Chapter 8: Joints – Structure, Classification, and Function

Joints (Articulations)

Introduction to Joints

Joints, also known as articulations, are the sites where two or more bones meet. They play a crucial role in providing the skeleton with mobility while also holding the skeleton together. Understanding the structure and function of joints is essential for comprehending movement and stability in the human body.

• Definition: A joint (articulation) is a point of contact between bones, or between bone and cartilage.

• Function: Joints allow for movement and provide mechanical support.

Classification of Joints

Structural Classification

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

• Fibrous Joints: Bones joined by dense fibrous connective tissue; no joint cavity.

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

• Synovial Joints: Bones separated by a fluid-filled joint cavity; freely movable.

Functional Classification

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

• Synarthroses: Immovable joints

• Amphiarthroses: Slightly movable joints

• Diarthroses: Freely movable joints

Additional info: Structural classification is more clear-cut and commonly used in anatomy.

Fibrous Joints

General Structure and Types

Fibrous joints are connected by dense fibrous connective tissue and lack a joint cavity. Most are immovable, though the degree of movement depends on the length of connective tissue fibers.

• Sutures: Rigid, interlocking joints found in the skull.

• Syndesmoses: Bones connected by ligaments; movement varies with fiber length.

• Gomphoses: Peg-in-socket joints, such as teeth in their sockets.

Sutures

Sutures are immovable joints that join skull bones into a single unit, protecting the brain. They contain short connective tissue fibers that allow for growth during youth. In middle age, sutures ossify and become synostoses (closed, immovable sutures).

• Example: Cranial sutures (e.g., coronal, sagittal, lambdoid sutures)

Syndesmoses

Syndesmoses are joints where bones are connected by ligaments. The length of the fibers determines the amount of movement possible.

• Short fibers: Little to no movement (e.g., inferior tibiofibular joint)

• Long fibers: Greater movement (e.g., interosseous membrane between radius and ulna)

Gomphoses

Gomphoses are peg-in-socket joints, such as the attachment of teeth to their sockets in the jaw. The connection is maintained by the periodontal ligament.

• Example: Tooth in alveolar socket

Cartilaginous Joints

General Structure and Types

Cartilaginous joints are united by cartilage and lack a joint cavity. They allow for more movement than fibrous joints but are generally not highly movable.

• Synchondroses: Bones united by hyaline cartilage; mostly immovable (synarthrotic).

• Symphyses: Bones united by fibrocartilage; strong, slightly movable (amphiarthrotic).

Synchondroses

Synchondroses are joints where hyaline cartilage unites the bones. Most are immovable.

• Example: Epiphyseal plate in growing children; joint between first rib and manubrium of sternum.

Symphyses

Symphyses are joints where fibrocartilage unites the bones, providing strength and flexibility. Hyaline cartilage is also present as articular cartilage on bony surfaces.

• Example: Intervertebral joints, pubic symphysis

Synovial Joints

General Structure

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

• Articular cartilage: Hyaline cartilage covering bone ends, preventing crushing.

• Joint (synovial) cavity: Small, fluid-filled space unique to synovial joints.

• Articular capsule: Two layers—external fibrous layer (dense connective tissue) and inner synovial membrane (loose connective tissue producing synovial fluid).

• Synovial fluid: Viscous, slippery filtrate of plasma and hyaluronic acid; lubricates and nourishes cartilage, contains phagocytes.

• Reinforcing ligaments: Capsular (thickened part of fibrous layer), extracapsular (outside capsule), intracapsular (deep to capsule).

• Nerves and blood vessels: Nerves detect pain and monitor joint position; capillaries supply filtrate for synovial fluid.

Other Features of Synovial Joints

• Fatty pads: Cushioning between fibrous layer and synovial membrane or bone.

• Articular discs (menisci): Fibrocartilage that improves fit, stabilizes joint, and reduces wear.

Bursae and Tendon Sheaths

Bursae and tendon sheaths are structures associated with synovial joints that reduce friction.

• Bursae: Fluid-filled sacs that act as lubricating ball bearings where ligaments, muscles, skin, tendons, or bones rub together.

• Tendon sheaths: Elongated bursae wrapped around tendons subject to friction.

Factors Influencing Stability of Synovial Joints

Stability Determinants

Three main factors determine the stability of synovial joints and help prevent dislocations:

• Shape of articular surface: Shallow surfaces are less stable than ball-and-socket joints.

• Ligament number and location: More ligaments provide stronger support.

• Muscle tone: Keeps tendons taut as they cross joints; most important for reinforcing shoulder, knee, and foot arches.

Movements Allowed by Synovial Joints

Types of Movement

Synovial joints allow various types of movement, classified by the number of axes:

• Uniaxial: Movement in one plane

• Biaxial: Movement in two planes

• Multiaxial: Movement in or around all three planes

General types of movement include:

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

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

• Rotation: Bone turns around its own axis (e.g., rotation of humerus or femur).

