Fundamentals of Anatomy & Physiology - Joints

Introduction to Joints

• Joints (articulations):
  • Locations where two bones meet.
  • Sites where body movement occurs.
• There is a trade-off between joint strength and mobility.

9.1 Classification of Joints

• Two classification schemes:
  • Structural (based on anatomy).
  • Functional (based on range of motion).
• Joint structure determines its function.

Structural classifications:

• Fibrous.
• Cartilaginous.
• Bony.
• Synovial.

Functional classifications:

• Synarthrosis (immovable joint).
• Amphiarthrosis (slightly movable joint).
• Diarthrosis (freely movable joint).

Synarthrosis (Immovable Joint)

• Very strong joints.
• Edges of bones may touch or interlock.
• May be fibrous or cartilaginous.
• Four types of synarthrotic joints:
  • Suture
  • Gomphosis
  • Synchondrosis
  • Synostosis

Suture

• Found only between bones of the skull.
• Edges of bones interlock.
• Bound by dense fibrous connective tissue.

Gomphosis

• Binds teeth to bony sockets.
• Fibrous connection via the periodontal ligament.

Synchondrosis

• Rigid cartilaginous bridge between two bones.
• Found between vertebrosternal ribs and the sternum.
• Also present in the epiphyseal cartilage of growing long bones.

Synostosis

• Created when two bones fuse.
• Example: metopic suture of the frontal bone.
• Also seen in epiphyseal lines of mature long bones.

Amphiarthrosis (Slightly Movable Joint)

• More movable than a synarthrosis.
• Stronger than a diarthrosis.
• May be fibrous or cartilaginous.
• Two types of amphiarthroses:
  • Syndesmosis: bones connected by a ligament.
  • Symphysis: bones connected by fibrocartilage.

9.2 Synovial Joints

• Synovial joints (diarthroses) are freely movable joints.
• Located at the ends of long bones.
• Surrounded by a joint capsule (articular capsule).
  • Contains a synovial membrane.
• Synovial fluid produced by the synovial membrane fills the joint cavity.
• Articular cartilage covers articulating surfaces to prevent direct contact between bones.

Synovial Fluid

• Has the consistency of egg yolk.
• Contains proteoglycans.
• Primary functions include:
  • Lubrication.
  • Nutrient distribution.
  • Shock absorption.

Stabilization of Synovial Joints

• Synovial joints are mobile but relatively weak.
• Stabilized by accessory structures:
  • Cartilages and fat pads.
  • Ligaments.
  • Tendons.
  • Bursae.

Cartilages

• Meniscus: fibrocartilage pad between opposing bones.

Fat Pads

• Adipose tissue covered by synovial membrane.
• Protect articular cartilages.

Ligaments

• Support and strengthen joints.
• Sprain: ligament with torn collagen fibers.

Tendons

• Attach to muscles around the joint.

Bursae

• Small pockets of synovial fluid.
• Cushion areas where tendons or ligaments rub against other tissues.

Factors Stabilizing Synovial Joints

• Prevent injury by limiting the range of motion with:
  • Collagen fibers of the joint capsule and ligaments.
  • Shapes of articulating surfaces and menisci.
  • Other bones, muscles, or fat pads.
  • Tendons attached to articulating bones.

9.3 Movements at Synovial Joints

• Movements are described in terms that reflect the plane or direction of movement, and the relationship between structures.
• Planes of movement:
  • Monaxial: 1 plane (e.g., elbow).
  • Biaxial: 2 planes (e.g., wrist).
  • Triaxial: 3 planes (e.g., shoulder).

Types of Movement at Synovial Joints

• Gliding movement.
• Angular movement.
• Circumduction.
• Rotational movement.
• Special movements.

Gliding Movement

• When two flat surfaces slide past each other.
• Example: between carpal bones.

