Biomechanics of Solids Devendra Bajaj Graduate Student, LAMP Department of Mechanical Engineering UMBC ENCH 484/693: Biomedical Engineering. (April 18, 2007)
Historical Facts Leonardo da Vinci 1452-1519 Drawings of the torso and the arms Studies of the human skull Anatomical studies of the Shoulder www.davinciandthebrain.org
Historical Facts Henry Grey 1827-1860 An illustration from 1918 edition Illustration of skull from the 1918 edition www.wikipedia.org
List of bones of Human Skeleton 1. Frontal bone 2. Parietal bone 3. Temporal bone 4. Occipital bone 5. Zygomatic bone 6. Superior and inferior maxilla 7. Mandible 8. Cervical vertebrae incl. atlas & axis thoracic vertebrae 9. Nasal bone 10. Sternum 11. Humerus 12. Ilna 13. Radius 14. Lumbar vertebrae 15. Pelvis 16. Sacrum 17. Hip joint 18. Femur 19. Patella 20. Tibia 21. Fibula 22. Greater trochanter of femur 23. Condyles of femur 25. Clavicle or collarbone 26. Condyles of humerus 27. Head of radius 28. Ribs 29. Scapula or shoulder blade www.wikipedia.org
Loads Transmitted Bone: Femur www.wikipedia.org
Major Concerns Bone health issues, such as osteoporosis, are expected to affect 50% of Americans by the year 2010* *Bone Health and Osteoporosis: A Report of the Surgeon General, HHS 2004
Fracture and Treatment of Hip Joints Statistics show that about 80 % of all failures that lead to the revision operation of a total hip are caused by aseptic loosening of the total hip. http://www.doereport.com/imagescooked/12945w.jpg
Human Tooth http://www.nlm.nih.gov
Restorative Dentistry Demineralization Excavation Restoration
Failure of Restored Teeth 500 µm Arola et al., J. Mat Sci.:Materials in Medicine, 1998 100 µm
The story so far.. Hard Tissues: Historical perspective Types of bones Load distribution Age related problems Failure and treatment of Hip joint Restorative dentistry Fracture of restored tooth Microstructure of BONE AND HUMAN TOOTH Influence of AGING?
Microstructure of Bone Nalla et al., Materials Science and Engineering, 2006
Bone: Influence of Aging Bone becomes more porous with age Young 34 years Aged 85 years Nalla et al., Materials Science and Engineering, 2006
Microstructure of Dentin Molar Occlusal Surface peritubular dentin intertubular dentin Collagen Debanding 67 nm Dentinal Tubule 5 µm ~10-12 nm Mineral Crystal Ten Cate, Oral Histology 1998 Porter et al., Biomaterials 2005
Dentin: Influence of Aging Dentin becomes less porous with age Young 17 years Aged 50 years Bajaj et al., Biomaterials 2006
Dentin v/s Bone Which one has reparative properties? BONE or DENTIN Teeth are exquisitely sensitive to local stress and trauma and we are conscious of our teeth in a way quite unlike our sense of what our bones are up to. - John D. Curry
Why study the MECHANICAL properties of hard tissues??? It is unfortunate that we have so little idea of the mechanical properties of many of the interesting bio materials. This is because of the small size and conventional shape of many of the structures in which they are found. Their histological structure shows that they are often very well designed mechanically, but, unfortunately, we usually cannot quantify them well. John D. Curry 10 nm Rouland, 2002 Structure Tubule orientation Aging Applications to other engineering materials (MEMS/NEMS) Ritchie et al., 2002
