Skeleton

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Biomechanical Characteristics of Bone - Bone
Tissue
Organic Components (e.g. collagen)
Inorganic Components (e.g., calcium and phosphate)
25-30 (dry wt)
65-70 (dry wt)
H2O (25-30)
ductile
one of the bodys hardest structures
brittle
viscoelastic
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Mechanical Loading of Bone
Compression Tension Shear Torsion
Bending
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Compressive Loading
Vertebral fractures cervical fractures spine
loaded through head e.g., football, diving,
gymnastics once spearing was outlawed
in football the number of cervical injuries
declined dramatically lumbar fractures weight
lifters, linemen, or gymnasts spine is loaded in
hyperlordotic (aka swayback) position
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Tensile Loading
Main source of tensile load is muscle tension
can stimulate tissue growth fracture
due to tensile loading is usually an
avulsion other injuries include sprains, strains,
inflammation, bony deposits when the tibial
tuberosity experiences excessive loads from
quadriceps muscle group develop condition known
as Osgood-Schlatters disease
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Shear Forces created by the application of
compressive, tensile or a combination of these
loads
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Bone Compressive Strength
From Biomechanics of the Musculo-skeletal
System, Nigg and Herzog
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Relative Strength of Bone
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Bending Forces
Usually a 3- or 4-point force application
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Torsional Forces
Caused by a twisting force produces shear,
tensile, and compressive loads tensile and
compressive loads are at an angle often see a
spiral fracture develop from this load
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Strength and Stiffness of Bone Tissue
evaluated using relationship between applied
load and amount of deformation LOAD -
DEFORMATION CURVE
Bone Tissue Characteristics
Anisotropic
Viscoelastic
Elastic
Plastic
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Stress Force/Area
Strain Change in Length/Angle
Note Stress-Strain curve is a normalized
Load-Deformation Curve
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Elastic Plastic responses

• elastic thru 3deformation
• plastic response leads to fracturing
• Strength defined by failure point
• Stiffness defined as the slope of the
• elastic portion of the curve

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Elastic Biomaterials (Bone)

• Elastic/Plastic characteristics
• Brittle material fails before
• permanent deformation
• Ductile material deforms
• greatly before failure
• Bone exhibits both properties

Load/deformation curves
elastic limit
ductile material
load
brittle material
bone
deformation (length)
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Anisotropic response behavior of bone is
dependent on direction of applied load
Bone is strongest along long axis - Why?
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Bone Anisotropy
From Biomechanics of the Musculo-skeletal
System, Nigg and Herzog
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Viscoelastic Response behavior of bone is
dependent on rate load is applied
Bone will fracture sooner when load applied slowly
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SKELETON

• axial skeleton
• skull, thorax, pelvis, vertebral column
• appendicular skeleton
• upper and lower extremities
• should be familiar with all major bones

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Purposes of Skeleton

• protect vital organs
• factory for production of red blood cells
• reservoir for minerals

• attachments for skeletal muscles
• system of machines to produce movement in
• response to torques

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Bone Vernacular

• condyle
• a rounded process of a bone that articulates with
  another bone
• e.g. femoral condyle
• epicondyle
• a small condyle
• e.g. humeral epicondyle

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Bone Vernacular

• facet
• a small, fairly flat, smooth surface of a bone,
  generally an articular surface
• e.g. vertebral facets
• foramen
• a hole in a bone through which nerves or vessels
  pass
• e.g. vertebral foramen

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Bone Vernacular

• fossa
• a shallow dish-shaped section of a bone that
  provides space for an articulation with another
  bone or serves as a muscle attachment
• glenoid fossa
• process
• a bony prominence
• olecranon process

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Bone Vernacular

• tuberosity
• a raised section of bone to which a ligament,
  tendon, or muscle attaches usually created or
  enlarged by the stress of the muscles pull on
  that bone during growth
• radial tuberosity

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Long Bones

• e.g. femur, tibia
• 1 long dimension
• used for leverage
• larger and stronger in lower extremity than upper
  extremity
• have more weight to support

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Short Bones

• e.g. carpals and tarsals
• designed for strength not mobility
• not important for us in this class

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Flat Bones

• e.g. skull, ribs, scapula
• usually provide protection

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Irregular Bones

• e.g. vertebrae
• provide protection, support and leverage

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Sesamoid Bones

• e.g. patella (knee cap)
• a short bone embedded within a tendon or joint
  capsule
• alters the angle of insertion of the muscle

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Long Bone Structure
cortical or compact bone (porosity 15)
periosteum outer cortical membrane
endosteum inner cortical membrane
trabecular, cancellous, or spongy, bone
(porosity 70)
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Long Bone Structure
epiphyseal plate cartilage separating metaphysis
from epiphysis
metaphysis either end of diaphysis filled with
trabecular bone
diaphysis shaft of bone
epiphysis proximal and distal ends of a long bone
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Biomechanical Characteristics of Bone
Physical Activity
Lack of Activity
Bone Tissue Remodeling/Growth
Gravity
Hormones
Bone Deposits (myositis ossificans)
Age Osteoporosis
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Longitudinal Bone Growth

