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

Joints (Articulations)

Overview and Purpose of Joints

Joints, or articulations, are the sites where two or more bones meet. They serve two primary functions: providing the skeleton with mobility and holding the skeleton together. Despite being the weakest parts of the skeleton, joints are essential for movement and stability.

• Mobility: Allow movement of the skeleton.

• Stability: Hold bones together, sometimes at the expense of movement.

Classification of Joints

Structural Classification

Joints are structurally classified 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; most movable type.

Functional Classification

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

• Synarthroses: Immovable joints.

• Amphiarthroses: Slightly movable joints.

• Diarthroses: Freely movable joints.

Fibrous Joints

General Features

Fibrous joints are connected by dense fibrous connective tissue and lack a joint cavity. Most are immovable, but some allow slight movement depending on the length of the connective fibers.

• Sutures: Seams between skull bones; immovable (synarthroses). Ossify in middle age to become synostoses.

• Syndesmoses: Bones connected by ligaments. Short fibers (e.g., tibia-fibula) allow little movement; longer fibers (e.g., radius-ulna) allow more movement.

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

Cartilaginous Joints

General Features

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

• Synchondroses: Bones united by a bar or plate of hyaline cartilage (e.g., epiphyseal plate, first rib-sternum joint); all are immovable.

• Symphyses: Bones united by fibrocartilage, which acts as a shock absorber and allows limited movement (e.g., intervertebral joints, pubic symphysis).

Synovial Joints

General Features

Synovial joints are characterized by a joint cavity filled with synovial fluid and are all freely movable (diarthroses). They include nearly all limb joints.

• Articular Cartilage: Hyaline cartilage covering bone surfaces to absorb compression and prevent crushing.

• Joint (Synovial) Cavity: Space containing synovial fluid.

• Articular Capsule: Double-layered capsule with an outer fibrous layer (strength) and inner synovial membrane (produces synovial fluid).

• Synovial Fluid: Viscous, egg-white-like fluid that lubricates and nourishes cartilage; thins during activity for reduced friction (weeping lubrication).

• Reinforcing Ligaments: Capsular (thickened fibrous layer), extracapsular (outside capsule), or intracapsular (inside capsule but outside synovial cavity).

• Nerves and Blood Vessels: Nerves monitor pain and joint position; blood vessels supply nutrients.

• Other Features: Fatty pads (hip, knee), articular discs/menisci (knee, jaw) for stability and fit, bursae and tendon sheaths to reduce friction.

Bursae and Tendon Sheaths

• Bursae: Flattened sacs lined with synovial membrane, reduce friction where structures rub together.

• Tendon Sheaths: Elongated bursae that wrap around tendons, common in crowded areas (e.g., wrist).

Factors Influencing Synovial Joint Stability

• Articular Surface Shape: Minor role in stability.

• Ligament Number: More ligaments increase stability.

• Muscle Tone: Most important factor; low-level muscle contraction keeps tendons taut.

Movements at Synovial Joints

Types of Movement

• Nonaxial: Slipping movements only (plane joints).

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

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

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

Specific Movements

• Gliding: Flat bone surfaces slide over each other (intercarpal, intertarsal joints).

• Angular Movements: Change the angle between bones.

  • Flexion: Decreases joint angle (e.g., bending elbow).

  • Extension: Increases joint angle (straightening).

  • Hyperextension: Extension beyond anatomical position.

  • Abduction: Movement away from midline.

  • Adduction: Movement toward midline.

  • Circumduction: Limb describes a cone in space (shoulder, hip).

• Rotation: Bone turns around its own long axis (e.g., atlas-axis, shoulder, hip).

• Special Movements:

  • Supination/Pronation: Rotation of forearm (palm up/down).

  • Dorsiflexion/Plantar Flexion: Upward/downward movement of foot at ankle.

  • Inversion/Eversion: Sole of foot turns medially/laterally.

  • Protraction/Retraction: Anterior/posterior movement (e.g., jaw).

  • Elevation/Depression: Lifting/lowering a body part (e.g., shoulders, jaw).

  • Opposition: Thumb touches tips of other fingers.

