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1.5

Terms in this set (100)

What are 5 functional demands of bone?
1) Act as rigid lever arms for movement/support (composite material: protect vital soft tissue, resist forces, be stiff)
2) Accommodate growth (expand, adapt)
3) Repair itself
4) Provide readily-accessible mineral reservoir for REMODELING******
5) AGE-dependent hemopoesis and immune differentiation
What are 5 kinds of forces that bone must resist?
Compressive, tensive, shear, bending, and torsional forces
What are the minerals embedded in bone?
Ca and phosphorus (as phosphate)
What spaces and bone provide the locus of hemopoesis and immune differentiation in bone?
Marrow spaces and cancellous bone
What are the 2 main materials that make up bone?
Collagen and apatite
Collagen is (stiff/floppy) in tension and (stiff/floppy) in compression
STIFF in tension, FLOPPY in compression
Apatite is (stiff/floppy) in tension and (stiff/floppy) in compression, but it is also _____ in tension
STIFF in tension AND compression, BUT WEAK in tension (susceptible to brittle fracture)
________ is critical for bone behavior
ORIENTATION is critical for bone behavior
The orientation of bone materials is ________ $$$
Anisotrophic (not equal in all directions) -> strength varies by force orientation.
The side of the bone shaft more in (tension/compression) is more susceptible to fracture (weaker)
More in TENSION = weaker
(Parallel/perpendicular) orientation of crystal-collagen array optimizes mass relative to strength and is stronger and stiffer
PARALLEL
"Stiff" resists _________
DEFORMATION
"Strong" resists _________
FAILURE under high load
TEST: Stress = _____/_____
Load/area
TEST: Strain = ______/_______
Deformation/original length
TEST: To calculate how STIFF a material is, what equation would you use?
Stress/strain (load/deformation)
TEST: What is the yield point of a material?
The onset of material failure
TEST: What is the strain at yield point for bone? (percentage) $$
1%
TEST: When does plastic deformation occur?
Beyond the yield point
TEST: The ultimate failure point, or ______ failure, is strain at bone fracture = ___% $$
STRUCTURAL failure

2%
(Cortical/trabecular) bone is remodeling MORE at any given time. This means ______ bone has LONGER life expectancy than ______.

What is the bone turnover rate (life expectancy) of trabecular vs. cortical bone respectively?
Remodeling at any given time: Trabecular (7%) vs. cortical (3%)

CORTICAL bone = longer life expectancy (undergoes remodeling less frequently)

Cortical: 20 yrs
Trabecular: 4 yrs
Bone remodeling is a function of what of the bone?
SURFACE AREA
TEST: Bone remodeling adapts to what? $$
Bone remodeling adapts to the cortical thickness, diameter, curvature, and attachment site morphology to habitual functional loads.
What are the 4 main functions of bone remodeling?

High yield!
1) repair structural damage (micro and macro fractures)

2) adapt architecture to ambient loads

3) liberate and sequester calcium and phosphate $

4) accomodate shape changes needed in growth
TEST: ______ bone is the primary calcium reservoir and has very (slow/rapid) turnover
ENDOSTEAL bone, very RAPID turnover
Like zombie worms, how to osteoclasts "eat bones"? In what direction do they work? What is their main function?
Secrete ACID and enzymes! They aim out from long axis of the haversian system. Main function is resorption of existing bone.
Which one is larger, osteoclasts or osteoblasts?
Osteoclasts
Osteoblasts deposit ______ during bone remodeling, which subsequently mineralizes and completes the process of bone remodeling
OSTEOIDS
TEST: Osteoblasts lay down osteoids within _______ in a (vertical/circumferential) pattern. What is an advantage of this pattern?
Within LAMELLAE in a CIRCUMFERENTIAL pattern

Advantage: Blunts cracks and microfractures
Bone responds to absence of loading by _______, while it responds to presence/magnitude of loading by ______ & _______
Absence of loading by RESORPTION

