1
Q
What is the concept of mechanostat?
A
- concept of bone density regualtion according to certain thresholds
- groups of cells within the bone work together to keep bone strain at an optimal level
2
Q
What is bone mechanotransduction?
A
- physiologic process that permits mechanstat
- four stages
- mechanocoupling
- biomechanical coupling
- transmission of biochemical signals
- the effector response
3
Q
What is the first stage of bone mechanotransduction?
A
mechanocoupling
- bone defomation causes changes in pressure gradients
- as fluid flows past the osteocytes it shears the cell membrane creating a cellular response
- degrees of deformation have different cellular responses
4
Q
What is the second stage of bone mechanotransduction?
A
biochemical coupling
- it is the coupling between bone and collagen matrix
- integrins connect the cell membrane to the collagen matrix
- the integrins transmit the biochemical force to the cytoskeleton and nucleus to alter gene expression
5
Q
What is the third stage of bone mechanotransduction?
A
- transmission of the biochemical signal
- mechanical strain through bone directly activates osteoblasts and bone lining cells
- intermediary biochemical compounds also work to communicate to osteoblasts
6
Q
What are some different theories of cellular mechnosensors?
A
- mechanically gaited ion channels
- integrens and focal adhesion
- G proteins
- linkage between cytoskeleton and certain phospholipase C isoforms
7
Q
What is the fourth stage of mechanotransduction?
A
The effector response
- the effector is the term for the portion of the bone that produces new bone in response to mechanical strain
- may be osteoblast or osteoclast that responds
8
Q
What is the time frame for cellular effector response?
A
- mechanical strain osteocytes and bone lining cells release prostacyclin
- after 5 minutes increased glucose 6-phosphate dhydrogenase
- 6-24 hours increased RNA synthesis and IGF-1 message of osteocytes
- 1-2 days osteoblast appears
- 3-5 days increased collagen and mineral apposition on bone surface
- 5-12 bone formation rate is increased
9
Q
What are the two types of stress generated electrical potentials in bone metabolism?
A
- piezoelectric potential- stretch or strain produces electrical potentials within the collagen and proteglycans (positive charges lead to regeneration and negatives lead to remodeling)
- streaming potentials- potential produces by electolyte fluid flow
10
Q
how do the electromechanical modeling mechanism for bone work?
A
- piezoelectric effect and streaming potentials convert mechanical energy to electrical energy
- (+) charged side bone under goes regeneration (-) side of the bone under goes remodeling
- positive chemical charge stimulates the osteoblastic activity
11
Q
What chemicals contribute to the bone modeling mechanism?
A
- vitamin D metabilities
- parathyroid hormone
- calcitonin
12
Q
What organ plays a role in the chemical modeling mechanism?
A
- kidney contributes to secretion and resorption of calcium and phophorus
- thyroid
13
Q
What are some intrinsic or extrinsic factots effecting bone synthesis?
A
- amount of weight bearing and use of antigravity muscles
- muscle contraction force on bone
- estrogen levels
- testosterone levels
- HGH
- thyroid hormone
- calcitonin levels
- androgens
- gludocorticoids
- nutrition levels (vitamin D)
14
Q
What factors are associated with bone disoluation or aborption?
A
- lack of weight bearing and decreased use of wt bearing muscles
- lack of stress to bone from muscle contraction
- glucocorticoids are produced by the adrenal cortex and fucntion to decrease absorption of calcium ions from the intestine
- excessive thryroid hormone
- levels of PTH which produce in response to low calcium and acidosis
15
Q
What does PTH do?
A
- enhances release of calcium from the large reservoir contained in the bones. Bone resorption is the normal destruction of bone by osteoclasts, which are indirectly stimulated by PTH. Stimulation is indirect since osteoclasts do not have a receptor for PTH; rather, PTH binds to osteoblasts, the cells responsible for creating bone.
- effects calcium absorption in the kidney to increase blood calcium levels
- Increases production of activated vitamin D for improving abrosption of Ca from the intestine
16
Q
What are the two basic phases of STEP?
A
- phase 1; pain free stage focusing on coordination and mobility around the physiologic axis throughout the ROM
- phase 2: restoration of function focusing on increasing tissue tolerances to demand of ADLs
17
Q
What is bone composed of?
A
5% water
70% mineral (Ca hydroxiapatite with phosphorus, magnesium, sodium and potassium)
30% organic compounds (type I collagen 5-10% non-collagenous protien)
18
Q
What are the three typs of bone cells?
A
osteocytes
osteoblasts
osteoclasts
19
Q
What is the function of osteoblast?
A
- produce structural protiens (type I collagen) and moncollengous protein (proteglycans) to rapidly build osteoid matrix
- produce regulating factors like cytokines, growth factor, protoglycans, and enzymes that degrade ECM and prepare it for calcification
20
Q
In what part of the bone are the osteoblasts located?
A
- only at the surface of the bone
2. only make up 5% of the bone cell surface lining
21
Q
What are osteocytes?
A
mature osteoblasts that have surrounded themselves with bone matrix and no longer have room for synthesis of osteoid matrix
22
Q
What is an osteoclast?
