Ball and Socket

• The ball and socket joints at the shoulder and hip provide more range of movement than any other joint. For example, in the shoulder the ball-shaped head of the humerus bone sits in the shallow ball-shaped cavity of the shoulder blade. Because the joint is loose and the socket is shallow, the shoulder moves freely and can rotate 360 degrees. The hip joint is more stable and less mobile than the shoulder. The ball-shaped head of the femur fits tightly into the deep socket of the hip bone, and the femur is also helped to stay in place by cartilage in the socket. The ligaments joining the femur and hip bone are the strongest in the human body.
  
  

Hinge

• The hinge joint is the simplest of all the joints. Found in the knees, elbows and the fingers and toes, the hinge allows movement in only one direction, much like a door hinge. The body's largest joints are the hinge joints at the knees. The knees each have four bones: the femur, also known as the thigh bone; the tibia and fibula in the lower leg; and the patella, commonly known as the knee cap. Knee joints differ from other hinge joints in that they are far more complex and move back and forth and swivel on their axes, allowing the feet to turn from side to side. The bones support the knee and provide the rigid structure of the joint, the muscles move the joint, and the ligaments hold the joint together.
  
  

Pivot

• Pivot joints allow rotation around an axis. The neck and forearms have pivot joints. In the neck, between the atlas and the axis, also known as the first and second cervical vertebrae, directly under the skull, the pivot joint allows for turning of the head from side to side. In the forearms, the radius and ulna twist around each other.
  
  

Knees

• According to the American Academy of Orthopaedic Surgeons, the knee joint is very prone to injury due to the constant pressure, pounding, bending and twisting it endures; almost five million people visit offices of orthopaedic surgeons annually because of knee problems. More than three million of the visits are injury-related; the remaining are due to arthritis and other disorders. When knees move, they do not simply bend and straighten, but flex and extend. Their ability to rotate slightly has only become recognized by the medical profession since the 1960s, which accounts for the large number of knee injuries about which little is still known and understood. Important in facilitating movement are the knee muscles which go across the knee joint: the quadriceps in the front of the knee and the hamstrings in the back. Ligaments hold the joint together.
  
  

Hips

• Older people are particularly susceptible to hip fractures, caused by deterioration in the effectiveness of the ball and socket joint, and diseases such as osteoarthritis and osteoporosis. Hip arthroplasty (replacement) is a common surgery for people with severe hip damage. The surgeon removes damaged cartilage and bone from the hip joint and replaces them with new, man-made parts.