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https://upload.orthobullets.com/topic/9015/images/myofibroblast.jpg
https://upload.orthobullets.com/topic/9015/images/loadelongationcurve2.jpg
Introduction
  • 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<subfibrils<fibrils<fascicles<tendon unit
    • insert into bone via 4 transitional tissues of increasing modulus)  
      1. tendon
        • type I and III collagen, elastin, proteoglycans, tendon fibroblasts
      2. 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
      3. 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
      4. 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 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
    • 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
    Organogenesis 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 mos.
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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(OBQ04.245) A load-elongation curve for a tendon is shown in Figure A. Which of the following statements accurately describes the region labeled "X"? Review Topic

QID: 1350
FIGURES:
1

The failure region which has crimped tendon fibers

1%

(3/427)

2

The linear region which has parallel oriented tendon fibers

7%

(32/427)

3

The linear region which has crimped tendon fibers

7%

(28/427)

4

The toe region which has parallel oriented tendon fibers

7%

(28/427)

5

The toe region which has crimped tendon fibers

77%

(329/427)

Select Answer to see Preferred Response

PREFERRED RESPONSE 5
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