Presence of loading by PROLIFERATION and DEPOSITION
Bone responds to tension with either an ________ or _________ response
OsteoLYTIC or osteoPROLIFERATIVE response
REVIEW: What is the function of osteocytes in the regulation of bone remodeling?
Sensing microfractures or mechanical load
What triggers pre-osteoclast differentiation? How is this regulated in the pre-activation stages bone remodeling?
Pre-osteoblasts trigger pre-osteoclast differentiation. This is regulated by RANK-L. If RANK-L is inhibited, this step of remodeling is inhibited.
REVIEW: What is the function of osteoclasts in the regulation of bone remodeling?
Secrete growth factors that trigger osteoblast differentiation and osteoclast apoptosis, reabsorb bone
REVIEW: What is the function of osteoblasts in the regulation of bone remodeling?
Block osteoclastic differentiation, and ultimately DIFFERENTIATE INTO OSTEOCYTES, forms bone
TEST: What are the 5 stages of active bone remodeling?
1) Activation (osteoBLAST dominated)
2) Resorption (osteoCLAST dominated)
3) Coupling
4) Formation
5) Mineralization
TEST: Explain the 4 steps of the activation stage of bone remodeling. What is it dependent upon?
1) Activated osteoblast bone lining cells produce collagenase.
2) Collagenase digests osteoid on bone surface exposing mineralized matrix.
3) Exposed bone matrix is chemotactic for osteoclasts.
4) Osteoclasts are then activated and migrate to exposed site.

$$ Main point: Activation is dependent upon osteoblasts
TEST: Explain the 4 steps of the resorption stage of bone remodeling. $$$ What is this stage dependent upon?
1) Form a sequestrum of dead bone $$$$$$$$
2) Secrete lysosomal enzymes, including acid phosphatase and proteolytic enzymes.
3) Bone mineral dissolved
4) Organic matrix phagocytized

Dependent upon osteoclasts
With osteoporosis, there is excessive (osteoblast/osteoclast) activity. What effect does this have?
Excessive osteoclast activity. Excessive bone resorption (breakdown).
TEST: What occurs during the coupling phase of bone remodeling? How long does this phase last?
Osteoblasts recruited to site.

1-2 week gap between osteoclast resorbing and bone formation onset.
TEST: What occurs during the formation phase of bone remodeling? How long does this last?
Osteoblasts line exposed bone surface, secreting osteoids

Duration: ~3 months
TEST: Mineralization starts how long after the onset of formation phase of bone remodeling? How long after bone formation completion does complete mineralization occur?
10-15 days after onset of bone formation.

Mineralization completes about 3-6 months after bone formation completion (=6-9 mo. from initiation of bone formation).
TEST: What are two classes of stimulators of bone remodeling? Why is this especially important to consider with osteoporosis patients? $$
1) Molecular: systemic hormones (vit D3, PTH, estrogen)

2) Mechanical: 4 cycles/day of weight-bearing strain sufficient to maintain bone mass. maximum load more important the repetitive load (like lifting weights).
- osteoporosis patients will feel weak and not want to move and we must educate them that moving can save them by stimulating bone growth! HY
One would hypothesize that inhibiting osteoclastic activity would improve the outcome of osteoporosis and bring bone mineral up. What are some disadvantages to this?
Impairs change of bone shape to accommodate localized loading conditions

Impairs microfracture HEALING

Impairs macroscopic bone injury repair (i.e. after dental extractions, req. osteoclastic activity)
If you are getting a dental extraction, you DO NOT want to inhibit _______
OsteoCLASTS! (you want that bone to remodel!)
TEST: A common osteoclast inhibitor is __________, which is associated with __________ (especially following dental extractions or bone injury)
Bisphosphonates

Associated OSTEONECROSIS (no remodeling, no circulation)
TEST: Osteoporosis shows up first in _______ bone, and the largest amount of this bone is located in our (cervical/thoracic/lumbar) vertebrae
Shows up first in CANCELLOUS bone, largest amount in LUMBAR vertebrae
TEST: One way of measuring bone mineral density is what?
Dual-energy X-ray absorptiometry (DEXA)
How is osteoporosis defined?
Low overall bone mass + structural deficiency
TEST/BOARD: Osteoporosis affects both ______ AND _______ function.
MINERAL and OSTEOID function.
Osteoporosis is caused by a mismatch between what?
Bone resorption and bone formation
$$ TEST: What common condition can underlie unexplained osteoporosis?
Hyperparathyroidism
Osteoporosis is a type of ________, which is defined as ____ bones
OSTEOPENIA, thin bones
What is osteomalacia? What is the juvenile form of this called?
Poorly mineralized bone

Juvenile form = RICKETS
TEST: What are the 2 types of PRIMARY osteoporosis?
1) Primary Type 1
- postmenopausal osteoporosis

