The Skeleton / What are Joints?

In order for physical movements to be carried out, the individual bones have to be able to move against each other and connected to each other. This is the function of the joints. According to the extent to which movement is facilitated, joints are distinguished between uni-axial, bi-axial and tri-axial to multi-axial joints. In addition to true joints (diarthroses), in which two bones always work within each other, whereby one of them is an articular head and the other an articular cavity, there are also false joints (synarthroses), such as fibrous or cartilaginous joints. In this case, the movements are more restricted; the sprung effect is predominant. Synovia reduces the friction between the articular surfaces. It is a fluid and is contained within an articular capsule, holding together the joint with its taut tissue.

A need for strength makes the bones rigid, but if the skeleton consisted of one solid bone, movement would be impossible. Nature has solved this problem by dividing the skeleton into many bones and creating joints where the bones intersect. Joints come in a variety of designs, each especially built for the limb it serves. Joints permit bodily movement and are held together by fibers called "ligaments". Joints are "oiled" continuously to prevent friction. Some joints, like those connecting the skull's series of bones, allow no movement. Others may permit only limited movement; the joints in the spine allow some movement in several directions. Most joints have a greater range of movement, and these are called "synovial" joints. The skeleton is made up of many kinds of movable joints. The bearing surface is made smooth by slippery cartilage to reduce friction. Larger joints are lubricated by "synovial" fluid. Connections called "synovial" joints are sturdy enough to hold the skeleton together while permitting a range of motions. The ends of these joints are coated with cartilages which reduce friction and cushion against jolts. Between the bones, in a narrow space, is the joint "cavity," which gives us freedom of movement. Ligaments then bind these bones to prevent dislocations and limit the joint's movements. The bones are held in position and controlled in movement by the ligaments. "Getting off on the wrong foot" or making a bad start is based on an ancient Roman superstition. They believed it to be bad luck to enter a house with the left foot first. The idea was so important to them that "footmen" were placed at the door to see that those entering would not misstep.

Joints connect the various bones within the skeleton, and can be divided into different categories.

Fibrous joints connect two bones by means of elastic or collagen (high proportion of protein) connective tissue.

Fibrous joints form, for example, interossei membranes between the calf bone (fibula) and the shin bone (tibia).

The sutures between the cranial bones of newborn babies are also fibrous joints. They contain connective tissue, which recedes completely by the end of the second year of life.

The way the teeth are fixed in the jaw is referred to as gomphosis. The tooth is held in place in the tooth socket by taut connective tissue.

Cartilaginous joints are permanent bone connections. A distinction is made between cartilaginous joints and fibrous cartilaginous joints. A cartilaginous joint is composed of hyaline cartilage. Examples are the connection between the first, sixth and seventh ribs (costa) and the breastbone (sternum). A fibrous cartilaginous joint is composed of fibrous cartilage and connective tissue. Examples of this are the pubic symphysis (shown on the right and below) and the intervertebral disks. Joints are lubricated by "synovial" fluid. Connections called "synovial" joints are sturdy enough to hold the skeleton together and at the same time permit a range of motions. The ends of these joints are coated with cartilages which reduce friction and cushion against jolts.

Synovial joints permit a large degree of movement. For this reason they are also termed freely mobile joints. This type of joint consists of at least two articular bodies. They are covered with cartilage.
 

This way the two articular bodies - the articular cavity and the articular head can glide against or over each other. The articular capsule is joined directly to the articular surfaces. It consists of two layers containing nervous fibers and blood vessels and is reinforced on the outside by strong lateral ligaments. In addition, synovia is constantly produced by the inner skin of the articular capsule, making it easier for the articular bodies to glide.
 

Joint Shapes

The articular head can not only turn in any direction, but also around its own axis, insofar as it is not impeded by fixed ligaments.

One example of a ball-and-socket joint is the shoulder joint. It facilitates not only up and down movements of the arm, but also sideways stretching out from the body, as well as circular movements.

The hip joint is another example of a ball-and-socket joint. Here the degree of movement is somewhat restricted, however, because the articular head is enclosed by the articular cavity by up to 2/3. This protects the hip from dislocation.

The saddle joint consists of two articular bodies which are concavely curved along one axis and convexly curved along the other.

One example of this is the carpometacarpal joint in the thumb (shown to the left).

Attached to a saddle-shaped articular surface on the trapezium (os trapezium) is the correspondingly shaped basis of the first metacarpal bone. The thumb can be bent and stretched around one axis, and "twiddled" around the other.

This joint facilitates complicated movements in the hand region (grip hand).

Level joints have two level articular surfaces, for example the interverte- bral joints within the cervical spine. The shape of this joint, falling flatly backwards, permits bowing movements, both forwards and backwards.

The rotational joints include the pivot and cone joints. They consist of a concave and a cylindrical articular body, whereby the articular axis runs through the longitudinal axis of the cylindrical body.

An example of a cone joint is the upper spoke bone ulnar joint; an example of a pivot joint is the lower spoke bone ulnar joint.

Hinged joints only permit movement in one direction.

The joint consists of an articular head with a cylindrical appearance and an articular cavity with the appearance of a hollow cylinder, largely enclosing the articular head. One example of a hinged joint is the upper arm elbow joint.

Ellipsoid joints permit movements around two mutually perpendicular axes.

In this case, the articular head is shaped like an egg and the articular cavity has the corresponding hollow form. The upper wrist is a particualrly obvious example of this. One plane permits stretching and bending movements, the other plane movements towards the thumb or little finger.

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