14 CPMS Biomechanics Tissue Mechanics II bone 2024.pdf

Biomechanics & Surgery: Tissue Mechanics II Bone At the completion of this topic, the student will learn: n Function, physiology and composition of bone tissue – Cortical / lamellar / compact – Trabecular / cancellous / spongy n Biomechanics of bone tissue – mechanical properties – Viscoelasticity n...

Biomechanics & Surgery: Tissue Mechanics II Bone At the completion of this topic, the student will learn: n Function, physiology and composition of bone tissue – Cortical / lamellar / compact – Trabecular / cancellous / spongy n Biomechanics of bone tissue – mechanical properties – Viscoelasticity n Readings – Required: Ding M, et al. Age variations in the properties of human trabecular bone. J Bone Joint Surg, 1997; 79-B:995-1002. Available on D2L Vassilios G. Vardaxis, Ph.D. Bone: Objectives At the completion of this topic the students should be able to understand: – Briefly review the basic bone biology, architecture and terminology – Describe the mechanical properties of the healthy bone tissue – Describe modes of mechanical failure of bone – Discus the clinical relevance of the bone mechanical properties Vassilios G. Vardaxis, Ph.D. Bone: Material composition Two major components – Organic matrix (~40% dw) n Type I collagen (90%) n Proteoglycans n Non-collagenous matrix proteins n Amorphous ground substance – Inorganic matrix (~60% dw) n Calcium hydroxyapatite Vassilios G. Vardaxis, Ph.D. Material composition: Collagen n 90% organic matrix Provides tensile strength to bone n Primarily type I n Structure n – Triple helix à fibril – Hole zones b/w fibrils – Pores b/w sides of parallel molecules n Minerals laid down in holes and gaps – X-linking decreases solubility and increases the tensile strength. Vassilios G. Vardaxis, Ph.D. Material composition: Proteoglycans n n n n Composed of glycosaminoglycans complexes Inhibit mineralization Numerous functions ranging from growth factors to binding properties Partially responsible for compressive strength of bone. Matrix Proteins (non-collagenous) n Promote mineralization and bone formation – Osteocalcin: Produced by osteoblasts, related to regulation of bone density, most abundant non-collagen matrix protein – Osteonectin: Secreted by platelets and osteoblasts, organization of mineral within matrix – Osteopontin: Cell binding protein Vassilios G. Vardaxis, Ph.D. Material composition: Inorganic Matrix n n 60% dry weight Almost entirely Calcium Hydroxyapatite Provides the compressive strength of bone Responsible for the mineralization of bone Primary mineralization occurs in holes and pores Vassilios G. Vardaxis, Ph.D. Bone: Microstructure Seven hierarchical levels 1. Isolated crystals & collagen fibrils 2. Mineralized collagen fibril 3. Mineralized fibril array 4. Fibril array patterns 5. Single osteon 6. Spongy and compact bone 7. Whole bone Vassilios G. Vardaxis, Ph.D. Bone: Cancellous vs. Cortical Vassilios G. Vardaxis, Ph.D. Bone: Structure Vassilios G. Vardaxis, Ph.D. Structure: Cortical bone Vassilios G. Vardaxis, Ph.D. Bone Growth: Osteon formation Vassilios G. Vardaxis, Ph.D. Secondary Osteons Skeletal Tissue Mechanics, Martin et al., 1998 Vassilios G. Vardaxis, Ph.D. Osteon Cutting Cone Skeletal Tissue Mechanics, Martin et al., 1998 Vassilios G. Vardaxis, Ph.D. Lamellar Bone: Structure Osteon Fibrils Skeletal Tissue Mechanics, Martin et al., 1998 Vassilios G. Vardaxis, Ph.D. Cortical Bone: types nlamellar – parallel layers of lamellae – mineralized collagen fibers are parallel within each lamella – direction of fibers may alternate between adjacent lamellae nwoven bone – quickly formed – poorly organized, fibers are more or less randomly arranged – more mineralized than lamellar – weaker than mineralized Vassilios G. Vardaxis, Ph.D. Compact Bone (cortical) n n n n n n n 80% skeleton Osteons connected by Haversian canals Cement lines define outer border of osteon Nutrition via intraosseous circulation. Slow turnover Relatively high Young’s modulus Higher resistance to torsion and bending than cancellous. Vassilios G. Vardaxis, Ph.D. Woven Bone n n n n Immature or pathologic Collagen arranged irregularly Isotropic à mechanical properties independent of orientation of stresses applied Exists in: – – – – – Fetal skeleton Fx callus Tooth sockets Bone forming tumors Stages of accelerated bone formation Vassilios G. Vardaxis, Ph.D. Bone: Trabecular Bone Vassilios G. Vardaxis, Ph.D. Bone: Trabecular Bone