Angular Movement

• Flexion and extension occur in the anterior-posterior plane.
  • Flexion: Decreases the angle between articulating bones.
  • Extension: Increases the angle between articulating bones.
  • Hyperextension: Extension past anatomical position.
• Abduction and adduction occur in the frontal plane.
  • Abduction: Movement away from the longitudinal axis.
  • Adduction: Movement toward the longitudinal axis.

Circumduction

• A complete circular movement without rotation.

Rotational Movement

• Rotation in reference to anatomical position.
  • Example: left or right rotation of the head.
• Limb rotation is relative to the longitudinal axis of the body.
  • Medial rotation (internal rotation toward the long axis).
  • Lateral rotation (external rotation away from the body).
• Pronation:
  • Rotates forearm so that the radius rolls across the ulna.
  • Results in the palm facing posteriorly.
• Supination:
  • Turns the palm anteriorly.
  • Forearm is supinated in the anatomical position.

Special Movements

• Inversion: Twists sole of foot medially.
• Eversion: Twists sole of foot laterally.
• Dorsiflexion: Flexion at the ankle (lifting toes).
• Plantar flexion: Extension at the ankle (pointing toes).
• Opposition: Movement of the thumb toward the palm or other fingers.
• Reposition: Opposite of opposition.
• Protraction: Anterior movement in the horizontal plane (forward).
• Retraction: Opposite of protraction (pulling back).
• Depression: Moving a structure inferiorly (down).
• Elevation: Moving a structure superiorly (up).
• Lateral flexion: Bending the vertebral column to the side.

Classification of Synovial Joints

• Plane (gliding).
• Hinge.
• Condylar (ellipsoid).
• Saddle.
• Pivot.
• Ball-and-socket.

Plane Joint (Gliding Joint)

• Flattened or slightly curved surfaces.
• Limited motion (nonaxial).

Hinge Joint

• Angular motion in a single plane (monaxial).

Condylar Joint

• Oval articular face within a depression.
• Motion in two planes (biaxial).

Saddle Joint

• Articular faces fit together like a rider in a saddle.
• Biaxial.

Pivot Joint

• Rotation only.
• Monaxial.

Ball-and-Socket Joint

• Round head in a cup-shaped depression.
• Triaxial.

9.4 Intervertebral Joints

• First two cervical vertebrae are joined by a synovial joint.
• Synovial joints lie between adjacent articular processes.
• Adjacent vertebral bodies form symphyses.

Intervertebral Disc

• Separates vertebral bodies.
• Anulus fibrosus:
  • Tough outer layer of fibrocartilage.
  • Attaches disc to vertebrae.
• Nucleus pulposus:
  • Elastic, gelatinous core.
  • Absorbs shocks.
• Vertebral end plates of cartilage:
  • Cover superior and inferior surfaces of the disc.

Damage to Intervertebral Discs

• Bulging disc:
  • Bulge in the anulus fibrosus.
  • Invades vertebral canal.
• Herniated disc:
  • Nucleus pulposus breaks through the anulus fibrosus.
  • Compresses spinal nerves.

Movement and Ligaments

• As the vertebral column moves, the nucleus pulposus compresses, and the disc shape conforms to motion.
• Intervertebral ligaments bind vertebrae together and stabilize the vertebral column.

Intervertebral Ligaments

• Ligamenta flava: Connect laminae of adjacent vertebrae.
• Posterior longitudinal ligament: Connects posterior surfaces of vertebral bodies.
• Anterior longitudinal ligament: Connects anterior surfaces of vertebral bodies.
• Interspinous ligaments: Connect spinous processes of adjacent vertebrae.
• Supraspinous ligament: Connects tips of spinous processes (C7 to sacrum); continuous with ligamentum nuchae (C7 to skull).

Vertebral Movements

• Flexion.
• Extension.
• Lateral flexion.
• Rotation.

