Biomechanical Characteristics of Bone - Bone Tissue
Organic Components
(e.g. collagen)
Inorganic Components
(e.g., calcium and phosphate)
25-30%
65-70%
(dry wt)
H2O
(25-30%)
(dry wt)
ductile
one of the
body’s hardest
structures
brittle
viscoelastic
1
Mechanical Loading of Bone
Compression Tension Shear
Torsion
Bending
2
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
3
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-Schlatter’s disease
4
Shear Forces
created by the application
of compressive, tensile or a
combination of these loads
5
Bone Compressive Strength
Material
Femur (cortical)
Compressive
Strength (MPa)
131-224
Tibia (cortical)
106-200
Wood (oak)
40-80
Steel
370
From: Biomechanics of the Musculo-skeletal System, Nigg and Herzog
6
Relative Strength of Bone
7
Bending Forces
Usually a 3- or 4-point
force application
8
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
9
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
10
Stress = Force/Area
Strain = Change in Length/Angle
Note: Stress-Strain curve is a normalized Load-Deformation Curve
11
Elastic & Plastic responses
plastic region
Stress (Load)
fracture/failure
elastic
region
•elastic thru 3%deformation
•plastic response leads to fracturing
Dstress
Dstrain
•Strength defined by failure point
•Stiffness defined as the slope of the
elastic portion of the curve
Strain (Deformation)
12
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
brittle material
bone
deformation (length)
13
Anisotropic response
behavior of bone is dependent
on direction of applied load
Bone is strongest along
long axis - Why?
14
Bone Anisotropy
trabecular
tension
compression
cortical
shear
tension
compression
0
50
100
150
200
Maximum Stress (MPa)
From: Biomechanics of the Musculo-skeletal System, Nigg and Herzog
15
Viscoelastic Response
behavior of bone is dependent
on rate load is applied
Bone will fracture sooner
when load applied slowly
Load
fracture
fracture
deformation
16
SKELETON
• axial skeleton
– skull, thorax, pelvis, &
vertebral column
• appendicular skeleton
– upper and lower
extremities
• should be familiar with
all major bones
17
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
18
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
19
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
20
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
21
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 muscle’s pull on that bone during
growth
• radial tuberosity
22
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
23
Short Bones
• e.g. carpals and
tarsals
• designed for
strength not mobility
• not important for us
in this class
24
Flat Bones
• e.g. skull, ribs,
scapula
• usually provide
protection
25
Irregular Bones
• e.g. vertebrae
• provide protection,
support and
leverage
26
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
27
Long Bone Structure
cortical or compact bone
(porosity ~ 15%)
periosteum
outer cortical membrane
endosteum
inner cortical membrane
trabecular, cancellous,
or spongy, bone
(porosity ~70%)
28
Long Bone Structure
epiphyseal plate
metaphysis
either end of diaphysis
filled with trabecular bone
cartilage separating
metaphysis from epiphysis
diaphysis
shaft of bone
epiphysis
proximal and distal
ends of a long bone
29
Biomechanical Characteristics of Bone
Physical Activity
Gravity
Lack of Activity
Bone Tissue
Remodeling/Growth
Bone Deposits
(myositis ossificans)
Hormones
Age &
Osteoporosis
30
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
31
Epiphyseal Closures
Vertebrae
Ribs
Humerus, prox.
Humerus, dist.
Ulna, prox.
Ulna, dist.
Tibia, prox.
Tibia, dist.
0
5
10
15
From: Biomechanics of Human Movement, Adrian and Cooper
20
25
30
32
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
33
Biomechanical Characteristics of Bone
Wolff’s Law
• 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
34
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
35
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
36
37
Typical Vertical GRF during running
30
Tip-Toe running pattern
Heel-toe running pattern
25
Fz (N/kg)
20
15
10
5
0
0
50
100
150
200
250
300
time (ms)
38
TVIS
Treadmill Vibration Isolation and
Stabilization System
39
Changes in bone over time
Early Years
• Osgood-Schlatter’s 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
40
Changes in bone over time
Early Years
• overuse injuries
– repeated stresses mold skeletal structures
specifically for that activity
– Little Leaguer’s Elbow
• premature closure of epiphyseal disc
– Gymnasts
• 4X greater occurrence of low back pathology in
young female gymnasts than in general
population (Jackson, 1976)
41
Changes in bone over time
Adult 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
42
Changes in bone over time
Adult Years
• hormonal influence
– estrogen to maintain bone minerals
– previously only consider after menopause
– now see link between amenorrhea and
decreased estrogen - Female Athlete Triad
disordered
eating
amenorrhea
low body fat
excessive training
osteoporosis
low estrogen
levels
43
Changes in Bone Over Time
Older 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
44
Bone Mass (g of Ca)
Age, Bone Mass and Gender
1000
500
0
25
50
Age (yr)
75
From: Biomechanics of Musculoskeletal Injury, Whiting and Zernicke
100
45
Osteopenia
Osteoporosis
Hormonal
Factors
Nutritional
Factors
Reduced BMD
slightly elevated risk
of fracture
Severe BMD reduction
very high risk of
fracture
(hip, wrist, spine, ribs)
Physical
Activity
28 million Americans affected – 80% of these are women
10 million suffer from osteoporosis
18 million have low bone mass
46
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%)
47
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 wedgeshaped vertebrae
– create a kyphotic curve known as Dowager’s Hump
• for some reason men’s vertebrae increase in
diameter so these effects are minimized
48
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)
49
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)
50