Bone Injury, Regeneration, and Repair

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Bone Injury, Regeneration, and Repair

• Gabriel Hommel, MD
• PGYII

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Bone Injury

• Injury to bone occurs can result from many
  different insults.
• Trauma, infection, tumor, and metabolism can all
  result in bone injury.
• Pathways involved in bone development also play
  an integral part in bone repair.

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Injury

• This presentation will discuss the basic science
  of bone repair and how we can alter and improve
  it.
• We will discuss two specific insults
  osteonecrosis and fracture.

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Osteonecrosis

• Osteonecrosis is broadly classified as traumatic
  or atraumatic.
• What these mechanisms have in common is a end
  result of vascular compromise.
• Mechanical disruption
• Arterial occlusion
• Injury to or pressure on arterial wall
• Venous outflow occlusion

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Osteonecrosis

• Mechanical disruption occurs with fracture
• Arterial occlusion occurs with embolism,
  thrombosis, nitrogen bubbles (the bends), or
  sickle cells.
• Injury or pressure to vessels can occur from
  extramural (blood, fat, marrow) or intramural
  (vasculitis, angiospasm, radiation) sources.
• Venous occlusion occurs with any condition
  causing local venous pressure to exceed arterial
  pressure.

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Osteonecrosis

• Three factors result in local thrombus formation
• Stasis
• Hypercoagulability
• Endothelial damage

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Stasis

• Some areas more susceptible than others
• Femoral head
• Humeral head
• Talus
• Carpus
• The microvascular anatomy has few collaterals and
  long, narrow arcades of end capillaries.
• This facilitates vascular stasis

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Hypercoagulable

• Increase in procoagulants (protein C and S)
• Vasoconstriction of subchondral arteriole bed
• Decreased endogenous fibrinolysis

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Endothelial Damage

• Exposure of subendothelial collagen leads to
  platelet aggregation and thrombosis.
• Exposure of tissue factors in endothelial walls
  activates intrinsic and extrinsic pathways.
• This leads to thrombosis and bone ischemia.

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Fat Embolism

• Platelet aggregation has been shown to occur on
  the surface of IV fat.
• This leads to thrombosis

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Intraosseous HTN

• May occur as result of excessive medullary venous
  stasis
• Unknown if this phenomenon is a cause or and
  effect of ON.

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ON Histology

• Histological changes can be seen as early as 10
  days.
• Necrosis of hematopoietic cells, capillary
  endothelial cells, and lipocytes
• Empty lacunae result from osteocyte death
• Necrosis of marrow contents result in increased
  water content, as evidenced by increased signal
  on T2 weighted MR images.

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ON Histology

• Bone remodeling occurs after necrosis just as in
  fracture.
• Hyperemia and fibrous tissue growth
• Creeping substitution vascularizes necrotic
  bone
• Mesenchymal cells differentiate and cutting cones
  are formed in necrotic cortical bone and osteiod
  is laid down in necrotic cancellous bone.

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Risk Factors

• Bacterial endotoxins cause Shwartzman reaction
  (DIC, hyperlipemia, fat emboli, thrombosis)
• The bends results from dysbaric phenomenon in
  deep sea divers.
• Hemoglobinopathies (SS dz, thalassemia)
• Exogenous glucocorticoids
• Alcohol abuse is thought to cause fat emboli,
  cortisol release, and altered lipid metabolism.

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Radiographic Changes

• MRI is able to pick up the early changes of
  increased water content.
• Later, remodeling results in subchondral cyst
  formation and increased size of trabeculae.
• Sclerosis develops over time.
• Subchondral bone necrosis and collapse results in
  classic crescent sign.
• Lastly, acetabular changes occur, indicating end
  stage disease.

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Classification
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Fracture Healing

• Unique, integrated, and highly reproducible
  process.
• Closely related to external factors (mechanics).
• Motion at the fracture site results in
  endochondral ossification (secondary bone
  healing)
• Stability at the fracture site results in
  intramembranous ossification (primary bone
  healing)
• Most of the time there is a combination of the
  two processes with one more prominent than
  another.

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Fracture

• Stepwise progression
• Hematoma phase
• Inflammation phase
• Soft callus phase
• Hard callus phase
• Remodeling phase

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Hematoma and Inflammation

• Immediately post-fracture, bleeding produces a
  hematoma
• Loss of stability, decreased local O2, and
  biochemical factor release all contribute to the
  initiation of inflammation (24-72hrs)
• Macrophages and degranulating platelets release
  cytokines (PDGF, TGF-ß, IL-1, IL-6, and PGE2).
• These cytokines play key role in initiation of
  repair by acting on key cell lines.

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Hematoma and Inflammation

• Periosteal preosteoblasts and osteoblasts
  (expressing osteocalcin) differentiate into bone.
• Mesenchymal cell proliferation (associated with
  FGF-1 and 2).
• FGFs stimulate endothelium (angiogenesis) and
  fibroblasts, chondrocytes, and osteoblasts
  (mitogenesis)
• These mesenchymal cells and fibroblasts
  infiltrate and replace the hematoma, producing
  granulation tissue.

