Anatomía del Miembro Inferior
Anatomy of the Coxal Bone and Pelvic Architecture
The coxal bone is a complex structure formed by the fusion of three distinct bones: the ilium (superior), the ischium (anteroinferior), and the pubis (posteroinferior). These segments meet at the acetabulum, a cup-shaped cavity that articulates with the head of the femur. The coxal bone connects to the vertebral column at the sacrum and coccyx through the sacrotuberous and sacrospinous ligaments, which are inserted into the tuberosity and spine of the ischium, respectively. The superior portion of the ilium is defined by the iliac crest, which terminates anteriorly at the anterosuperior iliac spine and posteriorly at the posterosuperior iliac spine. A notable lateral expansion of this crest is known as the tubercle of the iliac crest. The area where the ilium and pubis fuse is marked by a raised bone area called the iliopubic eminence.
The gluteal surface of the ilium is divided into regions by three curved lines: the inferior, anterior, and posterior gluteal lines. The inferior gluteal line originates above the anteroinferior iliac spine and curves toward the posterior border of the acetabulum. The anterior gluteal line begins at the lateral border of the iliac crest and curves down to the superior border of the greater sciatic notch. The posterior gluteal line descends from the iliac crest toward the posteroinferior spine. These lines serve as landmarks for muscle origins: the gluteus minimus originates between the inferior and anterior lines, the gluteus medius originates between the anterior and posterior lines, and the gluteus maximus originates posterior to the posterior gluteal line.
The Ischium, Pubis, and Acetabular Components
The ischial tuberosity is a substantial protuberance located posteroinferior to the acetabulum. It is divided by a transverse line into superior and inferior areas. The superior area is further divided: the medial portion serves as the origin for the semitendinosus and the long head of the biceps femoris, while the lateral portion is the attachment for the semimembranosus. The inferior area provides the origin for the adductor magnus. The medial border of the ischial tuberosity holds a sharp ridge for the sacrotuberous ligament. The middle part of the inferior area of the tuberosity is specifically designed to support the body's weight in a seated position. The pubic region includes the ischiopubic ramus and the body of the pubis, which serve as insertion points for the medial compartment muscles of the thigh, including the adductor magnus, pectineus, and gracilis.
The acetabulum itself comprises an articular lunate surface, which surrounds the anterior, superior, and posterior borders of the acetabular fossa. The acetabular fossa is the non-articular part of the cup and continues into the acetabular notch. The ligament of the head of the femur, also known as the ligamentum teres, inserts into this fossa. Anatomical protection in this region is critical, as pelvic fractures can lead to hemorrhages and pelvic hematomas that compress nerves, organs, and viscera. These injuries are classified into four types: Type 1 involves no rupture of the pelvic ring; Type 2 involves a single break; Type 3 involves a double fracture or bilateral ramus fractures; and Type 4 involves lesions related specifically to the acetabulum.
Osteology of the Femur and the Hip Joint
The femur, the sole bone of the gluteal and thigh regions, features a head that articulates with the acetabulum. The ligament of the head of the femur attaches to a small non-articular depression on the medial surface of the femoral head called the fovea (fosita). The femoral neck projects superomedially from the shaft at an angle of roughly degrees and is slightly posteriorly oriented. The proximal femur contains the greater and lesser trochanters, which are primary insertion sites for hip movers. The greater trochanter features the trochanteric fossa on its medial surface; the obturator externus inserts into an oval depression in this fossa. Other insertions on the greater trochanter include the gluteus minimus (anterolateral surface), gluteus medius (lateral surface), obturator internus with the gemelli (medial side of the superior face), and the piriformis (superior border).
Between the trochanters lie the intertrochanteric line and the intertrochanteric crest. The pectineal line is a spiral ridge continuing from the intertrochanteric line to the linea aspera on the posterior femur. The intertrochanteric crest is a smooth ridge featuring the quadrate tubercle for the insertion of the quadratus femoris. The femoral shaft descends at an angle of degrees from lateral to medial in the coronal plane. Fractures of the femoral neck are particularly dangerous as they can interrupt blood supply to the head, leading to femoral head necrosis. In contrast, intertrochanteric fractures run between the trochanters and generally do not affect the blood supply to the neck.
The hip joint is a multiaxial synovial ball-and-socket (enarthrosis) joint designed for stability and weight-bearing. It allows flexion, extension, abduction, adduction, medial and lateral rotation, and circumduction. The articular surfaces are protected by hyaline cartilage, except for the fovea. Stability is enhanced by the acetabular labrum (a fibrocartilaginous collar) and the transverse acetabular ligament, which bridges the acetabular notch. The joint capsule is reinforced by three ligaments oriented in a spiral: the iliofemoral (attached to the anteroinferior iliac spine and intertrochanteric line), the pubofemoral (attached to the iliopubic eminence), and the ischiofemoral (attached to the ischium and greater trochanter). These ligaments tighten during extension, reducing the muscular energy needed to maintain a standing posture.
