January 20

1
January 20

• Soft Tissue Injury Biomechanics
• Chapter 4

2
Review

• Basic biomechanics
• Bone
• Growth
• Anisotropy
• Viscoelasticity
• Response to stress
• Fracture and healing

3
Today

• Cartilage
• Tendon
• Ligament
• Muscle

4
Articular Cartilage

• Key structural ingredients
• Collagen
• Proteoglycans
• Fluid

5
Collagen fiber orientation in hyaline cartilage
Why?
6
Tension in cartilage

• Since collagen only resists force in tension, the
  varied orientation of collagen fibers in
  articular cartilage enables cartilage to resist a
  variety of different loading directions

7
Forces encountered in cartilage

• Shear at articular surface, as the two bone ends
  move past each other
• Cyclic loading and unloading
• Swelling, as hydrophilic proteoglycan aggregates
  repel each other and draw water into the
  extracellular matrix
• Compressive creep in weightbearing

8
Collagen Cartilage in tensile loading
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Lubrication Mechanisms

• Diarthrodial joints have been called natural
  bearings that are nearly frictionless and nearly
  wear resistant throughout our lives
• (Mow, Ratcliffe, Poole 1992)
• Two principal lubrication mechanisms are at work
  (we think)
• Boundary lubrication
• Fluid-film lubrication
• And maybe
• Squeeze film lubrication
• Boosted lubrication

10
Lubrication mechanism

• Like loading of collagen, the different methods
  are effective for different scenarios
• Surfaces gliding past one another
• Distributing force when weightbearing (Pressure)
• Reacting when fluid is forced out of the
  cartilage
• (Fluid Mechanics)

11
Articular Cartilage Adaptation

• Already extremely well adapted
• If used too much or too little, breakdown in
  quality can occur
• Proper use active loading facilitates diffusion
  of nutrients through the cartilage matrix (which
  is avascular in adults)
• Cartilage does adapt, but most adaptation is only
  degenerative

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Development, Maturation, and Aging

• Immature cartilage
• Blue-white, thicker
• Transition in content

Greater relative water content Highest
proteoglycan content
Greater relative collagen content Lowest
proteoglycan content
AGE
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Use vs. Disuse

• Exercise produces swelling of articular cartilage
• Dual, opposing outcomes

• Chondrocyte hypertrophy
• Increase in pericellular matrix
• Increased number of cells per unit of cartilage

• Wear and tear
• Osteoarthritis

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OA

• Material properties of bone and cartilage normal
• Applied loads are excessive

• Applied loads are reasonable
• Material properties of bone and cartilage are
  inferior

Among the factors that may contribute are
altered joint mechanics especially excessive
joint laxity due to previous ligament injury
pg.115 (Mankin et al. 1994)
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What about disuse?

• Reduced proteoglycan
• Increased surface fibrillation
• Degraded material properties
• When loaded
• More rapid deformation
• Fluid rapidly exudes from matrix

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Cartilage Injury

• Skipping to Ch. 5 page 139

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Injury

• Experimental data suggest that excessive joint
  loading leads to three types of articular damage
• Loss of cartilage matrix macromolecules,
  alteration of the macromolecular matrix, or
  chondrocyte injury
• Can occur with no detectable tissue disruption
• Isolated damage to the articular cartilage itself
  in the form of chondral fracture or flap tears
• Injury to the cartilage and its underlying bone
  (osteochondral fracture)

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Healing?

• Articular cartilage is unable to repair defects
  of any significant size
• No blood vessels
• Relatively few cells

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(No Transcript)
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Microfracture Surgery

• Developed in early 90s by Dr. Richard Steadman
• Small fractures induced in bone surface. Clot
  (w/stem cells) reduces chondrocytes
• Treatment du jour in NBA

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Fibrocartilage

• Transitional tissue at osteotendinous and
  osteoligamentous junctions
• Menisci interposed fibrocartilage pads
• Tibiofemoral
• Acromioclavicular
• Sternoclavicular
• Temporomandibular
• IV disk annulus fibrosus
• Injury will be discussed regionally

22
Tendons
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Tendon Material Properties

• UTS 50-100 MPa
• Ultimate load depends on cross-sectional area,
  which is widely variable
• Achilles or patellar tendon vs. proximal biceps
  tendon
• Viscoelastic (? strain rate ? ? stiffness)
• Less sensitive than bone

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Tendon Loading

• Recall changing moment arm during joint motion
• Changing muscle line of action changes angle
  between tendon and bone, but tendon itself is
  always in tension

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Tendon adaptation

• Not too much data about exercise-related
  adaptation of tendon
• A few results say exercise can increase
• Number and size of collagen fibrils
• Collagen synthesis in growing tendons
• Number of fibroblasts

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Tendon Injury (pg. 148)

• Injury to tendinous structures can restrict of
  even prevent normal movement and function
• Can occur at
• Body of tendon
• Connection with bone
• Connection with muscle

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Direct vs. Indirect

• Laceration by sharp instrument
• Hands and fingers

• Excessive tensile loads applied to the tendon
  structure
• Called strains

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Strains

• Mild
• Negligible structural disruption
• Local tenderness
• Minimal functional deficit
• Moderate
• Partial structural deficit
• Visible swelling
• Marked tenderness
• Some loss of stability
• Severe
• Complete structural disruption
• Marked tenderness
• Functional deficits that typically necessitate
  corrective surgical intervention

