Hard Tissue Repair and Replacement

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Hard Tissue Repair and Replacement

• MTC594
• 3/09/06

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The Skeletal SystemBone Tissue

• Skeleton composed of many different tissues
• Dynamic and ever-changing throughout life
• Cartilage, bone tissue, epithelium, nerve, blood
  forming tissue, adipose, and dense connective
  tissue

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Functions of Bone

• Supporting protecting soft tissues
• Attachment site for muscles making movement
  possible
• Storage of the minerals, calcium phosphate --
  mineral homeostasis
• Blood cell production occurs in red bone marrow
• Energy storage in yellow bone marrow

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Anatomy of a Long Bone

• Diaphysis shaft
• Epiphysis one end of a long bone
• Metaphysis growth plate region
• Articular cartilage over joint surfaces acts as
  friction shock absorber
• Medullary cavity marrow cavity
• Endosteum lining of marrow cavity
• Periosteum tough membrane covering bone but not
  the cartilage

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Cell Types of Bone

• Osteoprogenitor cells ---- undifferentiated cells
  
• can divide to replace themselves can become
  osteoblasts
• found in inner layer of periosteum and endosteum
• Osteoblasts--form matrix collagen fibers but
  cant divide
• Osteocytes ---mature cells that no longer secrete
  matrix
• Osteoclasts---- huge multinucleated cells
• function in bone resorption at surfaces such as
  endosteum

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Matrix of Bone

• Inorganic mineral salts provide bones hardness
• Hydroxyapatite (calcium phosphate) calcium
  carbonate
• Organic collagen fibers provide bones
  flexibility
• Their tensile strength resists being stretched or
  torn
• Remove minerals with acid rubbery structure
  results
• Calcification is hardening of tissue when mineral
  crystals
• deposit around collagen fibers
• Bone is not completely solid since it has small
  spaces for vessels and red bone marrow
• Spongy bone has many such spaces
• Compact bone has very few

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Bone Formation or Ossification

• Intramembranous bone formation formation of
  bone directly
• from mesenchymal cells.
• All embryonic connective tissue begins as
  mesenchyme.
• Endochondral ossification formation of bone
  from hyaline
• cartilage.

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Intramembranous Bone Formation
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Endochondral Bone Formation (1)
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Factors Affecting Bone Growth

• Nutrition
• Adequate levels of minerals and vitamins
• Calcium and phosphorus for bone growth
• Vitamin C for collagen formation
• Vitamins K and B12 for protein synthesis
• Vitamin D for calcium absorption
• Sufficient levels of specific hormones
• During childhood need insulinlike growth factor
• Promotes cell division at epiphyseal plate
• Need hGH (growth), thyroid (T3 T4) and insulin
• Sex steroids at puberty
• Growth spurt and closure of the epiphyseal growth
  plate
• Estrogens promote female changes -- wider pelvis

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Bone Remodeling

• Ongoing since osteoclasts carve out small tunnels
  and
• osteoblasts rebuild osteons.
• Osteoclasts dissolve bone matrix and release
  calcium and
• phosphorus into interstitial fluid (bone
  resorption)
• Osteoblasts take over bone rebuilding (bone
  formation)
• Continual redistribution of bone matrix along
  lines of
• mechanical stress
• Distal end of femur is fully remodeled every 4
  months

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Exercise Bone Tissue

• Pull on bone by skeletal muscle and gravity is
• mechanical stress .
• Stress increases deposition of mineral salts
• production of collagen (calcitonin prevents bone
• loss)
• Lack of mechanical stress results in bone loss
• Reduced activity while in a cast
• Astronauts in weightlessness
• Bedridden person
• Weight-bearing exercises build bone mass (walking
• or weight-lifting)

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Osteoporosis

• Decreased bone mass resulting in porous bones
• Those at risk
• White, thin menopausal, smoking, drinking female
  with
• family history
• Athletes who are not menstruating due to
  decreased
• body fat decreased estrogen levels
• People allergic to milk or with eating disorders
  whose
• intake of calcium is too low
• Prevention or decrease in severity
• Adequate diet, weight-bearing exercise,
  estrogen
• replacement therapy (for menopausal women)
• Behavior when young may be most important factor

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Hard tissue repair and replacement
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Hard tissue repair and replacement

• Design considerations
• Material properties
• Fatigue strength
• Corrosion resistance
• Shape
• Surface Characteristics
• Smooth/Rough
• Porous/nonporous
• Fixation of biological molecules

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Hard tissue repair and replacement

• Design considerations (cont.)
• Minimal friction
• Low production of wear products
• Biocompatibility
• Easily removed?

