Actions of the Knee Complex

The articulations between the femur, tibia, and patella form the knee joint and enable the following actions

• Flexion. Movement in the sagittal plane, decreasing the knee joint angle.
• Extension. Movement in the sagittal plane, increasing the knee joint angle.
• Medial rotation. Movement toward the midline in the transverse or axial plane.
• Lateral rotation. Movement away from the midline in the transverse or axial plane.

The lower limb is innervated by the ventral rami from nerve roots L1–S4, which form two separate networks of nerves that are referred to as the lumbar plexus (L1–L4) and the sacral plexus (L4–S4). (The sacral plexus is discussed in Chapter 35.) The lumbar plexus consists of ventral rami from the L2 to L4 levels of the spinal cord, which exit the intervertebral foramina and course along the posterior abdominal wall, en route to the anterolateral abdominal wall and lower limb. The ventral rami are divided into anterior and posterior divisions, corresponding to the anterior (ventral) muscles (flexors) and the posterior (dorsal) muscles (extensors), similar to those in the brachial plexus (Figure 36-3B).

Abdominopelvic Cutaneous Branches of the Lumbar Plexus

Branches from the lumbar plexus provide motor and sensory contributions not only to the anterior and medial compartments of the leg but also to the abdominal wall and pelvic areas. Branches are summarized as follows:

• Iliohypogastric and ilioinguinal nerves (L1). Emerge from the lateral border of the psoas major muscle and pierce the transverse abdominus muscle. The iliohypogastric and ilioinguinal nerves course anteriorly between the transverse abdominus and the internal oblique muscles, contributing to the motor innervation of the muscles of the abdominal wall (internal oblique and transversus abdominus muscles). In addition, the iliohypogastric nerve provides sensory innervation to the pubic region and the posterolateral gluteal skin. The ilioinguinal nerve (L1) provides sensory innervation to the superior medial thigh, root of the penis, anterior scrotum, mons pubis, and the labium majus.
• Genitofemoral nerve (L1–L2). Pierces the psoas major muscle and divides into two branches, the genital branch and the femoral branch. The genital branch enters the deep inguinal ring, providing motor innervation to the cremasteric muscle (male only) and sensory innervation to the skin of the anterior scrotum (or mons pubis) and the labium majus. The femoral branch passes behind the inguinal ligament to enter the femoral triangle, providing sensory innervation to the skin over the femoral triangle (superior and anterior region of the thigh).

Lower Limb Branches of the Lumbar Plexus

The lumbar plexus is divided into posterior and anterior divisions, according to posterior and anterior muscle groups (Figure 36-3C).

• Posterior division
  • Lateral cutaneous nerve of the thigh (L2–L3). Emerges from the lateral border of the psoas major muscle, crossing the iliacus muscle, and enters the thigh just medial to the anterior superior iliac spine, providing sensory innervation to the lateral thigh.
  • Femoral nerve (L2–L4). Emerges as the largest branch of the lumbar plexus, deep to the lateral border of the psoas major muscle, and passes deep to the inguinal ligament. The femoral nerve then enters the anterior compartment of the thigh, where it provides motor innervation to the quadriceps femoris muscle group, the sartorius muscle, and part of the pectineus muscles. In addition, the femoral nerve provides sensory innervation via the following sensory branches:
    • Anterior and medial cutaneous nerve of the thigh. Branches off the femoral nerve and supplies the skin of the anterior and medial thigh.
    • Saphenous nerve. Branches off the femoral nerve and courses with the femoral artery and vein into the adductor canal. The femoral artery and femoral vein traverse the adductor hiatus and enter the popliteal fossa. In contrast, the saphenous nerve exits the adductor canal and supplies the skin on the medial region of the leg.
• Anterior division
  • Obturator nerve (L2–L4). Emerges from the medial border of the psoas major muscle, passing posterior to the common iliac artery, and enters the medial compartment of the thigh through the obturator foramen. The obturator nerve provides motor innervation to the medial compartment of the thigh (excluding the pectineus and hamstring portion of the adductor magnus muscle). In addition, the obturator nerve provides sensory to the medial region of the thigh.

The blood supply to the lower extremity initiates from the descending aorta, which divides into the common iliac arteries. The common iliac arteries divide into the external and internal iliac arteries. The external iliac artery passes deep to the inguinal ligament to become the femoral artery, serving as the primary blood supply to the lower limb. The internal iliac artery gives rise to the obturator artery, which also contributes to blood supply of the lower limb.

Obturator Artery

Branches from the internal iliac artery exit the pelvis through the obturator foramen and bifurcate into anterior and posterior divisions. The two divisions circle the obturator foramen and supply blood, proximally, to the muscles inserting in the area. In addition, a branch enters the hip joint at the acetabular notch and travels through a conduit in the ligament of the head of the femur to supply blood to the femoral head.

