8.1 Joint Classification

• Anatomists classify joints based on either the histological structure of the joint (fibrous, cartilaginous, bony, or synovial) or the range of motion at the joint
• Three categories of joints:

  • Synarthrosis (immovable)
  • Amphiarthrosis (slightly movable)
  • Diarthrosis (freely moveable)
• A synarthrosis is a joint held together by dense, irregularly arranged connective tissue. Allow little or no movement
• At a synarthrosis, the bony edges are close together and may even interlock
• A suture is a fibrous synarthrotic joint found only between the bones of the skull
• The edges of these bones are bound together at sutures by the sutural ligament
• A synarthrosis allows forces to be spread easily from one bone to another with little or no joint movement, decreasing the chance of injury
• A gomphosis is a specialized fibrous synarthrosis that binds each tooth to its bony socket
• The fibrous connection for the gomphosis is the peridontal ligament
• In a growing bone, the diaphysis and epiphyseal ends are bound together by an epiphyseal cartilage, an example of cartilaginous synarthrosis, or a connection called synchondrosis
• When two separate bones fuse and the boundary disappears, it is called synostosis
• Amphiarthroses permits limited movement and the bones are farther apart than in synarthroses
• In fibrous amphiarthroses the bones are connected by collagen fibers
• Bones in a cartilaginous amphiarthroses are connected by a fibrous cartilage
• In a syndesmosis, a ligament connects the bones and limits movement at the joint
• At a symphysis, the bones are separated by a pad of fibrous cartilage
• In an amphiarthroses the bones of the joint are held together by fibrous cartilage, hyaline cartilage, or fibrous connective tissue. These joints allow limited movement
• An example of syndesmosis is the distal joint between the tibia and fibula or the membrane between the radius and ulna
• Diarthroses contain a fluid-filled cavity between the bones of the joint. Because these joints have a synovial membrane and contain synovial fluid, they are called synovial joints. Diarthroses are specialized for movement and have seven components
• A diarthrosis, or synovial joint, is specialized for movement and permits a wide range of motion
• The bony surfaces within a synovial joint are covered by articular cartilages and therefore are not in direct contact with one another
• Cartilages of diarthroses act as shock absorbers and help reduce friction with the joint
• Articular cartilage of diarthrosis have no perichondrium and matrix is more fluid than hyaline cartilage
• Synovial joints are found at the ends of the long bones of the upper and lower limbs
• Basic components of synovial joints:

  • joint capsule
  • articular cartilages
  • joint cavity filled with synovial fluid
  • synovial membrane lining the joint capsule
  • accessory structures
  • sensory nerves
  • blood vessels that supply the exterior and interior of the joint
• A synovial joint is surrounded by a joint capsule or articular capsule
• The joint capsule has an outer layer of thick, dense, regularly arranged connective tissue and an inner synovial membrane that lines the joint cavity
• Synovial fluid functions:

  • Lubrication and frictions reduction
  • Nutrient distribution
  • Shock absorption
• Menisci is an accessory structure
• Menisci are pads of fibrous cartilage that subdivide a synovial cavity
• Fat pads are an accessory structure
• Fat pads are found around the periphery of the joint and are lightly covered by the synovial membrane
• Capsular ligaments are thickenings of the joint capsule itself
• Extracapsular ligaments are an accessory structure
• Extracapsular ligaments are found outside the joint capsule
• Intracapsular ligaments are an accessory structure
• Intracapsular ligaments are inside the joint capsule
• Tendons are an accessory structure
• Bursae are an accessory structure
• Bursae are small fluid-filled pockets in connective tissue that reduce friction and act as shock absorbers