2) Primary Type 2
- senile osteoporosis (2x as common in males vs. females)
What are some reasons for secondary osteoporosis?
Chronic predisposing medical problems or disease, medical HARDWARE or prolonged use of certain meds (glucocorticoids)
TEST: Why is exercise/weight bearing important for our bones?
Stimulates osteoclasts, which secrete prostaglandins that INHIBIT APOPTOSIS of OSTEOBLASTS!
Loss of mechanical stimulation = loss of _________ activity
OSTEOBLASTIC
Hypocalcemia manifests itself as _____ in kids and ________ in adults, most commonly due to deficiency in what? What is the result?
RICKETS in kids

OSTEOMALACIA in adults

Mineral deficiency in vitamin D/Ca
- result is osseous sx related to mechanical deformation, structural weakness (no resistance to forces), and impaired growth.
T/F? Osteomalacia is preventable and reversible.
True!
What are two common defects seen in Rickets patients?
Bowing of legs due to mechanical alteration of bone during weight bearing: collagen normal, bone mineral deficient

Dental defects (not reversible) - brownish enamel, malocclusion, short roots, chronic periodontal dz
BOARD: 1 year old African American male presents with short stature for age and bilaterally bowed tibiae. His mother reports that he is EXCLUSIVELY breast fed. They are northern city dwellers with little outdoor exposure. Radiographic findings include:

- Enlarged rib ends
- Widened physes
- Widened frayed and cupped metaphyses
- Mild femoral and tibial bowing
- Osteopenia

Deformation of the long bone is due to deficiency of which of the following elements?

a) Osteoid
b) Type I collagen
c) Type II collagen
d) Hydroxyapatite
e) Overall bone mass
D) Hydroxyapatite
Cartilage is (well/poorly) vascularized
POORLY
What are 3 possible causes of osteonecrosis? What is Legg-Calve-Perthe's dz?
Caused by:
- inhibited remodeling
-circulatory disruption by various means (usu mechanical- overweight, male). May also be due to trauma, DM, alcoholism, sickle-cell dz.
- sepsis (can be circulatory or systemic)

Legg-Calve-Parthe: femoral head necrosis and collapse in juveniles. Left untreated, will heal in collapsed position
$$ The blood supply to the epiphysis in young bone is _____ from the diaphysis
Separate.

Bone growth is dictated by path of blood supply
What is the characteristic feature on radiographs in thalassemia?
Hair on end appearance of overgrowth of diploic space.
TEST: How are trabeculae oriented to each other? Why?
At right angles to each other, along ambient lines of force

One direction resists compression, the other direction resists tension
Femoral buttressing is seen in what part of the femur? Why?
The part of the femur NECK that is subjected the most to compression forces (cortex of the inferior curve of the femur head), because this is the area that will provide the most strength gains from the same amount of material (limited regeneration resources)
TEST/RECAP: Bone material is weaker in (tension/compression)
Weaker in TENSION than in compression
Part of the typical response to bone insult is lifting of what? 2nd response?
Lifting of the PERIOSTEUM and ENDOSTEUM. 2nd response is inflammatory reaction w/macrophage migration (critical to healing of bone).
Are bones hollow or solid? Why?
Hollow b/c material in center would give little strength but adds weight and metabolic cost $
Where are bone fractures most common?
Fractures most common in areas of high tensile loading (tendon/ligament insertions or femoral neck).
TEST: What is the main difference between a stable and unstable fracture?
If the vascular continuity can be reestablished between broken fragments, this is a STABLE fracture.

UNSTABLE fracture is when avascular repair blastema arises, which is characterized by a WEDGE of cartilage plugging the gap/fracture
In younger pts., DO NOT mistake a __________ for a fracture.
Growth plate - know where they are!!
What are the 3 stages of the healing sequence?
1) Soft callus formation
2) Hard callus formation
3) Remodeling
What occurs during soft callus formation? How long does this take?
Blood vessel invasion of fibrinogen clot, and the clot is replaced with disorganized collagen mass

Takes 3 weeks
What occurs during hard callus formation? How long does this take?
Replacement of soft callus by WOVEN BONE (resembles endochondral bone formation)