Structural Categories of Synovial Joints

Joint Type	Articular Surfaces	Movement	Examples
Plane	Flat	Nonaxial (gliding)	Intercarpal, intertarsal joints
Hinge	Cylinder and trough	Uniaxial (flexion/extension)	Elbow, knee, interphalangeal joints
Pivot	Sleeve and axle	Uniaxial (rotation)	Proximal radioulnar, atlantoaxial joints
Condylar	Oval	Biaxial (flexion/extension, abduction/adduction)	Metacarpophalangeal joints
Saddle	Concave and convex	Biaxial (flexion/extension, abduction/adduction)	Carpometacarpal joint of thumb
Ball-and-socket	Spherical head and cup	Multiaxial (all movements)	Shoulder, hip joints

Major Synovial Joints

Knee Joint

The knee is the largest and most complex joint, consisting of three joints in one: femoropatellar (plane), and lateral and medial tibiofemoral (hinge with some rotation). It is heavily reinforced by tendons and ligaments.

• Menisci: C-shaped fibrocartilage pads that deepen the tibial surface and absorb shock.

• Ligaments: Patellar ligament (anterior), fibular and tibial collateral ligaments (prevent lateral/medial rotation), oblique and arcuate popliteal ligaments (posterior stability), anterior and posterior cruciate ligaments (prevent anterior/posterior displacement).

• Bursae: At least a dozen present to reduce friction.

• Common Injuries: Collateral/cruciate ligament tears, meniscal injuries, especially from lateral blows or sudden direction changes.

Shoulder Joint

The shoulder (glenohumeral) joint is a ball-and-socket joint with the greatest range of motion. The shallow glenoid cavity is deepened by the glenoid labrum, but stability is mainly provided by the rotator cuff muscles and tendons.

• Ligaments: Coracohumeral and glenohumeral ligaments (weak).

• Rotator Cuff: Four muscle tendons encircle the joint for stability.

• Common Injuries: Dislocations and rotator cuff tears from excessive circumduction.

Elbow Joint

The elbow is a hinge joint allowing flexion and extension. The articular capsule is thin anteriorly and posteriorly, with strong collateral ligaments on the sides.

• Ligaments: Ulnar and radial collateral ligaments restrict side-to-side movement.

• Anular Ligament: Encircles the head of the radius.

Hip Joint

The hip is a ball-and-socket joint with a deep acetabulum, stabilized by strong ligaments and the acetabular labrum.

• Ligaments: Iliofemoral, pubofemoral, and ischiofemoral ligaments stabilize the femur.

• Ligamentum Teres: Ligament of the femur head; not a major stabilizer.

Temporomandibular Joint (TMJ)

The TMJ is where the mandible meets the temporal bone. It allows depression, elevation, and gliding movements.

• Articular Surfaces: Mandibular fossa and articular tubercle.

• Ligament: Lateral ligament provides stability.

Joint Disorders

Common Disorders

• Torn Cartilage: Usually refers to meniscal tears; poor healing, may require surgical removal (arthroscopy).

• Sprains: Stretched or torn ligaments; treatment may include sewing, grafts, or immobilization.

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

• Bursitis: Inflammation of bursae, often from trauma or friction; treated with rest, anti-inflammatories, or fluid removal.

• Tendonitis: Inflammation of tendons from overuse; treated with rest and anti-inflammatories.

Arthritis

• Osteoarthritis: Degeneration of articular cartilage, formation of bony spurs; usually irreversible.

• Rheumatoid Arthritis: Chronic autoimmune inflammation of synovial membrane, leading to scar tissue and joint deformity.

• Gouty Arthritis: Uric acid crystal accumulation in joints; can cause joint fusion.

• Lyme Disease: Bacterial infection from ticks; can cause joint inflammation.

Example: The knee joint is a classic example of a complex synovial joint, combining hinge and gliding movements, stabilized by multiple ligaments and menisci, and commonly affected by sports injuries.

Additional info: In clinical practice, joint injuries are often diagnosed with imaging (MRI, X-ray) and treated with a combination of physical therapy, medication, and sometimes surgery. Prevention includes proper training, equipment, and conditioning.