Skeletal System Articulations: Comprehensive Study Guide

Introduction to Articulations and Arthrology

  • Definition of Joints (Articulations):

    • Places of union between bones, bones and cartilage, or bones and teeth.

    • Bones are said to "articulate" at a joint.

  • Arthrology: The scientific study of joints.

  • Primary Functions based on Joint Type:

    • Fibrous joints: Primarily hold two bones together.

    • Cartilaginous joints: Primarily resist compression and tension stress; act as resilient shock absorbers.

    • Synovial joints: Primarily facilitate movement, making them all freely mobile.

General Classification of Joints

Structural Classification

Joints are categorized into three classes based on the materials binding the bones together and whether a space occurs between the articulating bones:

  • Fibrous Joint: Bones are held together by dense regular connective tissue; no joint cavity is present.

  • Cartilaginous Joint: Bones are joined specifically by cartilage; no joint cavity is present.

  • Synovial Joint: Bones are joined by ligaments with a fluid-filled joint cavity separating the bone surfaces.

Functional Classification

Joints are categorized into three classes based on the extent of movement they permit:

  • Synarthroses: Immobile joints. These can be structurally classified as either fibrous or cartilaginous joints.

  • Amphiarthroses: Slightly mobile joints. These can be structurally classified as either fibrous or cartilaginous joints.

  • Diarthroses: Freely mobile joints. All synovial joints fall into this functional category.

The Inverse Relationship of Mobility and Stability

  • Range of Motion: Motion ranges from no movement (immobile) to extensive movement.

  • Determinants of Movement: The unique structure of each joint determines its specific mobility and stability.

  • The Trade-off: There is an inverse relationship between mobility and stability.

    • Stable but Limited Mobility: The more stable a joint is, the less mobile it is (e.g., skull sutures are immobile but provide high stability).

    • Mobile but Less Stable: The more mobile a joint is, the less stable it is (e.g., the glenohumeral/shoulder joint is the most mobile but the least stable).

  • Stability Continuum (from most stable/least mobile to least stable/most mobile):

    1. Suture (Immobile, most stable).

    2. Interosseous membrane.

    3. Intervertebral joints (Slightly mobile).

    4. Knee joint.

    5. Glenohumeral joint (Shoulder) (Freely mobile, least stable).

Detailed Study of Fibrous Joints

General Characteristics

  • Connected by dense regular connective tissue.

  • Lacks a joint cavity.

  • Functional category: Immobile (synarthroses) or only slightly mobile (amphiarthroses).

Three Most Common Types

  • Gomphoses ("Peg in a socket"):

    • Location: Articulations of teeth (roots) with the alveolar processes (sockets) of the mandible and maxillae.

    • Structure: The tooth is held in place by fibrous periodontal ligaments (also called periodontal membranes).

    • Function: Synarthroses (immobile).

  • Sutures:

    • Location: Found exclusively between certain bones of the skull.

    • Structure: Composed of very short fibers with interlocking, irregular edges.

    • Benefits: The irregular edges increase stability and decrease the risk of fracture.

    • Function: Synarthroses (immobile).

    • Life Cycle: They allow for skull growth in childhood. In older adults, they become ossified and are then called synostoses.

  • Syndesmoses:

    • Structure: Bones are bound by a broad ligamentous sheet called an interosseous membrane (or interosseous ligament).

    • Location: Found between parallel long bones, specifically the radius and ulna in the forearm, and the tibia and fibula in the lower leg.

    • Function: Amphiarthroses (slightly mobile). They provide a pivot point for the two long bones.

Detailed Study of Cartilaginous Joints

General Characteristics

  • Bones are joined by either hyaline cartilage or fibrocartilage.

  • Lacks a joint cavity.

  • Functional category: Immobile (synarthroses) or slightly mobile (amphiarthroses).

Types of Cartilaginous Joints

  • Synchondroses:

    • Composition: Bones joined specifically by hyaline cartilage.

    • Examples:

      1. Epiphyseal plate in growing long bones (located beneath the rounded head of the bone).

      2. Joint between the first rib and the sternum.

      3. Costochondral joints (immobile joints between the ribs and their costal cartilage).

    • Function: Synarthroses (immobile).

  • Symphyses:

    • Composition: Pads of fibrocartilage between articulating bones.

    • Function: Resist compression and act as resilient shock absorbers.

    • Examples:

      1. Pubic symphysis (located in the middle of the inferior joint of the pubic bones).

      2. Intervertebral joints (specifically the intervertebral discs located between the bodies of vertebrae).

    • Function: Amphiarthroses (slightly mobile).

Clinical View: Costochondritis

  • Definition: Inflammation of the costochondral joints.

  • Symptoms: Localized chest pain.

  • Etiology: The cause is usually unknown, though it may result from chest trauma or infection.

  • Differential Diagnosis: The pain may be mistaken for that of a myocardial infarction (heart attack).

  • Treatment: Generally treated with nonsteroidal anti-inflammatory drugs (NSAIDs).

Anatomy of Synovial Joints

General Features

  • Bones are separated by a space called the joint cavity (articular cavity).

  • Most joints in the human body are synovial joints.

  • Functional category: Diarthroses (freely mobile).

Seven Common Structural Components

  1. Articular Capsule (Joint Capsule): A double-layered envelope encapsulating the joint cavity.

    • Outer Fibrous Layer: Formed from dense connective tissue. It strengthens the joint and prevents bones from being pulled apart.

