Chapter 09: Skeletal System - Articulations

Anatomy & Physiology: Skeletal System - Articulations

9.1 Classification of Joints

• Joints (articulations): Places of union between bones, bones and cartilage, or bones and teeth.

• Bones articulate at a joint.

• Arthrology: The study of joints.

• Classification Criteria: Joints can be classified by structure and function.

Structural Classes of Joints

• Fibrous Joint:

  • Bones held together by dense connective tissue.

• Cartilaginous Joint:

  • Bones joined by cartilage.

• Synovial Joint:

  • Bones joined by ligaments with a fluid-filled joint cavity separating bone surfaces.

Range of Motion at Joints

• Motion ranges from no movement to extensive movement.

• The structure of each joint determines its mobility and stability.

• Trade-off Relationship:

  • There is an inverse relationship between mobility and stability.

  • Example: Skull sutures are immobile but very stable.

Joint Classification Based on Function

• Synarthroses:

  • Immobile joints with no joint cavity.

• Amphiarthroses:

  • Slightly mobile joints.

• Diarthroses:

  • Freely mobile joints with a joint cavity (synovial joints).

Types of Joints

• Fibrous Joints:

  • Examples: Gomphosis, Suture, Syndesmosis

• Cartilaginous Joints:

  • Examples: Synchondrosis, Symphysis

• Synovial Joints:

  • Types include: Plane, Hinge, Pivot, Condylar, Saddle, Ball-and-socket.

The Relationship Between Mobility and Stability in Joints

• For every joint, there is a trade-off between mobility and stability.

  • Stable (yet limited mobility):

  • The more stable a joint, the less mobile it is.

  • Fibrous Joints: Primary function to hold two bones together, can be immobile or slightly mobile.

    • Examples: Sutures, syndesmosis (interosseous membrane).

  • Mobile (yet less stable):

  • The more mobile a joint, the less stable it is.

  • Cartilaginous Joints: Function to resist compression and tension stress, and act as resilient shock absorbers.

    • Example: Intervertebral joints.

  • Synovial Joints:

  • Primary function is movement; all are freely mobile.

    • Examples: Glenohumeral joint (shoulder) and knee joint.

9.4 Distinguishing Features and Anatomy of Synovial Joints

Components of Synovial Joints

• Articular Cartilage:

  • Hyaline cartilage on the bone surface at joint.

  • Functions:

    • Reduces friction during movement.

    • Acts as a cushion to absorb joint compression.

    • Prevents damage to articulating ends of bones.

    • Lacks a perichondrium and is avascular.

• Joint Cavity:

  • Space between articulating bones, lined by synovial membrane secreting synovial fluid.

  • Synovial Fluid: Viscous, oily substance.

    • Functions:

    • Lubricates articular cartilage on articulating surfaces.

    • Nourishes the chondrocytes of articular cartilage.

    • Acts as a shock absorber.

• Sensory Receptors and Blood Vessels:

  • Numerous in synovial joints to detect painful stimuli, report on movement, compressive forces, and stretch.

• Tendons:

  • Composed of dense regular connective tissue.

  • Not part of the synovial joint, instead attach muscle to bone, helping stabilize joints.

• Bursae:

  • Fibrous, saclike structures containing synovial fluid.

  • Lined internally by synovial membrane.

  • Associated with joint cavity or completely separate from it, alleviate friction where bones, ligaments, muscles, skin, or tendons rub together.

Knee Joint Bursae

• Example of the organization of bursae in the knee joint, showing locations and associated structures such as the patellar ligament, quadriceps femoris tendon, and various bursae (suprapatellar, prepatellar, infrapatellar).

Other Accessory Structures in Synovial Joints

• Tendon Sheaths:

  • Elongated bursae that wrap around tendons where friction is excessive, common in wrist and ankle.

• Fat Pads:

  • Act as protective packing material in joint periphery, can fill spaces when joint shapes change.

9.6 Synovial Joints and Levers

• Biomechanics: Application of mechanical principles to biology. Synovial joint movements can be compared to the mechanics of a lever system.

Types of Levers

• First-Class Levers:

  • Fulcrum is between effort and resistance.

  • Example: Atlanto-occipital joint of the neck, similar to a pair of scissors where effort is applied to the handle and the fulcrum is in the middle.

• Second-Class Levers:

  • Resistance is between fulcrum and effort. Rare in the body.

  • Example: Standing on tiptoe, where small force balances a larger weight.

• Third-Class Levers:

  • Effort applied between resistance and fulcrum. Most common in the body.

  • Example: At elbow joint where the joint acts as the fulcrum, the biceps tendon applies effort, and the weight in hand provides resistance.

Clinical View: Knee Injuries and Replacement Surgery

• Ligamentous injuries increase the risk of osteoarthritis of the knee.

• In severe cases, those affected may opt for knee replacement surgery where arthritic parts of bones are removed and replaced with metal or plastic components.