Human Anatomy Lesson 25
The thigh is the part of the lower limb between the hip joint and the knee. It is bordered superiorly by tissues of the abdominal wall, which terminate in the inguinal ligament, and posteriorly by the gluteal region. The sciatic nerve and other structures enter the thigh through the gluteal region. In addition, structures enter the thigh from the abdomen under the inguinal ligament, and from the pelvic cavity through the obturator canal. The thigh can be divided into three compartments: an anterior compartment containing muscles that extend the leg at the knee joint; a posterior compartment containing muscles that extend the thigh at the hip and flex the leg at the knee; and a medial compartment containing muscles that AD-duct the thigh at the hip joint. In this lesson, we will go over muscles, nerves, arteries, and veins in each compartment, in addition to how all of these structures are related to each other halfway down the thigh and at the knee joint.
- Describe muscle attachment and other relevant structural details on the pelvis, femoral shaft and distal femur, patella, and proximal tibia and fibula
- List and describe the origins, insertions, innervations, and functions of thigh muscles in the anterior, medial, and posterior compartments
- Trace the courses of 3 nerves (femoral, obturator, sciatic) through the thigh
- List and describe the branches of two important arteries (femoral, obturator) that run through the thigh, and one vein (great saphenous)
- List and describe membranes, ligaments, and functions of the knee joint
- Understand the relative positions of structures within the popliteal fossa
The larger muscles in the thigh generally originate on the pelvis and insert onto the femur or the proximal parts of the tibia and fibula.
- Femur: we previously covered the proximal femur with the hip joint, so this section will focus on the shaft and distal end of the femur. The femur is angled obliquely relative to the midline of the body from proximal to distal, so that the greater tuberosity and femoral neck lie further laterally than does the distal part of the femur at the knee. This angle is called the carrying angle or valgus angle, and positions the knee under the center of gravity, which aids in bipedal walking. The midshaft of the femur has a triangular cross-section, and the linea aspera points posteriorly. The linea aspera widens inferiorly to become continuous with the medial and lateral supracondylar lines. The medial supracondylar line terminates at the adductor tubercle on the superior aspect of the medial condyle, which forms the attachment area for the adductor magnus muscle. Just lateral to the lower end of the medial supracondylar line is a roughened area for attachment of the medial head of the gastrocnemius muscle. The distal end of the femur is characterized by two large condyles which articulate with the proximal head of the tibia. The condyles are separated posteriorly by an intercondylar fossa. There are small oval facets on the two intercondylar walls for attachment of the proximal ends of the anterior cruciate ligament and the posterior cruciate ligament. Collateral ligaments of the knee joint attach to epicondyles on the nonarticular outer surfaces of the condyles.
- Patella: this bone is formed with in the tendon of the quadriceps femoris muscle, which crosses anterior to the knee joint to insert on the tibia. The patella is triangular, and its apex is pointed inferiorly and forms attachment for the patellar ligament.
- Tibia: the proximal end of the tibia is expanded mediolaterally, and the medial condyle and lateral condyle articulate with corresponding condyles on the femur. The intercondylar region between the two condyles provides ample attachment area for the cruciate ligaments and interarticular cartilages (menisci) of the knee joint. The tibial tuberosity on the anterior side of the proximal tibia also provides attachment area for muscles and ligaments, including the patellar ligament. The tibial shaft is triangular in cross-section and has anterior, interosseous, posterior, and medial borders. The anterior border is sharp and easily palpable; the interosseous border has a small ridge where the interosseous membrane attaches on the lateral margin of the tibia; the medial border is sharp at midshaft and indistinct superiorly.
- Fibula: this long spindly bone does not participate in the knee joint or weightbearing, although the fibular collateral ligament attaches to its head. The biceps femoris muscle attaches on its head. The distal end forms the lateral malleolus which participates in the ankle joint.
Anterior Compartment Muscles
As noted above, muscles in the anterior compartment act on the hip and knee joints. We have seen two of these muscles - psoas major and iliacus - before, when we covered the abdominopelvic region. Two other muscles - sartorius and rectus femoris, cross both joints and act to flex the thigh at the hip and the leg at the knee. The vastus muscles act on the knee joint. Psoas major is innervated directly by the anterior rami of L1, L2, and L3 spinal nerves and the other muscles in the anterior compartment are innervated by the femoral nerve.
- Iliopsoas: formed from two muscles, psoas major and iliacus, which run from different parts of the posterior abdominal wall to the lesser trochanter of femur.
- Vastus medialis: runs from the medial part of the intertrochanteric line, pectineal line, linea aspera, and medial supracondylar line to the quadriceps femoris tendon and medial border of patella. Extends the leg at the knee joint.
- Vastus intermedias: runs from the upper 2/3 of anterior, medial, and lateral surfaces of the femur to the quadriceps femoris tendon and lateral margin of patella. Extends the leg at the knee joint.