Neurovascular Pathways and the Inguinal Region
There are four main entry and exit points between the abdomen/pelvis and the lower limb: the space deep to the inguinal ligament, the greater sciatic foramen, the obturator canal, and the lesser sciatic foramen. The space deep to the inguinal ligament and superior pubic ramus transmits the psoas major, iliacus, and pectineus muscles; the femoral nerve; the femoral branch of the genitofemoral nerve; the lateral femoral cutaneous nerve; and the femoral artery, vein, and lymphatics. This space is a weak area where abdominal contents may herniate into the thigh, specifically through the femoral canal (the medial compartment of the femoral sheath containing lymphatics).
The greater sciatic foramen is the passage for the piriformis muscle, as well as the sciatic, superior and inferior gluteal, and pudendal nerves. It also transmits the superior and inferior gluteal and internal pudendal vessels. The lesser sciatic foramen transmits the tendon of the obturator internus and the internal pudendal vessels and nerves. The obturator canal, formed in the superior part of the obturator foramen by the obturator membrane and muscles, carries the obturator nerve and vessels.
Neurology: Plexuses and Peripheral Nerves
The lower limb is innervated by the lumbar plexus () and the sacral plexus (), connected by the lumbosacral trunk (). The femoral nerve () innervates the anterior thigh compartment, iliacus, and pectineus, providing sensation to the anterior thigh, medial leg, and medial ankle. The obturator nerve () innervates the medial thigh (except parts of the adductor magnus and pectineus) and provides sensation to the medial thigh. The sciatic nerve () is the largest nerve, dividing into the common peroneal (fibular) and tibial nerves. The tibial portion innervates the posterior thigh and leg muscles, and the lateral foot/ankle skin. The common peroneal portion innervates the lateral leg and dorsal foot.
Gluteal muscles are served by the superior gluteal nerve (), which innervates the gluteus medius, minimus, and tensor fasciae latae, and the inferior gluteal nerve (), which innervates the gluteus maximus. Other nerves include the ilioinguinal (), genitofemoral (), lateral femoral cutaneous (), and the posterior femoral cutaneous (). Lower spinal levels can be assessed via dermatomes: (medial side of the 2nd toe), (medial side of the 5th toe), (posterior thigh), (gluteal fold), and (perineum). Motor function (myotomes) includes hip flexion (), knee extension (), knee flexion (), and plantar flexion (). Tendon reflexes like the patellar () and calcaneal () are used to evaluate unconscious patients.
Vascularization and Lymphatics
The principal artery is the femoral artery, a continuation of the external iliac artery. Its major branch is the profunda femoris (deep femoral artery), which gives off the medial and lateral circumflex femoral arteries and three perforating arteries. The gluteal region is supplied by the superior and inferior gluteal arteries (branches of the internal iliac). Venous drainage includes deep veins (femoral, gluteal, obturator) and superficial veins (great and small saphenous veins). The great saphenous vein originates medially in the dorsal venous arch of the foot, ascends the medial leg and thigh, and joins the femoral vein through the saphenous opening in the fascia lata. The small saphenous vein originates laterally, passes behind the lateral malleolus, and joins the popliteal vein posterior to the knee.
Valvular incompetence in these veins leads to varicose veins, often found at the saphenofemoral junction or at perforating vein locations ( above the medial malleolus). Deep vein thrombosis (DVT/TVP) is a critical concern, characterized by Virchow's Triad: venous stasis, vascular wall injury, and hypercoagulability. DVT can cause pulmonary embolism and death. Lymphatic drainage follows two systems: superficial inguinal nodes (receiving linfa from the gluteal region, lower abdomen, and perineum) and deep inguinal nodes (medial to the femoral vein). Both ultimately drain into the external iliac nodes. Popliteal nodes, located behind the knee, receive drainage from the small saphenous area and deep leg/foot, draining into the inguinal nodes.
The Thigh: Compartments and Muscles
The thigh is separated from the abdomen by the inguinal ligament and from the gluteal region by the gluteal fold. It contains three compartments: anterior (extensors), medial (adductors), and posterior (extensors of the hip/flexors of the knee). The anterior compartment is innervated by the femoral nerve and includes the sartorius (the most superficial), rectus femoris (with direct and reflected heads), and the three vastus muscles (medial, intermediate, and lateral). Together, the rectus femoris and vasti form the quadriceps femoris. The medial compartment is served by the obturator nerve and includes the gracilis, pectineus (innervated by the femoral nerve), adductor longus, adductor brevis, adductor magnus (partially innervated by the sciatic nerve), and obturator externus.