Femoral Artery

Several arteries originate from the femoral artery, including the following (Figure 36-4A and B):

• Deep artery of the thigh (profunda femoris artery). The largest branch of the femoral artery. The deep artery of the thigh branches posteriorly in the femoral triangle and travels between the adductor longus and brevis muscles and the adductor longus and magnus muscles. Perforating branches of the deep artery of the thigh pierce through the adductor magnus muscle and are the major supplier of blood to the three compartments of the thigh. Distally, the deep artery of the thigh anastomoses with branches of the popliteal artery. Other branches that originate from the deep artery of the thigh include the following:
  • Lateral circumflex femoral artery. Branches from the lateral side of the deep artery of the thigh (sometimes directly from the femoral artery) and travels laterally to branch into ascending, transverse, and descending branches.
    • Ascending branch. Ascends deep to the tensor fasciae latae muscle and connects with the medial femoral circumflex artery.
    • Transverse branch. Pierces the vastus lateralis muscle and wraps around the proximal shaft of the femur to anastomose with other vessels (i.e., medial femoral circumflex artery) to supply blood to the proximal femur and hip.
    • Descending branch. Descends laterally and pierces the vastus lateralis muscle. The descending branch distally connects with the popliteal artery.
  • Medial circumflex femoral artery. Branches medially from the deep artery of the thigh (may branch from femoral artery) and passes around the shaft of the femur. The medial circumflex femoral artery and branches of the lateral circumflex femoral artery supply the hip joint.

The main trunk of the femoral artery follows the adductor canal distally. It travels through an opening in the distal attachment of the adductor magnus (adductor hiatus) muscle and enters the popliteal fossa (posterior to the knee) to become the popliteal artery.

Coronary angioplasty is a procedure that is frequently performed on patients who have obstructed blood flow to the heart musculature or who are experiencing a myocardial infarction. The femoral triangle allows easy access to a major blood vessel, the femoral artery. Because of the easy access, small balloons can be threaded through the femoral artery, at the femoral triangle, to coronary vessels in the heart. The balloon is then expanded to open the blood vessel.

Popliteal Artery

The popliteal artery is the continuation of the femoral artery after it traverses the adductor hiatus (Figure 36-4A and B). The branches form an anastomotic vascular supply to the knee and are named according to their relationship to each other (superior lateral and medial genicular arteries and inferior lateral and medial genicular arteries).

Lymphatics of the Inguinal Area

The lymphatics of the thigh are organized into the following inguinal lymph nodes:

• Superficial inguinal nodes. Nine to eleven nodes that run parallel and distal to the inguinal ligament. The superficial inguinal nodes receive lymph from the external genitalia, anal canal, gluteal region, and the inferior abdominal wall; they receive most of the lymph from the superficial vessels of the lower limb. The superficial inguinal nodes drain to the external iliac nodes.
• Deep inguinal nodes. One to three nodes located medial to the femoral vein. The deep inguinal nodes receive lymph from deep vessels associated with the femoral vessels and glans penis or clitoris. Also, the deep inguinal nodes drain into the external iliac nodes.
• Popliteal nodes. Six small nodes located in the popliteal fat. The popliteal nodes receive lymph from the knee and the deep vessels associated with the tibial vessels of the leg. Vessels from the popliteal nodes ascend the thigh to drain into the deep inguinal nodes, eventually reaching the external iliac nodes.

Veins of the Thigh

The veins of the thigh consist of a superficial and a deep venous system. The superficial system consists of the great saphenous vein.

• Great saphenous vein. Originates from the dorsal venous arch in the foot on the medial side. The great saphenous vein ascends the leg and thigh on the medial side and pierces the fascia latae of the thigh, forming the saphenous opening to drain in the femoral vein.
• The deep venous system consists of as many as three veins that course with each artery. Most of the veins in the thigh drain into the femoral vein.

The knee complex consists of articulations between the femur and the tibia (tibiofemoral joint) and between the femur and the patella (patellofemoral joint). These articulations allow for static positions (standing) and dynamic movements (walking or running). The superior tibiofibular joint is not considered as part of the knee complex because it does not have the same capsule and is biomechanically linked with the ankle

The knee is composed of the following joints:

• Tibiofemoral joint. A synovial bicondylar joint with two degrees of motion. Articulations occur between the two condyles of the femur and the two tibial plateaus, producing flexion and extension. In addition, the tibiofemoral joint allows for minimal axial rotation with the pivot point, located medially on the medial tibial plateau.
• Patellofemoral joint. Articulation is between the intercondylar notch of the femur and the patella and shares the same joint capsule as the tibiofemoral joint. The patellofemoral joint directly serves the tibiofemoral joint; however, because of the vast differences in clinical problems and pathologies, the two joints will be discussed independently.

Patellofemoral disorder is one of the most common knee disorders seen in patients who visit orthopedic clinics. The disorder is usually caused by excessive pressure or malalignment between the patella and the femur, resulting in pain at the patellofemoral joint.

