 the ulna as the forearm is pronated or supinated.
Figure 8. Elbow joint
The Hip Joint
The coxal (hip) joint is the point where the head of the femur inserts into the acetabulum of the hip bone. Because the coxal joints bear much of the body’s weight, they have deep sockets and are much more stable than the shoulder joint. The depth of the socket is somewhat greater than you see on dried bones because of a horseshoeshaped ring of fibrocartilage, the acetabular labrum, attached to its rim. Dislocations of the hip are rare, but some infants suffer congenital dislocations because the acetabulum is not deep enough to hold the head of the femur in place. If detected early, this condition can be treated with a harness, worn for 2 to 4 months, that holds the head of the femur in the proper position until the joint is stronger.
Figure 9. Hip joint
The Knee Joint
The tibiofemoral (knee) joint is the largest and most complex diarthrosis of the body. It is primarily a hinge joint, but when the knee is flexed it is also capable of slight rotation and lateral gliding. The patella and patellar ligament also articulate with the femur to form a gliding patellofemoral joint. The joint capsule encloses only the lateral and posterior aspects of the knee joint, not the anterior. The anterior aspect is covered by the patellar ligament and the lateral and medial patellar retinacula. These are extensions of the tendon of the quadriceps femoris muscle, the large anterior muscle of the thigh. The knee is stabilized mainly by the quadriceps tendon in front and the tendon of the semimembranosus muscle on the rear of the thigh. Developing strength in these muscles therefore reduces the risk of knee injury. The joint cavity contains two C-shaped cartilages called the lateral and medial menisci (singular, meniscus) joined by a transverse ligament. The menisci absorb the shock of the body weight jostling up and down on the knee and prevent the femur from rocking from side to side on the tibia.
The posterior popliteal region of the knee is supported by a complex array of extracapsular ligaments external to the joint capsule and two intracapsular ligaments within it. The extracapsular ligaments include two collateral ligaments that prevent the knee from rotating when the joint is extended—the fibular (lateral) collateral ligament and the tibial (medial) collateral ligament—and other ligaments. The two intracapsular ligaments lie deep within the joint. The synovial membrane folds around them, however, so that they are excluded from the fluid-filled synovial cavity. These ligaments cross each other in the form of an X; hence, they are called the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL). These are named according to whether they attach to the anterior or posterior side of the tibia, not for their attachments to the femur. When the knee is extended, the ACL is pulled tight and prevents hyperextension. The PCL prevents the femur from sliding off the front of the tibia and prevents the tibia from being displaced backward. The ACL is one of the most common sites of knee injury.
Figure 10. Knee joint
The Ankle Joint
The talocrural (ankle) joint includes two articulations—a medial joint between the tibia and talus and a lateral joint between the fibula and talus, both enclosed in one joint capsule. The malleoli of the tibia and fibula overhang the talus on each side like a cap and prevent most side-to-side motion. The ankle therefore has a more restricted range of motion than the wrist.
The ligaments of the ankle include (1) anterior and posterior tibiofibular ligaments, which bind the tibia to the fibula; (2) a multipart medial (deltoid) ligament, which binds the tibia to the foot on the medial side; and (3) a multipart lateral (collateral) ligament, which binds the fibula to the foot on the lateral side. The calcaneal (Achilles) tendon extends from the calf muscles to the calcaneus. It plantarflexes the foot and limits dorsiflexion. Plantar flexion is limited by extensor tendons on the anterior side of the ankle and by the anterio