Skeletal System: Articulations

Chapter 09: Skeletal System - Articulations Anatomy & Physiology

9.1 Classification of Joints

  • Joints (articulations):

    • Definition: Places of union between bones, bones and cartilage, or bones and teeth.

    • Function: Bones articulate at a joint.

    • Arthrology: Study of joints.

    • Classification methods: By structure and function.

Structural Classes

  1. Fibrous Joint

    • Definition: Bones held together by dense connective tissue.

  2. Cartilaginous Joint

    • Definition: Bones joined by cartilage.

  3. Synovial Joint

    • Definition: Bones joined by ligaments with fluid-filled joint cavity separating the bone surfaces.

Functional Classes

  1. Synarthroses

    • Definition: Immobile joints. Can be either fibrous or cartilaginous joints.

  2. Amphiarthroses

    • Definition: Slightly mobile joints. Can be either fibrous or cartilaginous joints.

  3. Diarthroses

    • Definition: Freely mobile joints. All synovial joints.

Range of Motion at Joints

  • Motion ranges from no movement to extensive movement.

  • The structure of each joint determines its mobility and stability:

    • Inverse Relationship: There is a tradeoff between mobility and stability.

    • Example: Skull sutures are immobile but very stable.

Joint Classification by Structure

  • No Joint Cavity

    • Synarthroses (immobile) or Amphiarthroses (slightly mobile)

    • Fibrous Joints: Dense regular connective tissue binds bones.

    • Cartilaginous Joints: Cartilage between bones.

  • Joint Cavity

    • Diarthroses (freely mobile)

    • Synovial Joints: Capsule lined by synovial membrane.

Types of Joints

  • Fibrous Joints Types:

    • Gomphosis

    • Suture

    • Syndesmosis

  • Cartilaginous Joints Types:

    • Synchondrosis

    • Symphysis

  • Synovial Joints Types:

    • Plane, Hinge, Pivot, Condylar, Saddle, Ball-and-socket.

9.2 Fibrous Joints

  • Characteristics of Fibrous Joints:

    • Connected by dense regular connective tissue.

    • No joint cavity present.

    • Immobile or only slightly mobile.

    • Three common types:

    1. Gomphoses:

      • Description: “Peg in a socket.”

      • Example: Articulations of teeth with sockets of mandible and maxillae.

      • Function: Teeth held by fibrous periodontal ligaments (periodontal membranes).

      • Type: Synarthroses.

    2. Sutures:

      • Description: Found between skull bones; very short fibers.

      • Structure: Interlocking, irregular edges for stability.

      • Function: Increases stability, decreases fracture risk; allows growth in childhood; ossified synostoses in older adults.

      • Type: Synarthroses.

    3. Syndesmoses:

      • Description: Bound by interosseous membrane.

      • Example: Between radius and ulna, and tibia and fibula.

      • Function: Amphiarthroses; provides pivot for two long bones.

9.3 Cartilaginous Joints

  • Properties of Cartilaginous Joints:

    • Consist of either hyaline cartilage or fibrocartilage between bones.

    • Lack a joint cavity.

    • Immobile or slightly mobile.

Types of Cartilaginous Joints

  1. Synchondroses:

    • Definition: Bones joined by hyaline cartilage.

    • Type: Immobile (synarthroses).

  2. Symphyses:

    • Description: Pads of fibrocartilage between articulating bones.

    • Function: Resist compression, act as shock absorbers; allow slight mobility (amphiarthroses).

Clinical View: Costochondritis

  • Costochondritis:

    • Definition: Inflammation of the costochondral joints.

    • Symptoms: Localized chest pain; cause usually unknown (could be trauma or infection).

    • Diagnosis: May be mistaken for myocardial infarction.

    • Treatment: Nonsteroidal anti-inflammatory drugs (NSAIDs).

9.4a Distinguishing Features and Anatomy of Synovial Joints

  • Synovial Joints:

    • Bones separated by a joint cavity (articular cavity).

    • Include most joints in the body.

    • Type: Diarthroses (freely mobile) with common features.

Common Features of Synovial Joints

  1. Articular Capsule and Joint Cavity:

    • Structure: Joint capsule encapsulating joint cavity.

    • Double-layered:

      • Outer Fibrous Layer: Dense connective tissue for joint strength.

      • Inner Synovial Membrane: Composed primarily of areolar connective tissue; produces synovial fluid.

  2. Articular Cartilage:

    • Hyaline cartilage on bone surface at joint.

    • Functions:

      • Reduces friction during movement.

      • Acts as a cushion to absorb joint compression.

      • Prevents damage to articulating ends of bones.

      • Lacks perichondrium and is avascular.

