Joints in Human Anatomy and Physiology

CHAPTER 8: JOINTS

8-1 JOINTS

  • Definition: Joints, also known as articulations, are where two or more bones meet.
  • Functions of Joints:
    • Hold bones together: Joints stabilize the skeletal structure.
    • Allow for mobility: Joints enable movement between bones.
  • Classification of Joints:
    • Functionally: Based on the degree of movement allowed.
    • Structurally: Based on the material that binds the bones.

8-1 FUNCTIONAL CLASSIFICATION OF JOINTS

  • Synarthroses: Immovable joints.
  • Amphiarthroses: Slightly movable joints.
  • Diarthroses: Freely movable joints.

8-1 STRUCTURAL CLASSIFICATION OF JOINTS

  • Fibrous joints: Generally immovable.
  • Cartilaginous joints: Immovable or slightly movable.
  • Synovial joints: Freely movable.

8-2 FIBROUS JOINTS

  • Definition: Bones united by dense fibrous connective tissue.
  • Joint cavity: None present in fibrous joints.
  • Classification: All fibrous joints are classified as synarthrotic (immobile).
  • Types of Fibrous Joints:
    • Sutures: Joins cranial bones (e.g., sagittal suture).
    • Syndesmoses: Allows more movement than sutures but still classified as immobile; an example is the distal end of the tibia and fibula.
    • Gomphosis: A joint where a tooth is held in place by the periodontal ligament.

8-3 CARTILAGINOUS JOINTS

  • Definition: Bones connected by cartilage.
  • Joint cavity: None present.
  • Types of Cartilaginous Joints:
    • Synchondrosis: Hyaline cartilage; synarthrotic. Examples include the first rib with the sternum and the epiphyseal plate.
    • Symphysis: Fibrocartilage; amphiarthrotic. Examples include the pubic symphysis and intervertebral joints.

8-4 SYNOVIAL JOINTS

  • Definition: Articulating bones are separated by a joint cavity filled with synovial fluid.
  • Characteristics:
    • Include most limb joints and joints of the body.
    • All classified as diarthrotic (freely movable).
  • Six Distinguishing Features:
    1. Articular cartilage: Hyaline cartilage that prevents crushing of bone ends.
    2. Joint (synovial) cavity: Small, fluid-filled potential space.
    3. Articular capsule: Comprised of two layers:
    • External fibrous layer: Made of dense irregular connective tissue.
    • Inner synovial membrane: Made of loose connective tissue; produces synovial fluid.
    1. Synovial fluid: A viscous, slippery filtrate of plasma and hyaluronic acid that lubricates and nourishes articular cartilage, containing phagocytic cells to remove microbes and debris.
    2. Different types of reinforcing ligaments:
    • Capsular ligaments: Thickened parts of the outer fibrous layer.
    • Extracapsular ligaments: Located outside the capsule.
    • Intracapsular ligaments: Located deep to the capsule and covered by synovial membrane.
    1. Nerves and blood vessels: Nerve fibers detect pain, monitor joint position and stretch, while capillary beds supply filtrate for synovial fluid.

8-4 OTHER FEATURES OF SOME SYNOVIAL JOINTS

  • Fatty pads: Provide cushioning between fibrous layer and synovial membrane or bone.
  • Articular discs (menisci): Fibrocartilage that separates articular surfaces to improve “fit” of bone ends, stabilize the joint, and reduce wear and tear.
  • Bursae: Sacs lined with synovial membrane containing synovial fluid to reduce friction where ligaments, muscles, skin, tendons, or bones rub together.
  • Tendon sheaths: Elongated bursae wrapped completely around tendons subject to friction.

8-5 STABILIZING FACTORS FOR SYNOVIAL JOINTS

  • Shapes of articular surfaces: Play a minor role in joint stabilization.
  • Number and location of ligaments: Have a limited role in joint stabilization.
  • Muscle tendons that cross the joint: Most crucial factor; muscle tone keeps the tendons taut, significantly reinforcing the shoulder and knee joints, and the arches of the foot.

