joints

Definition of Arthrology: Arthrology is defined as the formal study of the structure, function, dysfunction, and treatment of joints.
Joint Nomenclature: Joints are typically named according to the specific bones they connect. An example of this naming convention is the atlanto-occipital joint, which connects the atlas (the first cervical vertebra) and the occipital bone of the skull.

Structural classification is determined based on the physical way bones are held together and whether or not a joint cavity is present.

Synovial Joints: These joints feature bones separated by a joint cavity. The cavity is lubricated by synovial fluid and the entire structure is enclosed within a fibrous joint capsule. Examples include:
- Shoulder, elbow, and carpal joints.
- Hip, knee, and tarsal joints.
- Interphalangeal joints.
- Joints located between the articular processes of cervical to lumbar vertebrae.
Fibrous Joints: In these joints, bones are held together by collagenous fibers that extend from the matrix of one bone directly into the matrix of the next. There is no joint cavity. Examples include:
- Skull sutures.
- Teeth in their sockets.
- Distal radioulnar joints.
- Tibiofibular joints.
Cartilaginous Joints: These joints have bones held together by cartilage and lack a joint cavity. Examples include:
- Epiphyseal plates of long bones.
- Costosternal joints.
- Pubic symphysis.
- Intervertebral discs.

Functional classification is determined by the relative mobility allowed by the joint.

Diarthroses: These are categorized as freely movable synovial joints. Examples include:
- Shoulder, elbow, and carpal joints.
- Hip, knee, and tarsal joints.
- Interphalangeal joints.
Amphiarthroses: These are defined as slightly movable joints. Examples include:
- Intervertebral discs.
- Joints between articular processes of cervical to lumbar vertebrae.
- Costosternal joints involving ribs $2$ through $7$ .
- Pubic symphysis.
- Distal radioulnar joints.
- Tibiofibular joints.
Synarthroses: These are joints that allow for little or no movement. Examples include:
- Skull sutures.
- Teeth in sockets.
- Epiphyseal plates of long bones.
- The first ( $1^{st}$ ) costosternal joint.
- Mental symphysis.

Suture: An immovable fibrous joint that closely binds skull bones to one another.
Gomphoses: A joint where a conical process fits into a socket, forming an immovable connection. A primary example is the tooth joint.
Syndesmoses: A joint where two bones are bound together by a ligament or an interosseous membrane. This represents the most movable type of fibrous joint. An example is the connection between the radius and the ulna.

Synchondroses: Joints where the bones are joined specifically by hyaline cartilage. Examples include the connection between the ribs and the sternum.
Symphyses: Joints where two bones are joined by fibrocartilage.
- Pubic Symphysis: Joins the two pubic bones.
- Intervertebral Disc: Composed of fibrocartilage, joining the bodies of adjacent vertebrae.
Anatomic Components of Cartilaginous Regions:
- Clavicle
- Sternum
- Costal cartilage
- Rib
- Body of vertebra

Bony Fusion (Synostosis): A synostosis occurs when two or more bones (or formerly separate regions of bone) become fused together with osseous tissue. This fusion can result from natural skeletal development or various pathologies.
Typical Synostoses (Developmental):
- Epiphyseal Plates of Long Bones: The transition from cartilage to bone as growth concludes.
- Acetabulum: The fusion of the pubic, ischial, and iliac bones.
- Sacrum: The fusion of the individual sacral vertebrae.
Pathological Synostoses:
- Craniosynostosis: The premature fusion of the skull bones.
- Bony Syndactyly: The pathological fusion of digits (fingers or toes).

Synovial joints consist of two bones separated by a space but held together by a joint capsule.

Joint Cavity: The space between the bones that contains synovial fluid for lubrication.
Articular Cartilage: A layer of cartilage covering the surfaces of the bones where they meet in the joint.
Joint Capsule: A structure connecting the bones, consisting of:
- Fibrous Capsule: The outer layer of the joint capsule.
- Synovial Membrane: The inner lining of the capsule that secretes synovial fluid.
Associated Structures:
- Ligaments: External connective tissues that surround and reinforce the capsule.
- Periosteum: The membrane covering the outer surface of the bones (e.g., the proximal and middle phalanx).

Synovial joints are classified by their shape and the types of movement they allow.

Ball & Socket Joint: This joint features one bone with a hemispherical surface that fits into a cup-like depression of the second bone.
- Example: Humeroscapular joint (between the head of the humerus and the scapula).
Hinge Joint: Features one bone with a convex shape that fits into a concave surface of another bone. These are mono-axial joints.
- Example: Humeroulnar joint (between the humerus and the ulna).
Saddle Joint: The joint surfaces are shaped like a saddle, allowing for a unique range of motion.
- Example: Carpometacarpal I joint (of the thumb, connecting a carpal bone and a metacarpal bone).
Pivot Joint: A mono-axial joint where one bone rotates around another.
- Example: Radioulnar joint (between the radius and ulna).
Gliding Joint: A joint where the articular surfaces are relatively flat, allowing adjacent bones to glide over each other.
- Example: Intercarpal joint (between the carpal bones of the wrist).
Condyloid Joint: This joint features an oval convex surface that fits into a corresponding oval concave surface. These are bi-axial joints.
- Example: Metacarpophalangeal joint (between a metacarpal bone and a phalanx).