Lecture 44 Biology of Fracture Healing Flashcards
(74 cards)
1
Q
What are the four phases of skeletal development?
A
- Migration (of preskeletal cells to sites of future skeltogenesis)
- Epithelial- mesenchymal interaction
- condensation (of mesenchymal cells)
- Differentiation (into odontoblasts and chondrocytes)
2
Q
What are the two types of bone formation?
A
- Endochondral bone formation
- Intramembranous Bone Formation
3
Q
What is endochondral bone formation?
A
- Indirect (mesenchyme forms cartilage template first which is later replaced by bone)
- occurs in most bones in the skeleton esp. bones that bear weight and have joints
- also occurs during fracture repair
4
Q
What is intramembranous bone formation?
A
- direct transformation of mesenchymal cells to osteoblasts (no cartilage intermediate)
- restricted to cranial vault, some facial bones, parts of the mandible and clavicle
- contributes to fracture repair
5
Q
When do secondary ossification centers appear?
A
- around the time of birth
6
Q
What produces VEGF and what does it do?
A
- hypertrophic chondrocytes
- attracts blood vessels that invade the cartilage model
7
Q
When does growth plate fusion occur?
A
- around age 14-20 in humans
8
Q
How do the flat bones of the skull form?
A
- intramembranous bone formation
9
Q
What is the process of intramembranous bone formation?
A
- mesenchymal cells condense to produce osteoblasts which deposit osteoid (unmineralized) bone matrix
- osteoid matrix calcifies/osteoblasts become arrangled along calcified region of the matrix
- some osteoblasts trapped in bone matrix- become osteocytes
- NO CARTILAGE MODEL PRECEDING THE BONE
10
Q
What is the immature bone that is produced first?
A
- woven bone aka primary bone (immature)
11
Q
Woven bone is produced when osteoblasts need to form bone rapidly in situations like:
A
- embryonic development
- fracture healing
- disease states (paget’s disease)
12
Q
What is immature woven bone remodeled and replaced with?
A
- Lamellar bone (aka secondary bone) (mature)
13
Q
What are the features of woven bone?
A
- disorganized structure
- collagen fibrils in random orientation (lower birefringence w/polarized light)
- increased cell density
- reduced mineral content
14
Q
What are the features of lamellar bone?
A
- highly organized
- bone lamellae concentrically arranged around central canal containing blood vessels and nerves
- collagen fibrils in parallel orientation (more birefringence w/polarized light)
- mechanically stronger
15
Q
What can lamellar bone be further classified into?
A
- Compact
(cortical/haversian) - cancellous (spongy/trabecular)
16
Q
What type of bone is the bone marrow located in?
A
- cancellous bone
17
Q
What is skeletal healing essential for?
A
- resolution of orthopedic trauma that has caused fractures
- healing of corrective surgeries where bony injuries are created intentionally to correct bone deformities
- bone regeneration in oral surgical procedures/tooth extractions
18
Q
Fracture healing requires coordinated activity of several cell types:
A
- inflammatory cells
- chondroprogenitors/chondrocytes
- osteoprogenitors
- osteoclasts
- vascular cells
19
Q
What is the timeline of the inflammatory phase?
A
- peaks at 48 hrs and is diminished by 1 week
20
Q
What is the timeline of the reparative phase?
A
- activated within a few days and persists for up to 2-3 months
21
Q
What is the timeline of the remodeling phase?
A
- can continue for several years
22
Q
John hunter (1935) described the 4 stages of fracture repair as what?
A
- Formation of vascular hematoma
- Formation of (fibrocartilage) callus
- Tissue metaplasia- callus replaced by mineralized bone
- Bone remodeling and turnover
23
Q
What are the cytokines that the hematoma releases in the hematoma formation/inflammation phase?
A
- Tumor necrosis factor a (TNF-a)
- Interleukins (IL-1, -6, -11 and -18)
24
Q
What do the cytokines do?
A
- lead to recruitment/infiltration of inflammatory cells
25
What do the inflammatory cells do?
- release more inflammatory cytokines and recruit mesenchymal stem cells (MSC)/osteogenic precursors to fracture site
26
What happens during formation of fibrocartilagenous callus?
- MSC/connective tissue stem cells/blood vessels invade hematoma
- hematoma degenerates/phagocytes clear debris
- fibrous connective tissue matrix laid down by fibroblasts (granulation tissue)
- some MSC differentiate towards chondrogenic/osteogenic lineages
- at borken ends of bones where blood supply was disrupted hypoxia/tissue necrosis occurs
- in hypoxic regions MSC differentiate into chondrocytes
- intramembranous bone may form in subperiosteal sites where vascular supply is intact= hard (external) callus
27
What initiates endochondral bone formation?
- MSC differentiating into chondrocytes
28
What are the cell sources of osteogenic precursors?
- periosteum
- muscle
- bone marrow
29
What are the cell types of osteogenic precursors?
- mesenchymal stem cell (MSC)
- pericyte
- muscle satellite cell
30
How does the bony callus form?
- intramembranous bone (formed where vascular supply is intact) contributes to bony callus
- **** cartilage under goes endochondral ossification
31
What is endochrondral ossification?
- hypertrophy -->calcification of cartilage --> removal by osteoclasts --> replacement with bone
32
When is the fracture considered healed?
- when bone stability is restored by bone tissue completely bridging the original fracture
"clinical union"
33
What is the timeline for formation of bony callus?
- several weeks up to 2-3 months
34
how long does formation of fibrocartilagenous callus take?
- 1 week
35
How quickly does the hematoma form at the fracture site?
- 0-2 days
36
T/F the remodeling phase of bone repair can last years
- T
37
What is the process of remodeling?
