Structure and Functions of Joints

Fibrous Joints

Fibrous joints are characterized by the lack of a synovial cavity, with articulating bones closely held together by dense irregular connective tissue. They allow little to no movement and include three types:

Sutures

• Composed of a thin layer of dense irregular connective tissue, sutures occur only between skull bones. An example is the coronal suture between the parietal and frontal bones.
• The interlocking edges provide strength and help absorb shocks.
• Immoveable in adults (synarthroses) but slightly movable in children (amphiarthroses).
• Can be replaced by bone in adulthood, forming a synostosis.

Syndesmoses

• These joints have a greater distance between the articulating surfaces and include ligaments (bundles of dense irregular connective tissue).
• Example: The distal tibiofibular joint, which provides slight movement (amphiarthrosis).
• Gomphosis is a specific type, connecting teeth to their sockets, allowing no movement (synarthrosis).

Interosseous Membranes

• A substantial sheet of dense irregular connective tissue binding neighboring long bones (e.g., radius and ulna, tibia and fibula) and allowing slight movement (amphiarthrosis).

Cartilaginous Joints

Cartilaginous joints lack a synovial cavity as well and allow little to no movement. They comprise two types:

Synchondroses

• These joints connect bones with hyaline cartilage. Example includes the epiphyseal plates in growing bones (immovable, or synarthroses).
• Also observe the first rib and the manubrium of the sternum, which ossifies into a bony joint (synostosis).

Symphyses

• Composed of hyaline cartilage and fibrocartilage, these joints connect bones with a broad disc, allowing slight movement (amphiarthrosis).
• Examples include the pubic symphysis and intervertebral joints.

Synovial Joints

Characteristics

• Distinct due to the presence of a synovial cavity, allowing considerable movement (diarthroses).
• Covered by articular cartilage for smooth movement and shock absorption.

Articular Capsule

• Surrounds the joint and consists of an outer fibrous membrane and an inner synovial membrane.
• The fibrous membrane provides tensile strength while allowing movement.
• The synovial membrane secretes synovial fluid, which lubricates the joint and supplies nutrients.

Synovial Fluid

• Reduces friction, absorbs shocks, and contains phagocytic cells that remove debris.
• Its viscosity changes with movement, increasing with activity.

Accessory Structures

• May include extracapsular ligaments (outside the capsule) and intracapsular ligaments (inside the capsule). Examples are the cruciate ligaments of the knee.
• Articular discs (menisci) in some joints serve to stabilize and absorb shock.
• The labrum in ball-and-socket joints deepens the socket and increases contact with the articulating surfaces.