Ligamentous and Capsular Support of the Knee

Ligament and capsule support of the knee are critical because of the incongruence of the joint, weight bearing of the joint, and the large range of motion with flexion and extension (Figure A–C). The ligaments and capsule provide support to the knee joint, as follows:

• Capsule. Surrounds the knee joint and includes the patellofemoral joint. The capsule extends from the distal femur to the proximal tibia and contains areas of laxity and recesses to allow for range of motion. The ligaments that support the capsule on all four sides of the joint are as follows:
  • Oblique popliteal ligament. Supports the posterior capsule and attaches from the posteromedial tibial condyle to the center of the posterior capsule.
  • Arcuate ligament. Supports the posterior capsule and attaches from the posterior fibular head to the intercondylar area of the tibia and the lateral epicondyle of the femur.
  • Medial collateral ligament (tibial collateral ligament). Resists valgus forces on the knee (tibia abducting on femur). The medial collateral ligament attaches from the medial epicondyle of the femur to the tibial condyle and proximal shaft. In addition, it has an attachment to the medial meniscus.
  • Lateral collateral ligament (fibular collateral ligament). Resists varus forces on the knee (tibia adducting on the femur). The lateral collateral ligament appears as a strong cord and attaches from the lateral femoral epicondyle to the fibular head.
  • Patellar ligament. Attaches from the patella to the tibial tuberosity on the proximal anterior surface of the tibia. In addition, superficial fibers of the patellar ligament fan out, attaching to the sides of the tibial tuberosity. These structures are referred to as the medial and the lateral patella retinacula. The patellar ligament transmits forces produced by the quadriceps muscles to the tibia.

In addition to the capsular ligaments, the knee complex also contains ligaments inside the capsule, called the cruciate ligaments. The cruciate ligaments are named by their location of attachment on the tibia.

• Anterior cruciate ligament. Attaches on the anterior intercondylar area of the tibia. The anterior cruciate ligament (known as the ACL) twists on itself as it ascends in a posterolateral direction to attach on the posteromedial side of the lateral femoral condyle. The ACL resists anterior translation of the tibia on the femur or posterior translation of the femur on the tibia.
• Posterior cruciate ligament. Stronger than the ACL and attaches to the posterior intercondylar area of the tibia. The posterior cruciate ligament ascends in an anteromedial direction to attach on the lateral surface of the medial femoral condyle. The posterior cruciate ligament resists posterior translation of the tibia on the femur or anterior translation of the femur on the tibia.

Menisci of the Knee Joint

The knee contains two fibrocartilaginous structures, one over the medial tibial plateau, the medial meniscus, and one over the lateral tibial plateau, the lateral meniscus (Figure A–C). Both are crescent shaped and do not complete a full circle. The menisci are also wedge shaped and, medially, are thin. However, laterally, the menisci are thicker, which increases the concavity of the articulating surface of the tibia.

• Medial meniscus. The anterior portion of the medial meniscus attaches to the anterior intercondylar area of the tibia and the posterior portion to the posterior intercondylar area. Its peripheral border is attached to the capsule, and a portion is attached to the medial collateral ligament.
• Lateral meniscus. Forms four-fifths of a complete circle. The anterior portion of the lateral meniscus attaches to the anterior intercondylar area of the tibia, and the posterior portion attaches to the posterior intercondylar area.

A tear of the ACL is usually seen in patients who participate in sports that require cutting movements with deceleration (e.g., soccer, football). The mechanism of injury is usually deceleration of the body on an outstretched leg with lateral rotation of the femur on a fixed tibia. Treatment is often surgery using an autograft (i.e., tissue is taken from the patient to replace the patient's ligament). Frequently, a midpatellar ligament graft is used.

Bursae Associated with the Knee Joint Complex

The knee joint has many bursae (sacs of synovial fluid) to decrease frictional forces. The most important bursae are as follows:

• Suprapatellar bursa. Located between the quadriceps tendon and the anterior femur.
• Subpopliteal bursa. Located between the popliteus muscle and the lateral femoral condyle.
• Gastrocnemius bursa. Located between the medial head of the gastrocnemius muscle and the medial femoral condyle.
• Prepatellar bursa. Located between the skin and the anterior patella.
• Subcutaneous infrapatellar bursa. Located between the patellar ligament and the tibial tubercle.

Prepatellar bursitis (commonly known as “housemaid's knee”) is caused by inflammation or bursitis of the superficial infrapatellar bursa between the skin and the patellar ligament. The mechanism of injury can be from direct impact or from an irritation to the knee that occurs over time. The condition is often seen in individuals whose occupations require them to place pressure on the knees, such as carpet layers or people who wash the floor on their hands and knees (thus the term housemaid's knee), resulting in pain over the patellar ligament.