Chapter 8: Joints (Articulations) – Structure, Function, and Movement

Joints (Articulations): Structure, Function, and Classification

Introduction to Joints

Joints, or articulations, are anatomical structures where two or more bones meet. They are essential for holding the skeleton together and providing mobility. Understanding joints is crucial for diagnosing and treating musculoskeletal injuries.

Classification of Joints

Joints are classified based on their structure and function:

• Structural classification: Based on the material binding bones together and the presence or absence of a joint cavity.

• Functional classification: Based on the amount of movement allowed at the joint.

Structural Types:

• Fibrous joints: Bones joined by fibrous tissue; no joint cavity.

• Cartilaginous joints: Bones joined by cartilage; no joint cavity.

• Synovial joints: Bones separated by a fluid-filled joint cavity.

Functional Types:

• Synarthroses: Immovable joints (mostly fibrous).

• Amphiarthroses: Slightly movable joints (mostly cartilaginous).

• Diarthroses: Freely movable joints (all synovial).

Fibrous Joints

General Features

Fibrous joints are connected by dense connective tissue, primarily collagen fibers. They lack a joint cavity and are mostly immovable (synarthrotic).

Types of Fibrous Joints

• Sutures: Found only in the skull; bones are tightly bound by minimal fibrous tissue. With age, sutures may ossify and become synostoses (bony junctions).

• Gomphoses: Peg-in-socket joints, such as teeth anchored in their sockets by the periodontal ligament.

• Syndesmoses: Bones connected by ligaments; movement depends on the length of the connecting fibers. Examples include the distal tibiofibular joint (little movement) and the interosseous membrane between the radius and ulna (more movement).

Cartilaginous Joints

General Features

Cartilaginous joints unite bones with cartilage and lack a joint cavity. They allow little to no movement.

Types of Cartilaginous Joints

• Synchondroses: Bones united by hyaline cartilage. Most are immovable (synarthrotic). Examples include the epiphyseal plates in growing bones and the joint between the first rib and the sternum.

• Symphyses: Bones united by fibrocartilage, which acts as a shock absorber and allows limited movement (amphiarthrotic). Examples include intervertebral discs and the pubic symphysis.

Synovial Joints

General Features

Synovial joints are characterized by a fluid-filled joint cavity and are freely movable (diarthrotic). They are the most common type of joint in the body, especially in the limbs.

Six Distinguishing Features of Synovial Joints

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

• Joint (articular) cavity: Space containing synovial fluid.

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

• Articular (joint) capsule: Two-layered capsule enclosing the joint cavity (outer fibrous layer and inner synovial membrane).

• Reinforcing ligaments: Strengthen and stabilize the joint (capsular, extracapsular, and intracapsular types).

• Nerves and blood vessels: Supply the joint, detect pain, and monitor joint position and stretch.

Other Structural Features

• Fatty pads: Cushioning structures between the fibrous layer and bone or synovial membrane (e.g., knee and hip joints).

• Articular discs (menisci): Fibrocartilage pads that improve the fit of bone ends, stabilize the joint, and reduce wear and tear.

• Bursae: Fluid-filled sacs that reduce friction where ligaments, muscles, skin, or tendons rub against bone.

• Tendon sheaths: Elongated bursae that wrap around tendons to decrease friction.

Stability of Synovial Joints

Three main factors influence the stability of synovial joints:

• Shape of articular surfaces: Minor role in stability.

• Ligament number and location: Limited role; more ligaments generally increase stability.

• Muscle tone: Most important factor; keeps tendons taut and stabilizes joints, especially in the shoulder, knee, and foot arches.

Movements at Synovial Joints

General Principles

All muscles attach to bone at two points: the origin (immovable) and the insertion (movable). Muscle contraction moves the insertion toward the origin, producing movement at the joint. Movements occur along transverse, frontal, or sagittal planes.

Types of Movement

• Gliding: Flat bone surfaces slide past each other (e.g., intercarpal joints).

• Angular movements: Change the angle between bones. Includes flexion, extension, hyperextension, abduction, adduction, and circumduction.

• Rotation: Turning a bone around its own long axis (e.g., between atlas and axis, shoulder, hip).

Movement	Definition
Gliding	Sliding the flat surfaces of two bones across each other
Flexion	Decreasing the angle between two bones, usually in the sagittal plane
Extension	Increasing the angle between two bones, usually in the sagittal plane
Abduction	Moving a limb away from the body midline in the frontal plane
Adduction	Moving a limb toward the body midline in the frontal plane
Circumduction	Moving a limb or finger so that it describes a cone in space
Rotation	Turning a bone around its longitudinal axis
Medial rotation	Rotating toward the median plane
Lateral rotation	Rotating away from the median plane

Range of Motion

• Nonaxial: Slipping movements only (plane joints).

• Uniaxial: Movement in one plane (hinge and pivot joints).

• Biaxial: Movement in two planes (condylar and saddle joints).

• Multiaxial: Movement in or around all three planes (ball-and-socket joints).

Types of Synovial Joints

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

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

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

• Condylar joint: Biaxial, flexion/extension and abduction/adduction (e.g., knuckle joints).

• Saddle joint: Biaxial, similar to condylar but with greater movement (e.g., thumb carpometacarpal joint).

• Ball-and-socket joint: Multiaxial, most freely moving (e.g., shoulder, hip).

Selected Synovial Joints: The Knee Joint

Structure and Function

The knee is the largest and most complex joint in the body, consisting of three joints in one cavity: the femoropatellar joint and the lateral and medial tibiofemoral joints. It is a modified hinge joint, allowing flexion, extension, and some rotation.

Ligaments and Stability

• Capsular and extracapsular ligaments: Prevent hyperextension and stabilize the joint (e.g., tibial and fibular collateral ligaments).

• Intracapsular ligaments: Cruciate ligaments (anterior and posterior) prevent anterior-posterior displacement of the tibia and femur.

• Menisci: Medial and lateral fibrocartilage pads that improve fit and absorb shock.

Common Knee Injuries

The knee is vulnerable to horizontal or lateral blows, especially in sports. Common injuries involve the three C's: collateral ligaments, cruciate ligaments, and cartilages (menisci).

Summary Table: Types of Synovial Joints and Movements

Joint Type	Movement	Example
Plane	Nonaxial (gliding)	Intercarpal joints
Hinge	Uniaxial (flexion/extension)	Elbow, knee
Pivot	Uniaxial (rotation)	Proximal radioulnar joint
Condylar	Biaxial (flexion/extension, abduction/adduction)	Knuckle joints
Saddle	Biaxial (greater movement)	Thumb carpometacarpal joint
Ball-and-socket	Multiaxial (all movements)	Shoulder, hip

Key Equations:

• Range of motion (ROM): The degree through which a joint can move, measured in degrees of a circle.

Additional info: Understanding joint structure and function is essential for clinical assessment and treatment of joint injuries, such as sprains, dislocations, and degenerative diseases like arthritis.