JOINTS

Joints and Movement Overview

  • Joints are integral to the musculoskeletal system, providing structural stability and mobility.

8.1 Classes of Joints

Structural Classification

  • Based on the major connective tissue type binding the bones:

    • Fibrous

    • Cartilaginous

    • Synovial

Functional Classification

  • Based on the degree of motion:

    • Synarthrosis: non-movable joints.

    • Amphiarthrosis: slightly movable joints.

    • Diarthrosis: freely movable joints.

8.2 Fibrous Joints

Characteristics

  • United by fibrous connective tissue.

  • No joint cavity present.

  • Exhibit little to no movement.

Types of Fibrous Joints

  1. Sutures

    • Opposing bones of the skull interdigitate.

    • Continuous periosteum between bones.

    • Sutural ligament: two periostea plus dense connective tissue in between.

    • May ossify to form synostosis in adults.

    • Fontanels: membranous regions between skull bones that allow for changes in head shape during childbirth and rapid brain growth post-birth.

  2. Syndesmoses

    • Bones are further apart and joined by ligaments.

    • Some movement possible, e.g., radioulnar joint connected by interosseous membrane.

  3. Gomphoses

    • Specialized joints where pegs fit into sockets.

    • Periodontal ligaments hold teeth in place.

    • Inflammation can lead to gingivitis and periodontal disease.

8.3 Cartilaginous Joints

Characteristics

  • Unite two bones by a pad of cartilage.

Types of Cartilaginous Joints

  1. Synchondroses

    • Contain hyaline cartilage.

    • Little or no movement; some are temporary and replaced by synostoses.

    • Examples include epiphyseal plates, sternocostal joints, and sphenooccipital joints.

  2. Symphyses

    • Featuring fibrocartilage that connects two bones.

    • Slightly movable; examples include the symphysis pubis, intervertebral disks, and the joint between the manubrium and body of the sternum.

8.4 Synovial Joints

General Characteristics

  • Contain synovial fluid.

  • Allow a considerable range of movement.

  • Most joints in the appendicular skeleton are synovial due to their greater mobility compared to axial skeleton joints.

Structure of Synovial Joints

  • Articular Cartilage: Hyaline cartilage providing a smooth surface.

  • Joint Cavity: Synovial cavity that encloses articular surfaces.

  • Capsule: Comprised of:

    • Fibrous Capsule: Dense irregular connective tissue, continuous with periosteum; may thicken to form ligaments.

    • Synovial Membrane: Lines inside of joint capsule (except at articular surfaces); secretes synovial fluid.

  • Synovial Fluid: Mixture of polysaccharides, proteins, fats, and cells; hyaluronic acid provides slipperiness.

  • Nerves in Capsule: Provide proprioceptive feedback to the brain regarding joint positions.

Accessory Structures of Synovial Joints

  • Bursae: Pockets of synovial membrane that extend from the joint, reducing friction.

  • Ligaments and Tendons: Stabilize and support the joint.

  • Articular Discs: Present in some joints like the temporomandibular and sternoclavicular joints.

  • Menisci: Fibrocartilaginous pads in the knee.

  • Tendon Sheaths: Enclose tendons as they pass over joints.

8.5 Types of Synovial Joints

Six Types of Synovial Joints

  1. Plane Joints: Example: Intervertebral joints (uniaxial, slight movement).

  2. Saddle Joints: Example: Carpometacarpal joint of the thumb (biaxial movement).

  3. Hinge Joints: Example: Elbow joint (uniaxial - flexion/extension).

  4. Pivot Joints: Example: Atlantoaxial joint (uniaxial rotation).

  5. Ball-and-Socket Joints: Example: Shoulder and hip joints (multiaxial).

  6. Ellipsoid Joints: Example: Atlantooccipital joint (biaxial).

8.6 Types of Movement

Key Movements of Joints

  1. Gliding: Occurs in plane joints; slight movement.

  2. Angular Movements: Angle between bones changes:

    • Flexion: Movement anterior to the coronal plane.

    • Extension: Movement posterior to the coronal plane.

    • Hyperextension: Beyond normal extension.

    • Plantar Flexion: Standing on toes.

    • Dorsiflexion: Foot lifted towards the shin.

    • Abduction: Movement away from the midline.

    • Adduction: Movement towards the midline.

  3. Circular Movements: Rotation around an axis:

    • Rotation: Turning around a long axis (e.g., head, arm).

    • Pronation: Palm facing posteriorly.

    • Supination: Palm facing anteriorly.

    • Circumduction: Appendage describes a cone.

8.7 Special Movements

Unique Movements

  • Elevation/Depression: Movement superior/inferior (e.g., shrugging).

  • Protraction/Retration: Gliding motion anterior/posterior at jaw or scapula.

  • Opposition/Reposition: Movement of thumb and little finger towards and away from each other.

  • Inversion/Eversion: Turning foot ankle medially/laterally.

8.8 Range of Motion**

  • Amount of mobility demonstrated at a joint:

    • Active: Movement via muscle contraction.

    • Passive: Movement by external force.

  • Influenced by:

    • Joint structure, cartilage shape, ligament strength, muscle bulk, fluid levels, and joint usage.

8.9 Abnormalities in Range of Motion

  • Dislocation (Luxation): Misalignment of articulating surfaces.

  • Subluxation: Partial dislocation.

  • Sprain: Damage to ligaments due to overstretching.

8.10 Description of Selected Joints

1. Temporomandibular Joint (TMJ)

  • Combination of plane and ellipsoid joint.

  • Fibrocartilage disk divides joint into superior/inferior cavities.

  • Allowed movements include: depression, elevation, excursion, protraction, and retraction.

  • Disorders can lead to chronic orofacial pain.

2. Shoulder Joint (Glenohumeral)

  • Ball-and-socket joint allowing for high mobility but less stability.

  • Movements: flexion, extension, abduction, adduction, rotation, circumduction.

  • Features glenoid labrum, bursae, and rotator cuff for stability.

3. Elbow Joint

  • Compound hinge joint comprising the humeroulnar, humeroradial, and proximal radioulnar joints.

  • Movement limits: mainly extension and flexion; allows pronation and supination.

4. Hip Joint (Coxal)

  • Ball-and-socket joint with acetabulum deepened by a fibrocartilage labrum.

  • More stable yet less mobile than the shoulder joint.

5. Knee Joint

  • Condyloid joint permitting flexion, extension, and small rotational movements.

  • Includes menisci and cruciate ligaments for stability and movement control.

6. Ankle Joint (Talocrural)

  • Highly modified hinge joint allowing dorsiflexion and plantar flexion; some inversion and eversion.

8.11 Effects of Aging on Joints

  • Slower tissue repair and reduced blood vessel development.

  • Articular cartilage wears down becoming more rigid.

  • Decline in synovial fluid production.

  • Ligaments/tendons shorten and lose flexibility, reducing range of motion.

  • Muscle weakness around joints.

8.12 Representative Joint Disorders

  1. Arthritis: Conditions like osteoarthritis (wear and tear) and rheumatoid arthritis (autoimmune).

  2. Gout: Metabolic disorder due to increased uric acid leading to joint and kidney issues.

  3. Tendinitis: Inflammation of tendon sheaths due to overuse.

  4. Sprain: Damage to joint ligaments.

  5. Dislocation: Misalignment of bones in a joint.