Overview
This lecture covers the anatomy, structure, and function of synovial (diarthrosis) joints, including their types, accessory structures, and stability versus flexibility trade-offs.
Synovial Joint Anatomy
- Synovial joints (diarthroses) are freely movable joints essential for movement.
- Long bone morphology includes compact bone (outer), spongy bone (inner), medullary cavity (marrow), and periosteum (outer covering).
- Articular cartilage (hyaline cartilage) covers bone ends at joints.
- Joint capsule merges with periosteum, providing protection.
- Synovial membrane lines the inside of the capsule and secretes synovial fluid.
- Joint cavity contains synovial fluid for lubrication, cushioning, and nutrient distribution.
Accessory Structures of Synovial Joints
- Bursae are fluid-filled sacs that reduce friction and cushion joints.
- Tendon sheaths are bursae that wrap around tendons for protection.
- Menisci (singular: meniscus) are fibrocartilage pads providing additional support.
- Fat pads offer extra cushioning.
- Ligaments stabilize joints.
- Joints also include nerves, blood vessels, and lymphatics.
Specific Synovial Joints and Movements
- Joint stability and flexibility are inversely related; more flexibility usually means less stability.
- Axes of movement: nonaxial (sliding only), monoaxial (one axis), biaxial (two axes), triaxial (three axes).
- Shoulder joint: triaxial, very flexible due to shallow glenoid cavity; less stable.
- Elbow joint: monoaxial, stable due to deep fit of ulna and strong ligaments.
- Hip joint: triaxial, deep acetabulum, strong ligaments (iliofemoral, pubofemoral, ischiofemoral), screw-home mechanism tightens joint during extension.
- Knee joint: monoaxial, stability aided by menisci and ligaments (ACL, PCL inside; LCL, MCL outside).
Diagnosing Ligament Injuries
- ACL and PCL prevent anterior and posterior translation of tibia, respectively.
- Drawer tests (anterior and posterior) assess ligament integrity by checking tibia movement relative to femur.
Key Terms & Definitions
- Diarthrosis â freely movable joint, synonymous with synovial joint.
- Synovial fluid â lubricates and nourishes joint cartilage.
- Bursa â fluid-filled sac reducing joint friction.
- Meniscus â fibrocartilage disk improving fit and stability of a joint.
- Ligament â dense connective tissue stabilizing joints.
- Monoaxial/Biaxial/Triaxial â joints with 1, 2, or 3 axes of movement.
- ACL (Anterior Cruciate Ligament) â prevents forward sliding of tibia.
- PCL (Posterior Cruciate Ligament) â prevents backward sliding of tibia.
Action Items / Next Steps
- Review diagrams/models of synovial joints, especially knee and shoulder.
- Learn key ligament names and functions for knee and hip.
- Prepare for lab identification of joint structures.