Chapter 9: Joints – Structure, Function, and Movement

Joints (Articulations)

Introduction to Joints

Joints, also known as articulations, are the locations where two bones meet and where body movement occurs. The structure of a joint determines its function, and there is a tradeoff between strength and mobility.

• Definition: A joint is the point of contact between two bones, allowing for movement or stability.

• Strength vs. Mobility: Joints that are very strong tend to be less mobile, while highly mobile joints are generally weaker.

Classification of Joints

Structural and Functional Classification

Joints are classified by their anatomical structure and their functional range of motion.

• Structural Classification: Fibrous, Cartilaginous, Bony, Synovial

• Functional Classification: Synarthrosis (immovable), Amphiarthrosis (slightly movable), Diarthrosis (freely movable)

Synarthrosis (Immovable Joints)

Synarthroses are very strong joints where the edges of bones may touch or interlock. They can be fibrous or cartilaginous.

• Suture: Found only between bones of the skull; edges interlock and are bound by dense fibrous connective tissue.

• Gomphosis: Binds teeth to bony sockets via the periodontal ligament.

• Synchondrosis: Rigid cartilaginous bridge between two bones (e.g., between vertebrosternal ribs and sternum).

• Synostosis: Created when two bones fuse (e.g., metopic suture of frontal bone).

Amphiarthrosis (Slightly Movable Joints)

Amphiarthroses allow more movement than synarthroses but are stronger than diarthroses.

• Syndesmosis: Bones connected by a ligament.

• Symphysis: Bones connected by fibrocartilage.

Diarthrosis (Freely Movable Joints)

Diarthroses are synovial joints, which are freely movable and found at the ends of long bones.

• Joint Capsule: Surrounds the joint and contains the synovial membrane.

• Synovial Fluid: Fills the joint cavity, lubricates, distributes nutrients, and absorbs shock.

• Articular Cartilage: Covers articulating surfaces, preventing direct contact between bones.

Accessory Structures of Synovial Joints

Stabilizing Structures

Synovial joints are stabilized by several accessory structures:

• Cartilages: Meniscus (fibrocartilage pad between opposing bones)

• Fat Pads: Adipose tissue covered by synovial membrane, protecting articular cartilages

• Ligaments: Support and strengthen joints; sprains occur when ligaments are torn

• Tendons: Attach muscles around the joint

• Bursae: Small pockets of synovial fluid that cushion areas where tendons or ligaments rub against other tissues

Movements at Synovial Joints

Planes and Types of Movement

Movements at synovial joints are described by the plane or direction of movement and the relationship between structures.

• Monaxial: Movement in one plane (e.g., elbow)

• Biaxial: Movement in two planes (e.g., wrist)

• Triaxial: Movement in three planes (e.g., shoulder)

Types of Movement

• Gliding: Two flat surfaces slide past each other (e.g., carpal bones)

• Angular: Includes flexion, extension, hyperextension, abduction, adduction, and circumduction

• Rotational: Includes medial and lateral rotation, pronation, and supination

• Special Movements: Includes inversion, eversion, dorsiflexion, plantar flexion, opposition, reposition, protraction, retraction, depression, elevation, and lateral flexion

Gliding Movement

• Definition: Linear motion where surfaces slide past each other.

• Example: Carpal bones.

Angular Movement

• Flexion: Decreases angle between articulating bones.

• Extension: Increases angle between articulating bones.

• Hyperextension: Extension past anatomical position.

• Abduction: Movement away from longitudinal axis.

• Adduction: Movement toward longitudinal axis.

• Circumduction: Circular movement without rotation.

Rotational Movement

• Medial Rotation: Internal rotation toward the long axis.

• Lateral Rotation: External rotation away from the body.

• Pronation: Rotates forearm so that radius rolls across ulna; palm faces posteriorly.

• Supination: Turns palm anteriorly; forearm is supinated in anatomical position.

Special Movements

• Inversion/Eversion: Twisting sole of foot medially/laterally.

• Dorsiflexion/Plantar Flexion: Flexion/extension at ankle.

• Opposition/Reposition: Movement of thumb toward/away from palm or fingers.

• Protraction/Retraction: Anterior/posterior movement in horizontal plane.