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Temporary internal fixation devices

• Wire (twisted knotted)
• Reattachment or additional stability
• Reduction in strength corrosion at deformed
  regions

Radial wires
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Temporary internal fixation devices

• Pins
• Bone fragment fixation, guidance of large screws
• Straight wires
• Holding power comes from elastic deformation
  or surrounding bone

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Temporary internal fixation devices

• Screws
• Bone fixation attachment of metallic plate to
  bone
• Cortical and cancellous screws
• Holding power affected by size of pilot drill
  hole, depth of screw engagement, outside diameter
  of screw and quality of bone

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Temporary internal fixation devices

• Plates
• Facilitate fixation of bone fragments
• Rigid/high elastic modulus (stainless steel)
  facilitates healing
• Flexible/low elastic modulus (titanium alloy)
  facilitates loading of bone
• Flexible plates allow micromotion and
  physiological loading of underlying bone
• Low contacting plates allow revascularization of
  underlying bone

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Temporary internal fixation devices

• Intramedullary nails
• Internal struts to stabilize long bone fractures
• Resist multidirectional bending
• Large contact surface between nail and internal
  cortex of bone required

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Permanent joint replacement

• Hip, ankle, shoulder, elbow, finger joint,
  intervertebral disc
• Materials
• CoCr alloys (femoral head, femoral stem, wire
  nails)
• Alumina (femoral head)
• Zirconia (femoral head)
• Ti alloy (femoral stem, screws, wire, flexible
  plate, nails)
• UHMWPE (acetabular cup)
• Monolithic cup
• Modular lining of a metallic shell
• Stainless steel (wire, pins, screws, rigid
  plates, nails)
• PLA/PGA (pins, screws)
• PE (femoral head)

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Permanent joint replacement
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Permanent joint replacement

• Total artificial hip
• Implant fixation methods
• Mechanical interlock
• Press-fitting
• PMMA-bone cement
• Threaded components
• Biological fixation
• Porous/textured surfaces
• Direct chemical bonding between implant and
  bone

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Permanent joint replacement

• PMMA bone cement
• Immediate stability and weight bearing
• Anchoring power depends upon ability to
  penetrate between bone trabeculae during
  insertion of device
• Viscoelastic properties
• Shock absorption
• Uniform transmission of load between implant
  and bone
• Prevent localized stress and fatigue failure

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Permanent joint replacement

• PMMA cement
• Two interfaces
• Bone-cement
• Cement-implant
• Potential problems
• Loosening at interfaces
• Macro/micromotion
• Bone loss
• Formation of fibrous membrane at interface
• Toxic effects of MMA monomers
• Inflammation/necrosis of bone
• Devascularization
• Accumulation of wear debris/inflammation

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Permanent joint replacement

• Porous ingrowth fixation
• 100-350 micron pores
• Requires immobilization for osseointegration (12
  weeks)
• Bone mass should be near normal for optimal
  integration
• Difficult removal
• Materials (porous coatings)
• Pure titanium
• Titanium alloy
• Tantalum
• HA

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Permanent joint replacement

• Porous ingrowth fixation
• Potential problems
• Lack of osseointegration and failure
• Uneven load sharing resulting in resumption of
  bone
• Flaking of coating resulting in inflammatory
  responses

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Interactions at the bone-implant interface

• Reaction of host to biomaterial implant ()
• Protein adsorption/Protein desorption
• Acute inflammation
• Adsorption of bone-inducing proteins
• Osteopontin/bone sialoprotein
• Formation of laminae limitans
• Osteointegration (conduction/induction)
• Extension of bone away from biomaterial
• Extension of bone towards biomaterial

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Interactions at the bone-implant interface

• Reaction of host to biomaterial implant (-)
• Bone resorption around implant and loosening
• High stress concentrations
• Stress shielding
• Micromotion
• Macromotion
• Necrosis, osteolysis, bone resorption, loosening
  of implant
• Fibrosis at implant-cement-bone interfaces
• Inflammatory responses, osteolysis and loosening
• Wear debris-UHMWPE
• Corrosion
• Friction at implant-bone interface

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Interactions at the bone-implant interface

• Reaction of implant to host
• Surface changes-oxidation of metallic implants
• Release of material-metal ions with local and
  systemic release
• Hypersensitivity reaction
• Chronic inflammation
• FBR
• Osteoclast activation
• Bone resorption
• Device failure