Biomechanics Of Bone.ppt

Biomechanics of Bone

Characteristics • Purpose of the skeletal system: to protect internal organs, provide rigid kinematic links and muscle attachment sites, and facilitate muscle action and body movement • Bone: • has unique structure and mechanical properties that allow it to carry out these roles. • among the body's hardest structures; only dentin and enamel in the teeth are harder. • A highly vascular tissue, an excellent capacity for self-repair and can alter its properties and configuration in response to changes in mechanical demand. – changes in bone density after periods of disuse and of greatly increased use – changes in bone shape during fracture healing and after certain operations – adapts to the mechanical demands placed on it.

Bone Composition and Structure • Normal human bone is composed of: • Mineral or inorganic portion: • consists primarily of calcium and phosphate, mainly in the form of small crystals resembling synthetic hydroxyapatite crystals with the composition Ca10(PO4)6(OH)2. • accounts for 60 to 70% of its dry weight • Water: 5-8% • Organic matrix: remainder of the tissue

Bone structure • Composition: a cellular component + an extracellular matrix. • The cellular component is made of • Osteoblasts: bone-forming cells, • Osteoclasts: bone-destroying cells, and • Osteocytes: bone-maintaining cells which are inactive osteoblasts trapped in the extracellular matrix. • Extracellular matrix: • responsible for the mechanical strength of the bone tissue • formed by an organic and a mineral phase. • organic phase: mainly composed of collagen fibres • mineral phase: composed of hydroxyapatite crystals. • a liquid component is also present.

Two main types of bone

Longitudinal section of human femur. The direction of principal stresses are shown in the scheme on the right

Characteristics • Osseous tissue: • primary tissue of bone • relatively hard and lightweight composite material, formed mostly of calcium phosphate in the chemical arrangement termed calcium hydroxylapatite • gives bones their rigidity. • Bone: • relatively high compressive strength but poor tensile strength (resists pushing forces well, but not pulling forces). • essentially brittle, but has a significant degree of elasticity, contributed chiefly by collagen. • consist of living cells embedded in the mineralized organic matrix that makes up the osseous tissue.

Compact bone or (Cortical bone) • The hard outer layer of bones is composed of compact bone tissue, so-called due to its minimal gaps and spaces. • This tissue gives bones their smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult skeleton. • Compact bone may also be referred to as dense bone.

Trabecular bone • It is an open cell porous network also called cancellous or spongy bone filling the interior of the organ • It is composed of a network of rod- and platelike elements that make the overall organ lighter and allowing room for blood vessels and marrow. • It accounts for the remaining 20% of total bone mass, but has nearly ten times the surface area of compact bone.

Cortical and Trabecular Bone

Sectional View of the Femur Head Section through the head of the femur, showing the outer layer of compact bone and the soft center of trabecular bone, filled with red bone marrow and a spot of yellow bone marrow (white bar = 1 centimeter)

Cancellous bone Illustration of a section through long bone, with spongy bone in its center. Light micrograph of cancellous bone, stained with hematoxylin and eosin, showing bone trabeculae (stained pink) and marrow tissue (stained blue).

Lamellar structure of osteons in cortical bone

Lamellar structure of osteons in cortical bone

Cortical bone is the more dense tissue usually found on the surface of bones. It is organised in cylindrical shaped elements called osteons composed of concentric lamellae

Trabecular bone

Trabecular structures in the L1 vertebra of a 24 year old

Trabecular structures in the calcaneus of a 24 year old

Trabecular bone is quite porous and it is organized in trabecules oriented according to the direction of the physiological load. The configuration of the trabecular structures is highly variable and it depends on the anatomical site.

Cross-section through a region of compact bone

This image scanned from a textbook, Basic Medical Anatomy, by Alexander Spence A cross-section through a region of compact bone, you will see rings of Haversian systems, each with a hole, the canal, in the center

The effect of aging Trabecular structures of vertebrae in a 36 year old woman

Trabecular structures of vertebrae in a 74 year old woman

Five types of bones in the human body • Long bones: – characterized by a shaft, the diaphysis, that is much greater in length than width. – comprised mostly of compact bone and lesser amounts of marrow, which is located within the medullary cavity, and spongy bone. – Examples: most bones of the limbs, including those of the fingers and toes. Exceptions are bones of the wrist, ankle and kneecap • Short bones – roughly cube-shaped, and have only a thin layer of compact bone surrounding a spongy interior. – Examples: bones of the wrist and, as are the sesamoid bones.

