Biomechanics of Bones

1. Biomechanics of bones Warsaw University of Technology 1 Farhod Turaev

2. Contents 2 Introduction The Skeleton System Types of the Bones Composition of Bones Functions of Bones Bone Biomechanics

3. Introduction 3 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 .

4. The Skeleton System 4 299 bones (baby), 209 bones (adults) 4 basic shapes

5. Types of the Bones 5 4 basic shapes: Long bones (femur)

6. Types of the Bones 6 4 basic shapes: Short bones (wrist, ankle)

7. Types of the Bones 7 4 basic shapes: Flat bones (skull, scapula)

8. Types of the Bones 8 4 basic shapes: Irregular bones (vertebrae)

9. Composition of Bones 9

10. Functions of Bones 10

11. Mechanical properties of bone 11 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 ).

12. 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!

13. Force Displacement Slope Stiffness = Force/Displacement Force, Displacement & Stiffness Basic Biomechanics

14. Basic Biomechanics Stress = Force/Area Strain Change Height ( L ) / Original Height(L ) Force Area L

15. Stress = Force/Area Strain = Change in Length/Original Length ( L/ L ) Elastic Modulus = Stress/Strain Stress-Strain & Elastic Modulus Basic Biomechanics

16. 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

17. Elastic Deformation Plastic Deformation Energy Energy Absorbed Force Displacement Plastic Elastic Basic Biomechanics

18. Stiffness-Flexibility Yield Point Failure Point Brittle-Ductile Toughness-Weakness Force Displacement Plastic Elastic Failure Yield Stiffness Basic Biomechanics

19. Flexible Ductile Tough Strong Flexible Brittle Strong Flexible Ductile Weak Flexible Brittle Weak Strain Stress

20. 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.

21. 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:

22. 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 Bone Compression < 212 N/m 2 Tension < 146 N/m 2 Shear < 82 N/m 2

23. 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.

24. 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.

25. Basic Biomechanics Bending Axial Loading Tension Compression Torsion Bending Compression Torsion

26. Fracture Mechanics Figure from: Browner et al: Skeletal Trauma 2nd Ed, Saunders, 1998.

27. Fracture Mechanics Bending load: Compression strength greater than tensile strength Fails in tension Figure from: Tencer. Biomechanics in Orthopaedic Trauma, Lippincott, 1994.

28. Fracture Mechanics Torsion The diagonal in the direction of the applied force is in tension – cracks perpendicular to this tension diagonal Spiral fracture 45 º to the long axis Figures from: Tencer. Biomechanics in Orthopaedic Trauma, Lippincott, 1994.

29. Fracture Mechanics Combined bending & axial load Oblique fracture Butterfly fragment Figure from: Tencer . Biomechanics in Orthopaedic Trauma, Lippincott, 1994.

30. 30 Stress-Strain-Strength properties; Elastic Deformation Analyses; Plastic Deformation Analyses; Hardness of the parts; Resistance of bones. Conclusion

31. No Questions ? 31 THANKS !!!