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Updated: Sep 18 2022


Images fibers.jpg
  • function
    • Ligaments function to
      • restrict joint motion
      • stabilize joint
      • have mechanoreceptors and free nerve endings that help with joint proprioception
  • Composition
    • Extracellular components consist of
      • water
      • Type I collagen (70% of dry weight)
      • elastin
        • higher elastin content than tendons
      • lipids
      • proteoglycans
      • epiligament coat
        • present in some ligaments, not all
        • analogous to epitenon of tendons
    • Cellular component
      • the main cell type in both tendons and ligaments is the fibroblast
      • both tendons and ligaments have low vascularity and cellularity
    • Ligaments vs. tendons
      • composition
        • compared to tendons, ligaments have
          • lower percentage of collagen
          • higher percentage of proteoglycans and water
          • less organized collagen fibers
          • rounder fibroblasts
  • Bone insertion
    • Two types of ligament bone insertion
      • indirect (fibrous insertion)
        • most common form of bone insertion
        • superficial fibers insert into the periosteum
        • deep fibers insert directly into bone via perforating collagen fibers called Sharpey fibers
        • at insertion, endotenon becomes continuous with periosteum
        • examples
          • MCL inserting into proximal tibia
      • direct (fibrocartilaginous insertion)
        • has both deep and superficial fiber insertion
        • deep fibers
          • have four transitional zones of increasing stiffness that allow for force dissipation and reduce stress concentration
            • Zone 1 (tendon or ligament proper)
              • consists of well aligned type I collagen fibers with small amounts of proteoglycan decorin
            • Zone 2 (fibrocartilage)
              • consists of types II and III collagen, with small amounts of type I, IX and X collagen, and proteoglycans aggrecan and decorin
            • Zone 3 (mineralized fibrocartilage)
              • consists of type II collagen, with significant amounts of type X collagen and aggrecan
            • Zone 4 (bone)
              • is made up of type I collagen, with high mineral content
        • examples
          • supraspinatus insertion
  • Blood Supply
    • Origin
      • receives blood supply at insertion site (different from tendons)
        • ACL (and PCL) receives blood supply from middle geniculate artery
      • have uniform microvascularity within ligament
  • Biomechanical Properties
    • Stress relaxation
      • decreased stress with time under constant deformation
    • Creep
      • increased deformation with time under constant load
    • Hysteresis (energy dissipation)
      • when tissue is loaded and unloaded, the unloading curve will not follow the loading curve
      • the difference between the 2 curves is the energy that is dissipated
    • Stress-strain (load-elongation) curve
      • toe region
        • significant deformation for given load
        • in this region, the crimped and relaxed fibers of the ligament straighted to take up load
      • linear region
        • fibers oriented longitudinal and parallel to load
        • constant load-elongation
        • stiffness = slope of load-elongation curve in this region
          • Young's modulus of elasticity
      • yield and failure region
        • nonlinear
        • yield point
          • transition from elastic (reversible) to plastic (irreversible) deformation
        • ultimate failure
          • point before steep decline in load-deformation curve
    • Ligament vs. tendons
      • stress-strain differences between tendons and ligaments
        • tendons carry higher loads, recruit fibers quickly
          • smaller toe region
        • ligaments recruit fibers gradually
          • elongated toe region
  • Ligament Failure
    • Mechanism
      • rupture of sequential series of collagen fibers
      • ligaments do not plastically deform
    • Failure site
      • usually midsubstance in adults
      • usally at bony insertion in children
        • ligament avulsion
          • occurs at junction of mineralized and unmineralized fibrocartilage layers
    • Classification
      • ligament injuries are classified into 3 grades
        • Grade I
          • corresponds to mild sprain
        • Grade II
          • corresponds to moderate sprain/partial tear
        • Grade III
          • corresponds to complete tear
  • Ligament Healing
    • Phases
      • inflammatory phase
        • occurs at 1-7days
        • influx of neutrophyils and macrophages
        • production of type III collagen
        • growth factors involved
          • TGF-β1
          • IGF
          • PDGF
          • BMPs -12 and -13
          • bFGF
      • proliferation phase
        • occurs at 7-21 days
        • gradually replaced by type I collagen
        • tendons and ligaments are weakest at day 5-21
      • remodeling phase
        • occurs at >14 days
      • maturation phase
        • up to 18 months
    • Factors that impair ligament healing
      • intra-articular
        • extra-articular ligaments (e.g. knee MCL) have a greater capacity to heal compared with intra-articular ligaments (e.g. knee ACL)
      • increasing age
      • immobilization
        • reduces strength of both intact and repaired ligament
      • smoking
      • NSAIDS
        • including indocin, celcoxib, parecoxib
      • diabetes
      • alcohol intake
      • decreased growth factors
        • bFGF, NGF, and IGF-1
      • decreased expression of genes involved with tendon and ligament healing
        • examples include
          • procollagen I
          • cartilage oligomeric matrix protein (COMP)
          • tenascin-C
          • tenomodulin
          • scleraxis
    • Factors that improve ligament healing (experimental)
      • extra-articular
        • extra-articular ligaments (e.g. knee MCL) have a greater capacity to heal compared with intra-articular ligaments (e.g. knee ACL)
      • compromised immune response
        • CD44 (receptor for lymphocyte activation) knockout mice have faster patellar tendon healing
        • Interleukin 10 (anti-inflammatory cytokine) improves patellar tendon healing in mice
        • Interleukin 1 (inflammatory mediator) receptor antagonist inhibits loss of mechanial properties in patellar tendons in rabbits
        • depletion of macrophages (source of TGF-β1 that stimulates fibrosis) improves ACL graft healing in mice (less scar, more fibrocartilage)
      • mesenchymal stem cells
        • improved healing of tendon graft in bone tunnel in rabbits and rats
        • promote healing of partial tears of digital flexor tendons in horses
        • insufficient for rat rotator cuff repair (shear stresses too high)
      • growth factors
        • PDGF-BB
          • increases cellular proliferation and limits adhesions in dog flexor tendon repairs, but provides no improvement in tensile strength
        • GCSF
          • improves tendon incorporation into bone tunnels in ACL reconstruction in dogs
        • BMP-2 and -12
          • improves healing in animal rotator cuff models
      • scaffolds to help primary ligament healing (instead of reconstruction)
        • collagen-platelet-rich plasma hydrogel helps primary ACL repair
          • but still inferior to native ACL strength
      • neuropeptides
        • denervation degrades tendons and ligaments
        • calcitonin gene-related peptide improves MCL healing in rabbits
    • Scarring
      • tendons and ligaments heal with scar tissue that
        • reduces ultimate strength
        • causes adhesions
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