Five types of bones in the human body • Flat bones – thin and generally curved, with two parallel layers of compact bones sandwiching a layer of spongy bone. – Examples: Most of the bones of the skull, as is the sternum.

• Irregular bones – – – –

do not fit into the above categories. consist of thin layers of compact bone surrounding a spongy interior. their shapes are irregular and complicated. Examples: bones of the spine and hips are irregular bones.

• Sesamoid bones: – bones embedded in tendons. – Since they act to hold the tendon further away from the joint, the angle of the tendon is increased and thus the force of the muscle is increased. – Examples: the patella and the pisiform

Long Bones

Mechanical Properties of Bone The different structures of cortical bone and trabecular bone result in different mechanical properties. Bone mechanical properties are highly variable according to species, age, anatomical site, liquid content, etc.

Ultimate strength (MPa) and ultimate strain (%) of cortical bone from the human femur as a function of age

Anisotropic Property of Cortical Bone Cortical bone is an anisotropic material, meaning that its mechanical properties vary according to the direction of load. Cortical bone is often considered an orthotropic material. Orthotropic materials are a class of anisotropic materials characterized by three different Young's moduli E1, E2, E3 according to the direction of load, three shear moduli G12, G13, G23 and six Poisson's ratios ν12, ν13, ν23, ν21, ν31, ν32.

Comparison between the mechanical behaviour of isotropic and anisotropic materials

Elastic constants of cortical bone from different anatomical sites

Average elastic constants of mandible bone in corpus and ramus

Average elastic constants of corpus cortical bone in inferior, lingual and buccal zones

Average elastic constants of human mandibular bone by tooth location

Young's modulus of trabecular bone as a function of density of bone. Bone density ρ is expressed in g/cm3 and Young's modulus E in MPa

The mechanical characterization of trabecular bone is even more difficult. The mechanical properties of trabecular bone as a whole are due to the mechanical characteristics of single trabecules and to its highly porous structure

Bone remodelling • Bone adapts and remodels in response to the stress applied. • Wolff's law: bones develop a structure most suited to resist the forces acting upon them, adapting both the internal architecture and the external conformation to the change in external loading conditions. This change follows precise mathematical laws. • When a change in loading pattern occurs stress and strain fields in the bone change accordingly. • Bone tissue seems to be able to detect the change in strain on a local bases and then adapts accordingly.

Bone Remodelling • The internal architecture is adapted in terms of change in density and in disposition of trabecules and osteons and the external conformation in terms of shape and dimensions. • When strain is intensified new bone is formed. – microscopic scale: bone density is raised – macroscopic scale: the bone external dimensions are incremented.

• When strain is lowered bone resorption takes place. – microscopic scale: bone density is lowered – macroscopic scale: the bone external dimensions are reduced

Effect of reduction (from A to B) and of intensification of strain (from B to A) on bone trabecules

Remodelling • When the change in strain is due to a change in direction of load • microscopic scale: the structure of trabecules and osteons is rearranged • macroscopic scale: a change in bone shape may occur. • Remodelling is carried out by the cellular component of bone. • Resorption: osteoclasts reabsorb collagen and mineral phase which are then taken into the circulatory system . • Deposition: osteoblasts group on the deposition surface and build the collagen network of bone. Mineralization takes place afterwards.

Bone resorption and deposition

Bone resorption

Bone resorption is the process by which osteoclasts break down bone and release the minerals, resulting in a transfer of calcium from bone fluid to the blood

Bone deposition

Equilibrium strain state • Bone resorption and bone deposition processes are always active in bone. • An equilibrium strain state exists in correspondence to which the two activities are perfectly balanced. • Strain intensity > the equilibrium strain: • deposition activity is more intense than resorption activity and net deposition occurs. • Strain intensity < the equilibrium strain: • deposition activity is less intense than resorption activity and net resorption occurs. • Dynamical equilibrium between resorption and deposition is again achieved when the equilibrium strain state is newly established.

