Chapter 8: Joints (Articulations) - Study Guide A&P

Joints (Articulations)

Definition and Classification of Joints

Joints, also known as articulations, are sites where two or more bones meet. They allow for movement and provide structural support to the skeleton. Joints are classified by their structure and function.

• Structural Classification: Based on the material binding bones and presence of a joint cavity.

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

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

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

• Functional Classification: Based on the degree of movement allowed.

  • Synarthroses: Immovable joints.

  • Amphiarthroses: Slightly movable joints.

  • Diarthroses: Freely movable joints.

Fibrous Joints

Fibrous joints are connected by dense connective tissue and lack a joint cavity. They are generally immovable.

• Sutures: Found only in the skull; bones are tightly bound by minimal fibrous tissue. Example: Coronal suture.

• Syndesmoses: Bones connected by ligaments; movement depends on length of ligament. Example: Distal tibiofibular joint.

• Gomphoses: Peg-in-socket joints. Example: Tooth in alveolar socket.

Cartilaginous Joints

Cartilaginous joints are united by cartilage and lack a joint cavity. They allow little to no movement.

• Synchondroses: Bones united by hyaline cartilage. Example: Epiphyseal plate in growing bones.

• Symphyses: Bones united by fibrocartilage. Example: Pubic symphysis, intervertebral discs.

Synovial Joints

Synovial joints are the most movable and structurally complex joints. They feature a fluid-filled joint cavity.

• Articular cartilage: Hyaline cartilage covers bone surfaces.

• Joint (synovial) cavity: Space containing synovial fluid.

• Articular capsule: Two layers—fibrous layer and synovial membrane.

• Synovial fluid: Lubricates and nourishes cartilage.

• Reinforcing ligaments: Support and strengthen the joint.

• Nerves and blood vessels: Provide sensation and nutrition.

Factors Stabilizing Synovial Joints

Joint stability is maintained by several natural factors:

• Shape of articular surfaces (minor role)

• Ligaments (moderate role)

• Muscle tone (most important factor)

Movements at Synovial Joints

Synovial joints allow a variety of movements:

• Flexion: Decreases the angle between bones.

• Extension: Increases the angle between bones.

• Abduction: Movement away from the midline.

• Adduction: Movement toward the midline.

• Rotation: Turning a bone around its axis.

• Circumduction: Circular movement combining flexion, extension, abduction, and adduction.

• Special movements: Includes pronation, supination, inversion, eversion, protraction, retraction, elevation, depression, opposition.

Types of Synovial Joints

Synovial joints are classified by the shape of their articular surfaces and the movements they allow.

Type	Movement	Example
Plane	Gliding	Intercarpal joints
Hinge	Flexion/Extension	Elbow, knee
Pivot	Rotation	Proximal radioulnar joint
Condyloid	Flexion/Extension, Abduction/Adduction	Metacarpophalangeal joints
Saddle	Flexion/Extension, Abduction/Adduction	Thumb carpometacarpal joint
Ball-and-Socket	All movements	Shoulder, hip

Major Synovial Joints

Several synovial joints are notable for their structure and function:

• Jaw (Temporomandibular Joint): Articulates mandible and temporal bone; allows hinge and gliding movements; prone to dislocation.

• Shoulder (Glenohumeral Joint): Ball-and-socket; humerus and scapula; most mobile, least stable.

• Elbow: Hinge; humerus, radius, ulna; stable, allows flexion/extension.

• Hip: Ball-and-socket; femur and pelvis; stable, allows all movements.

• Knee: Hinge; femur, tibia, patella; largest, complex, allows flexion/extension and slight rotation.

Common Joint Injuries

Joints are susceptible to various injuries:

• Sprains: Ligament stretching or tearing; pain, swelling, instability.

• Dislocations: Bones forced out of alignment; pain, loss of function.

• Cartilage injuries: Damage to articular cartilage; joint pain, limited movement.

Types of Arthritis

Arthritis is inflammation of joints, with several common types:

Type	Characteristics	Symptoms
Osteoarthritis	Degenerative; wear and tear	Joint pain, stiffness
Rheumatoid Arthritis	Autoimmune; chronic inflammation	Swelling, pain, deformity
Gouty Arthritis	Uric acid crystal deposition	Sudden pain, redness, swelling

Summary Table: Structural and Functional Classification of Joints

Structural Type	Functional Type	Example Location
Fibrous (Suture)	Synarthrosis	Skull
Fibrous (Syndesmosis)	Amphiarthrosis	Distal tibiofibular joint
Fibrous (Gomphosis)	Synarthrosis	Tooth in socket
Cartilaginous (Synchondrosis)	Synarthrosis	Epiphyseal plate
Cartilaginous (Symphysis)	Amphiarthrosis	Pubic symphysis
Synovial	Diarthrosis	Shoulder, knee, hip

Additional info:

• Muscle tone is the most important stabilizing factor for synovial joints, especially in the shoulder and knee.

• Synovial fluid is produced by the synovial membrane and contains hyaluronic acid for lubrication.

• Arthritis can lead to chronic disability if untreated.