Chapter 9: Skeletal System – Articulations

Key Topics to Know

• Structural vs. Functional Joint Classification

• Synovial Joint Components

• Movement Types

• Synovial Joint Types

• Knee Joint Anatomy

• Levers in the Body

Joint Classification

Structural and Functional Classification of Joints

Joints, or articulations, are classified based on their structure and function. Structural classification is determined by the material binding the bones together and whether a joint cavity is present. Functional classification is based on the amount of movement the joint permits.

• Structural Classification:

  • Fibrous Joints: Bones joined by dense connective tissue; no joint cavity (e.g., sutures of the skull).

  • Cartilaginous Joints: Bones joined by cartilage; no joint cavity (e.g., intervertebral discs).

  • Synovial Joints: Bones separated by a fluid-filled joint cavity; most movable type (e.g., knee, shoulder).

• Functional Classification:

  • Synarthrosis: Immovable joint (e.g., sutures).

  • Amphiarthrosis: Slightly movable joint (e.g., pubic symphysis).

  • Diarthrosis: Freely movable joint (e.g., most limb joints).

Synovial Joints

Components of Synovial Joints

Synovial joints are the most common and movable type of joint in the body. They have several key components:

• Articular Cartilage: Hyaline cartilage covering bone surfaces to reduce friction.

• Joint (Synovial) Cavity: Space filled with synovial fluid.

• Articular Capsule: Double-layered capsule enclosing the joint cavity.

• Synovial Fluid: Viscous fluid that lubricates and nourishes the joint.

• Ligaments: Bands of dense connective tissue that reinforce joints.

• Nerves and Blood Vessels: Supply the joint for sensation and nutrition.

Types of Synovial Joints

Synovial joints are classified by the shapes of their articulating surfaces and the movements they allow:

• Plane (Gliding) Joint: Allows sliding movements (e.g., intercarpal joints).

• Hinge Joint: Permits flexion and extension (e.g., elbow).

• Pivot Joint: Allows rotation (e.g., atlantoaxial joint).

• Condyloid (Ellipsoid) Joint: Allows movement in two planes (e.g., wrist).

• Saddle Joint: Allows greater movement; both surfaces are concave and convex (e.g., thumb carpometacarpal joint).

• Ball-and-Socket Joint: Allows movement in all axes (e.g., shoulder, hip).

Movement Types

Types of Movements at Synovial Joints

• Flexion and Extension: Decreasing or increasing the angle between bones.

• Abduction and Adduction: Movement away from or toward the midline.

• Rotation: Turning a bone around its own long axis.

• Circumduction: Circular movement combining flexion, extension, abduction, and adduction.

• Special Movements: Include supination, pronation, inversion, eversion, protraction, retraction, elevation, and depression.

Knee Joint Anatomy

Structure of the Knee Joint

The knee is the largest and most complex synovial joint in the body. It is a hinge joint that allows flexion and extension, as well as slight rotation.

• Articular Surfaces: Femur, tibia, and patella.

• Menisci: C-shaped fibrocartilage pads that improve fit and absorb shock.

• Ligaments: Include anterior and posterior cruciate ligaments (ACL, PCL), medial and lateral collateral ligaments (MCL, LCL).

• Synovial Membrane and Fluid: Lubricate and nourish the joint.

Levers in the Human Body

Classes of Levers

Levers are rigid structures that move around a fixed point (fulcrum) when a force is applied. The human body uses three classes of levers to facilitate movement:

• First-Class Lever: Fulcrum is between the effort and the load (e.g., neck extension).

• Second-Class Lever: Load is between the fulcrum and the effort (e.g., standing on tiptoe).

• Third-Class Lever: Effort is between the fulcrum and the load (e.g., biceps curl).

Ligaments vs. Tendons

Differences Between Ligaments and Tendons

• Ligaments: Connect bone to bone; provide joint stability.

• Tendons: Connect muscle to bone; transmit force to move bones.

Sample Questions and Answers

Multiple Choice Questions

1. The functional classification of a freely movable joint is:

   • A. Synarthrosis

   • B. Amphiarthrosis

   • C. Diarthrosis (Correct Answer)

   • D. Synchondrosis

2. The fluid that reduces friction in a joint is:

   • A. Interstitial fluid

   • B. Synovial fluid (Correct Answer)

   • C. Periosteal fluid

   • D. Cartilage secretion

3. Which joint is an example of a saddle joint?

   • A. Hip

   • B. Thumb (carpometacarpal) (Correct Answer)

   • C. Knee

   • D. Shoulder

Short Answer Questions and Key

1. Structural classification is based on anatomy; functional classification is based on movement permitted.

2. More mobility results in less stability; stable joints (e.g., sutures) move less.

3. Ligaments connect bone to bone; tendons connect muscle to bone.

4. First class: neck; second class: calf raise; third class: biceps curl.

Summary Table: Joint Types and Examples

Key Formula

Levers in the body can be described by the law of the lever:

Additional info: This study guide covers the essential concepts of joint classification, synovial joint structure, movement types, and the biomechanics of levers, which are foundational for understanding the musculoskeletal system in Anatomy & Physiology.