Physiology of bone -Rega Flashcards
What forces is collagen strong in? Weak in?
Strong/stiff in tension (doesn’t deform easily)
weak in compression (longitudinal force)
What forces is Apatite strong in? weak in?
stiff in compression and tension
weak in tension–> susceptible to brittle fracture (fracture with no plastic deformation–> does not deform before fx)
What does anisotrophic mean? How does the structure of trabecular allow for this?
strength varies by force orientation
-trabeculae are oriented parallel to the ambient loss
What does the combination of collagen and apatite in bone allow for? How is bone more likely to fail?
- stiff and strong in tension and compression
- weaker and more commonly fails in tension than compression
- likely to undergo brittle fracture. collagen blunts small cracks
- anisotropic
What does it mean for something to be stiff? Strong? Brittle?
Stiff=resists deformation
strong=resists failure under high load
brittle=undergoes sudden failure with little deformation
What is the yield point?
What force does bone have the lowest yield point?
What is plastic deformation?
yield point: onset of material failure (1% for bone==> plastic deformation)
lowest yield point: shear stress (< compressive)
plastic deformation: deformation beyond the yield point
What is ultimate failure?
What force has the lowest ultimate failure point?
ultimate failure=structural failure. strain at fracture for bone=2%
tensile failure < compressive failure
Does trabecular or cortical bone remodel faster? What does this allow for?
trabecular is faster
-allows for the remodeling of bone in response to stress–> will see pathology of bone here first
How do osteoclasts eat bone?
acid (produced by proton pumps) and enzymes (collagenase)
What are the two ways that bone can respond to tension?
osteolytic or osteoproliferative response
pits or tubercles to allow for increase Surface area for mm attachment
*What are the 4 steps in the activation stage of bone remodeling?
What cells are important and what is taking place in each step?
- activated osteoblasts lining cells produce collagenase
- collagenase digests osteoid on bone surface, exposing mineralized matrix
- exposed bone matrix is chemotactic for osteoclasts
- osteoclasts are activated and migrated to exposed site
*What are the adverse side effects of bisphosphonates and other osteoclast inhibitors?
- hypocalcemia from a decrease in bone remodeling (increase parathyroid hormone as a compensatory reaction)
- osteonecrosis of the jaw (inability to heal after injury)
both due to decreased osteoclasts –> inability to heal and release calcium
What is often the first indicator of osteoporosis?
persistent pain after a fall –> MRI may show compression fx due to osteoporosis
What is osteoporosis?
Low overall bone mass and structural deficiency due to a mismatch between bone resorption and formation. normally mineralized
affects mineral + osteoid
What is osteopenia?
thin bones, caused by osteoporosis, bit D deficiency, hyperthyroidism, or hyperparathyroidism
*What causes the bowing of the legs seen in pts with Rickets?
What is the structural problem with bone in Rickets pts?
Deformation due to mechanical alteration of bones during weight bearing because of low bone mineralization
collagen is normal, but bone is mineral deficient
What is rickets? What is the adult form of Rickets called?
low bone mineralization due to vitamin D3 deficiency in children –> structural weakness and impaired growth
- deformed rib cage from soft bones
- widening of physis at the wrist
- dental defects: hypoplastic, yellowish teeth
Adult form = osteomalacia
- 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 & tibial bowing
- Osteopenia
Deformation of the long bones is due to deficiency of which of the following elements? Osteoid a. Type I collagen b. Type II collagen c. Hydroxyapatite [Ca10xHx(PO4)6OH2 ] d. Overall bone mass
Rickets.
deformation of long bones due to a deficiency of:
c. Hydroxyapatite [Ca10xHx(PO4)6OH2 ]
What are some causes of osteonecrosis?
- Inhibited remodeling (bisphosphonates)
- Circulatory disruption by various means (endocrine, sickle cell, substance abuse)
- Sepsis
**What is femoral buttressing? With what condition(s) is this commonly seen?
seen in osteoarthritis with accompanying osteopenia
-limited bone mass will cause an overgrowth of the cortex where it is loaded –> more cortical bone is placed in attempt to make up for the decrease in bone mass to help strengthen
large cortical bone (heavily mineralized) and coarse trabecular
What is the difference between how a stable and an unstable fracture heal?
stable fracture maintains vascular continuity–> little or no callus formation –> direct remodel
unstable fracture: callus formation and wedge of cartilage that plugs the gap between fragments
What are the stages of healing?
Soft callus formation: Blood vessel invasion of fibragen clot; connective tissue cells invade and replace clot with disorganised collagen mass and cartilage islands. 3 weeks. Won’t see on x-ray because not mineralized
Hard callus formation: Replacement of soft callus by woven bone. Resembles endochondral bone formation. Less stiff and strong than lamellar/Haversian bone. 2-4 months.
Remodeling: Commences immediately after woven bone formation. Can take years.
What allows for enhanced healing in bone fractures of juveniles?
periosteum hinge formation
periosteum is thicker in juveniles than in adults
Where are avulsion fractures commonly seen?
in areas of high tensile loading (ligament/tendon insertion or the femoral neck)
What type of force generally causes a spiral fracture?
torsional forces
high forces ex: auto accident
skiing injuries are worsened by closed-chain kinematics
What generally causes compression fractures? What kind of bone is this more common in?
great force or underlying bone pathology
common in cancellous bone (spongiosa–> highly trabecular)
*What can result from compression fractures through epiphyses? (slide 68–> be able to identify)
growth plate disruption –> can cause premature fusion of bone and deformities later in life
Will an open reduction always have a soft callus formation?
NO!
this is direct remodeling because the bone is aligned and vasculature is maintained
What is a major concern in adults with bone fractures?
fat embolism due to marrow entering circulation after a fracture or bone surgery
What does the use of surgical hardware leave the bone at risk for when healing?
localized osteoporosis due to relieved bone stress caused by the surgical hardware
What part of the bone supports weight?
epiphysis
When could lines of arrested growth be seen on X-rays? What do they look like?
Lines of arrested growth occur when growth starts and stops–> collagen crystals change their arrangement forming lines that transverse the bones
this can occur with chronic illness or abuse/neglect
How does the growth spurt vary between males and females?
females hit their growth spurt about 2 years earlier (11 yo) but it is not as large in magnitude as males
What is important to look for in a child’s growth reference?
that the slope is relatively the same
if weight gain»height, it is concerning
What kind of growth can be seen in kids with illnesses?
“Catch-up” growth where the kid will have a huge growth spurt when the stressor (illness) is removed
can rebound
What are some characteristics of pediatric bone?
- less stiff and strong
- less mineralized
- rapid remodeling and greater volume remodeled at any given time
- higher proportion of woven bone and cartilage
- better at impact absorption, greater plastic defamation
- susceptible to: greenstick fracture, plastic failure, growth plate and cartilage damage, avulsion fractures
What are some characteristics of geriatric bone?
- slightly less stiff and strong than young bone
- less bone mass (cortical bone porous and trabecular thinning)
- bone strength decreases by the square of bone density
- remodeling times can be double of the young adult but the resorption rate can double –> bone formation is slower
