The body has bones to provide it with a supportive framework. Without bones we would be like jellyfish. In order to move however, we need joints, which are necessary to provide more or less movable connections between the bones. There are several different types of joint, the form of which indicates its particular function.
The least movable connections between bones are the suture and the fusion. These forms of joint allow no movement, but provide a tough and stable construction for protection or support. Examples of these forms of joints are the bones of the skull (suture), which protect the brain, and the bones of the pelvis (fusion) which support its superstructure and protect the viscera.
Many of a baby’s bones are joined by connective tissue, in order to allow expansion. The bones of the skull are joined by the fontanelles, a tough connective tissue, that allows the brain to grow. As the child grows older the plates of the skull grow until they eventually meet and are joined in sutures. Another form of fairly immovable joint is the cartilage connection (symphysis). One example is the pubic symphysis, which is the point at which the bodies of the two pubic bones of the pelvis meet. During pregnancy the cartilage connecting the two halves softens, thus allowing slight movement of the hip bones which is important during childbirth. The joints in the limbs are all freely movable. The range of the type of movement of these joints is variable – range is controlled by the shape of the articulat-
The freely movable joints are known as synovial joints and they all have a similar anatomical structure. In a synovial joint the ends of the bone are covered by a layer of smooth hyalin cartilage, to ensure the least possible friction.
The joint is enclosed in a fibrous capsule that surrounds a sac of synovial membrane. This membrane produces a sticky fluid, the synovial fluid, which lubricates the inside of the joint. The hinge joint allows movement along one axis – that is, backwards and forwards. The elbow and the knee are hinge joints, allowing bending and extension. The range of the movement is restricted by the ligaments. The rotation joint is also movable along one axis, which runs parallel to the direction of the bone. For that reason the movement is opposite to this direction. Rotation joints include the atlas and axis, enabling us to turn our head; and the head of the forearm bone pivotes on the upperarm bone and enables us to rotate the wrist and place the palm of our hand upwards or downwards.
The saddle joint allows movement along two axes, which are perpendicular to each other. A well-known example of this joint is the connection between thumb and metacarpal bone. We can hinge our thumb, but also move it across the palm to oppose it to each finger, which enables us to grasp small things. This is typical only in apes and man.
The ball-and-socket joint is movable in two planes, enabling to move backwards and forwards as.well as sideways and also allows the limb to rotate. We can see this in the shoulder joint and hip joint. The greater range of movement in the shoulder joint is only possible because the surfaces of the connective bones are shallow, and the ligaments allow greater range. The arm needs more mobility, whereas the leg has a more supportive function. That is why the shoulder joint also dislocates more easily. Most movements in the body are a combination of these basic movements. The use of many joints together allows great possibilities. We can see this in the wrist, where a large number of bones move over small distances, resulting in a large range of movement.
Movement of the joints is limited by strong fibrous structures called ligaments. They provide stability and if they are torn, the joint becomes unstable. Most ligaments reinforce outside the joint capsule, but in certain joints, for example the knee, there are also ligaments inside to prevent abnormal movements. As the knee joint reaches full extention the thigh bone rotates slightly and locks the shin bone to stiffen the leg from hip to ankle. The two cartilages inside the knee joint – the menisci – help the end of the shin bone to fit snugly with the end of the thigh bone, but still allow a little rotation. Sudden twisting movements under, such as turning while playing soccer, may tear these menisci.
Blood and nerve supply
Around each joint arteries form a large anastomosis: numerous branches supply the synovia, ligaments and bone with. The hyalin cartilage however has no vessels (and is therefore slow to heal). All joints have a very effective nerve supply. Many nerve endings in the synovia, ends of bone and muscles contain proprioceptive which enable us to know the position of our limbs. For example, the shoulder joint has a large number of proprioceptive nerve which inform the brain of the exact postion of the arm in relation to the body by detecting the degree of stretch of the in the joint. This enables us to lift the arm and place it accurately in any position without looking or touching. In fact movements in the shoulder joint are probably more precise than in the finger joints.
Pain fibres pass from the joint to the spinal cord, then to the brain. If these fibres are damaged, the joint loses its self-protective mechanism, and repeated minor damage can lead to serious injury to the joint.
One or more joints are used every time you move. Because of this, you soon notice any problem with them. It is perhaps not surprising that they sometimes go wrong. The elderly are probably most affected by joint disorders, although no age is immune. The disorders may be short-lived, or exceedingly chronic; agonizingly painful or just plain uncomfortable. Injuries to joints vary in severity from mild sprains to fractures and dislocations. A sprain is damage to ligaments, tendons and muscles that follow a sudden wrench to a joint – for example, by landing awkwardly on the side of the foot when jumping. More violent stresses may cause tears in the ligaments and tendons. Tears in the cartilage, such as those of the knee, can be more serious. This is an injury commonly sustained by athletes, and because the cartilages of the knee have little capacity for repair, they must be removed surgically.
Continual strain on one or several joints, inflicted during a particular sport or pastime or as a result of being overweight, most certainly subjects a joint to wear and tear, which sometimes causes a condition known as osteoarthritis. It develops most commonly in the larger weight bearing joints – chiefly the knees, hips or spine. In this disorder the smooth lining of the bones at the point where they come into contact begins to flake and crack. As the cartilage deteriorates the underlying bone is affected and may become thickened or distorted. Movement becomes painful and restricted, and as it does so the muscles that work the joint become atrophied through under-use and gradually waste away.
Joints can, of course, become swollen and painful as a rsult of an infection – for example, German measles (rubella), mumps or chickenpox – but these unvari-ably clear up when the causative infection is dealt with. In rare cases, arthritic symptoms may be produced by the bacterial invasi6n of a joint, which if left untreated, may become stiff and almost useless. In certain conditions modern orthopaedic surgery enables people who have one or more joints damaged by injury or severely crippled by an arthritic condition to have an artificial replacement.