Chapter 4 Tissue Biomechanics and Adaptation Modification of an organism or its parts that makes it more fit for existence under the conditions of its environment (Mish, 1984)

In vitro, in situ or in vivo? n In vitro: in a glass (artificial environment) n Allow direct measurements n Invasive n In situ: in its normal place n some elements of the natural environment are preserve n Artificial testing n In vivo: done within the living body (ideal) n Difficult to obtain (invasive), few human models

Testing Procedures n Same testing principles used for testing materials n Materials can be tested under: n compression n tension n torsion n bending n shear n Sample of material of known dimension is tested

Load-deformation curve n Elastic region n Proportional limit (yield point) n Elastic limit n Plastic region n Ultimate strength n Energy stored

Structural vs. material properties n Material properties are the characteristics of the material regardless of size, density etc. n The femur and phalange can have the same material properties but different structural properties (maximal load, bending stiffness)

Geometry n Moment of Inertia n I=mr 2 n Example A: smaller moment of inertia, bending will occur n Example B: larger (I) greater cross- sectional more stiffness A B

Bone geometry d= 2.0 d = 2.5 I II III Increase in stiffness without adding mass Why not solid bones?

Mechanical properties of cortical bone n Anisotropic n Stiffness: calcium/porosity n Poisson ratio( ) n High: < 0.6 n Absorbs  ME before fracture n Ductile: Allows deformation

Cortical Bone Properties n Viscolelastic n Strain-rate sensitive n rate  ultimate strength also  n Fatigue: cyclic loads n Remodeling outpaced by damage microcracks develop, stress fractures n Microcracks: most likely to occur in the highly mineralized part of the bone

Trabercular Bone n Mesh network: different densities and patterns n Nonlinear elastic modulus and strength n Marrow: Enhances Load bearing effect

Bone Adaptation n Modeling: addition of new bone n different rates n continuos n any bone surface n growing years (fast) n initiation ? n Remodeling: resorption and formation of bone n Activation, resorption and formation n Osteoclast resorption n new bone (osteoblast) n Longer process n Initiated n functional strain n fatigue damage theory (Burr)

Age n BMC: Bone mineral content n PHV: Peak height velocity (growth) n Period of bone weakness PHV and BMC n Maximal BMC years

Age n Men > BMC then women n cortical bone n 50’s decline in BMC n cortical same rate n women lose trabercular bone at a faster rate n rate  after menopause (3%) n Importance of reaching high BMC during adolescence

Osteoporosis n Reduction of bone mineral mass and changes in geometry leading to fractures (hip, spine, wrist) n Bone mass loss increases after menopause

Nutrition n Mineral balance n vitamin D metabolites n parathyroid hormone n calcitonin n 99% of Calcium is found in the skeleton (1% in extracellular fluid) n Vitamin D n calcium absorption n sun exposure n Dietary protein helps control urinary calcium handling n deficiency  calcium absorption, osteopenia n excess  calcium loss causing imbalance n excess dietary fats  calcium absorption

Physical Activity n Exercise can stimulate bone growth n growing bone: low- moderate activity n threshold n Moderate-intense  BMC (1-3%) in men and women n Intense activity 11% in tibia of young adults n Must continue exercise n depend of initial bone mass n Exercise related conditions n amenorrhea n oligomenorrhea n dietary restrictions n female triad n eating disorders n disrupted hormone levels n low BMC n Type of exercise n high intensity and impact

Bone exercise

Disuse n Immobilization, bed rest, space flight n Space flight: lack of loads n  deposition n  resorption n affect more weight bearing trabercular bones n Mostly reversible process: recovery is much slower n Early mobilization n fracture braces etc.

Articular Cartilage n Type II collagen n Different fibers orientation n Shear forces n tensile resistance to swelling n Creep n constant load n compression load n Cyclic loading n Benefits vs. damage

Articular Cartilage lubrication n Synovial joints n Low coefficients of friction n Theories of lubrication n Boundary n Fluid film n hydrodynamic (non deformable) n elastohydrodynamic n Squeeze Film n right angle movement n short duration molecules Fluid

Articular Cartilage lubrication n Boosted Lubrication n combination of elastohydrodynamic and squeeze n AC is deformed matrix fluid is forced out in the space between the surfaces  fluid viscosity Rigid Deformable

Articular Cartilage: Permeability n How easy a fluid flows through a permeable membrane n Inversely proportional to frictional drag n High loads decreases permeability of AC

Articular Cartilage: Adaptation n Active loading & unloading n Degenerative changes (OA) n Aging n  water content n  PG n  collagen content

Articular Cartilage: Use & Disuse n Exercise: swelling of AC, increase PG’s n Long term: wear & tear, degradation, OA n OA: cause ? n excessive loads n inferior biomaterials n Some Factors n heredity n chemical changes n altered joint mechanics (ACL- laxity) n obesity

Articular Cartilage: Use & Disuse n Disuse n atrophy n reduction of synthesis n  PG n  fibrillation n  mechanical properties n deforms rapidly n Changes are reversible Biological properties LackNonstrenuous Strenuous Control

Tendon & Ligament n Ultimate tensile stress of tendon considerably high ( MPa) n Viscoelastic behaviors n creep, stress-relaxation n strain rate sensitivity, different from bone n fast strain rate ligament injuries, slow rate (avulsion fracture) n Partial failure n Geometry

Tendon & Ligament n Age n before maturity: more viscous & compliant n maturity:  stiffness & modulus of elasticity n After middle age:  viscosity, less compliant, weak insertions (avulsion fractures)

Tendon & Ligament n Sensitive to training and disuse n Hypertrophy: increase in size and mechanical strength n Exercise can produce increases up to 20% in ligament strength n Increase in number of collagen fibrils and cross-sectional area of tendons, collagen synthesis n Disuse n deterioration of both mechanical and biochemical properties n  strength, GAG, water, collagen synthesis, mass

Skeletal Muscle n Force production n twitch n tetanus n depends on # cross-bridges n rate force: sarcomeres in series n High power output

Skeletal Muscle: strains n Tears n bone tendon junctions n muscle belly n myotendinous junction n Contracting muscles required more force and energy to reach failure

Skeletal Muscle Adaptation n # muscle fibers set at birth ? n Muscle length associated with addition of sarcomeres at myotendinous junction n Muscle adaptations in children  strength no increase in size (neural factors) n Maximal strength years n Plateau age 50 with a decline n Loss of strength  # fibers, fast twitch n Gender: women 75% total cross-sectional are n Same relative strength

Skeletal Muscle Adaptation n Hypertrophy vs. Hyperplasia n Neurological components n Specific demands n strength vs. endurance n Atrophy n immobilization n bed rest n sedentary life n weightlessness n Changes in fiber size n lower protein synthesis n increase degradation n Slow twitch fibers more affected