Bone.pptx

BONE Material Properties, Load Bearing Design of Vertebrae, Bony Remodeling Material Properties of Bone 30% organic material • 90-95% collagen • Provides toughness, resiliency • The rest is matrix 60% inorganic material • Hydroxyapatite crystals • Provides stiffness, hardness • Correlation betwe...

BONE Material Properties, Load Bearing Design of Vertebrae, Bony Remodeling Material Properties of Bone 30% organic material • 90-95% collagen • Provides toughness, resiliency • The rest is matrix 60% inorganic material • Hydroxyapatite crystals • Provides stiffness, hardness • Correlation between mineral content and strength • 25% decrease in bone loss results in 50% decrease in bone strength 10% water • H20!! Material Properties of Bone • Cortical Bone • Compact, dense shell • Cancellous Bone • Spongy, inner component Anisotro pic Bone is an Anisotropic Material Isotropic • Mechanical properties of a material are different in different directions • Strength and modulus of elasticity are higher in longitudinal direction than in the radial and circumferential directions • Bone is stronger in compression, followed by tension, weakest in shear • Mechanical properties of a material are the same regardless of orientation • Metal, ice • Mechanical properties of bone are time dependent Bone is Viscoelastic • Higher loading rate = bone is more stiff • Slow loading rate = bone is less stiff Vertebral Body • Exclusively designed for longitudinally applied loads • Compression and tension • Slow loading rates • 2000 N (450 lbs) cervical spine • 8000 N (1800 lbs) lumbar spine • Control of movement and stability are created by ligaments, muscles, and posterior elements of the vertebrae Load Bearing Design • Vertebral Body – Cortical Shell • Shell of cortical bone surrounding a cancellous cavity • What would happen if the vertebral body was a solid block of bone? • Vertebral Body – Cortical Shell Load Bearing Design • Solid block : crystalline structure tends to fracture with sudden forces • Crystalline structures cannot absorb and dissipate suddenly applied loads Load Bearing Design • Vertebral Body – Cancellous Core • Vertical struts • Act like solid bone if they can resist bending and be kept straight Load Bearing Design • Vertebral Body – Cancellous Core • Horizontal Cross-Beams • Connect the vertical struts and prevent vertebral body collapse Load Bearing Design • Vertebral Body – Cancellous Core • Vertical and transverse trabeculae • Trabeculae give the vertebral body weightbearing strength and resilience • Compressive load is sustained by combination of vertical pressure and transverse tension in the trabeculae Load Bearing Design • Overall design results in a strong but lightweight load-bearing structure • Constructed with the minimum use of material Load Bearing Design • Facets • Clinically important – direct source of pain • Important stabilizing structures • Carry load • 20% total compressive load in lumbar spine • 66% total compressive load in cervical spine • 45% torsional strength • 50% stability during flexion and extension Load Bearing Design • Pedicles • Only connection between posterior elements and vertebral body • Manage/resist sliding and twisting movements • Transmit tension and bending forces • Tension – created with the locking of articular process • Bending – created by muscle contraction that pulls downward on pedicles Load Bearing Design • Pedicles • Mostly hollow with bundles of trabeculae sweeping out of vertebral body, through pedicles, into posterior elements • Reinforce transverse and spinous processes • Oriented to resist the force and deformations that processes habitually sustain Load Bearing Design • Neural Arch • Strength is the same with degenerated and healthy discs • Most failures occur at the pedicles, one-third at pars interarticularis Stress Generated Potentials aka Wolff’s Law Bone is constantly changing and adapting in response to mechanical stress Bone growth and remodeling are electrically mediated by piezoelectricity (pressure electricity) or streaming potentials (ionic flow) Transduction • General term referring to the conversion of one type of energy or force into another • Most common: mechanical  electrical  chemical • In bone remodeling, bone tissue acts as a Piezoelectricity AKA Pressure Electricity When a piezoelectric substance (bone) deforms due to mechanical stress Distortion of bonds (H+) or change in orientation of hydroxyapatite crystals Electric polarization of bone surfaces Normal Mechanical Pressure = Healing Electronegativity increases osteoblastic activity Electropositivity increases osteoclastic activity Fracture long bone  pressure electricity causes bone to heal straight Abnormal Mechanical Pressure = Deformation As long as a bone is asymmetrically loaded polarization will persist Convex side = osteoclastic Concave side = osteoblastic Bone absorption Bone growth Osteophytes grow on concave side Degenerative Scoliosis Streaming Potentials Mechanical load Movement of extracellular fluid through the pores of trabeculae Solid-liquid interface of bone has a surface charge (Ca++) Captures negatively charged elements of fluid Results in “squeezing” of positive ions out of trabecular pore Generates polarization of bone similar to piezoelectric effect Narrow-waist Appearance • The Narrow-waist appearance of vertebra is a result of stress generated streaming potentials • Compressive force of gravity causes the positively charged extracellular fluid to move laterally • Lateral margins become positive, cause osteoclastic activity Bone Remodeling • Osteophytes, lipping, spurring, fusions of facets, etc are a direct result of abnormal stresses on spinal vertebrae Bone Response to Exercise • Some exercises will not stimulate bone formation, while activities above the force/loading rate threshold will • Increasing muscle tone only is not effective in changing the bone composition of the spine • Exercise must be weight bearing to have an effect on bone composition