Special Movements

• Supination/Pronation: Rotation of radius and ulna (supination: palms anterior, pronation: palms posterior).

• Dorsiflexion/Plantar flexion: Movement of the foot (dorsiflexion: toes up, plantar flexion: toes down).

• Inversion/Eversion: Sole of foot faces medially (inversion) or laterally (eversion).

• Protraction/Retraction: Movement of mandible forward (protraction) or backward (retraction).

• Elevation/Depression: Lifting (elevation) or lowering (depression) a body part.

• Opposition: Movement of thumb to touch other fingers.

Types of Synovial Joints

Classification by Shape and Movement

Synovial joints are classified into six types based on the shape of their articular surfaces and the movements they allow:

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

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

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

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

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

• Ball-and-socket: Multiaxial, all movements (e.g., shoulder, hip).

Selected Synovial Joints

Jaw (Temporomandibular Joint, TMJ)

The TMJ is a modified hinge joint allowing both hinge (depression/elevation) and gliding (side-to-side) movements. The mandibular condyle articulates with the temporal bone. The joint capsule thickens into a strong lateral ligament, but the TMJ is the most easily dislocated joint in the body due to its shallow socket.

• Movements: Opening/closing mouth, grinding teeth

• Homeostatic imbalance: Dislocation causes pain, muscle tenderness, and difficulty closing mouth.

Shoulder (Glenohumeral Joint)

The shoulder is the most freely moving joint in the body, sacrificing stability for mobility. It is a ball-and-socket joint with a large humeral head fitting into a shallow glenoid cavity of the scapula. The joint capsule is thin and loose, and stability is provided by ligaments and muscle tendons (especially the rotator cuff).

• Movements: Flexion, extension, abduction, adduction, rotation, circumduction

• Homeostatic imbalance: Dislocations are common, especially anteriorly and inferiorly.

Elbow Joint

The elbow is a hinge joint formed primarily by the trochlear notch of the ulna articulating with the trochlea of the humerus. It allows for flexion and extension only. Ligaments (ulnar and radial collateral) provide stability, and the annular ligament surrounds the head of the radius.

• Movements: Flexion and extension

Hip (Coxal Joint)

The hip is a ball-and-socket joint with a deep socket (acetabulum) that provides great stability but less range of motion than the shoulder. The acetabular labrum adds depth, and strong ligaments reinforce the joint.

• Movements: Flexion, extension, abduction, adduction, rotation

• Ligaments: Iliofemoral, pubofemoral, ischiofemoral

Knee Joint

The knee is the largest and most complex joint, consisting of three joints in one cavity: femoropatellar (plane joint), and lateral/medial tibiofemoral joints (hinge joints). It allows flexion, extension, and some rotation. Stability is provided by menisci, collateral and cruciate ligaments.

• Movements: Flexion, extension, slight rotation

• Ligaments: Patellar, fibular/tibial collateral, anterior/posterior cruciate

• Homeostatic imbalance: Common injuries involve the "unhappy triad"—ACL, tibial collateral ligament, and medial meniscus.

Common Joint Injuries and Disorders

Joint Injuries

• Cartilage tears: Often require arthroscopic surgery; removal may lead to instability and osteoarthritis.

• Sprains: Ligaments are stretched or torn; may require sewing, grafts, or immobilization.

• Dislocations (luxations): Bones forced out of alignment; subluxation is partial dislocation.

Inflammatory and Degenerative Conditions

• Bursitis: Inflammation of bursae, usually from trauma or friction; treated with rest and anti-inflammatory drugs.

• Tendonitis: Inflammation of tendon sheaths, typically from overuse; similar treatment to bursitis.

• Osteoarthritis: Irreversible, degenerative "wear-and-tear" arthritis.

• Rheumatoid arthritis: Chronic, inflammatory, autoimmune disease.

• Gouty arthritis: Deposition of uric acid crystals in joints, causing inflammation.

Summary Table: Structural and Functional Classification of Joints

Structural Type	Functional Type	Movement Allowed	Examples
Fibrous (Sutures)	Synarthroses	Immovable	Skull sutures
Fibrous (Syndesmoses)	Amphiarthroses	Slightly movable	Distal tibiofibular joint
Fibrous (Gomphoses)	Synarthroses	Immovable	Teeth in sockets
Cartilaginous (Synchondroses)	Synarthroses	Immovable	Epiphyseal plate, first rib-sternum
Cartilaginous (Symphyses)	Amphiarthroses	Slightly movable	Intervertebral discs, pubic symphysis
Synovial	Diarthroses	Freely movable	Shoulder, hip, knee, elbow

Key Equations and Terms

• Synovial Fluid Production: $\text{Synovial fluid} = \text{Plasma} + \text{Hyaluronic acid}$

• Types of Movement: $\text{Flexion},\ \text{Extension},\ \text{Abduction},\ \text{Adduction},\ \text{Rotation},\ \text{Circumduction}$