Bone and Cartilage

Question 1

Intramembranous Ossification

Answer 1

bones of skull, face, and clavicle

1. Ossification center forms in fibrous connective tissue and the mesenchymal cells mature into osteoblasts
2. Osteoblasts secrete osteoid which is mineralized and osteoblasts become osteocytes
3. Osteoid is about embryonic blood vessels and form trabeculae with periosteum surrounding it
4. Bony collar forms and red marrow appears
5. Appositional growth and remodeling will change the bone with age

Question 2

Endochondrial Ossification

Answer 2

bone replaces cartilage model

1. Formation of bony collar around cartilage
2. Cavitation of hyaline cartilage within the cartilage model
3. Chondrocytes enter forming bone and hypertrophy and calcify (not ossify like in intramembranous)
4. Chrondrogenic cells become osteoprogenitor cells which then become osteoblasts to secrete osteoid
5. Invasion of cavities by periosteal bud and spongy bone forms because bony collar prevents nutrients from supplying chondrocytes, which die and form spongy bone
6. Formation of medullary cavity as ossification continues and secondary ossification center appears in epiphysis (no bony collar at epiphysis)
7. Ossification of epiphysis with growth plate still consisting of hyaline cartilage

Question 3

Calcium is Required for Several Processes

Answer 3

Cell division
Blood coagulation
Gland and nerve cell secretions 
Nerve impulse transmission
Muscle contraction

Question 4

Role of RANK L

Answer 4

Osteoclasts have RANK ligand receptors and when RANK binds it promotes bone resorption
Osteoblasts secrete OPG, which bind to RANK ligands to inhibit binding onto RANK L receptors on osteoclasts in order to promote bone formation
When bone formation occurs, bone lining cells move and expose the bone and become osteoblasts and begin expressing RANK ligand, which binds to RANK on osteoclast precursors
RANK L binds to RANK to allow for bone resorption
Then osteoblasts form the bone for remodeling
OPG blocks RANK L binding to RANK receptors on osteoclasts and estrogen limits the amount RANK L expression by osteoclasts = regulators

Question 5

Wolff’s Law

Answer 5

Wolff’s law – A bone grows/remodels in response to forces or demands placed upon it

Long bones are thickest midway along the shaft (where bending stress is greatest)

Curved bones are thickest where they are most likely to buckle

Bony trabeculae form along lines of stress

Large, bony projections occur where active, robust muscles attach

Question 6

Basic Multicellular Unit (BMU)

Answer 6

Head of osteoclasts
Tail of capillaries, progenitor cells, and osteoblasts

Releasing H+ and breaking down mineral components and other organic components so that remodeling can occur
Nerves and osteoprogenitor cells to turn into osteoblasts that secrete matrix then turn into osteocytes
1-4 transverse cuts show the layers and cells that are present during this process

Question 7

Remodeling Time

Answer 7

I & II = Resorption of bone; 42 days
III to V = Formation of new bone; 22 days
IV = Mineralization lag time; 130 days

Around 192 days

Question 8

Microfractures

Answer 8

Small cracks within bones - occur all the time

Can accumulate faster than normal remodeling can take care of them = Stress fracture

Question 9

Nondisplaced vs. Displaced

Answer 9

Nondisplaced – bone ends retain normal position

Displaced – bone ends are out of normal alignment

Question 10

Complete vs. Incomplete

Answer 10

Complete – bone is broken all the way through

Incomplete – bone is not broken all the way through

Question 11

Linear vs. Transverse

Answer 11

Linear/longitudinal –fracture is parallel to long axis of bone

Transverse –fracture is perpendicular to long axis of bone

Question 12

Simple vs. Compound

Answer 12

Simple (closed) – bone ends do not penetrate the skin

Compound (open) – bone ends penetrate the skin

Question 13

Name a common bone broken in those under 75 years of age?

Answer 13

Distal Radius = Colle’s fracture

Question 14

Communited vs. Compression

Answer 14

Communited: brittle bones mostly in elderly people or osteogenesis imperfecta; bone fragments into three or more pieces

Compression: bones are crushed during a fall for example; bone is crushed

Question 15

Greenstick vs. Depressed

Answer 15

Greenstick: mostly in children because still isn’t completely calcified; bone breaks incompletely

Depressed: broken bone portion is pressed inward; typical of skull fracture

Question 16

Spiral vs. Epiphyseal

Answer 16

Spiral: most common in sports injuries; ragged break occurs when excessive twisting forces are applied

Epiphyseal: cartilage cells are dying and area is weaker; epiphysis separates from diaphysis along epiphyseal plate