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Spontaneous tendon rupture

• Weekend warriors
• Usually preceded by undetected tissue damage
• Research on Achilles Tendon and blood type
• Type O predisposed?
• Bilateral risk with B (90 in one study)

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Tendinitis

• Repetitive overloading triggers inflammatory
  response
• Acute
• Response to a limited session or event
• Chronic
• Repeated overuse
• Also possibly inflamed
• Peritenon
• Tendon sheath
• Accompanying bursa

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Surgical tendon repair

• Initial inflammatory response
• Collagen and glycosaminoglycan synthesis
• Immobilization needed until matrix integrity is
  restored
• 2nd and 3rd weeks post-op cyclic low loads align
  new fibers, limit muscle atrophy

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Stress Risers

• Areas of material geometry or heterogeneity in
  which stress is concetrated
• Typically first failure point
• Heterogeneity in tendons
• Osteotendinous junction
• Myotendinous junction

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Experiment

• Failure of an elastic material at a stress riser

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Grade 2 strain of the biceps femoris. Sagittal
T2-weighted fat-suppressed MR image through the
proximal thigh shows a feathery pattern of edema
and hemorrhage (arrows) around and within the
biceps femoris at the musculotendinous junction.
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Ligaments
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vs. Tendons

• Similar material constituents
• Difference in parallelility (!) of collagen
  fibers
• Tendons more parallel at rest
• Why?

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Effect on load-deformation curve

• Toe region reflects straightening of fibers

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Ligament Use/Disuse
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Ligament Material Properties

• More Similarity to tendons
• Also Viscoelastic (? strain rate ? ? stiffness)
• Also less sensitive than bone
• So, if an injury-causing tensile load is applied
  across a joint very quickly, the ________ will
  fail, but if its applied very slowly an _______
  might occur

ligament
avulsion
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Ligament Injury

• Sprain
• Susceptibility to injury affected by ligament
  class
• Intracapsular
• Capsular
• Extracapsular

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Progression toward failure

• Unusual load-deformation curve

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Potentially Painful Experiment

• Bundles of elastic fibers in progression toward
  failure

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Ligament healing

• Frank (1999) identified three phases
• Bleeding and inflammation
• Proliferation of bridging material
• Matrix remodeling

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Ligament healing

• Frank (1999) identified three phases
• Bleeding and inflammation
• Platelets promote clotting
• Fibrin clot deposited
• Growth factors released
• Local vasodilation
• Acute inflammatory cells infiltrate
• Fibroblastic scar cells arrive
• Proliferation of bridging material
• Matrix remodeling

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Ligament healing

• Frank (1999) identified three phases
• Bleeding and inflammation
• Proliferation of bridging material
• Generation of scar matrix
• Matrix remodeling

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Ligament healing

• Frank (1999) identified three phases
• Bleeding and inflammation
• Proliferation of bridging material
• Matrix remodeling
• Remodediling of scar matrix that does not produce
  normal ligament
• Smaller collagen fibers
• More haphazard fiber alignment

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Skeletal Muscle
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Muscle

• You know all the basics
• Muscle is unique because of its ability to
  develop tension
• Adaptation enormous capability for hypertrophy
  and atrophy
• Growth If bone gets longer, so must muscle
• Sarcomeres added at myotendinous junction

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Growth, Strength, Gender
Komi 1992
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Use vs. Disuse

• Note two training modes
• Strength / resistance training
• Endurance training
• have different implications for muscle injury
• Surprisingly, very little is known about how the
  myotendinous junction adapts to training.

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Understanding Muscle Strain Injury

• 1933 tears can happen to bone-tendon junctions,
  within the muscle belly, or at myotendinous
  junctions (McMaster)
• 1989 Also at sites within the muscle belly
  approx. 0.5 mm from the myotendinous junction
  (Tidball and Chan)
• But that was all passive muscle. What about
  active muscle?

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Passive vs. Active Muscle Research

• 1987 14-16 greater peak force at failure in
  active muscle (Garrett et al.)
• Failure at MTJ
• 1993 Failure at proximal MTJ in both (Tidball et
  al.)
• Stimulated required 30 more force and 110 more
  energy to reach failure

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Forms of Skeletal Muscle Injury

• Acute muscular strain
• Contusions
• Exercise-induced muscular injury

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Acute Muscular Strain

• Overstretching a passive muscle
• Dynamically overloading an active muscle
• Either concentrically or eccentrically
• Mild minimal structural disruption, rapid return
  to normal function
• Moderate partial tear in muscle tissue (often at
  or near MTJ), pain, and some loss of function
• Severe complete or near-complete tissue
  disruption and functional loss, hemorrhage,
  swelling

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Contusions

• Direct compressive impact causes intramuscular
  hemorrhage
• Common in contact sports
• Note loading mechanism other injuries are
  longitudinal tension
• Repeated insult may lead to secondary
  complications such as myositis ossificans
  (ossified mass within muscle)

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Exercise-induced muscular injury

• Connective and contractile tissue disruption
  following exercise
• A.K.A. Delayed-onset muscle soreness
• 24-72h after vigorous activity
• Esp. eccentric action
• Unfamiliar activity
• Symptoms and metabolic events similar to acute
  inflammation

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Muscle strain may also be caused by

• CRAMP!
• Caution stretchspasm more tensile force

58
Review

• Cartilage, Tendon, Ligament, Muscle
• Next time Exam 1