Chapter 8B: Joints (Synovial Joints)

Introduction

Synovial joints are a key focus in the study of human anatomy and physiology, providing the greatest range of motion among joint types. This section covers their structural characteristics, the types of movements they allow, and the classification of synovial joints based on their shapes and functions.

Learning Outcomes

• Describe the structural characteristics of synovial joints.

• Compare the structures and functions of bursae and tendon sheaths.

• List three natural factors that stabilize synovial joints.

• Name and describe common body movements.

• Name and provide examples of the six types of synovial joints and the movements they allow.

Movements Allowed by Synovial Joints

Range of Motion

Synovial joints allow a variety of movements, classified by the number of planes in which movement occurs:

• Nonaxial movement: Gliding movements only, with no axis of rotation.

• Uniaxial movement: Movement in one plane (one axis).

• Biaxial movement: Movement in two planes (two axes).

• Multiaxial movement: Movement in or around all three planes (three axes).

There are three general types of movements at synovial joints:

• Gliding

• Angular movements

• Rotation

Gliding Movements

Gliding occurs when the flat surfaces of two bones slide across each other. This movement is typically nonaxial.

• Examples:

  • Intercarpal joints (between carpal bones of the wrist)

  • Intertarsal joints (between tarsal bones of the ankle)

  • Between articular processes of vertebrae

Angular Movements

Angular movements change the angle between two bones and occur along the sagittal or frontal planes.

• Flexion: Decreases the angle of the joint (e.g., bending the elbow).

• Extension: Increases the angle of the joint (e.g., straightening the knee).

• Hyperextension: Extension beyond the anatomical position.

• Abduction: Movement away from the midline along the frontal plane (e.g., raising the arm sideways).

• Adduction: Movement toward the midline along the frontal plane (e.g., lowering the arm to the side).

• Circumduction: A circular movement that combines flexion, abduction, extension, and adduction, so the limb describes a cone in space.

Rotational Movements

Rotation is the turning of a bone around its own long axis, either toward or away from the midline.

• Medial rotation: Rotation toward the midline.

• Lateral rotation: Rotation away from the midline.

• Examples: Rotation between the first and second cervical vertebrae (C1 and C2), rotation of the humerus and femur.

Special Movements

• Supination and Pronation:

  • Supination: Rotation of the forearm so the palm faces anteriorly; radius and ulna are parallel.

  • Pronation: Rotation of the forearm so the palm faces posteriorly; radius rotates over ulna.

• Opposition: Movement of the thumb to touch the tips of other fingers on the same hand, enabling grasping.

• Dorsiflexion and Plantar Flexion (of the foot):

  • Dorsiflexion: Bending the foot toward the shin.

  • Plantar flexion: Pointing the toes downward.

• Inversion and Eversion (of the foot):

  • Inversion: Sole of the foot faces medially.

  • Eversion: Sole of the foot faces laterally.

• Elevation and Depression:

  • Elevation: Lifting a body part superiorly (e.g., shrugging shoulders).

  • Depression: Lowering a body part (e.g., opening the jaw).

• Protraction and Retraction:

  • Protraction: Moving a body part anteriorly (e.g., jutting the jaw forward).

  • Retraction: Moving a body part posteriorly (e.g., pulling the jaw back).

Types of Synovial Joints

Synovial joints are classified based on the shape of their articular surfaces and the types of movement they allow. There are six main types:

• Plane joints: Nonaxial, allow gliding movements (e.g., intercarpal joints).

• Hinge joints: Uniaxial, allow flexion and extension (e.g., elbow joint).

• Pivot joints: Uniaxial, allow rotation (e.g., proximal radioulnar joint).

• Condylar (ellipsoid) joints: Biaxial, allow flexion/extension and abduction/adduction (e.g., metacarpophalangeal joints).

• Saddle joints: Biaxial, allow similar movements as condylar joints but with greater freedom (e.g., carpometacarpal joint of the thumb).

• Ball-and-socket joints: Multiaxial, allow the greatest range of movement (e.g., shoulder and hip joints).

Summary Table: Types of Synovial Joints

Key Terms and Definitions

• Synovial joint: A freely movable joint characterized by a fluid-filled joint cavity.

• Bursae: Fluid-filled sacs that reduce friction between moving structures in a joint.

• Tendon sheaths: Elongated bursae that wrap around tendons, reducing friction.

Conclusion

Understanding the structure and function of synovial joints, as well as the types of movements they allow, is essential for comprehending human movement and joint health. Mastery of these concepts is foundational for further study in anatomy, physiology, and clinical practice.