What do you think when you hear the word biomechanics?

1. What do you think when you hear the word biomechanics?

2. What are some subdisciplines of bionechanics?

3. Advanced Biomechanics of Physical Activity (KIN 831) Lecture 1 Biomechanics of Bone

4. Single Joint System* Dr. Eugene W. Brown Department of Kinesiology Michigan State University * Material included in this presentation is derived primarily from two sources: Enoka, R. M. (1994). Neuromechanical basis of kinesiology. (2nd ed.). Champaign, Il: Human Kinetics. Nordin, M. & Frankel, V. H. (1989). Basic Biomechanics of the Musculoskeletal System. (2nd ed.). Philadelphia: Lea & Febiger.

5. Components of a Single Joint System • Rigid Link (Bone, Tendon, Ligament) • Joint • Muscle • Neuron • Sensory Receptor

6. Purpose of Bone?

7. Some Purposes of Bone • Provides mechanical support • Produces red blood cells • Protects internal organs • Provides rigid mechanical links and muscle attachment sites • Facilitates muscle action and body movement • Serves as active ion reservoir for calcium and phosphorus

9. Wolff’s Law “Every change in the form and function of a bone or of their function alone is followed by certain definitive secondary alteration in their external conformation, in accordance with mathematical laws”.

10. Composition and Structure of Bone • Consists of cells and an organic extracellular matrix of fibers and ground substance • High content of inorganic materials (mineral salts combined with organic matrix) • Organic component  flexible and resiliant • Inorganic component  hard and rigid • Mineral portion of bone primarily calcium and phosphate (minerals 65-70% of dry weight) • Bone is reservoir for essential minerals (e.g., calcium)

11. Composition and Structure of Bone • Collagen • Mineral salts embedded in variously oriented protein collagen (strength in various directions) in extracellular matrix • Tough and pliable, resists stretching • 95% of extracellular matrix (25-30%) of dry weight of bone

12. Schematic illustration of section of the shaft of long bone without inner marrow Concentric layers of mineralized matrix that surround a central canal containing blood vessels and nerves

13. Haversian canal – small canal at center of each osteon containing blood vessels and nerve cells • Lamellae - concentric layers of mineralized matrix surrounding haversian canal • Lacunae – small cavities at boundaries of each lamella containing one bone cell or osteocyte • Canaliculi – small channels that radiate from lacuna connecting lacunae of adjacent lamellae and reaching havesrian canal

14. Cement line • -limit of canaliculi • -collagen fibers in bone matrix do not cross cement line • -weakest portion of bone’s microstructure

16. Microscopic-macroscopic structure of bone. Data form Rho et al., 1998.

17. What are the types of bone?

18. Two Types of Bone • compact (or cortical) bone – outer shell, dense structure, surrounds cancellous bone • Cancellous (or trabicular) bone • Does not contain haversion canals • contains red bone marrow in spaces -------------------------------------------------------- • Biomechanical properties are similar; differ in porosity and density (see figure) • Quantity of compact and cancellous tissue in bone differs by function

19. Two Types of Bone

20. Two Types of Bone

21. Periosteum • Dense fibrous membrane that surrounds bone; outer layer permeated by blood vessels and nerve fibers that pass into cortex via Volkmann’s canals • Inner osteogenic layer contains osteocytes (generate new bone) and osteoblasts (bone repair)

23. Endosteum • Lines medullary cavityof long bones, filled with yellow fatty marrow • Contains osteoblasts and osteoclasts (resorption of bone)

24. Biphasic Behavior of Bone • Minerals  hard and rigid • Collagen and ground substance  resilient -------------------------------------------------------- Combination  stronger than either alone

25. Load Deformation Testing

26. Load Deformation Curve • B – max. load before deformation • D’ – deformation before structural change • Area under curve is force x distance = work= energy

27. Load Deformation Curve • Slope of elastic region defines stiffness • Area under curve defines energy that can be stored • Elastic region – return to original configuration once load is removed • Plastic region – deformation of material • Load deformation curve is usefull when determining comparative characteristics of whole structures (e.g., bone, tendon, cartilage, ligaments)

28. What is the function of normalization?

29. What is the function of normalization? • Independent of geometry of material • Permits comparison of different materials (e.g., bone, tendons, cartilage, ligaments)

30. What are some examples of normalization?

31. Normalizing Load • Stress – force/area • Strain – length change/initial length (unitless value) • Two types of strain • Linear – causes change in length • Shear – causes change in angular relations (radians)

32. Stress-Strain Relationships • Similar to load deformation curve

33. Stress-Strain Relationships Elastic modulus (Young’s modulus) – slope of the stress-strain curve in the elastic region (measure of stiffness) Plastic modulus – slope of the stress-strain curve in the plastic region Area under stress strain curve is measure of energy absorbed

34. Relationships of Age to Stress-Strain Characteristics of Bone • indirect relation between age and energy absorption

35. Cortical vs. Cancellous Bone • Cortical bone stiffer, withstand greater stress but less strain before failure • Cancellous bone fractures when strain exceeds 75% • Cortical bone fractures when strain exceeds 2% • Cancellous bone has larger capacity to store energy

36. Properties of Stiffness and Brittle/Ductile Interpretation?

37. Properties of Stiffness and Brittle/Ductile • Metal – large plastic region • Virtually no plastic region in glass • Stress-strain curve of bone not linear • Yielding of bone tested in tension caused by debonding of osteons at cement lines and microfractures

38. Ductile and Brittle Fracture • Young bone more ductile • Bone more brittle at higher loading rates

39. Load-deformation Relationships

40. Typical Response of Long Bone to Loads • greatest resistance to compression • weakest response to shear loads • intermediate strength for tension

41. Typical Response of Long Bone to Loads

42. Safety Factor • Safety factor - bones are 2 to 5 times stronger than forces they commonly encounter in activities of daily living; bone strength and stiffness are greatest in the direction in which loads are most commonly imposed (see figure)

43. Physiologic Area

44. What is Wolff’s Law?

45. Remodeling of Bone • Wolff’s Law • Remodeling – balance between bone absorption of osteoclasts and bone formation by osteoblasts • osteoporosis –increase porosity of bone, decrease in density and strength, increase in vulnerability to fractures • piezoelectric effect – electric potential created when collagen fibers in bone slip relative to one another, facilitates bone growth • use of electric and magnetic stimulation to facilitate bone healing

46. Factors Influencing the Dynamic Response of Bone • Mechanical properties of bone • Geometry • Loading mode • Rate of loading • Frequency of loading

47. Factors Influencing the Dynamic Response of Bone • Result of loading of bone in transverse and longitudinal directions dissimilar (anisotrophy) • Bone tends to be strongest in directions most commonly loaded

48.   Behavior of bone under tension, compression, bending, shear,torsion, and combined loading

49. Behavior of Bone Under Tension • under tensile loading structure lengthens and narrows • equal and opposite loads applied outward • maximum tensile stress occurs on a plane perpendicular to the applied load (see figure)

50. Tensile Loading