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Updated: Sep 18 2022



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  • Function
    • Transfer forces from muscle to bone to produce joint motion
    • Tendons orient themselves along stress
  • Types 
    • Paratenon covered tendons
      • e.g., patellar, achilles tendons
      • have rich vascular supply and thus heal better
      • often injured due to trauma and most often fail at the
        • musculotendinous junction
        • tendon-bone junction
    • Sheathed tendons
      • e.g., hand flexor tendons
      • less vascularized and have avascular areas that receive nutrition by diffusion
      • often injured due to laceration and at risk for adhesions
  • Anatomy
    • Composition
      • groups of collagen bundles (fascicles) separated by endotenon and surrounded by epitenon
      • composed of
        • water
          • tendons primarily composed of water
        • collagen
          • Type I collagen makes up 85% of dry weight of tendons
          • Type III collagen make up 0-5% of dry weight of tendons
        • proteoglycans
          • make up 0-5% of dry weight of tendons
          • decorin
            • is the most predominant proteoglycan in tendon
            • regulates collagen fiber diameter (length of 300nm, diameter of 1.5nm)
            • forms cross-links between collagen fibers and transfers loads between collagen fibers
          • aggrecan
            • is proteoglycan found in areas of tendon compression
    • Structure
      • has a highly ordered hierarchical structure
      • microfibrils
      • insert into bone via 4 transitional tissues of increasing modulus
        • tendon
          • type I and III collagen, elastin, proteoglycans, tendon fibroblasts
        • uncalcified fibrocartilage
          • aggrecan, types I, II and III collagen, fibrochondrocytes
          • tidemark - straight, basophilic line separating uncalcified and calcified fibrocartilage, a mechanical boundary between soft-hard tissue
        • calcified fibrocartilage (separated from fibrocartilage by tidemark)
          • type II collagen, aggrecan, types I and X collagen, fibrochondrocytes
          • irregular boundary, with interlocking of calcified fibrocartilage zone with bone
        • bone
          • osteocytes, osteoclasts, osteoblasts, type I collagen, apatite
    • Blood supply
      • the musculotendinous junction
      • the osseotendinous junction
      • vessels from various surrounding connective tissue (such as the paratenon, mesotenon and vincula)
        • FDS and FDP tendons have 2 vincula each (vincula longa and vincula brevia)
    • Cell biology
      • fibroblasts are predominant cell type
        • spindle shaped and arranged in parallel rows in direction of muscle loading
        • produce mostly type I collagen (85% of dry weight of tendons)
        • produce small amount of type III collagen (5% of dry weight)
        • responsible for healing process
      • Fibrous vs. Fibrocartilaginous Enthesis
      • Fibrous Enthesis
      • (Indirect Attachment)
      • Fibrocartilaginous Enthesis (Direct Attachment)
      • Attachment
      • Metaphysis and diaphysis of long bones
      • Epiphysis and apophysis
      • Composition
      • Perforating mineralized collagen fibers
      • 4 distinct zones (tendon, fibrocartilage, calcified fibrocartilage, and bone)
      • Angle of Insertion
      • Insertion angle changes slightly during motion
      • Insertion angle changes greatly during motion (thus prone to overuse injury)
      • Example
      • Deltoid-humerus attachment, adductor magnus-linea aspera attachment, pronator teres attachment
      • Rotator cuff, Achilles tendon
  • Material Properties
    • Characteristics
      • tendons contain more collagen and are less viscoelastic than ligaments
      • tendon enthesis contains free nerve endings responsible for nociception
      • viscoelastic behavior with nonlinear elasticity
        • the rate at which tendon sees force can influence the mechanical property
      • biomechanical effects
        • exercise has positive effect
        • immobilization has detrimental effect
        • age dependent
          • increase in strength from birth to maturity
          • decrease in strength after maturity
        • laser/heat causes tendons to shrink
        • vary with exposure to hydration, temperature, pH
          • tendons should be tested under physiologic relevant conditions
    • Advantages
      • strong in tension (can withstand 5-10% as opposed to 1-4% in bone)
    • Disadvantages
      • buckle in compression
      • demonstrate creep and stress relaxation
    • Load-elongation or stress-strain curve
      • toe region
        • initial nonlinear segment of curve during low loads due to tendons being "crimped"
      • linear region
        • intermediate loads
      • failure region
        • high loads
  • Tendon Healing
    • Stages of tendon healing
      • Stages of Soft Tissue Healing
      • (including tendons)
      • Hemostasis
      •  Platelets initiate coagulation cascade
      •  Fibrin clot and fibronectin interaction leading to chemotaxis to stabilize torn tendon edges
      • 5-15 minutes
      • Inflammation
      • Fibroblasts produce type III collagen
      • Macrophages help initiate healing and remodeling
      • 1-7 days
      • Proliferation
      • Tissue modeling via large amounts of disorganized collagen and angiogenesis
      • 7-21 days
      • Remodeling
      • Tissue remodeling replacing type III collagen to type I collagen
      • up to 18 months
  • Tendon Surgical Repair
    • Strength following repair
      • tendon repairs are weakest at 7-10 days
      • most of strength by 21-28 days
      • maximum strength at 6 months
        • final strength only reaches 2/3 of normal even years after repair
    • Early mobilization
      • allows earlier ROM but decreased tendon repair strength
      • beneficial for flexor tendon healing to prevent adhesion formation
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