Identifying and Analyzing Ball-and-Socket Joints

• 1). Check the shape of the joint articulations on the skeletal model. Ball-and-socket joints have a clear "ball" end and "socket", or bowl-shaped, end. The ball should fit loosely into the bowl-shaped articulation. For example, the hip joint is made of a distinctive "ball" at the end of the femur and a "socket" in the bones that shape the pelvis.
  
  
• 2). Check the range of motion. Ball and socket joints by definition have the greatest range of motion of any joint in the body, and allow for flexion, extension, adduction, abduction and internal and external rotation. For example, the shoulder allows the arm to move toward and away from the body (adduction and abduction), and in front of the body, behind the body and over the head (flexion and extension). When you hunch your shoulders forward, you are experiencing rotation. When you make large circles, you are performing a combination of all of these movements. In contrast, the hinge joints in the fingers, knees and elbows only allows for flexing and extending the fingers, legs and arms, respectively.
  
  
• 3). Note the depth of the socket. A deeper socket indicates a more stable joint, while a shallower socket indicates a less stable joint. For example, the hip joint has a deep socket; therefore, it is a rare and serious incident when the hip becomes dislocated. The shoulder has a much shallower socket in comparison. Shoulder dislocations, while very painful, are not uncommon and can usually be reset quickly by a doctor.