Joints are enclosed by the joint capsule. It provides the joint with protection, strength, and stability. Joint capsules can consist of both thin and tough fibers. In some cases, they are reinforced by ligaments.
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The inner wall of the joint capsule is lined with the synovial membrane. This membrane secretes synovial fluid (known technically as synovia) into the joint space. Synovial fluid resembles raw egg white in appearance and consistency. It is viscous when the joint is at rest and becomes more fluid when the joint moves.
In a healthy joint, there is usually just enough synovial fluid to coat the joint surfaces inside the joint cavity with a thin film. This ensures that the bones can move smoothly against one another and protects the joint from excessive friction. In a diseased joint, however, the amount of synovial fluid can increase to the point where visible swelling occurs.
The joint surfaces of the bones are covered with articular cartilage. Articular cartilage is important for reducing friction between the bones. Articular cartilage is nourished by synovial fluid, as it is not connected to the blood vessel system.
In addition, some joints contain fluid-filled sacs known as bursae, as well as cushion-like structures made of cartilage called menisci. The bursae usually serve as cushions to reduce friction between bones and other structures, such as tendons or muscles. In the knee joint and some other joints, the “menisci” divide the joint cavity into separate compartments. They contribute to joint stability and act as shock absorbers between the bones.
Joints can be moved in different directions. How a joint can be moved depends on its shape and structure. For example, there are uniaxial joints that can only move in one direction and multiaxial joints, such as the hip joint, that can move in many directions.
Joints are moved by the contraction or relaxation of muscles. Skeletal muscles extend from one bone to another, thus spanning 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 toward each other and can thus flex or extend the joints.
The muscles end in strands of firm connective tissue—the tendons—with which they are attached to the bones. The tendons transmit movement from the muscle to the bone. Some tendons, particularly those in the hand and ankle joints, are enclosed in structures called tendon sheaths, which are similar in structure to bursae and whose inner surface is coated with a film of synovial fluid. The tendon sheaths reduce friction as the tendons glide back and forth.
About 20 million people in this country suffer from joint pain. Pain is particularly often the result of wear and tear, such as osteoarthritis, joint inflammation, and rheumatic diseases.
Examinations of Joints and Joint Diseases—How to Stay Mobile to Prevent Disease
Arthroscopy allows for endoscopic visual diagnosis, including biopsy and therapeutic procedures within joints. In most cases, arthroscopy is indicated not solely for diagnostic reasons but primarily for therapeutic purposes.
In principle, all joints of the extremities are accessible via arthroscopy; however, for anatomical reasons, they are not equally suitable for this procedure. In some cases, a thick layer of soft tissue complicates instrumental manipulation (hip joint); in others, tight ligamentous structures or narrow compartments impair the surgeon’s view (ankle joint, wrist joint, elbow joint). The knee and shoulder joints are currently considered the primary domains of arthroscopy.
Under general or regional anesthesia, the affected joint is distended with fluid. The arthroscope is inserted through a targeted puncture incision, and the instruments for manipulation and the collection of biopsy specimens are inserted through a second puncture. The image captured by a video camera is transmitted to a monitor and can be documented accordingly.
Joint ultrasound is particularly important for the early diagnosis and monitoring of congenital hip dysplasia during the first year of life. Since it is safe, it can also be used without concern for preventive care and screening programs.
Ultrasound diagnostics is 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, thereby providing insights into their functional status.
Joint MRI, also known as magnetic resonance imaging, is particularly well-suited for visualizing soft tissue structures within joints.
Magnetic resonance imaging (MRI) uses a strong magnetic field to activate the atomic nuclei in body tissues and process the resulting electrical signals into an image. Unlike X-rays, current scientific knowledge indicates that the magnetic field is not expected to cause any harm.
The method provides particularly insightful information about soft tissue structures (brain and spinal cord, soft tissue tumors, soft tissue injuries, fluid accumulations in tissues and cavities, etc.). Unlike computed tomography, which allows for high-resolution imaging of tissue in only one plane, magnetic resonance imaging enables the computer-aided reconstruction and visualization of tissue in different planes.
Bone cannot be directly visualized in an MRI image; only non-bony, water-containing tissue types can be imaged. Bone is thus only indirectly recognizable as a void in the MRI image and therefore remains the domain of imaging techniques that use ionizing radiation.
Punctures of joints, bursae, tendon sheaths, abscesses, and the spinal canal provide information about the contents of cavities and swellings, as well as material for analysis.
Conclusions about the cause can be drawn from the nature of an effusion:
- clear serous effusions occur in mild inflammatory conditions (“irritation effusion”) in osteoarthritis, in cases of mechanical irritation (e.g., improper loading), or following minor trauma
- The presence of blood suggests an internal injury
- The presence of fibrin suggests an inflammatory origin
- A fibrinous-purulent consistency indicates infection (empyema)
- The presence of fat suggests bone injury or a fracture
Important information can also be obtained from the examination of the synovial fluid, particularly in cases of rheumatic diseases. All joint aspirations require strict adherence to aseptic technique!
Joints are movable connections between two or more bones and enable the mobility of the human body. True joints, also called diarthroses, have a joint space, whereas synarthroses have very little mobility or may even ossify, as is the case with the bones of the skull. Structures such as the articular head and socket, as well as the shape of the articular surfaces of the bones involved, ensure that joints function optimally. The joint capsule consists of an inner layer, the synovial membrane, and an outer layer, with synovial fluid supplying the cartilage with nutrients and preventing friction. This prevents bones from rubbing against each other while simultaneously distributing pressure to act as a shock absorber.
However, as we age, the risk of wear and tear and joint diseases such as rheumatoid arthritis or other joint disorders increases. Factors such as being overweight, improper weight-bearing, or a lack of exercise can further contribute to these processes and lead to pain and limited mobility, especially in high-impact areas such as the knees and hips, or even the elbows. To prevent this, regular exercise is crucial, as it strengthens muscles, helps stabilize joints, and can improve the mobility of certain joints. Exercise also ensures that the joints remain lubricated and improves blood flow through the blood vessels.
Anyone who wants to protect their joints should maintain a healthy body weight, avoid improper strain, and specifically strengthen their muscles. Healthy joints are best preserved when they are used regularly but not overloaded. This helps prevent degenerative joint diseases and maintain the function of the body’s joints in the long term.
What is a joint?
A joint is a movable connection between two or more bones. It enables movement and ensures that the body remains flexible.
How do joints work?
Joints function through the interaction of cartilage, joint surfaces, and synovial fluid. These structures prevent bones from rubbing against each other and enable smooth movement.
What types of joints are there?
There are true joints (diarthroses) and false joints (synarthroses). True joints are movable, while false joints have little or no movement.
What are common joint disorders?
The most common conditions include osteoarthritis, arthritis, and joint inflammation. These can cause pain and limited mobility.
How can you keep your joints healthy?
Joints stay healthy through regular exercise, a balanced diet, and avoiding improper strain.