• occurs at the
• epiphyseal or
• growth plate
• bone cells are produced on the diaphyseal side of
  the plate
• plate ossifies around age 18-25 and longitudinal
  growth stops

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Epiphyseal Closures
From Biomechanics of Human Movement, Adrian and
Cooper
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Circumferential Bone Growth

• growth throughout the
• lifespan
• bone cells are produced on the internal layer of
  the periosteum by osteoblasts
• concurrently bone is resorbed around the
  circumference of the medullary cavity by
  osteoclasts

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Wolffs Law
Biomechanical Characteristics of Bone

• bone is laid down where needed and resorbed where
  not needed
• shape of bone reflects its function
• tennis arm of pro tennis players have cortical
  thicknesses 35 greater than contralateral arm
  (Keller Spengler, 1989)
• osteoclasts resorb or take-up bone
• osteoblasts lay down new bone

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Bone Deposits

• A response to regular activity
• regular exercise provides stimulation to maintain
  bone throughout the body

• tennis players and baseball pitchers develop
  larger and more dense bones in dominant arm
• male and female runners have higher than average
  bone density in both upper and lower extremities
• non-weightbearing exercise (swimming, cycling)
  can have positive effects on BMD

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Bone Resorption

• lack of mechanical stress
• Calcium (Ca) levels decrease
• Ca removed through blood via kidneys
• increases the chance of kidney stones
• weightless effects (hypogravity)
• astronauts use exercise routines to provide
  stimulus from muscle tension
• these are only tensile forces - gravity is
  compressive

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Tip-Toe running pattern
Heel-toe running pattern
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TVIS Treadmill Vibration Isolation
and Stabilization System
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Changes in bone over timeEarly Years

• Osgood-Schlatters disease
• development of inflammation, bony deposits, or an
  avulsion fracture of the tibial tuberosity
• muscle-bone strength imbalance
• growth factor between bone length and muscle
  tendon unit (e.g., rapid growth of femur and
  tibia places large strain on patellar tendon and
  tibial tuberosity)
• during puberty muscle development (testosterone)
  may outpace bone development allowing muscle to
  pull away from bone

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Changes in bone over timeEarly Years

• overuse injuries
• repeated stresses mold skeletal structures
  specifically for that activity
• Little Leaguers Elbow
• premature closure of epiphyseal disc
• Gymnasts
• 4X greater occurrence of low back pathology in
  young female gymnasts than in general population
  (Jackson, 1976)

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Changes in bone over timeAdult Years

• little change in length
• most change in density
• lack of use decreases density
• DECREASE STRENGTH OF BONE
• activity
• increased activity leads to increased diameter,
  density, cortical width and Ca

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Changes in bone over timeAdult Years

• hormonal influence
• estrogen to maintain bone minerals
• previously only consider after menopause
• now see link between amenorrhea and decreased
  estrogen - Female Athlete Triad

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Changes in Bone Over TimeOlder Adults

• 30 yrs males and 40 yrs females
• BMD peaks (Frost, 1985 Oyster et al., 1984)
• decrease BMD, diameter and mineralization after
  this
• activity slows aging process

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Age, Bone Mass and Gender
From Biomechanics of Musculoskeletal Injury,
Whiting and Zernicke
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Reduced BMD slightly elevated risk of fracture
Osteopenia
Severe BMD reduction very high risk
of fracture (hip, wrist, spine, ribs)
Osteoporosis
Hormonal Factors
Nutritional Factors
Physical Activity
28 million Americans affected 80 of these are
women 10 million suffer from osteoporosis 18
million have low bone mass
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Osteoporosis

• age
• women lose 0.5-1 of their bone mass each year
  until age 50 or menopause
• after menopause rate of bone loss increases (as
  high as 6.5)

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Do you get shorter with age?

• Osteoporosis compromises structural integrity of
  vertebrae
• weakened trabecular bone
• vertebrae are crushed
• actually lose height
• more weight anterior to spine so the compressive
  load on spine creates wedge-shaped vertebrae
• create a kyphotic curve known as Dowagers Hump
• for some reason mens vertebrae increase in
  diameter so these effects are minimized

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Preventing Osteoporosis

• 13.8 billion in 1995 (38 million/day)
• Lifestyle Choices
• proper diet
• sufficient calcium, vitamin D,
• dietary protein and phosphorous (too much?)
• tobacco, alcohol, and caffeine
• EXERCISE, EXERCISE, EXERCISE
• 47 incidence of osteoporosis in sedentary
  population compared to 23 in hard physical labor
  occupations (Brewer et al., 1983)

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Osteoporosis, Activity and the Elderly
Rate of bone loss (50-72 yr olds, Lane et al.,
1990) 4 over 2 years for runners 6-7 over 2
years for controls However - rate of loss jumped
to 10-13 after stopped running suggest
substitute activities should provide high
intensity loads, low repetitions (e.g. weight
lifting)