  3. Joint Cavity:

    • Space between articulating bones lined by synovial membrane that secretes synovial fluid.

    • Function: Lubricate articular cartilage, nourish chondrocytes, acts as a shock absorber.

  4. Ligaments:

    • Definition: Dense regular connective tissue.

    • Function: Connect one bone to another; stabilize, strengthen, and reinforce synovial joints.

    • Types:

      • Extrinsic Ligaments: Physically separate from articular capsule.

      • Intrinsic Ligaments: Thickening of articular capsule itself.

  5. Sensory Receptors and Blood Vessels:

    • Numerous receptors in synovial joints detect painful stimuli, movement, compressive forces, and stretch.

  6. Tendons:

    • Composed of dense regular connective tissue; not part of synovial joint itself; attach muscles to bones; help stabilize joints.

  7. Bursae:

    • Saclike structures containing synovial fluid.

    • Lined internally by synovial membrane; alleviate friction.

    • Associated with joint cavity or separate from it; found in joints where bodies rub together.

  8. Accessory Structures:

    • Tendon Sheaths: Elongated bursae wrapping around tendons where friction is excessive (common in wrist and ankle).

    • Fat Pads: Protective packing material in joint periphery filling spaces during joint shape changes.

9.4b Classification of Synovial Joints

  1. Classes by Movements Allowed:

    • Uniaxial: Moves in one plane/axis.

    • Biaxial: Moves in two planes/axes.

    • Multiaxial/Triaxial: Moves in multiple planes/axes.

  2. Classes by Shape of Joint Surfaces:

    • Source: Least mobile to most mobile.

    1. Plane Joints

    2. Hinge Joints

    3. Pivot Joints

    4. Condylar Joints

    5. Saddle Joints

    6. Ball-and-socket Joints

Specific Joint Types

  1. Plane Joint: Articular surfaces flat; limited side-to-side gliding movement (uniaxial).

  2. Hinge Joint: Convex surface within concave depression (uniaxial); movement similar to door hinge.

  3. Pivot Joint: Rounded surface fits into a ligament ring (uniaxial); allows rotation.

  4. Condylar Joint: Oval, convex surface articulating with concave surface (biaxial).

  5. Saddle Joint: Convex and concave surfaces resembling saddle shape (biaxial).

  6. Ball-and-socket Joint: Spherical head fitting into cuplike socket (multiaxial); most freely mobile joint type.

9.5 The Movements of Synovial Joints

Four types of motion at synovial joints:

  1. Gliding

  2. Angular

  3. Rotational

  4. Special Movements

Gliding Motion

  • Definition: Two opposing surfaces slide back-and-forth or side-to-side.

  • Limited movement in any direction.

  • Example: Between carpals or tarsals.

Angular Motion

  1. Definition: Increases or decreases angle between two bones.

  2. Specific Types:

    • Flexion and Extension:

      • Flexion: Decreases angle between bones (e.g., bending finger).

      • Extension: Increases angle between articulating bones (e.g., straightening fingers after a fist).

    • Hyperextension: Extension beyond normal range (common in mobile joints or due to injuries).

    • Lateral Flexion: Trunk moving laterally in coronal plane (between vertebrae).

    • Abduction and Adduction:

      • Abduction: Moving body part away from midline (e.g., arm/thigh).

      • Adduction: Moving body part towards midline (e.g., arm/thigh).

    • Circumduction: Proximal end remains stationary while distal end makes a circular motion (e.g., drawing a circle on a board).

Rotational Motion

  • Definition: Bone pivots on its longitudinal axis.

  1. Types:

    • Lateral Rotation: Turns anterior surface laterally.

    • Medial Rotation: Turns anterior surface medially.

    • Pronation: Medial rotation of forearm (palm backward).

    • Supination: Lateral rotation of forearm (palm forward).

Special Movements

  • Depression: Inferior movement of a body part (e.g., opening mouth).

  • Elevation: Superior movement (e.g., closing mouth).

  • Dorsiflexion: Ankle joint action bringing foot upwards toward leg (e.g., digging heels).

  • Plantar Flexion: Points foot downwards (e.g., ballet dancers on tiptoe).

  • Eversion: Sole of foot turns laterally (intertarsal joints).

  • Inversion: Sole turns medially (intertarsal joints).

  • Protraction: Anterior movement from anatomical position (e.g., jutting jaw).

  • Retraction: Posterior movement from anatomical position (e.g., pulling in jaw).

  • Opposition: Thumb movement towards tips of fingers (grasping).

  • Reposition: Opposite of opposition.

9.6 Synovial Joints and Levers

  • Biomechanics: Applying mechanical principles to biology regarding synovial joint movements.