A
- large multinucleated cells originating form the fusion of monocytes in the bone marrow
- located near the boney surface with osteoblasts
- primarily function in bone disolustion and absorption to help maintain blood Ca levels
- produce acid to dissolve bone orgnaic matrix to release Ca and other minerals
23
Q
What are the resting bone cellls?
A
- flat, elongated, metabolically inactive osteoblasts that are found in inactive boney regions
- are capable of producing enzymes to help in process of bone matrix degredation
24
Q
What role do resting bone cells play in bone homeostasis?
A
they produce enzymes and enzyme regulating proteins such as collagenase and collagenase inhibitor which degrade bone
25
Wath are some examples of secondary bone resorbing cells and how do they cause resorption?
1.mast cells
2.moncytes
3.lymphocytes
they release heprin and cytokinins that work with bone resorbing hormones
26
What are the primary bone reabsorbing hormones?
1. PTH
2. vitamin D
3. prostaglandin E
27
How does the osteoclast cause bone re-absorption?
1. oseoblast must contract to allow access to bone surface area
2. neutral proteases are released to degrade the bone surface osteoid
3. once the mineralized surface is exposed the osteoclast is activated
4. integrins bind the osteoclast to the surface and the OC begin producing H+ t lower pH of the envrinement
5. decreased pH increases hydroxyapatite crystal solubility allowing acidic proteolytic digestion to remove it
6. secondary re-absorbing cells are stimulated and can assist process
28
What are the different types of bone marrow?
1. red- primarily found in sterum, vertebral bodies, ribs, clavical, pelvis, sacrum, skull and yound people in the proximal epiphysis of the femur and humerous
2. yellow- infilatration of adipocytes into the red bone marrow for energy and hormone storage. Also has reserve of hematrophic tissue in case of injury
29
What are the two basic types of bone tissue?
1.cortical- compact solid, dense
2.trabecular- spongy and porus
all bones have both types
30
What are the charateristics of trabecular bone
1. forms 3 branching lattices along lines of stress
2. high metabolic turnover compared to cortical
3. very strong for resisting compression
31
What are the charateristics of cortical bone?
1. makes up 80% fo the skeletal mass
2. structural unit is the osteon
3. typically made of successive layers of bone or lamelle
4. most responsive to bending or torsion
32
How does bone mass change in a person's life time?
typically increases until 21-40 years of age then begins to decrease by about 0.5% per year
33
What is the process for bone homeostasis?
1. it is a continuous process of new bone on the surface and breakdown of bone along the marrow cavity
2. happens o the macro and micro surfaces
3. osteoblast can communicate chemically with the osteocyte
4. osteocyte precieves stress adn strain and communicatess that information to the osteoblast
5. the process of re-absorption is linked to bone formation
6. osteoblast are in direct communication with osteocytes and recieve a majority of chemical and hormonal signals
34
What is the optimal stimulus for repair for bone?
1. biomechanical energy in the line of stress
2. below the level of pain and without shearing
3. force in excess of normal daily loads
35
How do electric potentials effect bone healing?
Weight bearing leads to stress generated electric potentials that help orient collagen fibers in closely packed bundles
(+) regeneration- distraction or pulling
(-) re-absorption or remodeling- compression side
36
How does the anti gravity environment affect bone density?
1. bone diameter growth is 40% slower in space
2. .5-2% total bone loss is permanent
3. bone loss is greatest at muscle insersions
37
What are some examples of dysfunctional bone remodeling?
1. pseudoarthorosis from stimulation of chondrocytes from shear forces
2. osteophytes- joint shear triggers bone formation for stabilization
3. bone spur- ligament attempts to stabilize
4. joint degeneration
38
Bone failure occurs with how much cyclic loading?
5K to 10K microstrains or with 106 cycles at 3K in uniaxial tension
-strenous activity only appears to cause 2K microstrains
39
How much strain is required to cause a stress fracture of the tibia? why is this important to know?
1. run 1K miles without any bone remodeling
| 2. stress fractures are not likely caused by repeatative stress alone
40
How does muscle fatigue contribute to bone loading?
1. loss of eccentric control increase one loading rates
| 2. loss of muscle control causes loss of mechanic for force attenuation
41
Base on Morshed literature review in 2008 what are the most common clinical criteria to assess proper fracture union?
1. absence of pain or tenderness at the fracture site in with weight bearing
2. abscence of pain or tenderness with palpation
3. the ability to bear weight
42
What type of loading maximizes bone density in children? and adults?
1. normal youth activity is 3x body weight but brief box jumping programs are up to 9x body weight and more effective in influencing bone density
2. walking does not provid sufficient stress, but progressive aerobics with steps and light jumping does
43
How does vibration influence bone density?
1. low magnitude high frequency vibration effects bone morphology
2. Studies have shown a 4.3% greater benefit form vibration progams of 6 one minute bout of vibration compared to 55 mintues of walking
44
How can endurance training for bone density be counter productive??
1. increased bat burning further reduces adipose stores that are necessary to conver porgesterone to estrogen
2. therefore more aggressive loading has show more promise form improving bone density
45
How doe you does resistence exercises for improving bone?
1. start at 0-60% of 1 RM to establish coordination for the exercise
2. progress to 80-90% of a 1 RM (8-12 reps) with 2 minute rest breaks
46
What dietary considerations should you make for bone health
1. increase Ca
2. vitamin D
3. avoid soft drinks and caffeine
