Study Guide: Joints (Articulations) in Human Anatomy & Physiology

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Joints (Articulations) lesson 8

Introduction to Joints

Joints, also known as articulations, are points where two or more bones meet. They play a crucial role in providing mobility and holding the skeleton together. The structure of a joint determines its function and the range of movement it allows.

Functional Classes of Joints

Overview of Functional Classification

Synarthroses: Immovable joints. Example: sutures in the skull.
Amphiarthroses: Slightly movable joints. Example: intervertebral discs.
Diarthroses: Freely movable joints. Example: shoulder, knee.

Structural Classes of Joints

Fibrous and Cartilaginous Joints

Joints are also classified based on the material binding the bones and whether a joint cavity is present.

Structural class	Made of what tissue?	Joint cavity present?	Mobility	Examples
Suture	Dense fibrous connective tissue	No	Immovable (synarthrosis)	Skull sutures
Syndesmosis	Ligament (fibrous tissue)	No	Slightly movable (amphiarthrosis)	Distal tibiofibular joint
Gomphosis	Periodontal ligament (fibrous tissue)	No	Immovable (synarthrosis)	Tooth in socket
Synchondrosis	Hyaline cartilage	No	Immovable (synarthrosis)	Epiphyseal plate, first rib-sternum
Symphysis	Fibrocartilage	No	Slightly movable (amphiarthrosis)	Pubic symphysis, intervertebral discs

Synovial Joints

General Characteristics

All synovial joints are diarthroses (freely movable).
They have a joint cavity filled with synovial fluid.
Articulating bone surfaces are covered with articular cartilage.

Structures of Synovial Joints

Articular cartilage: Hyaline cartilage covering bone ends, reduces friction and absorbs shock.
Joint (articular) cavity: Space containing synovial fluid.
Articular capsule: Double-layered capsule enclosing the joint cavity.
- Fibrous layer: Outer, strengthens joint.
- Synovial membrane: Inner, produces synovial fluid.
Synovial fluid: Lubricates, nourishes cartilage, and contains phagocytes.
Reinforcing ligaments: Strengthen and stabilize joints.
Articular disc (meniscus): Pads of fibrocartilage improving fit and stability.
Bursa: Fluid-filled sacs reducing friction between tissues.
Tendon sheath: Elongated bursa wrapping around tendons.

Innervation and Blood Supply

Synovial joints are innervated and supplied with blood vessels.
Nerves detect pain and monitor joint position and stretch.
Blood vessels supply the synovial membrane and ligaments.

Comparison: Articular Disc vs. Articular Cartilage

Articular cartilage: Covers bone ends, made of hyaline cartilage.
Articular disc: Pad of fibrocartilage, improves fit and stability.

Movements at Synovial Joints

Types of Movements

Movement	Description	Example 1	Example 2
Gliding	Sliding flat surfaces of two bones across each other	Gliding movement at the wrist
Flexion	Decreasing the angle between bones	Flexion of the neck	Flexion of the trunk
Extension	Increasing the angle between bones	Extension of the neck	Extension of the arm
Hyperextension	Extending more than normal	Extension of the arm	Extension of the neck
Abduction	Moving a limb away from the body midline	Raising your arm up
Adduction	Moving a limb toward the body midline	Bringing your arm to your side
Circumduction	Moving a limb so it describes a cone in space	Doing a circle with your hand
Rotation	Turning a bone around its longitudinal axis	Saying no (head rotation)
Medial rotation	Rotating toward the median plane
Lateral rotation	Rotating away from the median plane
Elevation	Lifting a body part superiorly	Chin raises up
Depression	Moving a body part inferiorly	Chin goes down
Protraction	Moving a body part in the anterior direction	Chin moves forward
Retraction	Moving a body part in a posterior direction	Chin goes back
Supination	Rotating the forearm so the palm faces anteriorly	Radius and ulna are parallel
Pronation	Rotating the forearm so the palm faces posteriorly	Radius rotates over ulna
Opposition	Moving the thumb to touch the tips of other fingers	Making OK sign with fingers
Inversion	Turning the sole of the foot medially
Eversion	Turning the sole of the foot laterally
Dorsiflexion	Lifting the foot so its superior surface approaches the shin
Plantar flexion	Depressing the foot (pointing the toes)	Elevating the heel

Types of Synovial Joints by Movement

Nonaxial: Adjoining bones do not move around a specific axis (gliding).
Uniaxial: Movement occurs around a single axis (flexion and extension).
Biaxial: Movement occurs around two axes (flexion/extension and abduction/adduction).
Multiaxial: Movement can occur around all three axes and along all three body planes (flexion, extension, abduction, adduction, rotation).