Terminology of Levers

  1. Lever: Elongated, rigid object rotating around a fixed point (fulcrum).

  2. Effort and Resistance:

    • Effort applied to one point; resistance located at another point.

    • Movement occurs when effort exceeds resistance.

  3. Effort Arm: Part of lever from fulcrum to point of effort.

  4. Resistance Arm: Part from fulcrum to point of resistance.

Types of Levers

  1. First-Class Levers: Fulcrum between effort and resistance (e.g., scissors, atlanto-occipital joint of neck).

  2. Second-Class Levers: Resistance is between fulcrum and effort (small force balancing larger load; rare in body, but seen in tiptoe standing).

  3. Third-Class Levers: Effort applied between resistance and fulcrum (most common in body; e.g., elbow joint). Effort from biceps applies at elbow while resistance is weight in hand.

9.7 Joint Analysis: Examples

1. Shoulder Joint Components

  • Sternoclavicular Joint: Saddle joint formed by manubrium and sternal end of clavicle.

    • Description: Fibrocartilaginous articular disc partitions joint into two parts forming two separate synovial cavities.

    • Movements: Elevation, depression, circumduction of clavicle possible.

    • Stability: Provided by fibers and ligaments.

2. Glenohumeral (Shoulder) Joint

  • Type: Ball-and-socket joint between head of humerus and glenoid cavity of scapula; greatest range of motion but most unstable and frequently dislocated.

  • Supporting Anatomic Features: Fibrocartilaginous glenoid labrum encircles socket; abundant bursae to reduce friction.

  • Ligaments and Muscles: Coracoacromial ligament, coracohumeral ligament, glenohumeral ligament, tendon of long head of biceps brachii, and rotator cuff muscles secure the joint.

    • Rotator Cuff Muscles: Subscapularis, supraspinatus, infraspinatus, teres minor; stabilize the head of the humerus in the glenoid cavity.

3. Elbow Joint

  • Type: Hinge joint composed of two articulations (humeroulnar and humeroradial joints) within a single articular capsule.

  • Stability Features: Thick articular capsule; bony surfaces interlock closely; multiple strong ligaments (e.g., radial collateral ligament, ulnar collateral ligament, annular ligament).

  • Common Injury: Subluxation of the Head of the Radius: Incomplete dislocation primarily occurring in children.

4. Hip Joint

  • Articulation: Head of femur and acetabulum of hip bone; acetabular labrum deepens socket.

  • Supportive Features: Articular capsule, ligaments, and muscles provide stability; retinacular fibers and intracapsular ligaments assist in support.

  • Common Injury: Fracture of femoral neck; associated with osteoporosis in older individuals.

5. Knee Joint

  • Type: Largest and most complex diarthroses, primarily a hinge joint capable of slight rotation and gliding.

    • Composed of two separate articulations: tibiofemoral and patellofemoral joints.

    • Strengthened by multiple ligaments and menisci.

  • Common Injuries: Tibial collateral ligament tear, ACL injury, meniscus injury, and the "unhappy triad" (injuries to tibial collateral ligament, medial meniscus, and ACL).

6. Talocrural (Ankle) Joint

  • Type: Highly modified hinge joint allowing for dorsiflexion and plantar flexion between tibia/fibula and talus.

  • Stability Features: Binds medial and lateral malleoli, various ligaments prevent excessive movement and sprains.

9.8 Development and Aging of the Joints

  • Development: Joints begin to form by the sixth week of development.

    • Fibrous joints: Mesenchyme differentiates into dense regular connective tissue.

    • Cartilaginous joints: Mesenchyme becomes fibrocartilage or hyaline cartilage.

    • Synovial joints: Mesenchyme forms articular capsule and supporting ligaments.

Aging Effects on Joints

  • Arthritis: Involves damage to articular cartilage, with subtypes including Osteoarthritis (common in aging) and Rheumatoid Arthritis (autoimmune).

  • Joint Health: Exercise can enhance synovial fluid flow to chondrocytes and strengthen supporting muscles, though excessive activity may worsen osteoarthritis.

Clinical Views - Common Conditions

Arthritis Overview

  • Symptoms: Joint swelling, pain, and stiffness. Can lead to:

    • Gouty Arthritis: Often in older males due to increased uric acid levels.

    • Osteoarthritis: Degenerative joint condition heavily affecting aging joints (fingers, knuckles, hips, knees, shoulders).

    • Rheumatoid Arthritis: Autoimmune disorder affecting younger/middle-aged adults, often women. Followed by scar tissue formation, ossification, and possible ankylosis.

    • Treatments: NSAIDs and corticosteroids for inflammation/pain reduce; DMARDs (both chemical and biological) assist in immune suppression.