Joints - structure, function and examinations

The bones are connected to each other by joints, which are strengthened and moved by ligaments, muscles and tendons.

The joint capsule

Joints are enclosed by the joint capsule. It provides the joint with protection, strength and stability. Joint capsules can consist of both thin and coarse fibers. Some of them are reinforced by ligaments.

The joint space and synovial fluid

The inner wall of the joint capsule is lined with the so-called synovial membrane. This secretes the synovial fluid into the joint space. The synovial fluid is similar in appearance and consistency to raw egg white. It is viscous when the joint is at rest and becomes thin when the joint moves.

In a healthy joint, there is usually just enough synovial fluid to cover the joint surfaces inside the joint cavity with a thin film. This ensures that the bones can move against each other and protects the joint from excessive friction. In a diseased joint, however, the amount of synovial fluid can increase to such an extent that visible swelling occurs.

The articular cartilage

The articular surfaces of the bones are covered with articular cartilage. Articular cartilage is important for reducing friction between the bones. The articular cartilage is nourished by the synovial fluid as it is not connected to the blood vessel system.

The bursa and the meniscus

Some joints also contain fluid-filled sacs, known as bursae, as well as cushion-like structures made of cartilage, known as the inter-articular disc or meniscus. The bursae usually serve as a cushion to reduce friction between bones and other structures, e.g. tendons or muscles. In the knee joint and some other joints, the "menisci" divide the joint cavity into separate spaces. They contribute to joint stability and act as shock absorbers between the bones.

How the joints are moved

Joints can be moved in different directions. How the joints can be moved depends on the shape and design of the joint. For example, there are uniaxial joints that can only move in one direction and multiaxial joints such as the hip joint, which can move in many directions.

The muscles and tendons

The joints are moved by tensing or relaxing the muscles. The skeletal muscles pull from one bone to another and thus span the joints. As a rule, a muscle crosses at least one joint and is attached to the bones that form the joint. When muscles contract or relax, they move the bones towards each other and can thus bend or stretch the joints.

The muscles end in strands of firm connective tissue, the tendons, with which they are attached to the bones. The tendons transmit the movement from the muscle to the bone. Some tendons, particularly in the wrist and ankle joints, are enclosed in so-called tendon sheaths, which are similar in structure to bursae and whose inner surface is covered with a film of synovial fluid. The tendon sheaths reduce friction when the tendons slide back and forth.

Joint pain

Around 20 million people in Germany suffer from joint pain. Pain is particularly common as a result of signs of wear and tear such as osteoarthritis, joint inflammation and rheumatic diseases.

Examinations of the joints

Arthroscopy

Arthroscopy allows endoscopic visual diagnostics including trial excisions and therapeutic surgery in joints. In most cases, arthroscopy is not only indicated for diagnostic reasons, but primarily for therapeutic purposes.

In principle, all extremity joints are accessible to the arthroscopic technique, but for anatomical reasons they are not equally suitable for this procedure. In some cases, a thick soft tissue sheath makes instrumental manipulation difficult (hip joint), while in others the tight ligament guidance or narrow compartmentalization impairs the overview (upper ankle joint, wrist, elbow joint). The knee and shoulder joints are currently considered the domain of arthroscopy.

Under general or regional anesthesia, the joint in question is expanded with fluid. The arthroscope is inserted through a targeted puncture incision and the instruments for manipulation and removal of examination material are inserted through a second puncture. The image received by a video camera is transmitted to a monitor and can be documented accordingly.

Joint ultrasound

Joint ultrasound is particularly important in the early diagnosis and follow-up of congenital hip dysplapsia in the first year of life. As it is harmless, it can also be used safely for preventive and screening purposes.

Ultrasound diagnostics are also used on other joints, particularly the shoulder, and for the differential diagnosis of various soft tissue changes. Its particular advantage lies in the fact that joints and muscles can be moved during the examination and thus insights into the functional situation can be gained.

Joint MRI

Joint MRI, also known as magnetic resonance imaging, is particularly suitable for visualizing soft tissue structures in joints.

Magnetic resonance imaging (MRI) uses a strong magnetic field to activate the atomic nuclei of the body tissue and process the resulting electrical phenomena into an image. In contrast to the use of X-rays, the magnetic field is not expected to cause any damage according to current knowledge.

The method provides particularly revealing information about soft tissue structures (brain and spinal cord, soft tissue tumors, soft tissue injuries, fluid accumulations in tissue and cavities, etc.). In contrast to computer tomography, which only allows high-resolution imaging of the tissue in one plane, magnetic resonance imaging allows the tissue to be reconstructed and imaged using computer technology in different planes.

Bone substance cannot be imaged directly in the MRI image, only the non-bony, water-containing tissue types. The bone is therefore only indirectly visible in the MRI image as a recess and thus remains the domain of those imaging procedures that use ionizing radiation.

Joint puncture

Punctures of joints, bursae, tendon sheaths, abscesses and the spinal canal are used to determine the contents of cavities and swellings and to obtain material for examination.

The nature of an effusion allows conclusions to be drawn about its cause:

• clear-serous hydrops occurs in blanched irritation conditions ("irritant effusion") in arthrosis, in mechanical irritation, e.g. incorrect loading, after minor trauma
• Blood content indicates an internal injury
• Fibrin content for inflammatory genesis
• fibrinous-purulent consistency for infection (empyema)
• Fat content for bone injury or fracture

Important information can also be obtained from the examination of the synovia, particularly in the case of rheumatic diseases. All joint punctures require strict adherence to asepsis!