior compartment of leg — dorsiflexion and toe extension) and superficial peroneal nerve (lateral compartment of leg — eversion and dorsum sensation).

Clinical correlations: Piriformis syndrome causes buttock pain radiating along the sciatic nerve distribution and results from sciatic nerve compression at the greater sciatic foramen, sometimes because of piriformis hypertrophy or anatomical variation. In the gluteal region, the nerve is at risk during intramuscular injections administered too medially — injections should always be given in the upper outer quadrant of the buttock. Sciatic nerve injury causes flaccid paralysis of hamstrings, all muscles below the knee, and intrinsic foot muscles, with sensory loss over the posterior thigh, posterior leg, and most of the foot. Common peroneal nerve palsy at the fibular neck causes foot drop (inability to dorsiflex or evert the foot), steppage gait, and sensory loss over the dorsum of the foot and lateral leg. Tibial nerve injury causes inability to plantarflex (loss of standing on toes) and sensory loss over the sole.

Knee Joint — Articular Surfaces and Supporting Structures

The knee joint is the largest and most complex joint in the body. It is a hinge-type synovial joint, though it permits some rotation and gliding. The articular surfaces include the femoral condyles (medial and lateral), the tibial condyles (medial and lateral), and the patella. The joint has three functional compartments: the medial femorotibial, the lateral femorotibial, and the patellofemoral.

The menisci — C-shaped wedges of fibrocartilage — sit on the tibial plateau. The medial meniscus is larger, less mobile (attached to the tibial collateral ligament), and more commonly torn. The lateral meniscus is more mobile and relatively protected. The menisci deepen the tibial plateau, distribute load, and absorb shock. They are supplied by the medial and lateral inferior genicular arteries; the peripheral one-third is vascular, the central two-thirds are avascular and heal poorly. The anterior and posterior cruciate ligaments are intracapsular but extrasynovial structures. The anterior cruciate ligament (ACL) prevents anterior translation of the tibia on the femur; the posterior cruciate ligament (PCL) prevents posterior translation. The ACL is most taut in extension and is the more commonly injured of the two.

The collateral ligaments are extracapsular. The medial collateral ligament (MCL) runs from the medial femoral epicondyle to the medial tibia; it resists valgus stress. The lateral collateral ligament (LCL) runs from the lateral femoral epicondyle to the fibular head; it resists varus stress. The MCL is biomechanically linked to the medial meniscus — injuries often occur together.

Synovial membrane lines the inner capsule and forms several recesses: the suprapatellar bursa (which extends 3–5 cm above the patella), the popliteal bursa (posterior), and the infrapatellar fat pad (Hoffa’s fat) anteriorly. The quadriceps femoris tendon, patella, and patellar ligament form the extensor mechanism.

Clinical correlations: ACL injuries are among the most common sports injuries, typically from non-contact pivoting mechanisms. The Lachman test is the most sensitive clinical test for ACL injury; the anterior drawer test and pivot shift test are also used. PCL injuries occur from direct posterior force on the tibia (dashboard injury). Meniscal tears produce joint line tenderness, locking, clicking, and a positive McMurray test. MCL injuries cause pain with valgus stress applied at 30 degrees of knee flexion; LCL injuries cause pain with varus stress. Collateral ligament injuries are graded I (stretch), II (partial tear), or III (complete tear). Knee effusion (synovial fluid accumulation) presents as a swelling of the suprapatellar pouch and causes a ballotable patella.

Extensor Mechanism and Patellofemoral Joint

The extensor mechanism consists of the quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius), the patella, and the patellar ligament. The vastus medialis obliquus (VMO) — the most distal component of vastus medialis — pulls the patella medially and prevents lateral patellar subluxation. The patella is the largest sesamoid bone,Embedded within the quadriceps tendon, and has seven articular facets on its posterior surface that articulate with the femoral trochlea. The patellofemoral joint experiences forces up to five times body weight during stair climbing.

The patellar ligament inserts on the tibial tuberosity. Osgood-Schlatter disease is an apophysitis of the tibial tuberosity in adolescents (from repeated quadriceps contraction during growth spurts), causing pain and swelling at the tuberosity. Patellar tendinitis (jumper’s knee) causes pain at the patellar ligament insertion on the inferior patella. Patellar dislocation is usually lateral, occurring when the foot is planted and the leg is rotated; it causes damage to the medial patellofemoral ligament (MPFL) and may require reduction.

Clinical correlations: The prepatellar bursa (between the skin and patella) can become inflamed from repetitive kneeling (“housemaid’s knee”). The infrapatellar bursa lies between the patellar ligament and the tibia. The anserine bursa — between the conjoined insertion of sartorius, gracilis, and semitendinosus on the medial tibia — is a common cause of medial knee pain in obese patients (anserine bursitis).