Biomechanics of Ligament and Tendon

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About Collagenous Tissues
Collagen Fiber
Strength of Tendons and Ligaments
Factors Affecting the Strength of Tendons and Ligaments

1. Nordin M, Lorenz T, Campello M (2001): Biomechanics of tendons and ligaments. In Nordin M & Frankel VH (eds): Basic Biomechanics of the Musculoskeletal System, 3^rd ed. Philadelphia, PA, USA: Lippincott Williams & Wilkins. pp.102-125.
2. Whiting, W.C. & Zernicke, R.F., 1998. Biomechanics of Musculoskeletal Injury. Champaign, IL, Human Kinetics. Chapter 2.

About Collagenous Tissues

Classification of collagenous tissues

• dense connective tissue
  • ligament： withstanding tensile stress
    • to augment capsule function for joint stability
    • to guide joint motions
    • to check excessive motion (static restraint)
  • tendon： withstanding tensile stress
    • to attach muscles to bone
    • to transit tensile loads from muscle to bone (dynamic restraint)
  • fascia or aponeurosis： fibrous ribbon-like membrane similar in composition to tendons

• loose connective tissues
  • capsule： withstanding tensile stress
    • to augment joint stability
    • to check excessive motion
  • skin： withstanding tensile stress
    • to protect internal structures
    • to check excessive motion
  • heel pad： withstanding shear stress
    • to provide shock absorption due to abundant adipose tissue inside
    • to resist shear stress

• cartilage
  • articular cartilage： withstanding compressive/ shear stress
    • to absorb the compressive loads
    • to allow motions between joint surfaces with minimal friction
    • to resist shear stress
  • fibrocartilage： withstanding compressive/ shear stress
    • to link two bony structure
    • to resist the compressive and/or shear loads

Components of Collagenous Tissues

• cell： ~20% of total volume
  • fibrobalst
  • chondrocyte

• extracellular matrix： ~80% of total volume
  • fiber
    • collagen fiber： for strength
    • elastin fiber： for flexibility
    • retin fiber： for mass
  • ground substance： PGs
    GAG bonded to a core protein, bind to a long hyaluronic acid (HA) chain
  • water： ~70% of extracellular matrix

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Collagen Fibers

Structure of collagen fiber

• the most abundant protein in the body (~1/3 of total protein in the body)

• tropocollagen： 3 procollagen polypeptide chains (a chains) coiled about each other into a left-handed triple helixes

collagen molecule： length: ~280 nm diameter: ~1.5 nm collagen fibril： parallel packing of several collagen molecules with cross-links diameter：110-120 nm in young adults

Types of collagen fiber

Tensile strength of collagen fiber

• closely associated with the number and quality of the cross-links within and between the collagen molecules
• stress-strain curve for an ideal collagen fiber
  • When the magnitude of the tensile strength is relatively small, a toe region is present because the relaxed, wavy collagen fiber is straightened
  • When the magnitude of the tensile strength is small, the elastic behavior of the collagen fiber follows Hooke's law
  • rupture as the tendon of the extensor digitorum longus is stretched by about 15% of its initial length or as the medial collateral ligament is stretched by about 20%　
  
• sources of tensile stress
  • for ligament： distraction of articular surfaces from mechanical actions
  • for tendon
    • passive increasing joint angle
    • active shortening of muscle fibers

Compressive Strength

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Strength of Ligaments and Tendons

Components of Connective Tissue

• cell： 20%
  • fibroblast
• matrix： 80%
  • water： 60-70% for ligaments
  • collagen： 70-80% of dry weight; molecular cross-link

Components of Connective Tissue

• cell： 20%
  • fibroblast
• matrix： 80%
  • water： 60-70% for ligaments
  • collagen： 70-80% of dry weight; molecular cross-link

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Factors Affecting Strength of Tendons and Ligaments

Age-Related changes

• before adolescent： ligament strength < bone strength
• maturation
  • increase in # and quality of cross-links
  • increase in diameter of collagen fibril
  • increase in tensile strength and stiffness
• aging
  • decrease in # of collagen fibers
  • collagen fibril concentration in the collagen fibers： controversial
  • decrease in tensile strength and stiffness

Pregnancy and the postpartum period

• increase in laxity of the tendons and ligaments in pubic area
• decrease in tensile strength of tendons and ligaments during later stages of pregnancy and the postpartum period
• decrease in stiffness during the early stage of postpartum period

Mobilization vs. immobilization

• remodeling in response to the mechanical demands placed upon it
• physical activity
  • mechanical strength： becomes stronger and stiffer
  • the diameters of the collagen fibers： increase
• immobilization
  • mechanical strength： weaker and less stiff
  • the diameters of the collagen fibers： controversial
• reconditioning after immobilization
  • do not return to normal at one year after injury

adapted from Noyes FR (1997). Clin Orthop 123, 210-242.

Steroids vs. nonsteroidal anti-inflammatory drugs (NSAID)

• steroid
  • inhibit collagen synthesis
  • decrease in stiffness, ultimate stress, and energy absorption ability
  • time- and dosage-dependent
• NSAID
  • increase tensile strength
  • increase cross-linkage of collagen molecules

Reconstruction surgery

• tendon graft： not the same as normal in mechanical properties

Pathological conditions

• diabetes mellitus
  
  

  pathology	proportion in DM
  tendon contracture	29%
  tenosynovitis	59%
  joint stiffness	40%
  capsulitis	16%

• hemodialysis
  
  

  pathology	proportion in hemodialysis
  tendon rupture	36%
  hyperlaxity of tendons or ligaments	74%
  patellar tendon elongation	49%
  articular hypermobility	51%

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