- Vastus lateralis: runs from the lateral margin of the intertrochanteric line, greater trochanter, gluteal tuberosity, and linea aspera to the quadriceps femoris tendon. Extends the leg at the knee joint.
- Rectus femoris: runs from two heads on the anterior inferior iliac spine and ilium just superior to the acetabulum to the quadriceps femoris tendon. Flexes the thigh at the hip joint and extends the leg at the knee joint.
- Sartorius: runs from the anterior superior iliac spine to the medial surface of the tibia inferomedial to the tibial tuberosity. Flexes the thigh at the hip joint and the leg at the knee joint, as when a tailor squats to hem a pair of pants.
Medial Compartment Muscles
Of the 6 muscles in this compartment, all but one (obturator externus) AD-duct the thigh at the hip joint, and adductor longus and adductor magnus also medially rotate the thigh. Obturator externus is a lateral rotator of the thigh at the hip joint.
- Gracilis: runs from the pubis, pubic ramus, and ischiopubic ramus to the medial surface of the proximal shaft of the tibia. Innervated by the obturator nerve. AD-ducts the thigh at the hip joint and flexes the leg at the knee joint.
- Pectineus: runs from the pectineal line of the pubis to the oblique line on the posterior surface of the proximal femur extending from the lesser trochanter to the linea aspera. Innervated by the femoral nerve. AD-ducts and flexes the thigh at the hip joint.
- Adductor longus: runs from the pubis inferior to the pubic crest to the linea aspera of the middle 1/3 of the femoral shaft. Innervated by the obturator nerve. AD-ducts and medially rotates the thigh at the hip joint.
- Adductor brevis: runs from the pubis and inferior pubic ramus to the posterior surface of the proximal femur and upper 1/3 of the linea aspera superior to the attachment of adductor longus. Innervated by the obturator nerve. AD-ducts the thigh at the hip joint.
- Adductor magnus: runs from the ischiopubic ramus and ischial tuberosity to the posterior surface of the proximal femur, linea aspera, medial supracondylar line, and the adductor tubercle. The "AD-ductor part" of adductor magnus is innervated by the obturator nerve and AD-ducts and medially rotates the thigh at the hip joint, whereas the "hamstring part" that attaches to the adductor tubercle is innervated by the tibial division of the sciatic nerve.
- Obturator externus: runs from the external surface of the obturator membrane and adjacent bone to the trochanteric fossa. Innervated by the obturator nerve. Laterally rotates the thigh at the hip joint.
Posterior Compartment Muscles
The three long muscles in the posterior compartment are collectively referred to as the hamstring muscles, and cross the hip and knee joints. They flex the leg at the knee joint and extend the thigh at the hip joint as well as rotating both joints. All three muscles are innervated by the sciatic nerve, and all three flex the leg at the knee joint and extend the thigh at the hip joint. In addition, biceps femoris laterally rotates the thigh and semitendinosus and semimembranosus medially rotate the thigh.
- Biceps femoris: runs from the ischial tuberosity and lateral lip of the linea aspera as two heads to the head of the fibula.
- Semitendinosus: runs from the ishcial tuberosity to the medial surface of the proximal tibia.
- Semimembranosus: runs from the ischial tuberosity to the medial tibial condyle.
As noted above, the femoral nerve is associated with muscles in the anterior compartment, the obturator nerve is associated with muscles in the medial compartment, and the sciatic nerve is associated with muscles in the posterior compartment.
- Femoral nerve: made up of L2-L4 fibers from the lumbar plexus which pass under the inguinal ligament to enter the thigh on the lateral side of the femoral artery. It supplies branches to iliacus and pectineus, then splits into anterior and posterior branches that run inferiorly as far as the foot. Branches include anterior cutaneous branches which supply skin on the front of the thigh and knee; motor nerves which supply quadriceps muscles and sartorius; and the saphenous nerve, which supplies skin on the medial side of the knee, leg, and foot.
- Obturator nerve: made up of L2-L4 fibers from the lumbar plexus which run in the psoas muscle in the abdomen and medial to psoas in the pelvis, entering the thigh by passing through the obturator canal to supply most of the adductor muscles and skin on the medial aspect of the thigh. It divides into an anterior and posterior branch which are separated by adductor brevis.
- Sciatic nerve: made up of L4-S3 fibers from the lumbosacral plexus which enter the posterior compartment of the thigh through the gluteal region. In the thigh, the sciatic nerve lies on the adductor magnus muscle and is crossed by the long head of biceps femoris. Proximal to the knee, the sciatic nerve divides into the tibial nerve, which innervates muscles in the posterior compartment (except for the short head of biceps femoris), almost all of the intrinsic muscles of the foot, and skin on the posterolateral side of the lower 1/2 of the leg and lateral side of the ankle, foot, and little toe; and the common fibular nerve, which innervates the short head of biceps femoris, muscles in the anterior and lateral compartments of the leg, one muscle on the dorsum of the foot and two in the sole, and skin over the lateral part of the leg, ankle, and dorsal parts of the foot and toes. The common fibular nerve used to be called the peroneal nerve, which is how it is labeled in the above figure.
