Classification of joints
A joint (or articulation) is a union of two or more bones connecting and articulating. Joints are supported and stabilized by various connective tissue structures. Articulations are classified by the type of origin of the tissue that holds them together or by the kind of movement they can provide. The main functions of joints are to provide a range of motion between bones and facilitate growth during development.
Joints can be divided into groups based either on the histological origin of the tissue or by the type of movement they provide. Histologically (depending on the tissue), there are three types of joints: fibrous, cartilaginous and synovial. Functionally (depending on the provided movement), there are also three types of joints: synarthrosis (doesn’t move), amphiarthrosis (moves slightly) and diarthrosis (moves freely).
Groups from both mentioned classifications correlate with one another. Free movement is possible in synovial joints, which are mainly diarthroses. Synarthroses and amphiarthroses are restricted in movement and correspond to fibrous and cartilaginous joints, respectively.
Fibrous joints connect bones with the help of fibrous connective tissue. The main component of fibrous tissue is collagen. These articulations allow little to almost no movement. They are fixed and have no joint cavity. Fibrous joints can be further divided into sutures and syndesmoses.
Sutures are joints that connect bones only in the skull. With age, usually beginning in the early twenties, the sutures ossify, and the gaps between bones become closed. This fusion of the bones is called synostosis.
Syndesmoses can move only slightly and can be classified as amphiarthroses. This type of joint connects bones with the help of an interosseous membrane or ligament. Syndesmoses are found between the distal parts of tibia and fibula (the inferior tibio-fibular joint), between the ulna and radius (the radio-ulnar joint), and also in the posterior region of the sacroiliac joint.
Cartilaginous joints are articulations where bones are connected with hyaline cartilage or fibrous cartilage. Depending on the type of cartilage involved, they can be divided into primary and secondary cartilaginous joints.
At the primary cartilaginous joints (also called synchondrosis), a bone meets hyaline cartilage. An example of the primary cartilaginous joint is the junction of ribs with their respective cartilage, as well as all epiphyses. An epiphysis is the end of a long bone and is related to the growth plate. The cartilaginous joints can be fixed (synarthroses) or with slight mobility (amphiarthroses). Synchondroses also tend to ossify with age in the process called synostosis.
Secondary cartilaginous joints (also called symphysis) consist of either hyaline cartilage or fibrocartilage. Fibrocartilage is like a disc in the middle of the hyaline cartilage covering two bones that articulate. Some examples of symphyses include the pubic symphysis and the manubriosternal joint (at the sternal angle). Intervertebral discs are also symphyses, but their fibrocartilage contains a gel-like substance (nucleus pulposus). Symphyses have limited mobility (they are amphiarthroses). Nevertheless, they are supposed to be extremely strong to endure stress due to tension and compression. Interestingly, all symphyses are located in the midline of the body.
Synovial joints are the main functional joints in the body. They are located mainly in the limbs and provide the broadest range of motion. Synovial joints are diarthroses. There are several main characteristics of these joints:
- Synovial joints have a cavity that is encased in a capsule.
- The articular capsule on the inside is covered by a synovial membrane.
- The joint cavity contains synovial fluid produced by the cells in the synovial membrane. Synovial fluid acts as a lubricant.
- Ligaments inside and outside the capsule stabilize it and strengthen it.
- The bones articulating in synovial joints are lined by hyaline cartilage.
Some synovial joints, like the knee joint, between their articulating surfaces, contain fibrocartilaginous structures called menisci.
Types of synovial joints
Synovial joints can be further divided according to the type of movements they provide:
- Plane joints
- Hinge joints
- Pivot joints
- Ellipsoid joints (also called condyloid)
- Saddle joints
- Ball and socket joints
Plane joints are joints where the movements are more of a sliding nature. These articulations are between flat bones that are similar in size. This joint type is found in intertarsal, some intercarpal, and acromioclavicular joints.
Hinge joints are uniaxial joints because they allow movement just in one plane. The articulation is between a convex end of one bone and a concave end of another bone. A good example of this type of joint is the humero-ulnar joint in the elbow.
Pivot joints are also uniaxial and allow rotational movement. In this case, the articulation occurs between a rounded end of one bone and another bone inside an osteo-ligamentous ring. Two examples are the atlanto-axial joint between the upper two cervical vertebrae (C1 and C2) and the proximal radio-ulnar joint.
Ellipsoid (or condyloid) joints move in two planes (they are biaxial), allowing flexion/extension and abduction/adduction (movements laterally and medially). The articular surfaces are rounded on one side and elliptical, slightly concave on the other side. This type is found in the metacarpophalangeal joints of the hand.
Saddle joints are seen in joints where the surfaces of both articulating bones have concave and convex regions. The concavity of the surface of one bone is matched to the opposing convexity of another bone. This type of joint is biaxial and is found in the first metacarpal bone of the thumb.
Ball and socket joints are multiaxial; they ensure flexion/extension, abduction/adduction and rotation. A rounded head of one bone (the “ball”) articulates with a convex cup-shaped end of another bone (the “socket”). The only examples of the ball-socket joints in the human body are the hip and shoulder joints.
Most common joint diseases
The most common acute joint conditions include traumatic injuries due to fractures, dislocations, sprain etc. The most common chronic diseases affecting the joints are various forms of arthritis. Arthritis essentially means inflammation of a joint.
The most widespread manifestation of arthritis is osteoarthritis (OA). It is considered an age-related “wear and tear” disease of the joints. It is found in the hands, knees, hips and spine (cervical and lumbar). Joint pain, stiffness and limited motion in the joints are the most common symptoms of OA.
Rheumatoid arthritis (RA) is another common joint condition. RA is a chronic joint inflammation most often found in the small joints of hands, wrists and feet. It leads to cartilage destruction, bone erosion and can result in severe disability if left untreated.
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