Bone Healing and Biomechanics

Question 1

describe bone structure, vascularity, and function

Answer 1

4 functions: support, motion (muscle and ligament attachment), storage (fat, marrow, mineral), and regeneration

structure:
-mineral component (65%): 95% hydroxyappetite, provides compressive strength
-organic: 2% cells and 98% matrix; provides tensile strength; TYPE I COLLAGEN

Question 2

what are the 5 bone shapes?

Answer 2

1. long: femur, humerus
2. short: carpal, tarsal
3. flat: scapula, some of skull, pelvis
4. sesamoid: facilitate change in direction of tensile forces
5. irregular

Question 3

what are the 2 jobs, 2 materials, and 2 types of bones?

Answer 3

2 jobs: compression and tension
2 materials: collagen and mineral
2 types: cortical/compact and trabecular/cancellous/spongey

Question 4

what are the 3 separate regions of long bones? describe inner and outer regions too

Answer 4

epiphysis, diaphysis (the shaft), and metaphysis

outer cortical bone: lamellar with haversion systems
inner medulla cancellous bone (more metabolic active)

Question 5

describe the structure of cortical/compact bone

Answer 5

lamellar, forms circumferentially; give strength in the longitudinal plane

Question 6

describe the structure of trabecular bones

Answer 6

seen more at bone ends, is more spongey and can distribute forces in more ways than longitudinal, can withstand more types of force than cortical

Question 7

describe the bone blood supply (4)

Answer 7

1. subchondral ateriole plexus from periostal and endosteal vasculature; forms subchondral capillaries: counter current exchange, pressure and flow gradient, prone to infarct and sepsis
2. nutrient artery: endosteal/marrow blood supply, enters middle of bone anlage; can be mistaken for a fracture!
3. periosteal arterioles: supply 1/3 of the outer cortex (if damaged can lead to necrosis); enter bones through Volkmann’s canals
4. metaphyseal and epiphyseal arteries: proximal and distal

Question 8

describe the periosteum

Answer 8

1. long bones are wrapped in periosteum except at joint surfaces
2. thin layer of osteogenic and fibroblastic cells; nerve and microvasculature network
3. 2 layers:
   -outer fibrous
   -inner layer with osteoblastic potential
4. supplies outer 1/3 cortical bone with blood (inner 2/3 from endosteum

Question 9

identify cell types within bone and how they contribute to bone healing

Answer 9

osteocytes: function is maintenance; terminally differentiated osetoblasts, cell body trapped in formed bone (lacunae), cytoplasmic processes extend through lacunae, communicate with neighboring cells, regulate osteoblasts and osteoclast activity, work via mechanosensors

osteoblasts: function is growth, deposit osteoid (type I collagen, ground substance/matrix), initiate mineralization, and mediate osteoclast activity

osteoclasts: function is resorption; multinucleated phagocytic cells utilize hydrogen pumps to acidify and dissolve mineral; enzymes promote degradation of mineral

Question 10

describe the composition of the extracellular matrix

Answer 10

1. TYPE I COLLAGEN makes up 95%!!! (will be asked this!!)
2. osteoid (non-collag protein)
3. hydroxyappetite

Question 11

describe bone modeling versus bone remodeling

Answer 11

bone modeling: new growth of juveniles during development; occurs via intramembranous ossification (no cartilage template) or endochondral ossification (cartilage template)

bone remodeling: resorption and formation occur on the same surface; maintenance of bone mass throughout life; 2 types
-cortical remodeling: a cutting cone within the cortex or on surface where osteoclasts tunnel through bone, followed by osteoblasts which lay down osteoid and form a circular osteon, haversian canal is filled with blood vessels and nerves
-cancellous remodeling: osteoclasts dig a trench rather than a tunnel, osteoclasts resorb bone, followed by osteoblasts which lay down osteoid (hemiosteons); no need for blood vessels/nerves within the osteons because they are directly adjacent to the marrow cavity

Question 12

describe the concept of load/deformation/stress/strain

Answer 12

force applied to bone = load

stress is force over an area
sheer stress is the load applied parallel to a surface
tension is positive normal stress
compression is negative normal stress

the bone lives in either elastic energy or plastic energy when stress is applied

physiologic strain within the elastic energy region can lead to changes that can be repaired, but if the strain passes the yield point and enters the plastic energy side, it is now pathologic strain and can continue past what bone can repair and lead to failure

Question 13

rank the forces under which bone is strongest; compare and contrast strength of cortical bone versus trabecular bone

Answer 13

1. compression
2. shear
3. tension

cortical bone is more brittle than trabecular; so it fails at a lower strain but higher stress; trabecular bone can store more energy prior to failure

Question 14

what are the 4 modes of failure, and what does each result in?