Schematic diagram of the Davy and Hart model for bone remodelling

Bone Fracture Types of bone fractures: Complete, Incomplete, Compound and Simple. complete fracture: the bone snaps into two or more parts incomplete fracture: the bone cracks but does not break all the way through. compound or open fracture: the bone breaks through the skin; it may then recede back into the wound and not be visible through the skin. simple or closed fracture: the bone breaks but no open wound in the skin.

Simple Fractures • Greenstick fracture: an incomplete fracture in which the bone is bent. This type occurs most often in children. • Transverse fracture: a fracture at a right angle to the bone's axis. • Oblique fracture: a fracture in which the break slopes. • Comminuted fracture: a fracture in which the bone fragments into several pieces. • An impacted fracture is one whose ends are driven into each other. This is commonly seen in arm fractures in children and is sometimes known as a buckle fracture. • Other types of fracture are pathologic fracture, caused by a disease that weakens the bones, and stress fracture, a hairline crack.

Bone Repair While the patient is pain-free (general or local anesthesia), an incision is made over the fractured bone. The bone is placed in proper position and screws, pins, or plates are attached to or in the bone temporarily or permanently. Any disrupted blood vessels are tied off or burned (cauterized). If examination of the fracture shows that a quantity of bone has been lost as a result of the fracture, especially if there is a gap between the broken bone ends, the surgeon may decide that a bone graft is essential to avoid delayed healing. If bone grafting is not necessary, the fracture can be repaired by the following methods: one or more screws inserted across the break to hold it. a steel plate held by screws drilled into the bone. a long fluted metal pin with holes in it, is driven down the shaft of the bone from one end, with screws then passed through the bone and through a hole in the pin.

Repair of a fractured bone An illustration of the repair of a fractured bone (a) is shown in this diagram. Blood infiltrates the damaged site, forming a hematoma (b), a soft callus of fibrocartilage forms around the hematoma to provide support (c), osteoblasts produce a hard callus to strengthen the soft callus (d), and finally, osteoclasts remove excess bone and callus (e).

What is Osteoporosis? A systematic skeletal disease characterized by low bone mass, increase of bone fragility and susceptibility to fracture. Osteoporosis can lead to irreversible deterioration of bone structure Symptoms Aches and pains Loss of height Fractures of the Hip, Spine, Wrist Disability

Risk Factors Age : > 45 yrs in Female and > 60 yrs in Male Lifestyle - lack of exercise Low Vitamin D in take Low calcium intake Smoking

Life style & osteoporosis prevention Be ON YOUR FEET Exercise at least 3 hours per week Take a meal rich in Calcium and Vitamin D Consume adequate calories Avoid Smoking AVOID TOBACCO & ALCOHOL

Rheumatoid arthritis Rheumatoid arthritis (RA) is a chronic, systemic autoimmune disorder that causes the immune system to attack the joints, where it causes inflammation (arthritis) and destruction. It can also damage some organs, such as the lungs and skin. It can be a disabling and painful condition, which can lead to substantial loss of functioning and mobility. It is diagnosed with blood tests (especially a test called rheumatoid factor) and X-rays. Diagnosis and longterm management are typically performed by a rheumatologist, an expert in the diseases of joints and connective tissues. Various treatments physical therapy and occupational therapy Analgesia (painkillers) and anti-inflammatory drugs, steroids, are used to suppress the symptoms disease-modifying antirheumatic drugs (DMARDs) are often required to reverse the disease process and prevent long-term damage.

Osteoarthritis Osteoarthritis (OA, also known as degenerative arthritis, degenerative joint disease), is a clinical syndrome in which low-grade inflammation results in pain in the joints, caused by abnormal wearing of the cartilage that covers and acts as a cushion inside joints and destruction or decrease of synovial fluid that lubricates those joints. Causes Aging Another disease or condition like obesity, repeated trauma or surgery to the joint structures, abnormal joints at birth (congenital abnormalities), gout, diabetes, and other hormone disorders. Crystal deposits in the cartilage can cause cartilage degeneration and osteoarthritis. Uric acid crystals cause arthritis in gout, while calcium pyrophosphate crystals cause arthritis in pseudogout. Hormone disturbances, such as diabetes and growth hormone disorders, are also associated with early cartilage wear and secondary osteoarthritis.