Structural Types of Synovial Joints

Type	Description	Functional classification	Types of movements allowed	Examples
Plane joint	Flat articular surface	Nonaxial movement	Gliding	Intercarpal joints
Hinge joint	Cylinder (bone) fits into trough (bone)	Uniaxial movement	Flexion and extension	Elbow, interphalangeal joints
Pivot joint	Sleeve (bone and ligament) and rounded bone	Uniaxial movement	Rotation	Proximal radioulnar joint, atlantoaxial joint
Condylar joint	Oval articular surface (condyle) fits into depression	Biaxial movement	Flexion, extension, abduction, adduction	Knuckle joints, wrist
Saddle joint	Articular surfaces are both concave and convex	Biaxial movement	Flexion, extension, abduction, adduction	Carpometacarpal joint of the thumb
Ball-and-socket joint	Cup (socket) spherical head (ball)	Multiaxial movement	Flexion, extension, abduction, adduction, rotation	Shoulder, hip

Stability of Synovial Joints

Factors Contributing to Stability

Articular surfaces: Shape determines how well bones fit together.
Ligaments: More ligaments = stronger joint. Ligaments resist excessive or abnormal movements.
Muscle tone: Continuous low-level contraction of muscles crossing the joint is the most important stabilizing factor.

Movements at Specific Joints

Examples of Movements

Elbow: Flexion and extension
Hip: Flexion, extension, abduction, adduction, rotation
Ankle: Dorsiflexion, plantar flexion, inversion, eversion
Metacarpophalangeal joint: Flexion, extension, abduction, adduction

Classification of Joints by Shape and Movement

Joint	Articulating bones	Structural type of synovial joint	Functional type of synovial joint	Types of movements allowed
Temporomandibular (TMJ)	Mandible and temporal bone	Modified hinge	Diarthrosis	Elevation, depression, protraction, retraction, lateral movement
Shoulder (glenohumeral)	Scapula and humerus	Ball-and-socket	Diarthrosis	Flexion, extension, abduction, adduction, rotation, circumduction
Elbow	Humerus, radius, ulna	Hinge	Diarthrosis	Flexion, extension
Wrist	Radius and carpals	Condylar	Diarthrosis	Flexion, extension, abduction, adduction
Hip	Femur and acetabulum of pelvis	Ball-and-socket	Diarthrosis	Flexion, extension, abduction, adduction, rotation
Knee (tibiofemoral)	Femur and tibia	Modified hinge	Diarthrosis	Flexion, extension, slight rotation
Ankle	Tibia, fibula, talus	Hinge	Diarthrosis	Dorsiflexion, plantar flexion

Knee Joint (Tibiofemoral Joint)

Structure and Function

The knee is the largest and most complex synovial joint.
It is a modified hinge joint allowing flexion, extension, and slight rotation.

Key Structures and Their Functions

Lateral meniscus & Medial meniscus: C-shaped fibrocartilage pads that deepen the articular surface and absorb shock.
Patellar ligament: Connects patella to tibia, stabilizes anterior knee.
Medial/lateral patellar retinaculum: Reinforce the joint capsule.
Fibular collateral ligament (lateral): Prevents lateral movement.
Tibial collateral ligament (medial): Prevents medial movement.
Oblique and arcuate popliteal ligaments: Reinforce posterior knee.
Anterior cruciate ligament (ACL): Prevents anterior displacement of tibia.
Posterior cruciate ligament (PCL): Prevents posterior displacement of tibia.

Locking Mechanism

The knee "locks" when fully extended, providing stability for standing.

Knee Injuries

Knee injuries are common in contact sports due to the joint's complexity and exposure to twisting forces.
The articular surfaces contribute little to stability; ligaments and muscle tone are more important.

Additional info: The knee's stability is highly dependent on the integrity of its ligaments and menisci. Damage to these structures can lead to instability and increased risk of osteoarthritis.