Joints and Movement

Introduction to Joints

Joints, also known as articulations, are integral components of the musculoskeletal system. They provide structural stability to the skeleton and enable movement. The type and structure of a joint determine its range of motion and functional capabilities.

• Definition: A joint is a location where two or more bones meet.

• Function: Joints allow for movement and provide mechanical support.

• Classification: Joints are classified by their structure and function.

Classification of Joints

Structural Classification

Structural classification is based on the major connective tissue that binds the bones and the presence or absence of a joint cavity.

• Fibrous Joints: Bones joined by fibrous connective tissue; no joint cavity.

• Cartilaginous Joints: Bones joined by cartilage; no joint cavity.

• Synovial Joints: Bones joined by a fluid-filled joint cavity.

Functional Classification

Functional classification is based on the degree of movement permitted at the joint.

• Synarthrosis: Non-movable joints.

• Amphiarthrosis: Slightly movable joints.

• Diarthrosis: Freely movable joints.

Fibrous Joints

General Characteristics

• United by fibrous connective tissue.

• No joint cavity present.

• Exhibit little to no movement.

• Types: Sutures, Syndesmosis, Gomphoses.

Sutures

Sutures are immovable joints found only between the bones of the skull.

• Opposing bones interdigitate (fit together like puzzle pieces).

• Periosteum of one bone is continuous with the periosteum of the other.

• Sutural ligament: Two periosteal plus dense, fibrous connective tissue between them.

• In adults, sutures may ossify completely, forming a synostosis.

• Fontanels: Membranous areas in the suture between bones in infants, allowing for skull deformation during birth and brain growth after birth.

Syndesmoses

• Bones are farther apart than in sutures and joined by ligaments.

• Allow some movement.

• Example: Radioulnar syndesmosis (interosseous membrane between radius and ulna).

Gomphoses

• Specialized joints where pegs fit into sockets.

• Example: Teeth fitting into the alveoli of the jaws.

• Periodontal ligaments: Hold teeth in place.

• Inflammation can lead to gingivitis and periodontal disease.

Cartilaginous Joints

General Characteristics

• Bones are united by cartilage (either hyaline or fibrocartilage).

• No joint cavity present.

• Allow more movement than fibrous joints but less than synovial joints.

• Types: Synchondroses, Symphyses.

Synchondroses

• Bones joined by hyaline cartilage.

• Allow little or no movement.

• Some are temporary (e.g., epiphyseal plates) and are replaced by bone (synostoses).

• Some are permanent (e.g., costal cartilage of the first rib and sternum).

• Some, like costochondral joints, may develop into synovial joints.

• Examples: Epiphyseal plates, sternocostal joint, spheno-occipital synchondrosis.

Symphyses

• Bones are united by fibrocartilage.

• Allow slight movement (amphiarthrosis).

• Examples: Symphysis pubis, intervertebral discs, joint between manubrium and body of sternum.

Synovial Joints

General Characteristics

• Contain a synovial cavity filled with synovial fluid.

• Allow considerable movement (diarthrosis).

• Most joints of the appendicular skeleton are synovial, reflecting greater mobility.

• Structurally more complex than fibrous or cartilaginous joints.

Structure of Synovial Joints

• Articular cartilage: Hyaline cartilage covering bone surfaces for smooth movement.

• Joint (synovial) cavity: Space that encloses the articular surfaces.

• Articular capsule: Double-layered; outer fibrous capsule (dense irregular connective tissue) and inner synovial membrane.

• Synovial membrane: Lines the inside of the joint capsule except at the articular cartilage; produces synovial fluid.

• Synovial fluid: Lubricates, nourishes, and reduces friction; contains hyaluronic acid.

• No blood vessels or nerves in articular cartilage; nutrients diffuse from nearby vessels and synovial fluid.

• Proprioceptors in the capsule help the brain sense joint position.

Accessory Structures

• Bursae: Synovial membrane sacs that reduce friction in areas of high movement.

• Tendon sheaths: Synovial sacs surrounding tendons as they pass over bone.

• Articular discs: Fibrocartilaginous pads (e.g., menisci in the knee) that improve fit and stability.

• Ligaments and tendons: Stabilize and support the joint.

Types of Synovial Joints

Synovial joints are classified by the shapes of their articulating surfaces and the types of movement they allow.

Types of Movement at Synovial Joints

Gliding Movements

• Occur in plane joints; bones slide past each other.

Angular Movements

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

• Extension: Increases the angle between bones (e.g., straightening the knee).