n Trabecular bone – formed by organization of plate- and rod-like struts called trabeculae – trabeculae are about 200 µm thick. Vassilios G. Vardaxis, Ph.D. Cancellous Bone (trabecular/spongy) n n n n n n n Less dense More remodeling along lines of stress (Wolff’s law) Much larger surface area Higher turnover Lower apparent modulus More elastic More resistance to compressive forces Vassilios G. Vardaxis, Ph.D. Wolff's Law Bone remodels in response to the stresses applied to it It is probable that this remodeling occurs to keep strain (not stress) between certain upper and lower limits If strain is too high, new bone is laid down along the lines of stress, and the bone becomes both thicker and denser If the strain is too low, bone is lost, making the bone less dense, this in the inactivity case may lead to osteoporosis Vassilios G. Vardaxis, Ph.D. Bone Anatomy – Wolff’s Law Vassilios G. Vardaxis, Ph.D. Bone: Growth / formation n customize the shape of bones during growth in accordance with mechanical needs – metaphyseal modeling to reduce bone diameter during growth – diaphyseal modeling to increase bone diameter n n addition of bone on the periosteum resorption of bone at endosteum – customized the shape of bones in accordance with mechanical needs n diaphyseal modeling to alter curvature n cross section drifts sideways relative to the ends of the bone Vassilios G. Vardaxis, Ph.D. Bone: Stress-Strain n n bone has nonlinear elastic behavior moderate plastic region Vassilios G. Vardaxis, Ph.D. Bone Mechanical Characteristics Vary according: Geometry Load mode applied Direction of load Rate of loading Frequency of loading Vassilios G. Vardaxis, Ph.D. Cortical Bone Sample comments Cortical bone is most brittle under tensile forces in the transverse direction It is tougher and stronger in compression Vassilios G. Vardaxis, Ph.D. Trabecular bone can absorb considerable energy while maintaining a minimum mass Vassilios G. Vardaxis, Ph.D. Bone: Anisotropic Behavior n bone exhibits different mechanical properties when loaded along different axes Vassilios G. Vardaxis, Ph.D. Stiffer, more plasticity Less Stiffness no plasticity Bone: Bending Load n n compression and tension Stress not equally distributed: increased magnitude farther from neutral axis Vassilios G. Vardaxis, Ph.D. Bone: Torsional load n n n Twist about an axis Shear stress is developed Magnitude of stress proportional to distance from axis Vassilios G. Vardaxis, Ph.D. Rate dependency: viscoelasticity Increased loading rate Increased stiffness Increased load Loading rate influences the fracture pattern and the soft tissue damage Fig 2-34, Nordin & Frankel, (2001) Vassilios G. Vardaxis, Ph.D. Reduced load effect on bone n n immobilized for 60 days bed rest reduces bone mass by 1%/wk Vassilios G. Vardaxis, Ph.D. Exercise effect on bones Growing bone responds to low or moderate exercise through significant addition of new cortical and trabecular bone usually through periosteal expansion. A threshold of activity exists above which some bones respond negatively by suppressing normal growth and modeling activity Moderate to intense physical training can generate modest increases (1-3%) in bone mineral content (BMC) in men and premenopausal women In young adults very strenuous training may increase BMC of the tibia by up to 11% and its bone density by 7%. Some evidence shows that exercise can also add bone mass to the postmenopausal skeleton, although the amounts are modest and site specific. After 1-2 yrs of intensive exercise, increases as high as 5-8% can be found, but usually less than 2%. The long-term benefits of exercise are retained only by continuing to exercise. The amount of bone mass that can be achieved appears to depend primarily on the initial bone mass, suggesting that individuals with extremely low initial bone mass may have more to gain from exercise than those with moderately reduce bone mass. Vassilios G. Vardaxis, Ph.D. Ageing Vassilios G. Vardaxis, Ph.D. Aging n n decreased strain decreased energy storage Vassilios G. Vardaxis, Ph.D. Aging variations in tibial trabecular bone n n n n n n Ding et al. 1997 Peak E 40-50 yo Peak Ultimate stress 40-50 yo Energy to fail decreases with age Strain decreased Tissue density did not change Apparent density (amount of bone decreased) Vassilios G. Vardaxis, Ph.D. young old Vassilios G. Vardaxis, Ph.D. Osteoporosis Vassilios G. Vardaxis, Ph.D. Vassilios G. Vardaxis, Ph.D. Vassilios G. Vardaxis, Ph.D.