Biomechanics II Lecture 11 PDF

‫السالم عليكم و رحمه الله و بركاته‬  Ahmed Torad, PhD, PT, Cairo university, 2020 Presente lecturer, Basic Science department, d by Faculty of Physical Therapy, Kafrelsheik  Do you rem stil e abo mbe l ut r str the ess s...

‫السالم عليكم و رحمه الله و بركاته‬  Ahmed Torad, PhD, PT, Cairo university, 2020 Presente lecturer, Basic Science department, d by Faculty of Physical Therapy, Kafrelsheik  Do you rem stil e abo mbe l ut r str the ess str - ain ?c u rv e Stress-strain curve  Stress-Strain curve of bone The bone is a viscoelastic tissue. The bone shows elastic behavior in response to increased stress till reaching the yield point then Plastic deformation starts. It is characterized by a plastic deformation till reaching the ultimate strength point of bone after which bone fractures.  Mechanical Properties of Bone Biomechanically the bone tissue consists of two phases: 1st phase: mineral 2nd phase: collagen A- mineral to collagen ratio: the collagen component is primarily responsible for elasticity and deformation of bone after yield. So, decreasing collagen ratio lead to decreased energy absorbing capacity of bone and become  Mechanical Properties of Bone B- Bone porosity and density: 1- bone porosity: is the state of being porous The trabecular network divides the interior volume of bone into pores of different dimensions, producing a structure of variable porosity and density. These spaces in the trabecular bone are filled with bone marrow. Bone marrow under pressure acts as an additional factor for strength as it absorb energy Mechanical Properties of Bone  Mechanical Properties of Bone 2- bone density: is defined as the mass of bone material per unit volume. Compact bone has low porosity and high density with more mineral contents while cancellous bone has high porosity. A strong correlation exists between bone density and stiffness and strength of trabecular bone. Maximum bone mineral density is men > women in age 20 to 30 year that decrease in both sexes as they grow older due to thinning of cortical bone thickness. Functionally the most important mechanical properties of the bone are: its strength and stiffness. To understand these properties we have to put the bone under loading. Loading leads to deformation or changes in bone dimensions. The deformation can be measured and plotted in load-deformation curve. (A) load, (B) deformation, (C) elastic region, (D) yield point, (E) plastic region, ( F) ultimate failure point  Load-deformation curve The elastic region in the curve reveals the elasticity of the bone which means, its capacity to return to its original shape after the load is removed. As loading continues, the outermost fibers of the bone begins to yield at some points, this yield points signals the elastic limit of the bone. As the load exceeds this point, the bone exhibits plastic behavior. The bone will no longer returns to its original shape after removal of the load still some residual deformation will be permanent.  Mechanical Properties of Bone 3- dynamic structure of bone: The skeleton is composed of cortical and cancellous bone. The difference between two types is not in the composite material but in proportion of porosity. The following is the difference between the stress strain curves of the cortical bone and cancellous bones. Cancellous (spongy) Cortical (compact) Compariso n High ( 30-90 %) Low ( 5-30 %) Porosity Low High Density Less stiff ( long plastic Stiffer (long elastic Stiffness region) region) Withstand less stress Withstand greater Function But, shock absorb stress Ductile Brittle Fracture type  Mechanical Properties of Bone 4- Age: children tend to have “greenstick” fractures involving considerable bending (plastic deformation)compared with adult who have more brittle failure of bone. With aging, bones have tendency to become less ductile and more brittle, less stiff and more fragile. Biomechanics of cartilage  Cartilage characteristics Cartilage is a connective tissue with high mechanical properties. 1. It has high tensile , as it is in tension even no stress is present. 2. It resists compressive and shear forces. 3. It gives resilience and elasticity. 4. It distributes weight especially in weight bearing joints.  Cartilage function 1- It is a self-lubricating tissue: Its primary function is to allow relative movement of the opposing joint surfaces with minimal friction and wear. Stresses affecting the cartilage during motion: A. Gravitational stresses B. Muscle contraction stresses  Cartilage function 2- the articular cartilage spreads the loads: Applied to the joints over large areas so the contact stresses decrease (stress= force ̷ area) So changing the contact areas during joint motion minimizes the stress. So the stress is not concentrated at one small area.  