Answer 14

1. tension: results in transverse fractures
2. compression: results in oblique fractures
3. bending: results in butterfly fractures
4. torsion: results in spiral fractures

Question 15

describe response of bone to load (2)

Answer 15

1. affected by rate of application of the load, bone is stiffer when loaded faster as it is storing more energy
2. cyclic loading: can cause failure secondary to fatigue over the course of progressive microdamage

Question 16

describe wolff’s law and limitations

Answer 16

law: bones adapt to the load they experiences; stress increases bone remodeling and bone becomes stronger, little or no stress can cause bone to lose density and become weaker

limitations (3):

1. excessive/improper stress: may damage bone (ex. excessive activity or weight bearing too soon after a fracture can result in poor healing or even re-fracture, or lead to fracture in the first place)
2. age and health:
   -young/healthy: bones adapt more readily to stress; old/unhealthy: bones may not respond as efficiently to stress
3. different bones: cancellous bone adapts faster while cortical bone adapts slower

Question 17

describe bone response to stress/strain (3)

Answer 17

1. strain leads to new bone formation
2. woven bone is laid down first; not as strong as compact bone; can fatigue and fracture
3. repetitive stress leads to microfractures and the change in hardness creates a stress riser (area of weakness that concentrates stress and makes fractures more likely)

Question 18

how does strain lead to bone adaptation?

Answer 18

1. cell desensitization occurs in response to strain, kicking off a period of optimization of bone, as new bone needs time to remodel compact bone (4-12 hours to recover), and active rest
2. adaptation around this looks like several shorter intervals of exercise per day rather than 1 sustained session when bone is recovering and gradually increasing stress over 4 months

Question 19

describe bone healing

Answer 19

similar to bone formation! 2 forms

1. primary (direct): contact healing, no callus, induce by anatomic reduction and rigid fixation to minimize interfragmentary strain
2. secondary (indirect): due to instability or a gap between the fractured bone fragments
   -inflammatory phase: disrupted osteocytes release enzymes and destroy organic matrix, necrotic material induces inflammation, hematoma forms, soft callus is MAINLY TYPE III COLLAGEN (lower strength than type I), angiogenesis occurs from the blood supply in the surrounding tissues
   -remodeling phase: mineralized cartilage is replaced by woven bone (will be further remodeled into lamellar bone), osteoclasts remove woven bone with cutting cones; WOLFFS LAW of weight bearing says the concave surface will have increased osteoblastic activity and the convex surface will have increase osteoclastic activity

Question 20

describe the use of plates and screws to repair fractures (3)

Answer 20

1. ideal for preventing forces (compression, shearing, rotation, bending)
2. the construct/bone-plate complex is the weakest against bending forces, so place the plate on the tension side of the bone!
3. can be used for almost any type of fracture as long as the bone is big enough to have the screws engaging both cortices

Question 21

how do we evaluate clinical healing of a fracture? (5)

Answer 21

1. may not see the fracture on a radiograph if the fracture is SUPER fresh! but
2. sharp fragment margin of the fracture, once formed, will be seen for 1 week
3. a wider fracture gap will be seen 2-3 weeks post injury as necrotic bone is removed
4. a callus will form after 3 weeks
5. bony union when callus disappears, fracture line is obilterated, and cortical bridging occurs

Question 22

when are fractures at their weakest and why?

Answer 22

2 weeks post injury because the fraction gap is widest since necrotic bone has been removed and the callus has not formed yet!

Question 23

what are 7 complications that can delay union or lead to nonunion of a fracture?

Answer 23

1. infection
2. necrosis/subsequent tissue damage
3. fixation failure
4. laminitis (equine)
5. angular limb deformity
6. patient factors: advanced age, comorbidities, metabolic disease, nutritional deficiencies