Question 17

Salter Harris Fractures

Answer 17

30% are associated with growth disturbance
2% result in disability
Mainly lower extremity due to weight bearing

Epiphyseal plate can be affected/fractured

Question 18

Salter Harris I

Answer 18

Point tenderness over physeal plate
X-rays initially often appear normal
One week later, films may show sclerosis relating to new bone reparative process
Good prognosis

Question 19

Salter Harris II

Answer 19

Fracture through growth plate and metaphysis
Most common SH fracture
May produce long bone shortening
Most significant if in the lower extremity

Question 20

Salter Harris III

Answer 20

Fracture through epiphysis and growth plate
Notice physeal widening
Prone to chronic disability because of articular involvement

Question 21

Salter Harris IV

Answer 21

Fracture affects epiphysis, metaphysis and growth plate

Prone to chronic disability because of articular involvement

Question 22

Salter Harris V

Answer 22

Compression or crush injury of growth plate
X-rays initially normal
Early closure of the growth plate
Poorer prognosis due to shortening and/or angulation

Question 23

Bone Repair Steps

Answer 23

1. Torn blood vessels hemorrhage, a mass of clotted blood (hematoma) forms at fracture site, site becomes painful, swollen, and inflamed
2. A few days after the fracture, a soft callus of highly vascular collagenous tissue (granulation tissue) is formed and capillaries grow into tissue and debris is cleared away by phagocytic cells
3. New bony trabeculae appear within fibro-cartilaginous callus; fibrocartilaginous callus is converted into a bony callus; bony callus appears 3-4 weeks post-injury, continues until bones are firmly united 2-3 months later
4. Bone Remodeling: Excess bone is removed along the exterior of the shaft and in the medullary cavity; compact bone is deposited, reconstructing walls of the shaft

Question 24

Fibrocartilaginous Callus Formation

Answer 24

Osteoblasts and fibroblasts migrate to the fracture and begin reconstructing bone
Fibroblasts secrete collagen fibers that connect broken bone ends
Osteoblasts begin forming spongy bone
Osteoblasts furthest from capillaries secrete an externally bulging cartilaginous matrix that later calcifies

Question 25

Bone Repair Process

Answer 25

The bone repair process is a combination of intramembranous and endochondral bone formation, coordinated with remodeling and resorption

Question 26

Cancellous Bone

Answer 26

(a.k.a. spongy, trabecular, medullary)
lined by endosteum with osteoprogenitor cells, osteoblasts and osteoclasts present
Haversian systems generally absent; osteocytes communicate with blood sinusoids in the marrow cavity via canaliculi
Found in medulla of membrane bones, ends of long bones, and filling center of irregular bones

Question 27

Woven Bone

Answer 27

(a.k.a. primary, immature)
No lamellar organization
First bone laid down, and also found during fracture healing and at some tendon attachments
Attach tendon to bone

Question 28

Sharpey’s Fibers

Answer 28

Attach other tissues to bone
Muscle to bone
Play large role in attaching the periodontal ligament and bone of tooth

Question 29

Hyaline Cartilage

Answer 29

Hyaline (clear; blueish in gross section)
Embryonic skeleton & portions of adult
Respiratory tree, nasal region, costal cartilages
Articular cartilage
Type II collagen
Diffuse fibers throughout matrix; amorphous
Normally surrounded by perichondrium
Outer fibrous layer
Inner chondrogenic layer
Exception - articular cartilage

Question 30

Repair of Cartilage

Answer 30

Not highly vascularized therefore hard to repair
Not innervated
Avascular
No lymphatic drainage
Receives nutrients by diffusion via surrounding connective tissue

Question 31

Elastic Cartilage

Answer 31

Elastic (yellowish in gross section)
Places needing structure and flexibility
Epiglottis, larynx, pinna (cauliflower ear if damaged), eustacian tube
Type II collagen + elastic fibers
Diffuse fibers throughout matrix; amorphous
Surrounded by perichondrium

Question 32

Fibrocartilage

Answer 32

Resists both compressive and shear forces
Pubic symphysis, menisci, intervertebral disks
Type I collagen
Thick bundles very obvious in matrix
Fewer chondrocytes than others
No perichondrium
Common injury = herniated disc

Question 33

Bone Lining Cells

Answer 33

Line the surface of bone (very spindle shaped)
Derived from osteoblast precursors or osteoblasts
Communicate with osteocytes via canaliculi
Serve as an ion barrier and appear to facilitate signaling for bone resorption and remodeling