Chapter 39 Bone Biomechanics and Fracture Biology Flashcards
Materials vs. composites vs. structures
A material may be composed of one or more elements
A composite is a material made from 2 or more primary materials
A structure may be composed with one or more materials or composites
Stress/strain vs load/deformation analysis
Load-deformation analysis is conducted on the body as a whole
Describes overall changes in geometry of the sample in response to an applied load
Stress-Strain analysis is conducted point-by-point within the body
Describes the materials response to loading
Difference between cancellous and cortical bone in load/deformation curve
Cortical bone = more dense, stiffer and stronger under compressive load
Structure and orientation of osteons make cortical bone resistance to deformation under axial compression
2% strain without failing
Becomes stronger and stiffer with higher strain rate
Cancellous bone = Can handle more strain
75% strain without failing
What are the 5 growth plate zones
Resting/reserve zone
Proliferative zone
Hypertrophy zone
Zone of calcification
Zone of ossification
Describe the resting/reserve zone of the growth plate
Only vascularized zone
Hyaline cartilage matrix with small chondrocytes – identical to hyaline cartilage
Describe the proliferative zone
Chondrocytes undergo mitosis
Align in longitudinal columns
Primarily type II collagen
Growth factors are in this zone
Describe hypertrophy zone
Chondrocytes hypertrophy and undergo apoptosis
Chondrocytes produce collagen X and decrease expression of type II collagen
Describe the zone of calcification and its result
Matrix mineralization
Chondrocytes release ALP and other enzymes that scavenge Ca and P
Result: Calcium=phosphate aggregates and matrix calcification
Describe the zone of ossification
Osteoblasts produce woven bone
Osteoclasts remodel woven bone to lamellar bone
Until activity within the zone of ossification > chondrocyte repopulation of the resting zone then no more growth plate
What is the force on the medial column of the femur during weight bearing
compressive
What forces are needed to generate an oblique fracture
oblique shear, transverse tensile, and compressive stress
Types of forces on transverse fracture
Concentric tensile loads
Oblique shear stress
Transverse compressive stress
Axial tensile stress
Types of forces on oblique fracture
Oblique shear stress - main
Concentric axial compressive loads
Transverse tensile stress
Types of forces on spiral fracture
Torsional load
Shear stress axially and transversely
Tensile stress
Compressive stress
Failure parallel to axis of the twist and propagates along tensile lines
Types of forces on butterfly segment and where would it fail
Bending moment
Induced by axial compressive loading = buckling
Compressive stress
Tensile stress
Failure on the tensile side
How does granulation tissue stabilize fracture
End result of inflammatory phase is a scaffold that remodels into granulation tissue to form external callus
Granulation tissue can withstand nearly 100% deformation - is primary stabilizer
How to achieve primary/direct bone healing
Anatomical reconstruction
Strain has been eliminated
Absolute rigidity and stability
How does primary/direct bone healing occur
Intermembranous ossification – osteoblasts and osteoclasts directly deposit bone at the fracture
Describe contact healing
Gap is < 0.01mm
Interfragmentary strain <2%
Lamellar bone deposited parallel to long axis
Describe gap healing
Gap < 1mm*
Interfragmentary strain <2%
Initially filled with fibrin matrix
Then remodeled with type I and type III collagen
Lamellar bone oriented transverse to the long axis
How does interfragmentary strain decrease in secondary bone healing
Osteoclasts resorb bone at fracture leading to a wider fracture gap*
Widened gap decreases strain*
Granulation tissue can now form and survive within the gap
External callus forms on the abaxial surface of bone
Bigger callus = increase in area moment of inertia = greater stability
How fast do osteoblasts get activated in bone injury
within 24 hours
Define Wolff’s law
Ability of bone to remodel adaptively in response to mechanical load
Describe elastic osteosynthesis
Hypothesis that distributing stress along the entire plate to limit stress at the screw-bone interface is more appropriate for juvenile patients
Best for animals < 5-6 months old
Compliance of bone-plate construct is increased to promote plate deformation within its elastic range to spare the weak bone-screw interface from overwhelming shear stresses
How to achieve elastic osteosynthesis
Minimal manipulation of fracture
Bridging plate – metaphysis to metaphysis
2-3 screws at each end of the bone plate
Screws should be diverging and not tapped
Define distraction osteogenesis
method of bone formation via intramembranous ossification
What is the fibrous interzone in distraction osteogenesis
Radiolucent zone in the osteotomy gap where in response to tension, the gap is filled with collagen type I that is deposited in a longitudinal manner by fibroblasts and mesenchymal stem cells in response to linear tensioning of the provisional fibrin matrix
What is platelet alpha granules role in bone healing
Hemorrhage and the resulting clot have activated platelets. Platelets are degranulated and platelet alpha granules release growth factors necessary for all wound healing especially bone healing. Alpha granules release IGF-1, PDGF, and TGF-beta which are osteopromotive (promotes de novo formation of bone).
Describe disadvantages of collagen X in the hypertrophy cartilage zone
Collagen X – stiffer and prevents nutrient diffusion – leading to tissue hypoxia
Structurally the weakest and predisposed to Salter-Harris fractures
