A joint is typically seen as the connection between two bones, but a joint consists of so much more than just bones. Without the surrounding structures of the bones, movement in the joint would not look as harmonious as usual, but rather “rigid”. The vast majority of joints in our body consist, apart from the bones involved, of articular cartilage, synovial fluid and the joint capsule.
Together, they all ensure the beautiful, gliding movements of a joint, which we all know from our own joints. The cartilage serves as a mechanical shock absorber on the ends of the bones so that they do not rub directly against each other, which would lead to their wear and tear quite quickly. Cartilage also compensates for inequalities between the bones in the joint.
The most famous example of this is the thigh and lower leg bones, because the bone ends only fit so perfectly together in the knee joint because of the often tearing meniscus cartilage. Since the blood supply to the end of the bone is very poor and the cartilage itself has much less blood vessels or even none at all, the cartilage must be fed differently, because the cartilage cells also need nutrients to fulfill their tasks. Their nutrition is provided by the synovial fluid, which in addition to its nutritional function is a further shock absorber and sliding layer.
This is also where the joint capsule comes into play. It surrounds the joints like a kind of bag and keeps all structures inside hidden and protected. It consists of different layers and cell types of which the innermost one is responsible for the production of synovial fluid.
Seen from the outside to the inside, the joint capsule consists of different layers and structures that have different tasks in the joint. The outermost layer is the so-called Membrana Fibrosa, or “fibrous skin”. It consists of very coarse fibers of connective tissue, which give the capsule a certain firmness.
Depending on the joint, the thickness of this skin can vary greatly, from very thin at small joints, such as the finger joints, to several millimeters thick in the hip joint. This layer of the joint capsule is in part reinforced to form really strong ligaments, which provide additional stability. Examples of this are the inner and outer ligaments in the knee joint as reinforcement of the knee joint capsule.
But even without the ligaments, this outer layer of the joint capsule gives a joint strength and can limit the range of motion of a joint. In addition to the connective tissue fibers, many nerve fibers also run within this outer layer. Among the nerve cells are cells that measure the position of the joint, so that we always know how the joint is positioned even when we cannot see the body parts.
This ability becomes noticeable, for example, when you close your eyes and can still tell exactly in which position your arms or legs are at the moment. But not only the nerve fibers for the sense of position, also those for the perception of pain are located in the joint capsule. This is why torn capsules and ligaments are so extremely painful, whereas pure cartilage damage is hardly noticeable, because cartilage is insensitive to pain.
The inner layer of the joint capsule forms the membrana synovialis. It is much more delicate than the outer layer and contains different types of cells. On the one hand, the cells that produce the synovial fluid (= synovia) are located on the inside of the capsule, on the other hand, there are cells that reabsorb the synovial fluid.
These cells thus ensure that the space in the joint capsule, i.e. also between the bones, is always free of abrasion products of the cartilage or bones and that clean fluid is constantly produced. In arthrosis, the wear and tear of the joint, the constantly increasing abrasion irritates the fluid-producing cells in the inner skin of the joint capsule in such a way that they constantly produce fluid and the balance between inflow and outflow of the joint fluid is massively disturbed. The capsule swells and restricts joint mobility.
Irritant substances in the fluid also cause inflammation and pain in the joint capsule, which is usually a source of pain for people suffering from arthrosis. In the long term, the inflammation leads to a shortening of the connective tissue and the mobility of the joint will thus be increasingly restricted until the joint has to be replaced in the final stage of arthrosis.