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Hematoma and Inflammation

• This granulation tissue expresses several BMPs
  (members of TGF-ß superfamily) which acts on cell
  growth, differentiation, and apoptosis.

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Soft Callus Phase (Proliferation)

• As granulation matures, new vessels infiltrate
  tissue and provide tissue with progenitor cells
  and growth factors.
• Hematoma and granulation tissue begins to develop
  cartilaginous matrix (collagen I and II)
• See expression of genes sox9 and col2 which leads
  to chondrocyte proliferation and differentiation.

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Soft Callus Phase (Proliferation)

• This forms a cartilaginous callus characterized
  by expression of Ihh.
• Chondrocytes differentiate, mature, and
  eventually hypertrophy
• Hypertrophic chondrocytes express collagen X and
  runx2 (transcription factor affecting
  differentiation through the ECM proteins
  osteocalcin and osteopontin)

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Hard Callus Phase (Maturation)

• Involves terminal chondrocyte differentiation,
  apoptosis, ECM degradation, angiogenesis, and
  osteogenesis.
• Cartilage calcifies at junction with newly formed
  woven bone
• Variety of genes expressed
• Osteoblastic (BMPs, TGF-ß, IGFs, and
  osteocalcin.)
• Collagen related (Types I, V, and XI)

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Remodeling Phase

• New woven bone then remodels through organized
  osteoblast/osteoclast activity.
• After remodeling, repair bone is
  indistinguishable from surrounding bone.

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Endochondral Ossification

• Also termed secondary bone healing
• A stepwise progression of new bone formation
  through a cartilage intermediate tissue.
• Requires some controlled motion (relative
  stability) at the fracture site. (IM nails,
  casting, ex-fix, locked bridge plating)

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Intramembranous Ossification

• Also known as primary bone healing
• Fractures heal without an intermediate cartilage
  tissue.
• Requires rigid fixation (absolute stability),
  minimal fracture motion, and intimate contact b/w
  fragments (Interfrag lag screws, compression
  plating)

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Biological Requirements

• Certain biological requirements must be present
  for fracture repair
• Skeletal progenitor cells present at right place
  and time
• ECM available for scaffolding and growth factor
  repository
• Certain molecules and downstream effectors at fx
  site
• Intact vasculature

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Progenitor Cells

• Evidence suggests that pluripotent mesenchymal
  stem cells are recruited from surround tissue and
  blood stream.
• Unknown as to the origin of these stem cells.
• Periosteum also a significant source of these
  cells.

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ECM

• Maintains structure and physical properties of
  cartilage and bone.
• A dynamic structure, constantly changing.
• Composed mainly of collagens
• Types II, IX, XI, and X in cartilage
• Types I and VI in bone
• Also contains proteoglycans which can bind and
  store growth factors.

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ECM

• Glycoproteins such as fibronectin, laminin,
  tenascin-C, thrombospondins, osteocalcin,
  osteopontin, and osteonectin help bind other ECM
  components and progenitor cells to the ECM.
• Constantly remodeled by matrix metalloproteinases
  (MMPs) like collagenase, gelatinase, and
  stomelysins.

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Molecules
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Vasculature

• Repair process inherently requires angiogenesis
• This process is regulated by several angiogenic
  regulators
• VEGF, PTH, TGF, BMP, FGF, IGF, and PDGF.
• VEGF acts directly on endothelial cells while
  molecules like BMPs affect angiogenesis by
  upregulating VEGF.

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Biomechanics of Fixation IM nails

• Are load-sharing devices that act as an internal
  splint.
• Provide good alignment and early weight bearing.
• Inserted away from zone of injury, avoiding
  disrupting fracture biology.

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IM nail properties

• Bending rigidity is proportional to r4
• Concept of unsupported length is the length of
  the nail not contacting bone.
• Negligible when two fracture ends contact one
  another, but with comminution and bone loss the
  unsupported length can be large.
• Amount of interfrag strain is proportional to the
  unsupported length squared.

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Biomechanics of Fixation Plates

• Load sharing between the plate and native bone
  seen as a frictional force. Three interfaces.
• Screw-bone interface
• Screw-plate interface
• Plate-bone interface
• Disruptions at any of these can drastically
  change construct rigidity.

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Plate properties

• Good a resisting bending forces.
• Bending rigidity proportional to the plate
  thickness to the third power.
• Working length refers to the distance from the
  closest screws on either side of the fracture.
  The closer the better.
• Bending deformation of the plate is proportional
  to working length2.

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Plate properties

• Since plates resist bending loads, they are best
  on the tensile (convex) side of the bone. This
  is the concept of the tension band.
• Stress shielding is a drawback to plating as
  bones typically do not reach intact strength
  after healing as the plate shields the bone
  from stress

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Biomechanics of Fixation External Fixation

• Endless options and configurations
• Allows for length, alignment, and rotation with a
  minimally invasive approach.
• Can be either load bearing or load sharing
  depending on fracture configuration.

