4.7 Joints

4.7.0.1 Joints can be classified functionally or structurally

• Functional classification depends on the amount of relative motion… less convenient
  • Large motions – diarthrodial joint
• Structural classification depends on joint mechanism

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4.7.1 Structural classification of joints

• Fibrous (or Immovable) joints
  • Held together by a thin layer of strong connective tissue
  • No movement between the bones such
    • Sutures of the skull
    • Teeth in their sockets
• Cartilaginous joints
  • Bones attached by white fibrocartilaginous discs and ligaments
  • Limited degree of movement
    • Intervertebral discs
    • The cartilage in the symphysis (binds the pubic bones together at the front of the pelvic girdle)
    • The cartilage between the sacrum and the hip bone

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• Synovial joints (subset of diarthrodial joints)
  • Covered with a layer of smooth articular (hyaline) cartilage
  • Enclosed by a bag-like capsular ligament which constrains the joint and helps contain the synovial fluid
  • The capsular ligament is lined with a synovial membrane. This membrane secretes synovial fluid into the cavity as a lubricant
  • In addition to the capsule, the bones are constrained by other strong ligaments
  • Very low friction coefficient
    • human knee – \(\mu=0.005-0.02\)
    • human hip – \(\mu=0.01-0.04\)
    • Lower than most engineered structures

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4.7.2 Synovial joints

@Bartel2006

• Ball and socket
• Condylar joints
• Multiple bone joints (Compound Joints)

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4.7.3 Ball and socket

Figure 4.1: AP Hip, AP Shoulder, and Axial Shoulder X-rays ⁶

• The rounded head interfaces the cup-shaped socket of another
  • Hip
    • Femoral head (ball) and acetabulum (socket)
  • Shoulder
    • Humeral head (ball) and glenoid (socket)
    • The glenoid is nearly flat… thus the shoulder requires significant soft tissue for stabilization

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4.7.4 Bi-condylar joints

⁷

• The condyles are the enlarged regions at knuckles of any joint.
  • They are enlarged, in part, to minimize the stresses of load transfer.
  • Examples: medial and lateral condyles of the femur and tibia

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4.7.5 Bi-condylar joints

• “Bi” - Pairs of articulating surfaces
• Two curved condyles articulate against relatively flat surfaces of the mating bone
• There is little kinematic constraint, thus, extensive soft tissue is required for stability.

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Unlabelled Image Missing

• Having two condyles allows for resistance to moments in planes perpendicular to the plane of motion.
  • The two condyles and their supporting soft tissues only allow small angular rotations in planes perpendicular to the intended plane
    • Less muscular restraint required about these planes

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• These joints have extensive range of motion in one plane
  • ie the knee has extensive flexion/extension in the sagittal plane

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4.7.6 Multiple bone joints

Figure 4.2: AP Hip, Oblique, and Lateral X-rays of the Foot ⁸

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4.7.7 Multiple bone joints

• More difficult to describe than ball-and-socket or bi-condylar joints
• Examples: wrist (8 carpal bones), ankle (7 tarsal bones)
• Bound by lots of ligaments and have synovial fluid
• Small motions between individual joints add up to larger motions

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6. http://medical-imaging2012.blogspot.com/2012/02/how-to-take-x-rays.html↩︎

7. https://classes.kumc.edu/som/radanatomy/image.asp?Image=7105-001.jpg&Film=7105&Features=1↩︎

8. http://orthopedicgallery.com/portfolio/foot-x-ray/↩︎