8-6 STABILIZING FEATURES: KNEE JOINT

  • Description: The knee joint is the largest and most complex joint in the body.
  • Joint structure: Composed of three joints surrounded by a single joint cavity:
    • Femoropatellar joint: A plane joint that allows gliding motion during knee flexion.
    • Lateral and medial tibiofemoral joints: Involves the femoral condyles with lateral and medial menisci of the tibia; facilitates flexion, extension, and some rotation.
  • Associated bursae: At least 12 bursae are associated with the knee joint.
  • Joint capsule: Reinforced by muscle tendons (e.g., quadriceps and semimembranosus tendons); it is thin and sometimes absent anteriorly.
  • Ligaments:
    • Extracapsular ligaments: Prevent hyperextension of the knee (e.g., fibular and tibial collateral ligaments, oblique popliteal ligament, arcuate popliteal ligament).
    • Intracapsular ligaments: Prevent anterior-posterior displacement; reside outside synovial cavity but within the joint capsule. Includes the anterior cruciate ligament (attaches to anterior tibia) and posterior cruciate ligament (attaches to posterior tibia).

8-6 KNEE JOINT INJURIES

  • Injury type: Absorbs significant vertical force.
  • Vulnerability: Prone to horizontal blows, especially laterally; injuries include:
    • Collateral ligaments
    • Cruciate ligaments
    • Cartilages (menisci).

8-7 STABILIZING FEATURES: SHOULDER JOINT

  • Composition: Reinforced by ligaments and muscle tendons:
    • Coracohumeral ligament: Supports the weight of the upper limb.
    • Glenohumeral ligaments: Three ligaments that are generally weak and sometimes absent.
    • Rotator cuff tendons: Four tendons encircle the shoulder joint (subscapularis, supraspinatus, infrapinatus, tener minor) and are prone to tears during activities such as baseball pitching.

8-8 STABILIZING FEATURES: ELBOW JOINT

  • Articulation details: Joint formed by the radius and ulna articulating with the humerus; classified as a hinge joint allowing primarily flexion and extension.
  • Anular ligament: Surrounds the head of the radius.
  • Capsular ligaments: Two ligaments limit side-to-side movement:
    • Ulnar collateral ligament
    • Radial collateral ligament.

8-9 STABILIZING FEATURES: HIP (COXAL) JOINT

  • Structure: A ball-and-socket joint formed by the head of the femur articulating with the acetabulum.
  • Range of motion: Good range of motion, but limited by the deep socket.
  • Acetabular labrum: Fibrocartilaginous rim that enhances depth of socket, making dislocations rare.
  • Reinforcing ligaments:
    • Iliofemoral ligament
    • Pubofemoral ligament
    • Ischiofemoral ligament
    • Ligamentum teres.

8-10 STABILIZING FEATURES: TEMPOROMANDIBULAR JOINT

  • Articulation details: The joint formed by the mandibular condyle articulating with the temporal bone; allows two types of movement:
    • Hinge movement: Depression and elevation of the mandible.
    • Gliding movement: Such as side-to-side grinding of teeth.
  • Characteristics: The articular capsule thickens to form lateral ligament and is considered the most easily dislocated joint in the body.

8-11 SYNOVIAL JOINTS: MOVEMENTS ALLOWED

  • Muscle attachments: All muscles attach to bone or connective tissue at no fewer than two points.
    • Origin: Attachment to immovable bone.
    • Insertion: Attachment to movable bone.
  • Movement mechanics: Muscle contraction causes the insertion to move toward the origin.
  • Movement planes: Movements occur along transverse, frontal, or sagittal planes.
  • Types of movements:
    • Nonaxial: Slipping movements only.
    • Uniaxial: Movement occurring in one plane.
    • Biaxial: Movement in two planes.
    • Multiaxial: Movement in or around all three axes.

8-11 TYPES OF SYNOVIAL JOINTS: SHAPE AND MOVEMENT

  • Classification by joint shape:
    • Plane joint: Nonaxial movement – flat articular surfaces that facilitate gliding (e.g. intercarpal joints).
    • Hinge joint: Uniaxial movement – allows flexion and extension (e.g. elbow joint).
    • Pivot joint: Uniaxial movement around a vertical axis (e.g. proximal radioulnar joints).
    • Condylar joint: Biaxial movement, including flexion, extension, adduction, and abduction (e.g. metacarpophalangeal joints).
    • Saddle joint: Biaxial movement; articular surfaces are both concave and convex (e.g. carpometacarpal joints of the thumbs).
    • Ball-and-socket joint: Multiaxial movement, allowing a wide range of motion (e.g. shoulder and hip joints).

8-12 FOUR GENERAL TYPES OF MOVEMENTS AT SYNOVIAL JOINTS

  1. Gliding: One flat bone surface glides over another.
  2. Angular movements: Includes flexion, extension, hyperextension, abduction, adduction, and circumduction.
  3. Rotation: Medial and lateral rotation around a bone’s long axis.
  4. Special movements: Specific to joints (e.g., mandible, foot, thumb).

