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

Joints: An Overview

Definition and Function

Joints, also known as articulations, are sites where two or more bones meet. They play a crucial role in holding the skeleton together and providing mobility to the body.

• Hold bones together to maintain structural integrity.

• Allow for mobility by permitting various types of movement.

Classification of Joints

Joints are classified in two main ways:

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

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

Functional Classification of Joints

• Synarthroses: Immovable joints (e.g., sutures of the skull).

• Amphiarthroses: Slightly movable joints (e.g., intervertebral discs).

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

Structural Classification of Joints

• Fibrous joints: Bones joined by dense fibrous connective tissue; generally immovable.

• Cartilaginous joints: Bones joined by cartilage; immovable or slightly movable.

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

Fibrous Joints

General Features

Fibrous joints are united by dense fibrous connective tissue and lack a joint cavity. All are synarthrotic (immovable).

• Sutures: Found between cranial bones; allow for growth during youth and fuse in adulthood.

• Syndesmoses: Bones connected by ligaments; allow more movement than sutures but still limited (e.g., distal tibiofibular joint).

• Gomphoses: Peg-in-socket joints (e.g., tooth in alveolar socket, held by periodontal ligament).

Cartilaginous Joints

General Features

Cartilaginous joints are united by cartilage and lack a joint cavity. They are either synarthrotic or amphiarthrotic.

• Synchondroses: Bones united by hyaline cartilage; mostly immovable (e.g., epiphyseal plates, first rib and sternum).

• Symphyses: Bones united by fibrocartilage; slightly movable (e.g., pubic symphysis, intervertebral discs).

Synovial Joints

General Features

Synovial joints are the most common and movable type of joint in the body. They have a joint cavity filled with synovial fluid and are always diarthrotic (freely movable).

• Articular cartilage (hyaline) covers bone ends to prevent crushing.

• Joint (synovial) cavity contains synovial fluid.

• Articular capsule encloses the joint cavity, consisting of an outer fibrous layer and an inner synovial membrane.

• Synovial fluid lubricates and nourishes cartilage, contains phagocytes.

• Reinforcing ligaments (capsular, extracapsular, intracapsular) stabilize the joint.

• Nerves and blood vessels supply the joint, detect pain, and monitor position.

Additional Features in Some Synovial Joints

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

• Articular discs (menisci): Fibrocartilage pads that improve fit, stabilize, and reduce wear.

• Bursae: Fluid-filled sacs that reduce friction where structures rub together.

• Tendon sheaths: Elongated bursae that wrap around tendons.

Stabilizing Factors for Synovial Joints

• Shape of articular surfaces (minor role).

• Number and location of ligaments (limited role).

• Muscle tone (most important): Keeps tendons taut and stabilizes joints, especially in the shoulder, knee, and foot arches.

Key Synovial Joints: Structure and Stabilization

Knee Joint

The knee is the largest and most complex joint, consisting of three joints within a single cavity. It allows flexion, extension, and some rotation.

• Stabilized by muscle tendons (quadriceps, semimembranosus), ligaments (patellar, collateral, cruciate), and menisci.

• Contains at least 12 bursae.

• Vulnerable to injury, especially from lateral blows ("unhappy triad": torn tibial collateral ligament, medial meniscus, anterior cruciate ligament).

Shoulder Joint (Glenohumeral Joint)

A ball-and-socket joint with the greatest range of motion but less stability. Stabilized by the rotator cuff muscles, glenohumeral ligaments, and the coracohumeral ligament.

• Rotator cuff muscles: Subscapularis, supraspinatus, infraspinatus, teres minor.

• Glenoid labrum deepens the socket.

Elbow Joint

A hinge joint formed by the articulation of the humerus with the radius and ulna. Allows flexion and extension only.

• Stabilized by the anular ligament (encircles the head of the radius), ulnar collateral ligament, and radial collateral ligament.

Hip (Coxal) Joint

A deep ball-and-socket joint formed by the head of the femur and the acetabulum of the pelvis. It is highly stable due to the depth of the socket and strong ligaments.

• Acetabular labrum deepens the socket.

• Reinforced by iliofemoral, pubofemoral, ischiofemoral ligaments, and ligamentum teres.

Temporomandibular Joint (TMJ)

The TMJ is a modified hinge joint between the mandibular condyle and the temporal bone. It allows both hinge (depression/elevation) and gliding (side-to-side) movements. It is the most easily dislocated joint in the body.

Types of Synovial Joints by Shape and Movement

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

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

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

Movements Allowed by Synovial Joints

• Gliding: Flat bone surfaces slide over 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 long axis (e.g., atlas and axis, shoulder, hip).

• Special movements: Include supination/pronation (forearm), dorsiflexion/plantar flexion (foot), inversion/eversion (foot), protraction/retraction (jaw), elevation/depression (mandible), opposition (thumb).

Joint Injuries and Clinical Considerations

• Cartilage tears: Often involve menisci; poor healing due to avascularity.

• Sprains: Ligaments stretched or torn; slow healing.

• Dislocations (luxations): Bones forced out of alignment; require reduction.

• Bursitis: Inflammation of a bursa, usually from trauma or friction.

• Tendonitis: Inflammation of tendon sheaths, often from overuse.

• Lyme disease: Bacterial infection causing joint pain and arthritis.

Types of Arthritis

• Osteoarthritis (OA): Degenerative, wear-and-tear arthritis; most common in elderly.

• Rheumatoid arthritis (RA): Autoimmune, chronic inflammation of joints; often bilateral.

• Gouty arthritis: Uric acid crystal deposition in joints; more common in men.

Developmental Aspects of Joints

• By embryonic week 8, synovial joints resemble adult joints.

• Joint structure is modified by use and activity.

• Aging leads to shortening and weakening of ligaments/tendons, increased risk of herniated discs, and higher prevalence of osteoarthritis.

• Regular full-range-of-motion exercise helps maintain joint health.