Ligaments vs. Tendons vs. Cartilage

Ligaments, Tendons, Cartilage, and Meniscus Comparison

• Ligaments:

  • Extracellular Matrix (ECM): Fibroblasts are the dominant cell type, with a low cell density and poor blood supply (hypovascular).
  • Poor healing response due to low vasculature, resulting in reparative tissue formation and poorly organized ECM.
  • Composed of hierarchical structures: tropocollagen (1.5 nm), microfibril (3.5 nm), subfibril (10-20 nm), fibril (50-500 nm), fascicle (5-30 µm) with crimp patterns.
  • Crimp straightening allows for force absorption and distribution to prevent injury.
  • Low elastin content, typically less than 3% by dry weight.
  • High collagen content (type 1), ranging from 75-80% by dry weight.
  • Locations include joints.

• Tendons:

  • ECM: Fibroblasts are the dominant cell type, with a low cell density and poor blood supply (hypovascular).
  • Healing is limited by a low cell count and less vasculature, leading to reparative scar tissue and an irregularly organized ECM.
  • Composed of hierarchical structures: tropocollagen (1.5 nm), microfibril (3.5 nm), subfibril (10-20 nm), fibril (50-500 nm), fascicle (5-30 µm) with crimp patterns.
  • Straightening of crimp patterns allows tendons to absorb and distribute forces efficiently.
  • Lower elastin content compared to ligaments (typically less than 5% by dry weight)
  • High collagen content (type 1), approximately 75-85% by dry weight.
  • Locations include connecting muscles to bones.

• Hyaline Cartilage:

  • ECM: Low cell density (hypocellular) and avascular (no blood supply), hindering repair.
  • Chondrocytes reside in a low-nutrient, hypoxic environment.
  • Hierarchical structure: nano (type II collagen and proteoglycans), micro (four cartilage zones), tissue levels.
  • Primarily composed of type II collagen (50-75% by dry weight)
  • High proteoglycan content (20-30% by dry weight).
  • High water content (55-65% by wet weight), providing viscoelastic properties and shock absorption.
  • Multiple zones exist within the cartilage structure—superficial, middle, deep, and calcified. Different zones/layers have distinct collagen and water content that help support the cartilage functions.
  • Locations include articular surfaces of joints, and certain structural elements (ie. nose, ribs).

• Meniscus:

  • Function: Load distributor for the femoral condyles across the tibial plateau, increasing the surface area, and significantly reducing stress on the articular cartilage.
  • Composition: Composed of type I collagen, arranged circumferentially, to bear hoop stress.
  • Structure: Several distinct zones, each with a specific collagen alignment for absorbing and distributing forces to the cartilage.
  • Zones of cartilage—superficial, middle, deep, and calcified that help with absorbing and distributing stress.
  • Moderate elastin content to allow for proper stretching and recoil.

Osteoarthritis

• Stress: Force divided by area.
  • Lower contact area means higher stress. Increased contact area disperses the stress.
• Meniscus Function in the Joint:
  • Load distributor; large area means reduced stress.
• Effects of Repetitive Heavy Mechanical Loading on Cortical Bone:
  • Weight-bearing exercise increases cortical bone thickness.
• Effects of Aging on Cancellous Bone:
  • Stress on osteocytes increases bone formation.

Intervertebral Disc

• Structure: Nucleus pulposus (water and proteoglycans) surrounded by the anulus fibrosus (collagen and elastin).
• Function:
  • Nucleus pulposus bears most of the compressive load; anulus fibrosus absorbs tensile and shear stresses, and maintains disc stability. Lamellae help resist shear.
• Types of Muscle Fibers:
  • Aerobic (slow-twitch): Endurance, ATP production with oxygen.
  • Anaerobic (fast-twitch): Rapid contraction, ATP without oxygen.
  • Fast-twitch oxidative glycolytic (2A): Intermediate.
  • Fast-twitch glycolytic (2B): Fastest contraction, fatigues quickly.

Blood Pressure

• Blood pressure: Systolic pressure/Diastolic pressure (e.g. 120/80 mmHg) influenced by heart rate, output, and Frank-Starling mechanism.
• Vascular Wall Layers:
  • Tunica intima: Inner layer (endothelial cells and basal lamina)
  • Tunica media: Middle layer—smooth muscle, elastin and type III collagen.
  • Tunica adventitia: Outer layer (fibroblasts, type 1 collagen, form attachments)

Muscle Contraction

• Action Potential:
  • Release of Ca²⁺ from sarcoplasmic reticulum.
  • Ca²⁺ facilitates actin-myosin binding.
  • Myosin head pulls actin (with ATP utilization).

Skeletal System Response

• Bone Adaptation:
  • Longitudinal growth associated with cartilage growth plate during development.
  • Radial expansion through bone resorption (osteoclasts) and formation (osteoblasts).

Bone Cells and Signals

• Bone remodeling: Repair, remove old/damaged bone, and replace it with new.
• Osteocytes: Strain gauges that guide remodel processes
• Wolff's Law: Bone adapts to mechanical environment.
• Mechanostat Theory: Mechanical loading regulates bone's mechanical behavior.

Load Sharing and Stress Shielding in Implants

• Implants need to have similar elasticity for loading sharing (and less stress shielding—less bone loss). Different materials have differing modulus of elasticity; need to match or come close for use as bone implants.

Adjacent Segment Disease (ASD)

• ASD is spinal segment degeneration/increased stress
  • This appears adjacent to segments repaired for trauma (surgery—e.g. fusion).
• Vascular Stents and Adjacent Vascular Distension: Stents can affect surrounding blood vessels (proximal = before; distal = after

Explore the differences and similarities between ligaments, tendons, cartilage, and meniscus. This quiz delves into their structural components, healing properties, and functions within the body. Test your understanding of these essential connective tissues!