Composition of articular cartilage 1- Collagen fibrils (15-22%): Function: 1- it has high tensile stiffness and strength properties. 2- it provides resistance to stresses and strains of the articulation. 2- Proteoglycan (4-7%): Function: It acts as a binding substance to the collagen molecules, as when stresses are applied to the cartilage, there is instantaneous deformation, because of change of Proteoglycan shape. Biomechanical properties of the articular cartilage 1- Viscoelasticity: Cartilage is perfectly elastic for a small load which is only applied for a short time.  Cartilage becomes deformed when exposed to constant stresses. It resumes its shape during rest. This explains why persons are taller during the morning compared to at night. Biomechanical properties of the articular cartilage 2- Anisotropy : It is like bone, it resists compression, tensile, torsion, bending, and shear stresses. Because the varying collagen fibers arrangements within the planes parallel to the articular surface. 3- permeability of the cartilage: Articular cartilage is a material of high porosity (80%). Permeability is a measure of the ease with which fluid can flow through a porous material. It is the frictional resistive force which is generated by the interaction of the interstitial fluid and the pore walls of the porous material. During joint motion or when the articular cartilage is compressed, some of the fluid content of the cartilage is exudated through the pores in the outermost layer. When the applied force increases, the cartilage will appear stiffer and it will be more difficult to cause fluid exudation, so cartilage permeability decreases.  Biomechanical properties of the articular cartilage 4- manner of nutrition: Articular cartilage nourishment depends on cyclic loading and unloading of cartilage which squeeze water out and back into the cartilage. The free flow of fluid is essential for the survival of the cartilage and function of reducing friction. Biomechanical properties of the articular cartilage 5- pressure and deformation: Under normal conditions, cartilage is extensible and compressible to a considerable degree. The greater elasticity develops always in the direction of joint motion and where the joint pressure is concentrated the tension lines always run radially from the point of greatest pressure. 5- pressure and deformation: 6- lubrication of the cartilage: There two types of lubrication; A.Boundary lubrication: involves single monolayer of lubricant molecules on each bearing surface. B. Fluid film lubrication: a thin fluid film provides greater surface to surface separation. Pathomechanics of the articular cartilage  Factors affecting cartilage degeneration 1- magnitude of total load on the joint: Stress concentration may occur due to menisectomy, tearing of meniscus, osteoarthritis. For example: Tearing of meniscus causes it to be redundant and as a result it does not provide resistance against extruding forces. So there is high stress concentration during load transmission.  Factors affecting cartilage degeneration 2- frequency of total loads on the joint: Articular cartilage degeneration increases by increasing the frequency of total loads on the joint. This explains why people in certain occupations experience high incidence of degeneration as in “football players” knees.  Factors affecting cartilage degeneration 3- prolonged immobilization: Articular cartilage often undergoes degenerative changes following prolonged immobilization. The reason is that immobilization interferes with the nutrition of the cartilage. Articular cartilage has a limited capacity for repair and regeneration  Factors affecting cartilage degeneration 4- various disorders of collagen metabolism or the structure of collagen\ proteoglycan matrix: These disorders weaken the cartilage and then the cartilage is destroyed by stresses of normal magnitude and frequency.  Wear of the articular cartilage Wear is removal of material from the solid surfaces by a mechanical action. Wear is classified into two types: 1- Interfacial wear: this occurs due to interaction of the bearing surfaces if the bearing surfaces come into contact with no lubricant film separating them  Wear of the articular cartilage 2- fatigue wear:  This occurs due to microscopic damage in a material when it is repetitively stressed leading to a fatigue failure. Although the magnitude of the applied stresses may be much less than the material`s ultimate strength, failure will occur.