- osteoclasts resorb woven bone in fracture callous then osteoblasts lay down new lamellar bone (haversian) which is mechanically stronger
38
T/F same sequence of fracture healing events occurs for healing of alveolar bone in tooth socket after tooth extraction
- T
39
Early phase of fracture healing include?
- Formation of hematoma
- recruitment of MSC
- cell proliferation
- initiation of chondrogenesis/osteogenesis
- vascular ingrowth/angiogenesis
40
Signaling molecules important in the fracture healing?
- pro- inflammatory cytokines
- TGFB superfamily members
- angiogenic factors
41
Who secretes the pro-inflammatory cytokines?
- macrophages, mesenchymal cells, inflammatory cells
42
Mice null for TNFa receptor show?
- impaired fracture healing
43
What are the pro-inflammatory cytokines?
- Tumor necrosis factor (TNFa)
| - Interluekins
44
What is the function of the pro-inflammatory cytokines?
- recruit other inflammatory cells/promote MSC recruitment
- induce apoptosis of hypertrophic chondrocytes
- recruit fibrogenic cells/promote formation of granulation tissue/ECM formation
- can promote osteoclast formation
45
What is the TGFB superfamily members job in fracture healing?
- promote ECM synthesis and assembly/initiation of callus formation
- promote osteogenic differentiation
- GDF-8 has a role in cell proliferation
46
What are the TGFB superfamily members involved in fracture healing?
- Transforming growth factor B (TGFB)
- Bone morphogenetic protein-2 BMP2 (also 5,6)
- Growth and differentiation factor (GDF-8)
47
What produces the TGFB superfamily members?
- produced by hematoma (platelets)/granulation tissue/differentiating MSC/periosteal callus
48
What are the angiogenic factors?
- VEGF- Vascular endothelial growth factor
- PDGF - platelet derived growth factor
- ANGPT- Angiopoietin
49
What do the angiogenic factors do?
- promote vascular ingrowth vessels in periostum (brings oxygen/osteogenic precursors [pericytes])
50
VEGF:
- promotes chemotaxis of osteoprogenitors
- up regulated in regions of hypoxia (under control of transcription factor HIF1a)
- HIF1a overexpressing mice show enhanced bone regeneration
51
Why is vascularization important? (why are angiogenic factors important)
- critical for fracture repair/bone formation
| - brings in calcium and phosphate for mineralization
52
What dictates why type of healing will occur?
- fracture stability (mechanical environment)
53
If strain <2% _______ bone healing will occur
- intramembranous
54
If strain is >2% <10% _________
- endrochondral bone healing will occur
55
High strain > 15% promotes
- fibrous tissue
56
Bone repair could be enhanced by:
- improving vascularization
- attracting progenitor cells
- accelerating bone formation
- accelerating remodeling
57
How have BMPS been used to help grow bone?
- appear to be effective alternative to autologous bone graft for repair of fracture non union/open tibial fractures
- controversy about clinical use due to cost effectiveness and potential safety drawbacks
58
Why have platelet rich plasma been tried for osteogenesis?
- contains multiple growth factors
- evaluated in preclinical and clinical trials
- appears effective in promoting bone heling
59
Why is FGFs being used for osteogenesis?
- FGF signaling is important in skeletal healing
- FGF2 shown to enhance fracture healing in various in vivo experiments dating back to 1990s
- continued elevation of FGF2 may impair mineralization so timing of treatment needs to be optimized
60
What are the cell based therapies for bone growth/healing?
- autologous bone marrow - collected from iliac crest/injected into non-union site (increases # of progenitor cells)
- purified stem cell sources (MSC- mesenchymal stem cells, EPC- endothelial progenitor cells)
61
Other approaches to bone healing treatments?
- anti resorptives (bisphosphonates, denosumab)
- bone anabolic agents (sclerostinn abs, teriparitide)
- gene therapy (still experimental)
62
Sclerostin
- inhibitor of Wnt/b catenin signaling
| - anitbodies to sclerostin being developed as anabolic treatment for osteoporosis
63
_____ showed sclerostin abs enhaced bone regeneration in rat model of periodontitis
- Taut
64
_______ a patient with a BMD> 2.5 standard deviations below average for a young healthy male or female
- osteoporosis definition
65
Is osteoporosis associated with menopause and or aging?
- Yes
66
Osteoporosis facts
- causes > 8.9 million fractures annually
- worldwide 1 in 3 women and 1 in 5 men over 50 will experience osteoporotic fractures
- Hip fractures with mortality rates of up to 20-24% in first year after fracture. greater risk of dying may persist for at least 5 years
67
What are the anti resorptive medecines? (prevent further bone degredation)
- amino bishosphonates
- hormone replacement therapy
- selective estrogen receptor modulators (SERMS) e.g raloxifene
- Denosumab
- Cathepsin K inhibitors
68
What are the anabolic agents?
- PTH 1-84, Teriparitide (PTH 1-34)
| - Anti sclerostin antibodies
69
_____ preferentially bind to hydroxyapatite. Inhibit activity of osteoclasts by inhibiting mevalonate pathway important for prenylation of GTPases important for vesicular trafficking
-Amino bisphosphonates
70
__________ restores hormone levels following menopause
- Hormone replacement therapy
71
Serms
- work as a partial antagonists of estrogen receptor but mechanism of action not fullly understood
72
Denosumab
- antibody against Rankl (inhibits osteocalst formation)
| - comparable efficacy as bishosphonates
73
Cathepsin K inhibitors
- currently in clinical trails (inhibit gone degrading enzyme Cathepsin K)
74
PTH 1-84, Teriparitide (PTH 1-34)
- intermitten administration of teriparatide stimulates bone formation, partly through inhibition of sclerostin
