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

Joints (Articulations)

Introduction to Joints

Joints, or articulations, are sites where two or more bones meet. They play a crucial role in holding the skeleton together and providing mobility. Understanding the structure and function of joints is essential for diagnosing and treating joint injuries, such as ankle sprains.

Classification of Joints

Structural Classification

Joints are classified structurally based on the presence or absence of a joint cavity and the material binding the bones together:

• Fibrous Joints: No joint cavity; bones connected by fibrous tissue.

• Cartilaginous Joints: No joint cavity; bones connected by cartilage.

• Synovial Joints: Joint cavity present; bones connected by a fluid-filled cavity.

Functional Classification

Joints are also classified by the amount of movement they allow:

• Synarthroses: Immovable joints (most fibrous joints).

• Amphiarthroses: Slightly movable joints (some cartilaginous joints).

• Diarthroses: Freely movable joints (all synovial joints).

Fibrous Joints

General Features

Fibrous joints are mostly synarthrotic (immovable) and lack a joint cavity. Bones are connected by collagen fibers. There are three main types:

• Gomphosis

• Suture

• Syndesmosis

Gomphosis

A peg-in-socket fibrous joint, found only in the teeth. The periodontal ligament holds the tooth in its socket.

Suture

Sutures are joints held together with very short, interconnecting fibers, and bone edges interlock. They are found only in the skull. With age, sutures may ossify and become synostoses (bony junctions).

Syndesmosis

Bones are connected by a ligament. The amount of movement depends on the length of the ligament. 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 lack a joint cavity and unite bones with cartilage. They are either synarthrotic or amphiarthrotic. Two main types are:

• Synchondroses: Bones united by hyaline cartilage.

• Symphyses: Bones united by fibrocartilage.

Synchondroses

These joints are united by hyaline cartilage and are typically immovable. Examples include the epiphyseal plates in growing bones and the joint between the first rib and the sternum.

Symphyses

Symphyses are strong, flexible, and amphiarthrotic joints where bones are united by fibrocartilage. Examples include the intervertebral discs and the pubic symphysis.

Synovial Joints

General Features

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

Six Distinguishing Features of Synovial Joints

• Articular cartilage: Hyaline cartilage covering bone surfaces.

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

• Synovial fluid: 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: Capsular, extracapsular, and intracapsular ligaments strengthen the joint.

• Blood vessels and nerves: Supply nutrients 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.

Friction-Reducing Structures

• 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

Factors Influencing Stability

• Shape of articular surfaces: Minor role in stability.

• Ligament number and location: Limited role.

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

Movements Allowed by Synovial Joints

Attachment Points and Movement

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

Range of Motion

• Nonaxial: Slipping movements only.

• Uniaxial: Movement in one plane.

• Biaxial: Movement in two planes.

• Multiaxial: Movement in or around all three planes.

Types of Movements

• Gliding: Sliding flat surfaces of bones across each other.

• Angular movements: Increase or decrease the angle between two bones (flexion, extension, hyperextension, abduction, adduction, circumduction).

• Rotation: Turning a bone around its own long axis (medial and lateral rotation).

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

Types of Synovial Joints

Six 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, movement in all axes (e.g., shoulder, hip).

Selected Synovial Joints: The Knee Joint

Structure and Function

The knee joint 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 provide stability (e.g., tibial and fibular collateral ligaments).

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

• Menisci: Medial and lateral menisci improve fit, stabilize, and absorb shock.

Common Knee Injuries

The knee absorbs significant vertical force but is vulnerable to horizontal or lateral blows. 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 joint
Ball-and-socket	Multiaxial (all movements)	Shoulder, hip