The femoral artery supplies most of the lower limb, but the obturator artery and inferior gluteal artery also enter the thigh and supply adjacent muscles and structures.
- Femoral artery: the external iliac artery passes under the inguinal ligament to enter the anterior aspect of the upper thigh, at which point it becomes the femoral artery and runs down the thigh through the adductor canal, passing through the adductor hiatus to become the popliteal artery in the knee. The femoral artery gives off the deep artery of thigh (profunda femoris artery), which travels inferiorly between the adductor longus and adductor magnus muscles to anastomose with branches of the popliteal artery behind the knee. Deep artery of thigh gives off (1) a lateral circumflex femoral artery deep to sartorius and rectus femoris; (2) a medial circumflex femoral artery between pectineus and iliopsoas/obturator externus and adductor brevis; (3) perforating arteries which penetrate adductor magnus near its attachment to the linea aspera.
- Obturator artery: branches off the internal iliac artery in the pelvic cavity and enters the medial compartment through the obturator canal, after which point it splits into an anterior branch and posterior branch. These branhces supply the obturator membrane and adjacent muscles and anastomose with the inferior gluteal and medial circumflex femoral arteries.
- Inferior gluteal artery: we covered this artery with the gluteal region - it enters the gluteal region through the greater sciatic foramen and anastomoses with the medial femoral circumflex artery.
Deep veins in the thigh follow the course of arteries and share their names - popliteal vein, femoral vein, obturator vein, inferior gluteal vein, etc. Superficial veins lie in the superficial fascia.
- Great saphenous vein: runs from a venous arch on the dorsum of the foot to medial side of the proximal thigh, at which point it passes through the saphenous ring to connect with the femoral vein.
The knee joint is the largest synovial joint in the body. The condyles of the femur sit on top of the interarticular cartilages (medial meniscus and lateral meniscus) which sit on top of the condyles of the tibia, and the joint is reinforced by an anterior cruciate ligament and posterior cruciate ligament. The patella articulates with the femur via the quadriceps femoris tendon and the tibia via the patellar tendon. The popliteus tendon and muscle "lock" the knee joint to reduce the muscular energy required to keep the joint extended during standing.
- Patellar ligament: also called the patellar tendon, this ligament is the extension of the quadriceps femoris tendon that attaches to the tibial tuberosity.
- Collateral ligaments: fibular and tibial collateral ligaments on the lateral and medial sides of the knee, respectively, strengthen and support the knee joint.
- Cruciate ligaments: anterior and posterior cruciate ligaments run from the medial borders of the medial and lateral condyles in the intercondylar area obliquely to the tibial condyles.
- Locking mechanism: we'll talk more about this in the next lesson, since the popliteus muscle and tendon lock and unlock the knee.
Like the cubital fossa anterior to the elbow, the popliteal fossa is an important area of transition, in this case from the thigh into the leg posterior to the knee joint. The popliteal fossa's margins are the distal ends of the semitendinosus and semimembranosus muscles superiorly and medially and the distal end of the biceps femoris muscle superiorly and medially and the medial and lateral heads of the gastrocnmeius muscle and the plantaris muscle inferiorly. The floor of the fossa is formed by the capsule of the knee joint and surrounding structures, and the roof is formed by deep fascia which is continuous with fascia lata of the thigh. The tibial and common fibular nerves, popliteal artery, and popliteal vein course through the popliteal fossa.
- Tibial and common fibular nerves: these two major branches of the sciatic nerve enter the popliteal fossa under the margin of biceps femoris, and immediately split at the upper inverted "V" made by the semitendinosus/semimembranosus medially and the biceps femoris laterally. The tibial nerve continues its vertical course to enter the posterior compartment of the leg deep to plantaris. The common fibular nerve runs laterally along the margin of the biceps femoris tendon over the lateral margin of the popliteal fossa and swings around the neck of the fibula to enter the lateral compartment of the leg.
- Popliteal artery and vein: the femoral artery and vein run through the anterior compartment of the thigh and course through the adductor hiatus to enter the posterior compartment just deep to semimembranosus and semitendinosus, where they run medial to the tibial nerve. In the leg, the popliteal artery splits into anterior and posterior tibial arteries.
- Roof of popliteal fossa: As noted above, the popliteal fossa is covered by superficial fascia and skin. The small saphenous vein ascends vertically from the lateral side of the dorsal venous arch of the foot in the superficial fascia and penetrates deep fascia to join the popliteal vein in the popliteal fossa. The posterior cutaneous nerve of thigh pierces the superficial fascia superiorly in the popliteal fossa to run with the small saphenous vein to innervate skin on the upper 1/2 of the back of the leg.
- Human Anatomy Lesson 26
This lesson covers the three compartments of the leg and the ankle joint.
© 2014 Robert McCarthy