biomechanics of bones
TRANSCRIPT
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BIOMECHANICS OF BONES
Warsaw University of Technology
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Farhod Turaev
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Contents
Introduction The Skeleton System Types of the Bones Composition of Bones Functions of Bones Bone Biomechanics
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Introduction
Bone tissue, or osseous tissue, - a type of connective tissue used in forming bones. Bones protect the vital organs and help support the body. Bone is composed mainly of collagen, or ossein, fibers, and bone cells called osteocytes. There are two types of bone tissue, referred to as cortical bone and cancellous bone.
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The Skeleton System
299 bones (baby), 209 bones (adults) 4 basic shapes
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Types of the Bones
4 basic shapes: Long bones (femur)
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Types of the Bones
4 basic shapes: Short bones (wrist, ankle)
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Types of the Bones
4 basic shapes: Flat bones (skull, scapula)
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Types of the Bones
4 basic shapes: Irregular bones (vertebrae)
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Composition of Bones
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Functions of Bones
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Mechanical properties of bone
Relatively hard; Lightweight; Composite material; high compressive strength of about
170 MPa (1800 kgf/cm²); poor tensile strength of 104–121 MPa; very low shear stress strength (51.6 MPa).
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Basic Biomechanics
Material Properties Elastic-Plastic Yield point Brittle-Ductile Toughness
Independent of Shape!
Structural Properties Bending Stiffness Torsional Stiffness Axial Stiffness
Depends on Shape and Material!
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Force
Displacement
Slope Stiffness = Force/Displacement
Force, Displacement & Stiffness
Basic Biomechanics
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Basic Biomechanics
Stress = Force/Area Strain Change Height (L) / Original Height(L0)
Force
Area L
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Stress = Force/Area
Strain = Change in Length/Original Length (L/ L0)
Elastic Modulus = Stress/Strain
Stress-Strain & Elastic Modulus
Basic Biomechanics
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Elastic Modulus (GPa) of Common Materials in Orthopaedics
Stainless Steel 200 Titanium 100 Cortical Bone 7-21 Bone Cement 2.5-3.5 Cancellous Bone 0.7-
4.9 UHMW-PE 1.4-4.2
Basic Biomechanics
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Elastic Deformation
Plastic Deformation
Energy Energy Absorbed
Force
Displacement
PlasticElastic
Basic Biomechanics
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• Stiffness-Flexibility• Yield Point• Failure Point• Brittle-Ductile• Toughness-Weakness
Force
Displacement
PlasticElastic
FailureYield
Stiffness
Basic Biomechanics
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FlexibleDuctile ToughStrong
FlexibleBrittleStrong
FlexibleDuctileWeak
FlexibleBrittleWeak
Strain
Stress
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Basic Biomechanics
Load to Failure Continuous
application of force until the material breaks (failure point at the ultimate load).
Common mode of failure of bone and reported in the implant literature.
Fatigue Failure Cyclical sub-
threshold loading may result in failure due to fatigue.
Common mode of failure of orthopaedic implants and fracture fixation constructs.
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Basic Biomechanics
Anisotropic Mechanical
properties dependent upon direction of loading
Viscoelastic Stress-Strain
character dependent upon rate of applied strain (time dependent).
Material properties of bones:
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Bone Biomechanics
Bone is anisotropic - its modulus is dependent upon the direction of loading.
Bone is weakest in shear, then tension, then compression.
Ultimate Stress at Failure Cortical BoneCompression < 212 N/m2
Tension < 146 N/m2
Shear < 82 N/m2
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Bone Biomechanics
Bone is viscoelastic: its force-deformation characteristics are dependent upon the rate of loading.
Trabecular bone becomes stiffer in compression the faster it is loaded.
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Bone Mechanics
Bone Density Subtle density
changes greatly changes strength and elastic modulus
Density changes Normal aging Disease Use Disuse
Cortical Bone
Trabecular Bone
Figure from: Browner et al: Skeletal Trauma
2nd Ed. Saunders, 1998.
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Basic Biomechanics
Bending Axial
Loading Tension Compressi
on Torsion
Bending Compression Torsion
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Fracture Mechanics
Figure from: Browner et al: Skeletal Trauma 2nd Ed, Saunders, 1998.
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Fracture Mechanics
Bending load: Compression
strength greater than tensile strength
Fails in tension
Figure from: Tencer. Biomechanics in Orthopaedic
Trauma, Lippincott, 1994.
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Fracture Mechanics
Torsion The diagonal in the direction of the applied
force is in tension – cracks perpendicular to this tension diagonal
Spiral fracture 45º to the long axis
Figures from: Tencer. Biomechanics in Orthopaedic
Trauma, Lippincott, 1994.
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Fracture Mechanics
Combined bending & axial load Oblique fracture Butterfly fragment
Figure from: Tencer. Biomechanics in Orthopaedic
Trauma, Lippincott, 1994.
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• Stress-Strain-Strength properties;
• Elastic Deformation Analyses;
• Plastic Deformation Analyses;
• Hardness of the parts;• Resistance of bones.
Conclusion
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