Joint (anatomy and classification)
An anatomical joint is the contact point between bones that permits movement and provides support. This article explains joint structure, types (structural and functional), common examples, development, and clinical importance.
A joint is the anatomical site where two or more bones meet. Joints permit a range of functions from rigid protection to free movement and are essential to posture, locomotion, and many daily tasks. Most movable joints are shaped and supported so that forces transmit across them while tissues reduce friction and absorb shock.
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Basic structure and components
Many joints share common elements: articular cartilage caps the contacting bone surfaces, a surrounding capsule encloses the space, and a synovial membrane (in movable joints) produces fluid for lubrication. Ligaments connect bones and stabilize the joint; tendons transmit muscle forces across a joint. Additional structures such as menisci, labra, or bursae can improve fit, distribute load, or reduce friction.
Structural and functional classification
Joints are described in two complementary ways. The structural classification is based on how bones are joined:
- Fibrous — bones united by dense connective tissue (e.g., skull sutures).
- Cartilaginous — connected by cartilage, allowing limited motion (e.g., intervertebral joints).
- Synovial — a fluid-filled cavity allows free movement; most limb joints fall into this group.
The functional classification describes how much movement is permitted: synarthrosis (immovable), amphiarthrosis (slightly movable), and diarthrosis (freely movable).
Common examples and differences
- Knee — a large synovial hinge with menisci and ligaments, bearing body weight.
- Hip — a ball-and-socket synovial joint providing stability and wide range of motion.
- Shoulder — highly mobile ball-and-socket, less stable than the hip.
- Skull sutures — fibrous, largely immobile joints that protect the brain.
Development, evolution and clinical importance
Joints originate in embryonic mesenchyme where cells differentiate to form cartilage, membranes and capsules. Across vertebrate evolution, joint types adapted to diverse functions: load-bearing, speed, and fine manipulation. Clinically, joints are central to musculoskeletal health; common problems include osteoarthritis (degenerative cartilage loss), inflammatory arthritis, sprains, and dislocations. Understanding joint anatomy guides rehabilitation, surgery, and preventive care.
For further general references on bone and cartilage structure see cartilage resources and introductory anatomy summaries at bone and joint overviews. More detailed classification schemes and functional descriptions can be found in standard anatomy texts and reviews (structural classification, functional classification).
Non-genuine joints (synarthroses)
Unreal joints, also called synarthroses or adhesions, include:
- cartilaginous bone joints (juncturae or articulationes cartilagineae)
- Synchondrosis: connection via hyaline cartilage, for example the rib cartilage, sternum
- Symphysis: connection via fibrocartilage, for example pelvic symphysis, intervertebral discs
- connective tissue bone connections (articulationes fibrosae)
- Suture (seam), for example between the bones of the skull
- Syndesmosis (ligamentous adhesion), for example between ulna and radius
- Gomphosis (wedging), exclusively for anchoring a tooth in the tooth socket (Articulatio dentoalveolaris)
- bony fusions (synostoses), for example sacrum, coccyx: immobile.
- Synsarcosis: connection of bones by skeletal muscles
Real joints (diarthroses)
In true joints (also called diarthroses or juncturae synoviales or articulationes synoviales), there is a gap between the ends of the bones, the joint space. The joint surfaces are covered by articular cartilage. A joint capsule is located around the joint. It consists of a
- outer membrana fibrosa (tight connective tissue)
- inner membrana synovialis (an epithelium-like bandage of connective tissue)
Reinforcements of the membrana fibrosa form the articular or capsular ligaments. However, joint ligaments can also be independent connective tissue tracts, whereby these can be located outside the joint capsule (extracapsular ligaments, for example the external ligament of the knee joint), in the wall of the joint capsule (for example the internal ligament of the knee joint) or inside the joint cavity (intracapsular ligaments, for example the cruciate ligaments of the knee joint). The latter are covered by a layer of membrana synovialis, which is connected to the joint capsule. Thus, strictly speaking, the cruciate ligaments are intracapsular, but since they cannot be reached directly from the joint cavity without passing through the membrana synovialis, this can also be considered extraarticular.
The joint capsule encloses the joint cavity without gaps. It lies flaccidly against the joint bodies and keeps appropriate reserve folds ready for the joint excursions (very typically observed on the finger joints). The joint capsule normally contains only a few milliliters of a viscous fluid, the synovia ("synovial fluid"), which is a product of the synovial membrane of the joint capsule. In the case of inflammation, a joint can fill up bulging with synovia, which is painful and can be treated, for example, by a puncture. If a joint fills with blood after an accident, this is called haemarthrosis, which must be treated immediately to suppress the damaging effect of the blood on the cartilage.
Joint forms
According to the shape of the articular surfaces, the true joints can be further subdivided:
- Spherical joint (Articulatio sphaeroidea): triaxial, for example shoulder joint, hip joint, metacarpophalangeal joints of the fingers (with the exception of the thumb); can be moved about the three perpendicular spatial axes: Flexion - extension, abduction and adduction, external and internal rotation.
- Ellipsoid or egg joint (articulatio ellipsoidea): biaxial, for example the first head joint between atlas and skull; proximal wrist joint between radius and carpal bones; allows both flexion-extension movement and side-to-side movement.
- Saddle joint (articulatio sellaris): biaxial, for example the joint between the carpal bone and the metacarpal bone below the thumb; can be moved about two spatial axes: Flexion and extension, abduction and adduction.
- Cylinder joint (Articulatio cylindrica), generic term for uniaxial joints of the hinge and roller joint type
- Hinge joint (Ginglymus): uniaxial, can only be bent (flexion) and extended (extension), for example the elbow joint (between humerus and ulna); the finger joints with the exception of the metacarpophalangeal joints.
- Roll, wheel or pivot joint (articulatio trochoidea): uniaxial, for example radioulnar joint (between radius and ulna) and atlantoaxial joint
- Planar joint or swivel joint (articulatio plana): no geometric center of motion, also called planar joint, for example between vertebral processes. Special feature here: two translational degrees of freedom.
- Bicondylar joint or condylar joint (Articulatio bicondylaris): Knee joint, biaxial, connection of two condyles with different curvatures: Flexion - extension, external and internal rotation (only possible when the knee joint is flexed).
- Tight joint (amphiarthrosis): for example sacroiliac joint
Joint Cracking
The cause of the "cracking" of joints (for example, finger cracking) is not yet fully understood. The most common explanation is gas bubbles in the synovium, which cause a noise by forming bubbles when pressure is equalized (cavitation). Unevenness in the surface of the knuckles is also conceivable as a possible cause.
Habitual finger cracking is not considered very harmful by experts. It cannot cause arthritis, but may reduce grip strength.
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Author
AlegsaOnline.com Joint (anatomy and classification) Leandro Alegsa
URL: https://en.alegsaonline.com/art/50956
Sources
- emedicinehealth.com : "Joint definition"