A joint is a movable joint that connects at least two bones in such a way that movement becomes possible. Here, there are different joint shapes as well as the possibility that joint-like connections exist, which are then called false joints.
What are joints?
In anatomy, joints, called articulationes, are described as being a joint between two bony or cartilaginous parts of the skeleton, separated by a joint space. However, the human skeleton has several different joint forms that vary in structure and function. Typical joint shapes are:
- Ball and socket joint
- Egg joint
- Saddle joint
- Hinge joint
- Wheel joint
Anatomy and structure
Inauthentic joints, known as synchondroses or articulationes cartilaginae, are bony connections that have no interruption, i.e., no joint space, and, as connective tissue or cartilaginous connections, also have limited mobility. Often, these are growth zones or connections between bone parts which, despite considerable stability, should also have a certain degree of mobility. Examples include the cartilaginous connections between the ribs and the sternum, the pubic symphysis or the connective tissue connection between the ulna and radius. If, after completion of growth, the cartilage completely transforms into bony structures, these connections are called synostoses. True joints, diarthroses, or discontinuous joints, have an interruption between the bones involved in the joint. This interruption is called the joint space, which is more or less developed depending on the function of the joint. Here, the joint space separates the joint surfaces covered with articular cartilage. The real joint is surrounded by a joint capsule, which is composed of two layers. The inner membrana synovialis and the outer membrana fibrosa form the capsule, which tightly surrounds the joint and forms a closed cavity on the inside, the joint cavity. The joint capsule is often reinforced by ligaments, the so-called capsular ligaments. To allow mobility and joint play, the joints are filled with a viscous fluid, the synovial fluid, within the capsule. Examples of true joints include the ankle joint, knee joint, or hip joint.
Functions and tasks
Furthermore, joints can also be differentiated according to their shape, function or mobility. Thus, hinge joints such as the elbow joint can only be moved within one body axis, for bending or stretching. The knee joint, on the other hand, which is classified as a rotational gliding joint, can be moved biaxially. Here, in addition to flexion and extension, internal and external rotation are also possible. If one speaks of ball and socket joints, then the hip and shoulder joints are relevant as examples, which can be moved in all three body axes. Here, in addition to the movements of flexion and extension, rotation as well as splaying and tightening are also possible. Depending on the type of joint, different forms of movement are possible and executed. Functionally, the joints are limited by the structures surrounding them, such as muscles, bones or even the capsule. The thumb saddle joint plays a special role here. It distinguishes the function of the human hand from that of other mammals. This is because the structure and mobility of the joint here enable the so-called opposition, which is also known as the pincer grip. Due to further movement possibilities of this joint, the mobility resembles a ball and socket joint.
Diseases
Known diseases of the joints are arthrosis, in which the joint shows wear in a way that is not appropriate for the age. Here, there are several possibilities by which arthrosis can be caused. Thus, in many cases, a constant overload or incorrect load of a joint is responsible for the complaints. Examples are often found in the knee and hip joints. Inflammations or injuries can also cause arthrosis, whereby the wear and tear of the joint is a consequence of the previous illness. Capsule or ligament injuries as a result of an accident or trauma are also frequently referred to as joint injuries.The best-known example is the cruciate ligament rupture in the knee joint, where the actual joint is not damaged, but only structures within the joint. Direct joint involvement often occurs in the context of fractures when the joint surfaces are also affected and the articular cartilage is injured or parts of the joint surfaces break away due to the impact of force. Other joint diseases, arthropathies, are found in the area of joint inflammation, known as arthritis. Since here the causes are often unclear and must be clarified differentialdiagostisch, there are different names, such as polyarthritis or also gout. Special forms of joint diseases are, for example, hallux valgus, the pathological obliquity of the big toe, or chondropathia patellae, in which the cartilaginous back of the kneecap is degeneratively changed and causes pain as well as movement restrictions.
Typical and common disorders
- Osteoarthritis
- Joint inflammation
- Joint pain
- Joint swelling
- Rheumatoid arthritis
