Properties of Biological Materials -- Collagenous Tissues 柴惠敏 台灣大學 物理治療學系

主要參考書 Nordin, M. & Frankel V.H., 2001. Basic Biomechanics of the Musculoskeletal System, 3rd ed. Philadelphia, PA, Lea & Febiger. Chapter 3 & 4. Chaffin & Addersson, 1999. Biomechnics is a discipline of science, newly developed and in the process of becoming established.

Biomechanics of Collagenous Tissues Basic Concepts

Collagenous Tissues Dense connective tissues Loose connective tissues ligament: tensile load tendon: tensile load Loose connective tissues capsule: tensile load Skin: tensile load Cartilage articular cartilage: compressive/ shear fibrocartilage: compressive/ shear

Components of Collagenous Tissues Cells: chondrocytes or fibroblasts Extracellular Matrix Fibers: collagen fiber -- strength elastin -- felxibility reticulin -- mass Ground substance Biomechnics is a discipline of science, newly developed and in the process of becoming established.

Collagen Fiber the most abundant protein in the body to resist tensile stress tropocollagen: 3 procollagen polypeptide chains ( chains) coiled about each other into a right-handed triple helixes  chain 1.4 nm

Types of Collagen Fiber Type I found in bone, tendon, ligament, and skin Type II found in articular cartilage, nasal septum, and sternal cartilage

Tensile Strength of Collagen Fiber strain stress microtrauma physiological loading clinical test injury failure toe region

Compression Strength of Collagen Fiber only able to resist low compression loads buckle under compression load slenderness ratio ratio of length to thickness

As load remains constant Creep Phenomenon Progressive deformation of a viscoelastic structure with time as the amount of load remains constant time strain As load remains constant

As deformation remains constant Load Relaxation Progressive decrease in load with time as the deformation of the structure remains constant time stress As deformation remains constant

Hysteresis Energy stored in a viscoelastic material when a load is given and then relaxed. strain stress loading unloading energy stored

Aged Heel Pad strain stress young aged

Biomechanics of Collagenous Tissues Basic Concepts Biomechanics of Connective Tissues

Functions of Connective Tissues Ligament to connect two adjacent bones to augment stability of joints to check excessive motions Tendon to attach muscle to bone to transmit forces from muscle to bone

Components of Connective Tissues cell: fibroblast 20% matrix: 80% water: 60-70% for ligaments collagen: 70-80% of dry weight molecular cross-link: for strength

Factors Influencing Mechanical Properties of Connective Tissues structure of the orientation of the collagen fibers properties of the collagen and elastin fibers proportion between collagen and elastin fibers ligamentum flava: 60% of elastin fibers ACL: 94% of collagen tendon ligament capsule

Strength-Stress Curve of Collagen Fiber plastic region elastic toe strain stress failure ultimate strain = 6-8%

Modulus of Elasticity Young’s modulus or stiffness E =  /  where  = stress = F / A (unit: Pa)  = strain (unit: %)

Strength-Stress Curve of Elastin Fiber strain stress failure ultimate strain ~ 70%

Effect of Age before adolescent: maturation aging ligament strength < bone strength maturation # and quality of cross-links increases collagen fibril diameter increases aging # of collagen fibers decreases

Other Factors Influencing Mechanical Behaviors of Connective Tissues pregnancy and postpartum period increase laxity decrease tensile strength mobilization vs. immobilization steroid vs. NSAID diabetes mellitus hemodialysis grafts

Mobilization vs. Immobilization

Biomechanics of Collagenous Tissues Basic Concepts Biomechanics of Connective Tissues Biomechanics of Articular Cartilage

Components of Articular Cartilage cell: chondrocyte <10% matrix: water: 65-80% collagen fibers: to resist tensile stresses proteoglycans: ~10%; to resist compression

Heterogeneous Zones of Articular Cartilage superficial tengential zone (10-20% of total thickness) transitional area (40-60% of total thickness) deep layer (30% of total thickness) calcified cartilage subchondral bone tidemark

Proteoglycans link protein hyaluronic acid protein core compression recovery

Biphasic Viscoelastic Properties in Compression Biphasic creep response creep + fluid exuadation up to 50% of fluid be squeezed out Biphasic load relaxation stress increased as fluid exudation stress decreased as fluid redistribuation

Permeability under Compressive Strain low load high load compressive strain

Lubrication of Articular Cartilage Boundary lubrication a single layer of lubricant molecules absorbed onto each bearing surface depends on the chemical property of lubricants Fluid film lubrication a thicker fluid film provides greater surface separation

Fluid Film Lubrication on Deformable Materials hydrodynamic squeeze film lubrication lubrication

Failure of Articular Cartilage mechanical loading and unloading prevent cartilage degeneration limited ability to remodel itself if articular cartilage is damaged loads leading to wear acute: active loading or impact loading chronic: interfacial or fatigue loads