Updated: 10/25/2018

Ligaments

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https://upload.orthobullets.com/topic/9016/images/sharpey fibers.jpg
https://upload.orthobullets.com/topic/9016/images/creep.jpg
https://upload.orthobullets.com/topic/9016/images/hysteresis.jpg
https://upload.orthobullets.com/topic/9016/images/stressstrain.jpg
Introduction
  • 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 amoutns 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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Questions (6)
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(OBQ05.158) Ligaments attach to bone by both direct insertion and indirect insertion. Which of the following most accurately describes the order of the four transition zones of direct insertion? Review Topic

QID: 1044
1

Ligament > fibrocartilage > mineralized fibrocartilage > bone

62%

(677/1091)

2

Ligament > mineralized fibrocartilage > fibrocartilage > bone

4%

(44/1091)

3

Ligament > mineralized fibrocartilage > periosteum > bone

11%

(119/1091)

4

Ligament > Sharpey's fiber > periosteum > bone

22%

(235/1091)

5

Ligament > periosteum > fibrocartilage > bone

1%

(8/1091)

ML 3

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