Mechanical Properties Of Bone

Mechanical Properties of Bone

1. Mechanical Effects on Bone Subject : Applied Anatomy, Kinesiology & Pathomechanics Subject code : MPT 19102 Presented by – Shubhankar Whaval (Sports) SRM IST

2. Contents Bone properties Strength Stiffness Anisotropic Characteristics Viscoelastic Characteristics Loads applied to the bone Compression Forces Tension Forces Shear Forces Bending Forces Torsional Forces Combined Loading

3. Bone properties Strength - The strength of bone or any other material is defined by the failure point or the load sustained before failure. Strength of bone is provided by the mineralization of its tissue: the greater the tissue mineral content, the stiffer and stronger the material. If bone becomes too mineralized, however, it becomes brittle and does not give during impact loading. Strength is assessed in terms of energy storage or the area under a stress–strain curve. Stiffness - Stiffness, or the modulus of elasticity, is determined by the slope of the load deformation curve in the elastic response range and is representative of the material’s resistance to load as the structure deforms. The stress–strain curve for ductile, brittle, and bone material is shown in Figure.

4. Bone properties Anisotropic Characteristics – Bone tissue is an anisotropic material, which means that the behavior of bone varies with the direction of the load application. Tissue of long bones can handle the greatest loads in the longitudinal direction and the least amount of load across the surface of the bone. Viscoelastic Characteristics - Bone is viscoelastic, meaning that its response depends on the rate and duration of the load. The collagen content gives the bone the ability to withstand tensile loads. Bone is also brittle, and its strength depends on the loading mechanism. The brittleness of bone is provided by the mineral constituents that provide bone with the ability to withstand compressive loads.

5. Loads applied to the bone

6. Load – Compression Type of Force - Presses ends of bones together to cause widening and shortening Source - Muscles, weight bearing, gravity or external forces Stress/Strain - Maximal stress on the plane perpendicular to the applied load. Compression Forces

7. Compression Forces Example If a force is applied against the top of the head when it is in this position, the cervical vertebrae are loaded along the length of the cervical vertebrae by a compressive force, creating a dislocation or fracture–dislocation of the facets of the vertebrae. Compression fractures in the lumbar vertebrae of weight lifters, football linemen, and gymnasts who load the vertebrae while the spine is held in hyperlordotic or sway-back position

8. Tension Forces Load – Tension Type of Force - Pulls or stretches the bone to cause lengthening and narrowing Source - Usually pull of contracting muscle tendon Stress/Strain - Maximal stress on the plane perpendicular to the applied load

9. Tension Forces Example Figure 2-26 shows an example of collagen alignment at the tibial tuberosity. This figure also illustrates the influence of tensile forces on the development of apophyses.

10. Shear Forces Load – Shear Type of Force - Force applied parallel to surface, causing internal deformation in an angular direction Source - Compressive or tension force application or external force Stress/Strain - Maximum stress on the plane parallel to the applied load

11. Shear Forces Example In developing children, this shear force can create epiphyseal fractures, such as in the distal femoral epiphysis.

12. Bending Forces Load – Bend Type of Force - Force applied to the bone having no direct support from the structure Source - Weight bearing or multiple forces applied at different points on the bone Stress/Strain - Maximum tensile forces on the convex surface of the bent member and maximum compression forces on the concave side

13. Bending Forces Example A force is usually applied perpendicular to the bone at both ends of the bone and a force applied in the opposite direction at some point between the other two forces. The bone will break at the point of the middle force application, as is the case in a ski boot fracture shown in Figure.

14. Torsion Forces Load – Torsion Type of Force - Twisting force Source - Force applied with one end of the bone fixed Stress/Strain - Maximum shear stress on both the perpendicular and parallel to axes of bone with tension and compression forces also present at an angle across the surface

15. Torsion Forces Example An example of the mechanism of spiral fracture to the humerus in a pitcher is shown in Figure.

16. Combined Loading Tension, compression, shear, bending, and torsion represent simple and pure modes of loading. It is more common to incur various combinations of loads acting simultaneously on the body. For example, the lower extremity bones are loaded in multiple directions during exercise.

17. References Biomechanical Basis of Human Movement 4 th Ed. by Joseph Hamill Joint Structure And Function 4 th Ed. by Pamela K. Levangie

18. Thank you