Explanation Classification of joints Joints may be classified as immovable joints, partially movable joints and movable joints. Immovable joints Immovable joints are fixed joints and allow no movement. The fibrous connective tissues present between the bones hold them firmly in position. Immovable joints are seen in the sutures of the skull. Partially movable joints Partially movable joints allow slight (partial) movement. Cartilaginous pads separate the bones from each other. Partially movable joints are seen in the joints between the vertebrae, which allow little movement while acting as shock absorbers. Freely movable joints Freely movable joints exhibit varying degrees of movement between the two bones forming the joint. The different types of freely movable joints are hinge joint, pivot joint and ball and socket joint. In hinge joint, the movement is restricted to one plane only as seen in the elbow or knee joint. In ball and socket joint, the ball-like end of one bone fits into the cup-like cavity of the other bone. This allows movement in any direction as seen in the shoulder joint and the hip joint In the shoulder joint, the ball-like head of the humerus fits into the cup of the glenoid cavity of the pectoral girdle. In the hip joint, the ball-like head of the femur (thigh bone) fits into the acetabulum cavity of the pelvic girdle. In pivot joint, a rounded or pointed surface of one bone articulates within the ring formed by another bone. One bone is rotated over the pivot like end of another bone as seen in the joint between the first two cervical vertebrae- atlas and axis. This permits the rotation of the head from side to side. Synovial joint Freely movable joints are also called synovial joints because the articulating ends of the bones are present within a synovial capsule. Synovial joints have a space between the articulating bones called synovial cavity. The synovial cavity is filled by a lubricating liquid called synovial fluid that helps the joint to move smoothly and avoid friction. The bones at a synovial joint are held together by strong fibres called ligaments. The bones, where they rub together are covered by a slippery layer of cartilage. The synovial membrane around the joint keeps the synovial fluid in place, so that it does not drain away. Lever systems The movement of bones at a joint can be described in terms of levers. Lever is a simple machine, which minimises the effort required to move loads. Each joint functions as a fulcrum or the pivot point. The force used to move a lever is called the effort. The effort is provided when the muscle contracts. The load or force to be overcome is called the resistance. This is provided by the action of gravity on the mass of the body. Levers are classified as first class, second class and third class levers, according to the relative position of the effort, resistance and fulcrum. First Class Lever First class lever has the fulcrum between the effort and resistance. An example of first class lever is seen in the movement of the skull on the spinal column. The resistance is the facial portion of the skull, the fulcrum is the joint between the atlas and the occipital bone of the skull and the effort is provided by the contraction of the back muscles of the neck. Second Class Lever Second class lever has the resistance between the fulcrum and effort. An example of second class lever is seen when raising the body on toes. The resistance is the body and the fulcrum is the ball of the foot and the effort is provided by the contraction of the calf muscles to pull the heel upwards. Third Class Lever Third class lever has the effort between the fulcrum and load. An example of third class lever is seen in the flexing of the forearm. The resistance is the weight of the forearm and the hand and the fulcrum is the elbow joint. The effort is provided by the contraction of the biceps. Arthritis Arthritis is a disorder that affects the joints. Arthritis is caused by the deterioration of the cartilage between bones or sometimes due to microbial infection. The membrane surrounding the joint becomes inflammed resulting in a stiff joint that is painful to move. The small joints in the hands and feet are most susceptible and often develop a gnarled appearance due to arthritis.