Joints

A joint connects two bones to each other. There are about 140 true joints in the human body and no movement would be possible without them. Learn more about true and false joints. What forms are there and how are they structured?

What are joints?

Did you know that the ball joint that connects our hip and thigh is our most mobile joint?
Joints are moveable connecting elements between two or more bones or cartilage structures, providing both stability and mobility. They consist of the following essential components:

  • Joint head and socket: These fit into each other, almost like two puzzle pieces
  • Articular cartilage: This coats the joint head and socket
  • Joint space with joint fluid or synovial fluid: This prevents abrasion of the joint head and socket
  • Joint capsule: This encloses the entire joint
  • Ligaments, tendons and muscles (for joint stabilisation): These provide additional support for the joint capsule

Since joints do a lot of work, signs of wear such as osteoarthritis or injuries such as sprains or luxation may occur.

Joints dampen impacts and are exposed to powerful forces in doing so. Joint injuries should be treated according to the RICE-rule to prevent further damage and to support the natural healing process.

The RICE-rule (rest, ice, compression, elevation) summarises the treatment method for muscle and joint injuries to minimise damage.

Function and types of joints

In anatomy, one differentiates between true and false joints that provide mobility for the human body.

True joints

True joints have a joint space that allows for greater freedom of movement in the joint. The knee, hip and ankle joints are examples of true joints. There are around 140 true joints in the human body.

False joints

False joints do not have a joint space. They are connections between bones formed by cartilage or connective tissue and only permit limited movement. The intervertebral discs and breastbone are examples of false joints. There are about 70 false joints in the human body.

Types of joints

Joints are classified into five different types according to their movement potential, degree and direction:

Ball-and-socket joint

With this joint, a ball-shaped joint head lies in a matching socket. A ball-and-socket joint permits forward, backward, sideways and rotating movements – mobility in six directions. This is the most mobile joint in our body. The hip and shoulder joints are examples of ball-and-socket joints.

Saddle joint

The saddle joint consists of a concave joint head and a socket with a shape resembling a saddle used for riding. This joint can move in four directions. It permits forward, backward and sideways movements, but rotation is not possible. A saddle joint is found in the thumb, for example.

Ellipsoid joint

Unlike the ball-and-socket joint, the joint head and socket have an oval cross-section in this joint type. It permits movement in four directions: Forward, backward and sideways. The wrist is an example of an ellipsoid joint.

Hinge joint

The hinge joint consists of a cylindrical joint head and a trough-shaped socket. It only permits forward and backward movement (flexion and extension), for just two directions of mobility. It is like a door on hinges that can be opened and closed. The elbow joint is an example.

Rotary joint

In the rotary joint, the joint head is conical and the socket is groove-shaped. This structure only permits a rotary movement. The radioulnar joint is an example.

All of these are mobile true joints. Since our body is built for our various types of joints, motion sequences can be executed entirely unconsciously. Our joint types dictate how we walk, stand, sit and so on.

Structure of a joint

Cartilage tissue is an essential element of well-functioning joint mobility and therefore the entire human body.

The joint cartilage sets itself apart through elasticity and an extremely low coefficient of friction on the surface. Cartilage tissue is like a microscopic web of protein fibres, similar to connective tissue. The cartilage cells as such only account for 1% of the joint tissue volume. Cartilage is entirely saturated by joint fluid (synovial fluid). Most of the joint cartilage volume consists of water.

The ability of cartilage tissue to bind water is central to its function in the joint. This high water retention of the cartilage tissue is accomplished by a special class of protein called aggrecan. It is produced by cartilage cells. The unique surface of the cartilage tissue withstands millions of load cycles without any wear.

Humans are effectively “walking on water” when they move about. However, this resilience is only given as long as the network of connective tissue fibres in the cartilage tissue is stable and the surface is intact.

The connective tissue matrix of the cartilage is made of proteins such as chondroitin and hyaluronic acid. Cartilage regeneration depends on the vitality of the cartilage cells. When cartilage cells die, they cannot be replaced by the body through cell division.

The water films of the cartilage surfaces absorb more than 90% of the loads in the human joint during movement. Only 10% is carried by the hyaline connective tissue. The construction of the sliding surfaces in the joints is several times more slippery than a smooth ice surface. As long as the cartilage-forming cells (chondrocytes) are active and healthy, they continuously regenerate the water-retaining cartilage tissue.

The cartilage cells themselves have a hard life: After the end of the human growth phase, they are no longer connected to blood circulation but supplied only through the passive distribution of nutrients in the tissue. Passive nutrition occurs through the flow of fluids in the fluid-filled matrix. This is referred to as bradytrophic (low nutrition) tissue. The bradytrophic situation influences the regeneration of cartilage cell, for instance after an accident.

Low nutrition limits the metabolic activity of cartilage cells to a very low level. This means the response in case of damage or trauma is limited as well. The cartilage cells are highly active in spite of the low nutrient supply: They continuously synthesise the connective tissue proteins and the enzymes that regulate the formation and breakdown of the cartilage tissue.

Similar to the bone tissue, the cartilage tissue maintains a regulated balance between cartilage formation and breakdown. The enzymes that break down cartilage are called metalloproteinases. When their formation is increased, for example, due to inflammation, a loss of cartilage caused by the inflammation can occur in the joint.

Similar to a lot of other connective tissue, the molecular network of the cartilage tissue can be permanently altered after an accident or due to biological age. Shearing movements are particularly damaging here.

Cartilage is subject to biological ageing. This ageing process is particularly noticeable for us in the face and on the skin. Connective tissue ageing also takes place the same way in the joints. The elasticity of cartilage continuously decreases after age 30.

The spine has shock absorbers between the vertebrae that evenly distribute the pressure resting on them. They consist of 80% to 85% water and allow the spine to bend in all directions. The intervertebral discs lose water in the course of the day so a person is 1 to 3 centimetres shorter in the evening. At rest, the intervertebral discs recover and absorb fluid and nutrients. This tells us that the quality of our bed is vital.

Joints

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