• Hyperextension: Extension beyond anatomical position.

• Abduction: Movement away from the midline.

• Adduction: Movement toward the midline.

• Plantar flexion: Pointing the toes downward.

• Dorsiflexion: Lifting the foot upward toward the shin.

Rotational Movements

• Rotation: Turning a bone around its long axis (e.g., turning the head).

• Medial rotation: Rotation toward the midline.

• Lateral rotation: Rotation away from the midline.

• Pronation: Rotating the forearm so the palm faces posteriorly.

• Supination: Rotating the forearm so the palm faces anteriorly.

Circumduction

• Combination of flexion, extension, abduction, and adduction; the limb describes a cone in space.

Special Movements

• Elevation: Moving a structure superiorly (e.g., shrugging shoulders).

• Depression: Moving a structure inferiorly (e.g., opening the mouth).

• Protraction: Moving a structure anteriorly (e.g., jutting the jaw forward).

• Retraction: Moving a structure posteriorly (e.g., pulling the jaw back).

• Excursion: Lateral movement of the mandible.

• Opposition: Movement of the thumb to touch the fingers.

• Reposition: Returning the thumb to anatomical position.

• Inversion: Turning the sole of the foot medially.

• Eversion: Turning the sole of the foot laterally.

Range of Motion (ROM)

Range of motion refers to the amount of mobility demonstrated at a given joint.

• Active ROM: Movement produced by muscle contraction.

• Passive ROM: Movement produced by an external force.

• Factors affecting ROM: shape of articular surfaces, cartilage, ligaments, tendons, muscle bulk, fluid, pain, and use/disuse.

Abnormalities in ROM

• Dislocation (luxation): Articulating surfaces are moved out of proper alignment.

• Subluxation: Partial dislocation.

• Sprain: Damage to ligaments, ranging from stretching to complete tear.

Selected Joints and Their Features

Temporomandibular Joint (TMJ)

• Combination plane and ellipsoid joint.

• Fibrocartilage disk divides joint into superior and inferior cavities.

• Allows depression/elevation, protraction/retraction, and lateral excursion.

• TMJ disorders are a common cause of chronic orofacial pain.

Shoulder (Glenohumeral) Joint

• Ball-and-socket joint; high mobility, reduced stability compared to hip.

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

• Glenoid labrum deepens the socket.

• Stabilized by rotator cuff muscles and ligaments.

• Common injuries: dislocations, rotator cuff tears.

Elbow Joint

• Compound hinge joint (humeroulnar, humeroradial, proximal radioulnar joints).

• Allows flexion/extension and pronation/supination.

• Stabilized by ulnar and radial collateral ligaments, annular ligament.

Hip (Coxal) Joint

• Ball-and-socket joint; deep acetabulum for stability.

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

• Strong joint capsule and ligaments (e.g., iliofemoral ligament).

• Ligament of head of femur carries a nutrient artery.

Knee Joint

• Modified hinge joint; allows flexion/extension, slight rotation.

• Menisci (fibrocartilage discs) deepen the articular surface.

• Stabilized by anterior and posterior cruciate ligaments (ACL, PCL), collateral ligaments, and muscle tendons.

• Common injuries: tears of ACL, medial meniscus, and collateral ligaments.

Ankle (Talocrural) Joint

• Highly modified hinge joint; allows dorsiflexion/plantar flexion, limited inversion/eversion.

• Stabilized by strong ligaments and the shape of the articular surfaces.

Effects of Aging on Joints

• Tissue repair slows; reduced blood vessel development.

• Articular cartilage wears down; matrix becomes more rigid.

• Production of synovial fluid declines.

• Ligaments and tendons become shorter and less flexible, decreasing ROM.

• Muscles around joints weaken; decreased activity leads to less flexibility.

Representative Joint Disorders

• Arthritis: Inflammation of joints.

• Osteoarthritis: Degenerative "wear and tear" arthritis.

• Rheumatoid arthritis: Autoimmune or post-infectious inflammation.

• Lyme disease: Bacterial infection causing chronic arthritis.

• Gout: Metabolic disorder with uric acid crystal deposition in joints.

• Bursitis: Inflammation of a bursa.

• Bunion: Deformation of the first metatarsal, often from tight shoes.

• Tendinitis: Inflammation of tendon sheaths, often from overuse.

• Dislocation: Bones move out of alignment.

• Sprain: Stretching or tearing of ligaments.

• Chondromalacia patellae: Inflammation and softening of cartilage under the patella.