Chapter 8: Articulations – Joints (BIO 135 Study Guide)

Joints (Articulations)

Definition and Functions

Joints, or articulations, are sites where two or more bones meet. They serve two primary functions: providing mobility to the skeleton and holding the skeleton together. Joints are classified both functionally and structurally.

• Articulation: The connection between bones.

• Mobility: Joints allow movement of the skeleton.

• Stability: Joints maintain the integrity of the skeleton.

Functional Classification of Joints

Based on Movement Allowed

Functional classification is determined by the amount of movement permitted by the joint.

• Synarthroses: Immovable joints (e.g., sutures in the skull).

• Amphiarthroses: Slightly movable joints (e.g., intervertebral discs).

• Diarthroses: Freely movable joints (e.g., most limb joints).

Structural Classification of Joints

Based on Material and Joint Cavity

Structural classification depends 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; mostly immovable.

• Cartilaginous Joints: Bones united by cartilage; no joint cavity; not highly movable.

• Synovial Joints: Bones separated by a fluid-filled joint cavity; all are freely movable.

Fibrous Joints

Types of Fibrous Joints

• Sutures: Short, interconnecting fibers; found only in the skull; immovable for brain protection.

• Syndesmoses: Bones connected by ligaments; movement varies with fiber length (e.g., tibiofibular joint, interosseous membrane of radius and ulna).

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

Cartilaginous Joints

Types of Cartilaginous Joints

• Synchondroses: Bones united by hyaline cartilage (e.g., epiphyseal plate, first rib and sternum); all are immovable.

• Symphyses: Bones united by fibrocartilage; hyaline cartilage present as articular cartilage; strong, flexible, slightly movable (e.g., intervertebral discs, pubic symphysis).

Synovial Joints

General Structure and Features

Synovial joints are the most common and freely movable joints in the body. They are characterized by several distinguishing features:

• Articular cartilage: Hyaline cartilage covers bone ends, preventing crushing.

• Joint (synovial) cavity: Small, fluid-filled space between bones.

• Articular capsule: Two layers—external fibrous layer (dense irregular connective tissue) and inner synovial membrane (loose connective tissue, produces synovial fluid).

• Synovial fluid: Viscous, slippery fluid that lubricates and nourishes cartilage; contains phagocytic cells.

• Reinforcing ligaments: Capsular (thickened part of fibrous layer), extracapsular (outside capsule), intracapsular (deep to capsule).

• Nerves and blood vessels: Nerve fibers detect pain and monitor joint position; capillary beds supply filtrate for synovial fluid.

Other Features of Synovial Joints

• Fatty pads: Cushioning between fibrous layer and synovial membrane or bone.

• Articular discs (menisci): Fibrocartilage separates articular surfaces, improves fit, stabilizes joint, reduces wear.

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

• Tendon sheaths: Elongated bursae wrapped around tendons subjected to friction.

Stabilizing Factors at Synovial Joints

Joint Stability

• Articular surface shape: Minor role in stability.

• Ligament number and location: Limited role.

• Muscle tendons: Most important; muscle tone keeps tendons taut, especially in shoulder, knee, and foot arches.

Movements at Synovial Joints

Types of Movements

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

• Angular movements: Change angle between bones (flexion, extension, hyperextension, abduction, adduction, circumduction).

• Rotation: Bone turns around its own axis (e.g., C1-C2 vertebrae, humerus, femur).

• Special movements: Supination/pronation, dorsiflexion/plantar flexion, inversion/eversion, protraction/retraction, elevation/depression, opposition.

Types of Synovial Joints

Classification by Shape and Movement

• Plane: Nonaxial gliding (e.g., intercarpal joints).

• Hinge: Uniaxial flexion/extension (e.g., elbow).

• Pivot: Uniaxial rotation (e.g., proximal radioulnar joint).

• Condylar: Biaxial flexion/extension, adduction/abduction (e.g., knuckle joints).

• Saddle: Biaxial (e.g., thumb carpometacarpal joint).

• Ball-and-socket: Multiaxial (e.g., shoulder, hip).

Major Synovial Joints

Knee Joint

The knee is the largest and most complex joint, consisting of three joints surrounded by a single cavity. It allows flexion, extension, and some rotation.