The posterior compartment contains the hamstring muscles: biceps femoris (long head and short head), semitendinosus, and semimembranosus. These are largely innervated by the sciatic nerve, though the short head of the biceps is served by the common peroneal division. The "pata de ganso" (pes anserinus) is a combined insertion of the sartorius, gracilis, and semitendinosus on the medial tibial shaft. The femoral triangle is a wedge-shaped depression bounded by the inguinal ligament (base), adductor longus (medial), and sartorius (lateral). Its floor includes the pectineus, adductor longus, and iliopsoas. The triangle contains the femoral nerve, artery, vein, and canal.
Anatomy of the Leg and Knee
The leg consists of the tibia (medial, weight-bearing) and the fibula (lateral). They are joined by a fibrous interosseous membrane and the tibiofibular joints. The tibia features medial and lateral condyles, an intercondylar eminence, and the tibial tuberosity for the patellar ligament insertion. The fibula features a head, neck, and lateral malleolus. The common peroneal nerve passes laterally around the neck of the fibula, making it susceptible to injury. The leg is divided into anterior (dorsiflexors), posterior (plantarflexors), and lateral (evertors) compartments.
The posterior leg has superficial (gastrocnemius, soleus, plantaris) and deep (popliteus, flexor hallucis longus, flexor digitorum longus, tibial posterior) groups. All are innervated by the tibial nerve. The gastrocnemius and soleus combine to form the calcaneal (Achilles) tendon. The anterior compartment includes the tibial anterior, extensor hallucis longus, extensor digitorum longus, and fibularis tertius. The lateral compartment contains the fibularis longus and brevis, innervated by the superficial peroneal nerve. The popliteal fossa, a diamond-shaped space behind the knee, contains the popliteal artery/vein and the sciatic nerve division. Its floor is the popliteus muscle and its roof is the deep fascia.
The Foot: Structure and Function
The foot comprises tarsal bones (talus, calcaneus, navicular, cuboid, and three cuneiforms), metatarsals, and phalanges. The calcaneus is the largest tarsal bone and features the sustentaculum tali. The tarsal tunnel, located on the posteromedial ankle, allows passage for the tibial posterior, flexor digitorum longus, and flexor hallucis longus tendons, along with the posterior tibial artery and tibial nerve. The foot has intrinsic muscles arranged in layers. The first layer includes the abductor hallucis, flexor digitorum brevis, and abductor digiti minimi. The dorsal surface contains the extensor digitorum brevis and extensor hallucis brevis, innervated by the deep peroneal nerve. Foot blood supply comes from the posterior tibial and dorsalis pedis arteries. The dorsalis pedis pulse is the most distal palpable pulse, essential for evaluating peripheral circulation.
Questions & Discussion
Anatomy Quiz and Self-Evaluation
Q: What nerve innervates the anterior and lateral compartments of the leg? A: The peroneal portion of the sciatic nerve.
Q: Where can one palpate the femoral artery? A: Under the inguinal ligament, midway between the anterosuperior iliac spine and the pubic symphysis.
Q: What is the largest sesamoid bone in the body? A: The patella (rótula).
Q: What muscles form the "pata de ganso"? A: Sartorius, gracilis, and semitendinosus.
Q: Which nerve is most susceptible to injury at the lateral neck of the fibula? A: The common peroneal (common fibular) nerve.
Q: What is Virchow's Triad for thrombosis? A: Venous stasis, vascular wall injury, and state of hypercoagulability.
Q: What happens if the valves in the veins become incompetent? A: Additional pressure is exerted on distal valves, making them incompetent and leading to dilated, tortuous veins called varices.
Q: Which muscle is the only one in the medial compartment of the thigh not solely acting on the hip? A: The gracilis, which crosses both hip and knee.
Q: What is the Trendelenburg sign? A: A physical exam finding in patients with weak or paralyzed abductor muscles (gluteus medius and minimus).
Q: What are the structures inside the popliteal fossa from medial to lateral? A: Popliteal artery, popliteal vein, tibial nerve, and common peroneal nerve.
Q: Describe the four types of pelvic fractures mentioned. A: Type 1: No pelvic ring rupture. Type 2: Single break in the pelvic ring. Type 3: Double fracture in the ring or bilateral pubic ramus fractures (risk of urethral injury). Type 4: Injury involving or relative to the acetabulum.
Q: What are the most frequent masses found in the popliteal fossa? A: Popliteal (Baker’s) cyst, popliteal aneurysm, and arterial adventitial cyst.
Q: What are the boundaries of the adductor canal? A: Anteriorly/medially by the sartorius, laterally by the vastus medialis, and posteriorly by the adductor longus and magnus.
Q: Name the components of the tarsal tunnel in order from anteromedial to posterolateral. A: Tibial posterior tendon, flexor digitorum longus tendon, posterior tibial artery and veins, tibial nerve, and flexor hallucis longus tendon.