8-12 TYPES OF MOVEMENTS: GLIDING MOVEMENTS

  • Definition: Movement where one flat bone surface glides or slips over another similar surface.
  • Examples: Intercarpal joints, intertarsal joints, and joints between articular processes of vertebrae.

8-12 ANGULAR MOVEMENTS

  • Definition: Movements that increase or decrease the angle between two bones:
    • Movement along the sagittal plane:
    • Flexion: Decreases the angle of the joint.
    • Extension: Increases the angle of the joint.
    • Hyperextension: Movement beyond the anatomical position.
  • Movement along the frontal plane:
    • Abduction: Movement away from the midline.
    • Adduction: Movement toward the midline.
    • Circumduction: A circular movement describing a cone in space involving flexion, abduction, extension, and adduction.

8-12 ROTATION

  • Definition: Turning of a bone around its own long axis:
    • Movements: Toward midline or away from it (medial and lateral rotation).
  • Examples: Between C1 and C2 vertebrae, rotation of the humerus and femur.

8-12 SPECIAL MOVEMENTS AT SYNOVIAL JOINTS

  • Sprained movements:
    • Dorsiflexion and plantar flexion of the foot.
    • Inversionand eversion of the foot.
    • Protractionand retraction of the jaw.
    • Elevationand depression of the mandible.
    • Opposition of the thumb: Movement of the thumb to touch the other fingers.

8-13 DAMAGES TO JOINTS

  • Joint injuries include:
    • Cartilage tears: Usually affect the meniscus (partial or complete); treated with arthroscopic repair as cartilage is avascular and rarely heals completely, leading to instability.
    • Sprains: Stretching or tearing of ligaments commonly affecting ankles, knees, and lumbar spine; may take time to heal due to inflammation.
    • Dislocation (luxation): Bones forced out of alignment, often due to falls or sports injuries; must be treated to restore normal alignment, frequently accompanied by sprains and inflammation.

8-13 INFLAMMATORY DAMAGE TO JOINTS

  • Common inflammatory injuries include:
    • Bursitis: Inflammation of bursae generally caused by a blow or friction; treated with anti-inflammatory medications.
    • Tendonitis: Inflammation of tendon sheaths, treated similarly; an example is tennis elbow, often resulting from repetitive use.
    • Lyme disease: Caused by bacteria from tick bites, resulting in joint pain and potentially arthritis; usually treated with extended courses of antibiotics.

8-13 DEGENERATIVE (INFLAMMATORY) DAMAGE TO JOINTS: CLINICAL FORMS OF ARTHRITIS

  • Osteoarthritis (OA): Most common chronic arthritis, linked to normal aging processes and wear and tear on the joints.
  • Rheumatoid arthritis (RA): An autoimmune condition where the immune system attacks joints; characterized by bilateral inflammation in specific joints (often hands) and can lead to deformities.
  • Gouty arthritis (gout): Caused by deposition of uric acid crystals in joints; more common in men and generally manageable through dietary control.

8-14 DEVELOPMENTAL ASPECTS OF JOINTS

  • Joint development timeline: By embryonic week 8, synovial joints exhibit adult characteristics.
  • Changes with use and age: Size, shape, and flexibility can be modified by use; aging results in shortened and weakened ligaments and tendons, increased likelihood of herniation of intervertebral discs, with many individuals in their 70s exhibiting some degree of OA.
  • Preventative measures: Engaging in full-range-of-motion exercises is essential for postponing joint issues.

LEARNING OBJECTIVES

  • Be prepared to:
    • 8-1 Define joints and distinguish functional and structural classifications.
    • 8-2 Describe fibrous joints, including types and examples.
    • 8-3 Explain cartilaginous joints, including types and examples.
    • 8-4 Describe synovial joints, their features, and additional components.
    • 8-5 Identify stabilizing factors for synovial joints.
    • 8-6 Detail features of knee joint stabilization.
    • 8-7 Outline stabilizing features of shoulder joint.
    • 8-8 Identify stabilizing features of elbow joint.
    • 8-9 Recognize stabilizing features of hip joint.
    • 8-10 Detail stabilizing features of temporomandibular joint.
    • 8-11 Describe types of synovial joints by shape and movement; provide examples.
    • 8-12 Explain movements of synovial joints; provide examples.
    • 8-13 Understand key aspects of joint damage, injury types, and inflammatory effects.
    • 8-14 Discuss developmental aspects of joints.