• Femoropatellar joint: Plane joint, gliding motion.

• Lateral and medial tibiofemoral joints: Flexion, extension, some rotation.

• Menisci: Medial and lateral fibrocartilage discs.

• Ligaments: Capsular, extracapsular (fibular/tibial collateral, oblique popliteal, arcuate popliteal), and intracapsular (anterior/posterior cruciate).

• Bursae: At least 12 associated bursae.

Knee Joint Injuries

The knee absorbs great vertical force and is vulnerable to lateral blows, especially when extended. Common injuries involve the 'three C's': collateral ligaments, cruciate ligaments, and cartilages (menisci).

Shoulder (Glenohumeral) Joint

The shoulder is a ball-and-socket joint, offering the greatest range of motion but sacrificing stability. The head of the humerus articulates with the glenoid cavity of the scapula.

• Ligaments: Coracohumeral, glenohumeral (weak, sometimes absent).

• Muscle tendons: Long head of biceps brachii, four rotator cuff tendons (subscapularis, supraspinatus, infraspinatus, teres minor).

Elbow Joint

The elbow is a hinge joint formed by the articulation of the radius and ulna with the humerus. It allows only flexion and extension.

• Anular ligament: Surrounds head of radius.

• Ulnar collateral ligament: Medial support.

• Radial collateral ligament: Lateral support.

Hip (Coxal) Joint

The hip is a ball-and-socket joint, with the head of the femur articulating with the acetabulum. It has a good range of motion, limited by the deep socket and reinforced by ligaments.

• Acetabular labrum: Rim of fibrocartilage deepens the socket.

• Ligaments: Iliofemoral, pubofemoral, ischiofemoral, ligamentum teres.

Temporomandibular Joint (TMJ)

The TMJ is the articulation between the mandibular condyle and the temporal bone. It allows both hinge (depression/elevation) and gliding (lateral excursion) movements. It is the most easily dislocated joint in the body.

Common Joint Injuries

Cartilage Tears

Cartilage tears are caused by compression and shear stress. Fragments may cause the joint to lock or bind. Cartilage rarely repairs itself and is often treated with arthroscopic surgery.

Sprains

Sprains involve stretching or tearing of reinforcing ligaments. Partial tears heal slowly due to poor vascularization. Complete tears may require surgical repair, grafts, or immobilization.

Dislocations (Luxations)

Dislocations occur when bones are forced out of alignment, often accompanied by sprains and inflammation. Subluxation is a partial dislocation.

Inflammatory and Degenerative Conditions

Bursitis and Tendonitis

• Bursitis: Inflammation of a bursa, usually caused by blow or friction; treated with rest, ice, and anti-inflammatory drugs.

• Tendonitis: Inflammation of tendon sheaths, typically caused by overuse; similar symptoms and treatment as bursitis.

Arthritis

Arthritis refers to over 100 types of inflammatory or degenerative diseases that damage joints. Symptoms include pain, stiffness, and swelling.

• Osteoarthritis (OA): Common, irreversible, degenerative arthritis; more cartilage destroyed than replaced; bone spurs form; treatment includes moderate activity and mild pain relievers.

• Rheumatoid Arthritis (RA): Chronic, inflammatory, autoimmune disease; immune system attacks own cells; symptoms include joint pain, swelling, anemia, osteoporosis, muscle weakness, cardiovascular problems; treatment includes anti-inflammatory drugs and immune suppressants.

• Gouty Arthritis: Deposition of uric acid crystals in joints; more common in men; typically affects the base of the great toe; untreated cases may lead to joint immobilization.

Lyme Disease

Lyme disease is caused by bacteria transmitted by tick bites. Symptoms include skin rash, flu-like symptoms, and joint pain. Treatment involves a long course of antibiotics.

Developmental Aspects of Joints

Joint Development and Aging

• By embryonic week 8, synovial joints resemble adult joints.

• Joint size, shape, and flexibility are modified by use.

• With aging, ligaments and tendons shorten and weaken; intervertebral discs are more likely to herniate; most people in their 70s have some degree of OA.

• Full-range-of-motion exercise is key to postponing joint problems.