9.5 Elbow and Knee Joints

Elbow Joint

• Hinge joint.
• Articulations involve the humerus, radius, and ulna.
• Humero-ulnar joint:
  • Largest, strongest joint at the elbow.
  • Between the trochlea of the humerus and the trochlear notch of the ulna.
  • Limited movement.
• Humeroradial joint:
  • Smaller joint.
  • Articulation between the capitulum of the humerus and the head of the radius.
• Biceps brachii muscle:
  • Attaches to the radial tuberosity.
  • Controls elbow motion.
• Elbow ligaments:
  • Radial collateral.
  • Anular.
  • Ulnar collateral.

Knee Joint

• Complex hinge joint that transfers weight from the femur to the tibia.
• Three articulations:
  • Two femur-tibia articulations at the medial and lateral condyles.
  • One between the patella and the patellar surface of the femur.
• Medial and lateral menisci:
  • Fibrocartilage pads at the femur-tibia articulations.
  • Cushion and stabilize the joint.
• Seven major supporting ligaments:
  1. Patellar ligament (anterior).
  2. & 3. Two popliteal ligaments (posterior).
  3. & 5. Anterior and posterior cruciate ligaments (inside joint capsule).
  4. & 7. Tibial collateral ligament (medial) and fibular collateral ligament (lateral).

9.6 Shoulder and Hip Joints

Shoulder Joint

• Glenohumeral joint.
• Ball-and-socket diarthrosis between the head of the humerus and the glenoid cavity of the scapula.
• Greatest range of motion of any joint.
• Most frequently dislocated joint.
• Supported by skeletal muscles, tendons, and ligaments.
• Glenoid labrum:
  • Rim of fibrocartilage that extends beyond the bony rim and deepens the socket of the glenoid cavity.
• Acromion and coracoid process of the scapula:
  • Project laterally, superior to the humerus.
  • Help stabilize the joint.
• Shoulder ligaments:
  • Acromioclavicular.
  • Coracoclavicular.
  • Coraco-acromial.
  • Coracohumeral.
  • Glenohumeral.
• Shoulder separation: Partial or complete dislocation of the acromioclavicular joint.
• Muscles of the rotator cuff:
  • Supraspinatus.
  • Infraspinatus.
  • Teres minor.
  • Subscapularis.
• Shoulder bursae:
  • Subdeltoid.
  • Subcoracoid.
  • Subacromial.
  • Subscapular.

Hip Joint

• Between the head of the femur and the acetabulum of the hip bone.
• Strong ball-and-socket diarthrosis with a wide range of motion.
• Acetabular labrum:
  • Rim of fibrocartilage that increases the depth of the joint cavity and seals in synovial fluid.
• Ligaments of the hip joint:
  • Iliofemoral.
  • Pubofemoral.
  • Ischiofemoral.
  • Transverse acetabular.
  • Ligament of the femoral head.

9.7 Effects of Aging on Joints

• Rheumatism: Pain and stiffness in the musculoskeletal system.
• Arthritis (joint inflammation): All rheumatic diseases that affect synovial joints.
• Osteoarthritis: Caused by wear and tear of joint surfaces or genetic factors affecting collagen formation; generally affects people over age 60.
• Rheumatoid arthritis: An inflammatory condition where the immune system attacks joint tissues.
• Gouty arthritis: Crystals of uric acid form within synovial fluid.
• Degenerative changes:
  • Can be caused by joint immobilization, which reduces the flow of synovial fluid and can cause symptoms of arthritis.
  • Treated by continuous passive motion (CPM).
• Aging:
  • Bone mass decreases.
  • Bones weaken.
  • The risk of fractures increases.

9.8 Integration with Other Systems

• Living bones undergo remodeling that involves bone formation (osteoblasts) and bone recycling (osteoclasts).
• Factors affecting the balance between bone formation and recycling:
  • Age.
  • Physical stresses.
  • Hormone levels.
  • Calcium and phosphorus uptake and excretion.
  • Genetic or environmental factors.
• Other systems interact with the skeletal system:
  • Muscles attach to bones.
  • Bones are controlled by the endocrine system.
  • Digestive and urinary systems provide calcium and phosphate minerals to bones for growth.
  • The skeleton serves as a reserve for calcium, phosphate, and other minerals.