Juvenile Arthritis Juvenile idiopathic arthritis (JIA), formerly known as juvenile rheumatoid arthritis (JRA) is not a degenerative disease such as osteoarthritis. It can be classified as an auto-immune disease and is therefore caused by the immune system attacking the body. The normal function of the immune system is to ward off outside enemies such as viruses, but in auto-immune diseases, the body turns on itself. Juvenile arthritis is also known as Juvenile chronic arthritis (JCA) It affects children sixteen years old or under. JRA can be divide into three distinct types: Pauciarticular, Polyarticular and Systemic.

Juvenile Arthritis • Pauciarticular onset juvenile idiopathic arthritis (JIA) or pauciarthritis – subset of JIA that includes patients with involvement of fewer than five joints. – most common subgroup, constituting about 50 percent of cases of JIA

• Polyarticular onset juvenile idiopathic arthritis – subset of juvenile idiopathic arthritis (JIA) that is defined by the presence of more than four affected joints during the first six months of illness. – comprises 20 to 30 percent of patients with JIA.

• Systemic onset JRA – referred to patients with rash and intermittent fever, in addition to arthritis of any number of joints. – It is responsible for about 10 to 15 percent of JRA cases.

Autoimmune Diseases • Occurs when the body’s immune system attacks and destroys healthy body tissue by mistake. There are more than 80 types of autoimmune disorders. • The immune system does not distinguish between healthy tissue and antigens. As a result, the body sets off a reaction that destroys normal tissues. • Exact cause of autoimmune disorders is unknown. – some microorganisms (such as bacteria or viruses) or drugs may trigger changes that confuse the immune system. – may happen more often in people who have genes that make them more prone to autoimmune disorders

• Result: The destruction of body tissue, Abnormal growth of an organ, Changes in organ function • Areas affected: Blood vessels, connective tissues, endocrine glands such as the thyroid or pancreas, joints, muscles, red blood cells, skin

Biomechanical Properties of Bone • Biomechanically, bone tissue may be regarded as a twophase (biphasic) composite material; with the mineral as one phase and the collagen and ground substance as the other. • In such materials (a non-biological example is fiberglass) in which a strong, brittle material is embedded in a weaker, more flexible one, the combined substances are stronger for their weight than is either substance alone. • Functionally, the most important mechanical properties of bone are its strength and stiffness.

Typical Load-Deformation Curve

Load-deformation curve for a structure composed of a somewhat pliable material. If a load is applied within the elastic range of the structure (A to B on the curve) and is then released, no permanent deformation occurs. If loading is continued past the yield point (B) and into the structure's plastic range (B to C on the curve) and the load is then released, permanent deformation results. The amount of permanent deformation that occurs if the structure is loaded to point D in the plastic region and then unloaded is represented by the distance between A and D. If loading continues within the plastic range, an ultimate failure point (C) is reached.

Testing of Bone

Standardized bone specimenin a testing machine The strain in the segment of bone between the two gauge arms is measured with a strain gauge. The stress is calculated from the total load measured.

Stress-strain curve for a cortical bone sample tested in tension (pulled), Yield point (B)

Stress-Stress Curves in Compression

Example of stress-strain curves of cortical and trabecular bone with different apparent densities, Testing was performed in compression. The figure depicts the difference in mechanical behavior for the two bone structures.