    • Inner Synovial Membrane: Composed primarily of areolar connective tissue. It covers all internal joint surfaces not covered by cartilage and is responsible for producing synovial fluid.

  2. Joint Cavity: The space between articulating bones lined by the synovial membrane.

  3. Synovial Fluid: A viscous, oily substance secreted by the synovial membrane.

    • Function 1: Lubrication: Reduces friction on the articular cartilage during movement.

    • Function 2: Nutrient Distribution: Nourishes the chondrocytes (cartilage cells) of the avascular articular cartilage.

    • Function 3: Shock Absorption: Distributes stress evenly across the joint surfaces.

  4. Articular Cartilage: A layer of hyaline cartilage covering the bone surfaces at the joint.

    • Characteristics: Lacks a perichondrium and is avascular (lacks blood vessels).

    • Functions: Reduces friction, acts as a cushion to absorb compression, and prevents damage to the articulating ends of bones.

  5. Ligaments: Composed of dense regular connective tissue.

    • General Function: Connect one bone to another; stabilize, strengthen, and reinforce the joint.

    • Extrinsic Ligaments: Physically separate from the articular capsule.

    • Intrinsic Ligaments: Represent thickenings of the articular capsule itself; include structures both inside and outside the capsule.

  6. Sensory Receptors: Numerous receptors that detect painful stimuli and report on joint movement, compressive forces, and stretch.

  7. Blood Vessels: Plentiful supply to provide nutrients and waste removal for the joint tissues.

Non-Joint Stabilizer: Tendons

  • Composition: Dense regular connective tissue.

  • Anatomy: Attach muscle to bone.

  • Note: Tendons are not part of the synovial joint itself but are crucial for joint stabilization.

Accessory Structures of Synovial Joints

  • Bursae:

    • Structure: Fibrous, saclike structures containing synovial fluid and lined with a synovial membrane.

    • Location: Found where bones, ligaments, muscles, skin, or tendons rub together.

    • Function: Alleviate friction.

    • Example (Knee Joint): Includes the suprapatellar bursa, prepatellar bursa, and infrapatellar bursae.

  • Tendon Sheaths:

    • Structure: Elongated bursae that wrap around tendons.

    • Location: Common in areas of excessive friction, such as the wrist and ankle.

  • Fat Pads:

    • Function: Act as protective packing material in the joint periphery and fill spaces created when joint shapes change during movement.

Classification of Synovial Joints

Classes by Movement (Axes)

  • Uniaxial: Bone moves in only one plane or axis.

  • Biaxial: Bone moves in two planes or axes.

  • Multiaxial (or Triaxial): Bone moves in multiple planes or axes.

Classes by Shape (Least Mobile to Most Mobile)

  1. Plane Joint: Articular surfaces are flat. Simplest synovial articulation. Uniaxial (limited side-to-side gliding in one plane).

  2. Hinge Joint: Convex surface of one bone fits into a concave depression of another. Uniaxial (like a door hinge).

  3. Pivot Joint: A bone with a rounded surface fits into a ring formed by another bone and a ligament. Uniaxial (rotation on a longitudinal axis).

  4. Condylar Joint: An oval, convex surface articulates with a concave surface. Biaxial.

  5. Saddle Joint: Articulating surfaces have both convex and concave regions, resembling the shape of a saddle. Biaxial.

  6. Ball-and-Socket Joint: A spherical head of one bone fits into a cuplike socket of another. Multiaxial (movement in three planes). This is the most freely mobile joint type.

Section Review: Questions & Discussion

Section 9.1: Classification of Joints

  1. What is the relationship between mobility and stability in a joint?

    • There is an inverse relationship; as mobility increases, stability decreases.

  2. Are all fibrous joints also synarthroses? Explain why or why not.

    • No. While sutures and gomphoses are synarthroses, syndesmoses are classified as amphiarthroses (slightly mobile).

Section 9.2: Fibrous Joints

  1. Where are gomphoses located, and what type of movement do they allow?

    • They are located between the roots of teeth and the alveolar processes of the mandible/maxillae. They allow no movement (synarthroses).

  2. What is the composition of a suture, and where in the body is it found?

    • They are composed of very short dense regular connective tissue fibers and are found exclusively in the skull.

  3. What type of movement is allowed at a syndesmosis?

    • Slight movement (amphiarthroses), specifically providing a pivot for two parallel long bones.

Section 9.3: Cartilaginous Joints

  1. What is the composition of a synchondrosis, and where is it located in the body?

    • Composition is hyaline cartilage. Locations include the epiphyseal plates of growing bones and costochondral joints.

  2. Into what functional category is a symphysis placed? Why is it in this category?

    • It is an amphiarthrosis. It is in this category because it allows for slight mobility while acting as a shock absorber.

Section 9.4: Synovial Joints

  1. Diagram and label the general components of a synovial joint.

    • (Students should identify: Articular capsule [fibrous layer/synovial membrane], joint cavity, synovial fluid, articular cartilage, and ligaments).

  2. What are the three primary functions of synovial fluid in the joint?

    • Lubrication of articular cartilage, nourishment of chondrocytes, and shock absorption.

  3. What types of movements do each of the six kinds of joints allow?

    • Plane (uniaxial gliding), Hinge (uniaxial), Pivot (uniaxial rotation), Condylar (biaxial), Saddle (biaxial), Ball-and-Socket (multiaxial).