• Depression/Elevation: Moving a structure inferiorly/superiorly.

• Lateral Flexion: Bending vertebral column to the side.

Classification of Synovial Joints

Types of Synovial Joints

Synovial joints are classified by the shape of their articulating surfaces and the type of movement they allow.

Joint Type	Movement	Examples
Plane (Gliding)	Nonaxial, limited gliding	Intercarpal joints, vertebrocostal joints
Hinge	Monaxial, angular	Elbow, knee, ankle
Condylar (Ellipsoid)	Biaxial, angular	Radiocarpal, metacarpophalangeal
Saddle	Biaxial, angular	First carpometacarpal (thumb)
Pivot	Monaxial, rotation	Atlanto-axial, proximal radioulnar
Ball-and-Socket	Triaxial, angular, circumduction, rotation	Shoulder, hip

Intervertebral Joints

Structure and Function

The vertebral column contains both synovial joints (between articular processes) and symphyses (between vertebral bodies).

• Intervertebral Disc: Separates vertebral bodies; consists of anulus fibrosus (outer fibrocartilage) and nucleus pulposus (elastic core).

• Vertebral End Plates: Cartilage covering superior and inferior surfaces of disc.

Intervertebral Ligaments

Ligaments bind vertebrae together and stabilize the vertebral column.

• 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.

• Supraspinous Ligament: Connects tips of spinous processes; continuous with ligamentum nuchae.

Elbow and Knee Joints

Elbow Joint

The elbow is a hinge joint involving the humerus, radius, and ulna.

• Humero-ulnar Joint: Largest, strongest joint at elbow.

• Humeroradial Joint: Smaller joint between capitulum of humerus and head of radius.

• Ligaments: Radial collateral, anular, ulnar collateral.

Knee Joint

The knee is a complex hinge joint transferring weight from femur to tibia.

• Three Articulations: Two femur–tibia (medial and lateral condyles), one patella–femur.

• Menisci: Fibrocartilage pads at femur–tibia articulations.

• Ligaments: Patellar, popliteal, cruciate (anterior/posterior), tibial collateral, fibular collateral.

Shoulder and Hip Joints

Shoulder Joint (Glenohumeral Joint)

The shoulder is a ball-and-socket diarthrosis between the head of the humerus and the glenoid cavity of the scapula.

• Glenoid Labrum: Rim of fibrocartilage deepening the socket.

• Ligaments: Acromioclavicular, coracoclavicular, coraco-acromial, coracohumeral, glenohumeral.

• Rotator Cuff Muscles: Supraspinatus, infraspinatus, teres minor, subscapularis.

• Bursae: Subdeltoid, subcoracoid, subacromial, subscapular.

Hip Joint

The hip is a strong ball-and-socket diarthrosis between the head of the femur and the acetabulum of the hip bone.

• Acetabular Labrum: Rim of fibrocartilage increasing depth of joint cavity.

• Ligaments: Iliofemoral, pubofemoral, ischiofemoral, transverse acetabular, ligament of the femoral head.

Effects of Aging on Joints

Degenerative Changes

• Rheumatism: Pain and stiffness in the musculoskeletal system.

• Arthritis: Inflammation of joints; includes osteoarthritis (wear and tear), rheumatoid arthritis (immune system attack), and gouty arthritis (uric acid crystals).

• Joint Immobilization: Reduces flow of synovial fluid, causing symptoms of arthritis.

• Bone Mass: Decreases with age, increasing risk of fractures.

Integration with Other Systems

Functional Relationships

The skeletal system interacts with other body systems for bone formation, recycling, and mineral balance.

• Bone Remodeling: Involves osteoblasts (formation) and osteoclasts (recycling).

• Factors: Age, physical stress, hormone levels, calcium/phosphorus uptake, genetic/environmental factors.

• Interactions: Muscles attach to bones; endocrine system controls bones; digestive and urinary systems provide minerals; skeleton serves as mineral reserve.

Summary Table: Joint Classification

Structural Type	Functional Type	Example
Fibrous	Synarthrosis	Suture, gomphosis
Cartilaginous	Synarthrosis, Amphiarthrosis	Synchondrosis, symphysis
Bony	Synarthrosis	Synostosis
Synovial	Diarthrosis	Shoulder, hip, knee