Anatomy & Physiology of Joints: Structure and Function

Joints: Structure and Function

Overview of Joints (Articulations)

Joints, or articulations, are sites where two or more bones meet. They play a crucial role in holding the skeleton together and providing mobility. Joints are classified both structurally and functionally, which determines their movement and stability.

• Structural classification is based on what material binds the bones together and whether a joint cavity is present.

• Functional classification is based on the amount of movement allowed by the joint.

Classification of Joints

Structural Classification

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

• Cartilaginous Joints: Bones united by cartilage; no joint cavity; slightly movable.

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

Functional Classification

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

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

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

Fibrous Joints

Types of Fibrous Joints

• Sutures: Interlocking seams found only in the skull; immovable (synarthrotic).

• Syndesmoses: Bones connected by ligaments; movement varies depending on fiber length (e.g., distal tibiofibular joint).

• Gomphoses: Peg-in-socket joints (e.g., teeth in alveolar sockets); immovable.

Cartilaginous Joints

Types of Cartilaginous Joints

• Synchondroses: Bones united by a bar or plate of hyaline cartilage; mostly immovable (e.g., epiphyseal plates, first rib and sternum).

• Symphyses: Bones united by fibrocartilage; provide strength with flexibility (e.g., pubic symphysis, intervertebral joints).

Synovial Joints

General Structure

Synovial joints are the most common and most movable type of joint in the body. They have a unique structure that allows for a wide range of movements.

• Articular cartilage: Hyaline cartilage covering bone surfaces to reduce friction and absorb shock.

• Joint (synovial) cavity: Space filled with synovial fluid.

• Articular capsule: Double-layered capsule enclosing the joint cavity (outer fibrous layer and inner synovial membrane).

• Synovial fluid: Lubricates and nourishes articular cartilage.

• Reinforcing ligaments: Strengthen and support the joint (capsular, extracapsular, intracapsular).

• Nerves and blood vessels: Supply the joint, detect pain, and monitor joint position.

Associated Structures

• Bursae: Fluid-filled sacs that reduce friction where structures such as ligaments or tendons rub together.

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

Stability of Synovial Joints

Factors Affecting Stability

• Articular surfaces: Shape and depth can enhance stability (e.g., ball-and-socket joints).

• Ligaments: More ligaments generally increase stability, but overstretched ligaments may not return to original length.

• Muscle tone: The most important stabilizing factor; keeps tendons taut across joints.

Movements at Synovial Joints

Types of Movement

• Gliding: One flat bone surface glides or slips over another (e.g., intercarpal joints).

• Angular movements: Increase or decrease the angle between two bones (e.g., flexion, extension, abduction, adduction).

• Rotation: Turning of a bone around its own long axis (e.g., atlas and axis, shoulder, hip).

• Special movements: Include supination, pronation, inversion, eversion, protraction, retraction, elevation, and depression.

Types of Synovial Joints

Classification by Shape and Movement

Type	Shape	Movement	Example
Plane	Flat surfaces	Gliding	Intercarpal joints
Hinge	Cylinder and trough	Flexion/Extension	Elbow
Pivot	Rounded end into ring	Rotation	Proximal radioulnar joint
Condyloid	Oval articular surfaces	All angular motions	Wrist
Saddle	Concave and convex	Greater freedom of movement	Thumb carpometacarpal joint
Ball-and-socket	Spherical head and cup	Multiaxial	Shoulder, hip

Major Joints of the Body

Shoulder (Glenohumeral) Joint

• Ball-and-socket joint: head of humerus and glenoid fossa of scapula.

• Stabilized mainly by rotator cuff muscles and the coracohumeral ligament.

• Allows the greatest range of motion but is less stable.

Elbow Joint

• Hinge joint: trochlea of humerus with trochlear notch of ulna.

• Stabilized by ulnar and radial collateral ligaments and the anular ligament.

Hip (Coxal) Joint

• Deep ball-and-socket joint: head of femur and acetabulum of pelvic bone.

• Stabilized by strong ligaments (iliofemoral, pubofemoral, ischiofemoral) and the ligamentum teres.

Knee Joint

• Largest and most complex joint; primarily a hinge joint.

• Composed of three joints: femoropatellar, lateral tibiofemoral, and medial tibiofemoral.

• Stabilized by menisci, cruciate ligaments (anterior and posterior), and collateral ligaments (tibial and fibular).

Temporomandibular Joint (TMJ)

• Modified hinge joint: mandibular condyle and mandibular fossa of temporal bone.

• Contains an articular disc dividing the joint cavity.

• Allows both hinge (depression/elevation) and gliding (side-to-side) movements.

• Most easily dislocated joint in the body.

Additional info: Understanding the structure and function of joints is essential for comprehending how the body moves and maintains stability. Damage to joint structures can lead to instability, pain, and decreased mobility, highlighting the importance of joint health in overall musculoskeletal function.