7.3 Joints

A joint (articulation) is a location where two bones, or a bone and cartilage, connect. The scientific study of the structure and function of joints is called Arthrology.

Joints are classified based on their structure and the degree of movement they permit.

7.3.1 Fibrous Joints

Fibrous joints are connections where adjacent bones are directly linked by dense, collagen-rich fibrous connective tissue. They lack a joint cavity and permit no movement.

• Examples:
  • Sutures: Found between most bones of the skull.
  • Joints between the shaft regions of long bones (e.g., in the forearm and leg).
  • The connection between the root of a tooth and its socket in the mandible or maxilla.

7.3.2 Cartilaginous Joints

In cartilaginous joints, adjacent bones are united by either hyaline cartilage or fibrocartilage. These joints also lack a cavity and allow for only a small amount of movement.

• Examples:
  • Costal cartilages: Attach the ribs to the sternum.
  • Pubic symphysis: Connects the two pubic bones of the pelvis.
  • Intervertebral discs: Found between the vertebrae of the spine.

For more on the vertebral column, see 7.1 Human Skeleton→(/biology-12/07-skeletal-system-of-man/7-1-human-skeleton).

7.3.3 Synovial Joints

Synovial joints are the most common type of joint and are characterized by their free mobility.

• Key Structures:
  • Articular Cartilage: A layer of smooth hyaline cartilage covering the ends of the bones.
  • Joint Capsule: A tube-like structure of dense fibrous tissue that encloses the joint.
    • Outer Layer: Composed of ligaments for stability.
    • Inner Layer: The synovial membrane, which secretes synovial fluid.
  • Synovial Fluid: A viscous fluid that lubricates the joint, reduces friction, and nourishes the articular cartilage.
  • Joint Cavity: The space between the bones that is filled with synovial fluid.

• Types and Examples:
  • Hinge Joints: Allow movement in one plane (e.g., elbow, knee, finger joints).
  • Ball-and-Socket Joints: Allow for a wide range of motion (e.g., shoulder and hip joints).
  • Pivot Joints: Allow for rotational movement (e.g., between the first two vertebrae in the neck).
  • Gliding Joints: Allow for sliding or gliding movements (e.g., carpal joints in the wrist, tarsal joints in the ankle).

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Possible Questions and Answers

• Q: What is arthroplasty? A: Arthroplasty is the medical term for joint replacement surgery, where an artificial joint (prosthesis) made of metal and plastic is used to replace a damaged joint. It can be a partial or total replacement.

• Q: How do the skeleton and musculature support bipedal posture in humans? A: The curvatures of the vertebral column help balance the body. Intervertebral discs absorb shock. The structure of the pelvis allows for an upright stance and locomotion on two legs. Additionally, specific posture muscles work against gravity to maintain this stance.

For more on muscle functions, see 7.7 Muscles→(/biology-12/07-skeletal-system-of-man/7-7-muscles).

For common joint injuries, see 7.6 Injuries to Joints→(/biology-12/07-skeletal-system-of-man/7-6-injuries-to-joints).