Mechanical Properties of Bone • Mechanical properties differ in the two bone types. Cortical bone is stiffer than cancellous bone, withstanding greater stress but less strain before failure. • Cancellous bone in vitro may sustain up to 50% of strain before yielding, while cortical bone yields and fractures when the strain exceeds 1.5-2%. Cancellous bone has a large capacity for energy storage • The physical difference between the two bone tissues is quantified in terms of the apparent density of bone, which is defined as the mass of bone tissue present in a unit of bone volume (g/cc)

Schematic stress-strain curves for three materials Metal has the steepest slope in the elastic region and is thus the stiffest material. The elastic portion of the curve for metal is a straight line, indicating linearly elastic behavior. The fact that metal has a long plastic region indicates that this typical ductile material deforms extensively before failure. Glass, a brittle material, exhibits linearly elastic behavior but fails abruptly with little deformation, as indicated by the lack of a plastic region on the stress-strain curve. Bone possesses both ductile and brittle qualities demonstrated by a slight curve in the elastic region, which indicates some yielding during loading within this region.

Mechanical Properties of Selected Biomaterials Ultimate Strength (MPa)

Modulus (GPa)

Elongation(%)

Cast

600

220

8

Forged

950

220

15

Stainless steel

850

210

10

Titanium

900

110

15

Polymers - Bone cement

20

2.0

2-4

Ceramic - Alumina

300

350

<2

Cortical bone

100-150

10-15

1-3

Trabecular bone

8-50

Tendon, ligament

20-35

Metals - Co-Cr alloy

Biological 2-4 2.0-4.0

10-25

Fracture of Ductile and Brittle Materils When pieced together after fracture, the ductile material will not conform to its original shape whereas the brittle material will. Bone exhibits more brittle or more ductile behavior depending on its age (younger bone being more ductile) and the rate at which it is loaded (bone being more brittle at higher loading speeds) Fracture surface of sample, of a ductile and a brittle material. The broken lines on the ductile material indicate the original length of the sample. before it deformed. The brittle material deformed very little before fracture.

Anisotropic behavior of cortical bone

Anisotropic behavior of cortical bone specimens from a human femoral shaft tested in tension (pulled) in four directions: longitudinal (L), tilted 30° with respect to the neutral axis of the bone, tilted 60°, and transverse (T).

Stress-Strain Behaviour of Trabecular Bone Example of tensile stress-strain behavior of trabecular bone tested in the longitudinal axial direction of the bone.

Trabecular or cancellous bone is approximately 25% as dense, 5 to 10%, as stiff, and five times as ductile as conical bone.

Schematic representation of various loading modes The mechanical behavior or bone - its behavior under the influence of forces and moments - is affected by its mechanical properties, its geometric characteristics, the loading mode applied, direction of loading, rate of loading, and frequency of loading Forces and moments can be applied to a structure in various directions, producing tension, compression, bending, shear, torsion, and combined loading. Bone in vivo is subjected to all of these loading modes.

Rate dependency of cortical bone The biomechanical of bone behavior varies with the rate at which the bone is loaded. Rate dependency of cortical bone is demonstrated at five strain rates. Both stiffness (modulus) and strength increase considerably at increased strain rates. The figure shows cortical bone behavior in tensile testing at different physiological strain rates. As can be seen from the figure, the same change in strain rate produces a larger change in ultimate stress (strength) than in elasticity (Young's modulus). The data indicates that the bone is approximately 30% stronger for brisk walking than for slow walking.

Influence of Muscle Activity on Stress Distribution in Bone

Calculated stresses on the anterolateral cortex of a human tibia during walking

Calculated stresses on the anterolateral cortex of a human tibia during jogging

Summary • Bone is a complex two-phase composite material. One phase is composed of inorganic mineral salts and the other is an organic matrix of collagen and ground substance. The inorganic component makes bone hard and rigid, whereas the organic component gives bone its flexibility and resilience. • Microscopically, the fundamental structural unit of bone is the osteon, or haversian system, composed of concentric layers of a mineralized matrix surrounding a central canal containing blood vessels and nerve fibers. • Macroscopically, the skeleton is composed of cortical and cancellous (trabecular) bone. Cortical bone has high density while trabecular bone varies in density over a wide range.

Summary… • Bone is an anisotropic material, exhibiting different mechanical properties when loaded in different directions. Mature bone is strongest and stiffest in compression. • Bone is subjected to complex loading patterns during common physiological activities such as walking and jogging. Most bone fractures are produced by a combination of several loading modes. • Muscle contraction affects stress patterns in bone by producing compressive stress that partially