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Ex-Fix properties

• Pins are weakest link, bending rigidity is r4
• High bending forces may lead to loosening at pin
  bone interface b/c of resorption.
• Bar to bone distance is directly proportional to
  pin deflection, and inversely proportional to
  construct stiffness.

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Bone Graft Substitutes and Growth Factors

• Allograft
• Calcium Sulfate
• Calcium Phosphate
• Collagen-calcium phosphate composite
• Polymer
• BMP with matrix carrier

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Allograft

• Fresh frozen, freeze dried, or demineralized.
• Fresh frozen more immunogenic but retain more of
  their original mechanical properties.
• Freeze drying reduces immunogenicity but reduces
  graft strength 50.
• Can transmit HIV, Hep B and C, and prions.

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Allograft

• Has osteoconductive and weak osteoinductive
  properties.
• Relies on the host bed for cellular and hormonal
  components.
• Comes in particulate (Cancellous chips, crushed
  cortical) or structural (cortical) forms.
• Particulate has little to no structural
  properties but incorporates faster.
• Cortical has good structual properties but
  incorporates very slowly.

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Allograft

• Demineralized allograft can be combined with a
  carrier compound to produce DBM (DBX Synthes,
  West Chester, PA)
• This releases matrix-bound osteoinductive
  glycoproteins that may activate host cells.

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Calcium Sulfate

• Plaster of Paris
• Dissolves in vivo in 30-60 days. Weak structual
  support.
• Osteoconductive
• Osteoblasts attach to the crystals and
  osteoclasts resorb CaSO4.

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Tricalcium Phosphate Ceramics, Calcium Phosphate
Cements

• Hydroxyapatite and tricalcium phosphate falling
  out of favor due to poor fatigue characteristics
  and lack of moldability.
• Newer CaPO4 cements like Norian SRS (Synthes -
  West Chester, PA) contains mono and tri calcium
  phosphate, calcium carbonate, and sodium
  phosphate in an injectable paste.
• No heat generation while setting. Hardens into a
  dahllite (carbonated hydroxyapatite)
• Undergoes similar in vivo remodeling as normal
  bone.

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Collagen-Ceramic Composites

• Collagen coated with thin layer of soluble
  hydroxyapatite and tricalcium phosphate.
• Products have similar success of ceramics without
  the disadvantages.
• Healos (DePuy Spine, Raynham, MA) and Collagraft
  (Zimmer, Warsaw, IN)

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Polymers

• Polylactic and polyglycolic acid.
• Osteoconductive and moldable.
• Used for bioabsorbable fixation with suture
  anchors, screws, and spinal interbody cages.

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BMP

• Both osteoconductive and inductive.
• Current products on the market are BMP-2 and 7.
• High doses requires for efficacious activity.
• As effective as iliac crest autograft so far in
  the research.
• Holds significant promise for future use.

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Adjuncts

• Electrical stimulation
• Three types
• Inductive coupling (IC)
• Capacitive coupling (CC)
• Direct current (DC)
• Treatment based on observed electrical fields in
  bone under mechanical strain.
• Cells lay down bone in electronegative regions of
  compression and resorb bone in electropositive
  areas of tension.

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E-stim

• IC external current-carrying coils powered by a
  signal generator produce magnetic field that
  induces secondary electric field at the fracture
  site.
• Is not attenuated by a cast.
• Promotes angiogenesis, chondrogenesis, and
  osteogenesis.

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E-stim

• CC stimulation is similar.
• Electrodes with conductive gel placed on skin and
  connected to external AC signal generator to
  produce electrical field at the fx site.
• Transmembrane calcium translocation via
  voltage-gated Ca channels.
• This activates calmodulin and upregulates bone
  growth factors.

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E-stim

• DC current stimulation is produced from implanted
  electrodes at the fx site.
• The reaction at the cathode reduces oxygen
  concentration and increases pH, stimulating
  osteblastic activity and inhibiting osteoclastic
  activity.
• Also thought to upregulate BMPs.

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Ultrasound

• Transmits mechanical energy in the form of high
  frequency acoustical pressure waves.
• Energy is absorbed and attenuated as it passes
  through the tissue.
• Has been shown to be effective in treating
  fractures in a cast, however, has not been shown
  to be helpful when used on tibia fxs treated with
  IM nail.
• A direct example of Wolffs law.

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Julius Wolff

• "Remodeling of bone ... occurs in response to
  physical stresses - or to the lack of them - in
  that bone is deposited in sites subjected to
  stress and is resorbed from sites where there is
  little stress"

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References

• Einhorn, T.A., OKeefe, R.J. Buckwalter, J.A.
  2007. Orthopaedic Basic Science Foundations of
  Clinical Practice. 3rd ed. Rosemont, IL AAOS.
• Rüedi, T.P., Murphy, W.M. 2000. AO Principles of
  Fracture Management. Stuttgart, NY Thieme.