Testing of Hard Tissues Tensile 120 100 σ ut Stress (MPa) 80 60 40 20 0 0.000 0.005 0.010 0.015 0.020 0.025 Strain Sano et al., J Dent Mat. 1984
Testing of Hard Tissues Flexure 200 P(t) P(t)/2 P(t)/2 Bend Stress (MPa) 150 100 50 Strength Elastic Modulus Energy to Fracture 0 0.000 0.005 0.010 0.015 0.020 Strain (m/m) Arola and Reprogel, Biomaterials 2006
Tubule Orientation θ=0 θ=90 Stress (MPa) 160 140 120 100 80 60 40 20 θ=0 o θ=90 o 0 0.000 0.005 0.010 0.015 0.020 Strain θ=0 θ=90 Arola and Reprogel, Biomaterials 2006
Tubule Orientation θ=0 θ=90 Stress amplitude, σ a (MPa) 80 70 60 50 40 30 20 10 θ = 0 ο θ = 90 ο 0 10 2 10 3 10 4 10 5 10 6 10 7 Cycles, N θ=0 θ=90 Arola and Reprogel, Biomaterials 2006
Dentin: Influence of Aging Young 18 years 140 120 Stress (MPa) 100 80 60 40 Old Old 66 years 20 Young 0 0.000 0.005 0.010 0.015 0.020 Strain (m/m) Arola and Reprogel, Biomaterials 2005
Dentin: Influence of Aging Young 18 years Stress amplitude, σ a (MPa) 80 70 60 50 40 30 20 10 Young Old 0 10 2 10 3 10 4 10 5 10 6 10 7 Cycles, N Old 66 years Arola and Reprogel, Biomaterials 2005
Fatigue Testing of Hard Tissues Possible specimen Primary section a 2.0 da/dn (mm/cycle) 10-4 10-5 10-6 C Low Crack Growth Resistance I II III m High Crack Growth Resistance 2.0 1.0 4.0 1.0 10-7 0.8 0.9 1.0 ΔK th ΔK (MPa m 0.5 ) 6.0 Bajaj et al., Biomaterials 2006
Fatigue Crack Growth Dentin 10-3 da/dn (mm/cycle) 10-4 10-5 10-6 Young Old 10-7 0.5 0.6 0.7 0.8 0.9 1.0 2.0 ΔK (MPa m 0.5 ) Bajaj et al., Biomaterials 2006
Mechanisms of Crack Growth Bajaj et al., J Biomed Mat Res, 2007b (submitted)
Fatigue Crack Growth Bone Nalla et al., Biomaterials, 2005
The story so far.. Microstructure of BONE and DENTIN Influence of AGE on structure Testing of hard tissues Tensile Flexure Fatigue Fatigue in DENTIN Fatigue in BONE Why do we see differences in properties with AGING? STRUCTURE CHEMISTRY
Changes in Structure & Chemistry Incidence Electron Beam X-Rays BSE SE Specimen Surface Binary Depth of the source of secondary electron information (~5nm) Depth of the source of Backscattered electron information (~1/3 total depth) Threshold 1.0 μm Depth of the source of X-Ray information (~3µm) Energy Dispersive x-ray Analysis Bajaj, Masters Thesis, 2006
Influence of Structure Dentin Area of Lumen / Area of Tubule 0.25 0.20 0.15 0.10 0.05 0.00 10 20 30 40 50 60 70 80 90 Age (years) Crack Growth Exponent (m) 30 25 20 15 10 5 0 r = 0.799 P = 0.0028 0.00 0.05 0.10 0.15 0.20 Area of Lumen/Area of Tubule Bajaj, Masters Thesis, 2006 Area of Area of Lumen Tubule = Degree of Occlusion
Influence of Chemistry Dentin Ca / P 2.6 2.5 2.4 2.3 2.2 2.1 2.0 10 20 30 40 50 60 70 80 90 Age (years) Crack Growth Exponent (m) 30 25 20 15 r = 0.878 P = 0.0056 10 2.0 2.1 2.2 2.3 2.4 2.5 2.6 Ca/P Bajaj, Masters Thesis, 2006
Influence of Structure Bone Nalla et al., Materials Science and Engineering, 2006
Influence of Chemistry Bone Ager et al., J Biomed Optics, 2005
Conclusions Aging results in changes in properties of hard tissues Changes in structure and chemistry are responsible for changes in properties Enamel (E) Dentin (D) Pulp 2 mm D DEJ E 250 µm What is the biological seed to aging? How does the mineral and non-mineral component contribute to strength? How much damage accumulates in these tissues per day? Can we arrest these cracks mechanically or chemically?