Joints (Articulations)

Definition and General Concept

A joint (also called an articulation) is any point where two bones meet, regardless of whether the bones are movable at that interface. Joints are essential for movement, stability, and the structural integrity of the skeletal system.

• Articulation: The location at which bones connect.

• Joints may be classified by their structure or by their function (degree of movement).

• Example: The flexibility seen in gymnasts is due to the range of motion at various joints.

Joint Classification

Structural and Functional Classification

Joints are named after the bones involved and classified according to how the adjacent bones are bound to each other. There are four major types:

• Bony joints (synostosis): Immovable joints formed when two bones fuse.

• Fibrous joints (synarthrosis): Bones are bound by collagen fibers.

• Cartilaginous joints (amphiarthrosis): Bones are linked by cartilage.

• Synovial joints (diarthrosis): Bones are separated by a synovial cavity; most movable and complex.

Note: The suffix -sis in joint types can be replaced with -dial (except for bony joints).

Bony Joints (Synostosis)

Characteristics and Examples

A synostosis is an immovable joint formed when the gap between two bones ossifies, resulting in a single bone. This can occur in either fibrous or cartilaginous joints.

• Examples:

  • Frontal and mandibular bones in infants

  • Formation of sternum, coxal bone, sacrum, coccyx

  • Epiphyseal line in post-adolescents

  • Cranial sutures in elderly

  • Attachment of first rib and sternum in elderly

Fibrous Joints (Synarthrosis)

Definition and Types

A fibrous joint is a point at which adjacent bones are bound by collagen fibers that emerge from one bone, cross the space between them, and penetrate into the other. These joints are generally immovable or only slightly movable.

• Three types of fibrous joints:

  • Sutures

  • Syndesmoses

  • Gomphoses

Sutures

Sutures are immovable or slightly movable fibrous joints that closely bind the bones of the skull to each other. They are classified by their shape:

• Serrate: Interlocking wavy lines (e.g., coronal, sagittal, and lambdoid sutures)

• Lap (squamous): Overlapping beveled edges (e.g., temporal and parietal bones)

• Plane (butt): Straight, nonoverlapping edges (e.g., palatine processes of the maxillae)

Example: The sutures of the skull provide protection and stability to the brain by tightly binding cranial bones.

Syndesmoses

Syndesmosis is a fibrous joint where bones are bound by longer collagenous fibers, allowing for more mobility than sutures.

• Example: The interosseous membrane between the radius and ulna allows for supination and pronation of the forearm.

Gomphoses

Gomphosis is the attachment of a tooth to its socket, held in place by the fibrous periodontal ligament. This joint allows slight movement under the stress of chewing.

• Example: The connection between teeth and the maxillae or mandible.

Additional info: Fibrous joints provide stability and limit movement, which is crucial for protecting vital organs and maintaining posture.