Takes 2-4 months
When does Remodeling commence? How long does it take?
Begins immediately after woven bone formation. Can take years.
Tibial fractures typically occur where?
At the distal diaphysis (narrowest part)
ONCE MORE: Bone fails first in (tension/compression)
TENSION
What are 2 fracture types by tension?
1) Hairline fracture (failure in tension side of bend without compression side failure)
2) Greenstick (pediatric, bone less brittle)
What are 3 configurations of bone fractures?
Transverse, oblique, or spiral
Clinical: Explain the fracture terms of: site, extent, configuration, relationship of fragments, relationship to external environment, complications.
Site: epiphyseal, diaphyseal, metaphyseal, growth plate
Extent: complete, incomplete
Configuration: transverse, oblique, spiral
Relationship of fragments: displaced, undisplaced
Relationship to external environment: open (simple), closed (compound)
Complications: sequalae, iatrogenic
TEST: Avulsion fracture commonly results from failure in (compression/tension) from _______
Avulsion = Failure in TENSION from TRACTION (due to high tensile loading)
TEST: Spiral fractures most commonly results from ______ forces
TORSIONAL forces: closed-chain kinematics (fixed foot)
What goes on during closed-chain kinematics? Is the proximal or distal limb fixed?
Distal limb fixed, body moves towards it
What goes on during open-chain kinematics? Is the proximal or distal limb fixed?
Proximal limb/body fixed, distal limb moves towards it
TEST: Compression fracture is more common in (compact/cancellous) bone
CANCELLOUS bone (spongiosa)
TEST: What are 3 types of compression fractures?
1) Comminuted (multiple pieces)
2) Butterfly
3) Buckle (pediatric)

NOTE: compression fractures shouldn't happen under normal circumstances (more common in spongiosa (cancellous bone))
Compression fractures often occur through the (epiphyses/diaphyses). When imaging for such fractures what do you have to be weary of?
Epiphyses

plain film x-ray may not always show a fracture when there is one present. Can be cryptic, especially with compression fractures
TEST: What kind of fracture healing would produce little to no soft callus formation?
Open reduction with internal fixation (ORIF) (repair by means of remodeling). Risk of malunion if not reduced.
What does malunion refer to in terms of fracture healing?
Bone ends in non-anatomical position (common in subluxation and spiral fractures)
What does non-union refer to in terms of fracture healing?
Fracture gap filled with dense fibrous tissue and cartilage
What are some complications of fractures?
-Ossification to adjacent CT or mineralization build up of surrounding tissue can lead to abnormal bone growth
- Displacement of fat --> fat embolism; examine lungs!!
TEST: A 29 yo male presents in the ER with closed transverse fractures of the distal left tibia & fibula with lateral displacement after being struck by a car. The orthopedic surgeon elects to perform an open surgical reduction with FIXATION of tibia, leaving fibula alone. How should the subsequent osseous healing of the tibia differ from the closed reduction fracture healing process in the fibula?

A) The soft callus will surround the tibial fixation hardware, resulting in a larger hard callus on the tibia than on the fibula
B) The tibial soft callus will be less likely to break and cause subsequent hard callus discontinuity and pseudoarthrosis than the soft callus of the fibula
C) The tibia will heal by means of direct remodeling with little callus, while the fibula will manifest soft/hard callus formation prior to remodeling.
D) The tibial hard callus will remodel at a slower rate and remain more sclerotic than that of the fibula, due to presence of hardware.
E) Healing in both will be identical
C) The tibia will heal by means of direct remodeling with little callus, while the fibula will manifest soft/hard callus formation prior to remodeling
TEST: The healing tibial fracture site (with surgical hardware) is susceptible to what IATROGENIC condition?

A) Retardation of remodeling across fracture site due to internal fixation
B) Subluxation of bone ends during hematoma stage of healing
C) Pathological fracture of proximal tibia
D) Failure of union due to bending at fracture site during soft-callus phase
E) Localized osteoporosis due to relieved bone stress caused by surgical hardware
E) Localized osteoporosis due to relieved bone stress caused by surgical hardware (bone remodeling is not preceding as it normally would)
How do lines of arrested growth (LAGS) form in child bone development?
When the growth slows down, then speeds up again causing the collagen fibers to grow in a different direction than before (often a result of infection, child neglect, or some other pathology that interrupts growth)
The onset of peak velocity of bone growth is at ____ yo for males and ____ yo for females.

What is used to track a child's growth?
15 yo for males and 12-13 yo for females.

Females experience a growth spurt earlier than males

Growth tracked with a growth curve
BE AWARE: When looking at bones of kids on an x-ray, what can resemble a fracture?
GROWTH PLATE! (physes)
Pediatric bone is better at ______ absorption, but subject to greater ______
Better at IMPACT absorption, but subject to greater DEFORMATION
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